diff --git a/autocleanup.go b/autocleanup.go
index 0b1e4c8..72add80 100644
--- a/autocleanup.go
+++ b/autocleanup.go
@@ -46,6 +46,12 @@ func NewBackgroundTask(name string, interval time.Duration, taskFunc func(), log
 	if len(wg) > 0 {
 		externalWG = wg[0]
 	}
+	// run() uses time.NewTicker(interval), which panics for d <= 0 inside
+	// the started goroutine (autocleanup.go:186), crashing the whole
+	// process on a misconfigured interval. Normalize here (R149).
+	if interval <= 0 {
+		interval = time.Minute
+	}
 	return &BackgroundTask{
 		name:       name,
 		interval:   interval,
@@ -88,8 +94,20 @@ func (bt *BackgroundTask) Start() {
 			return
 		}
 
-		// Reserve the task slot immediately when starting
-		registry.cb.OnTaskStart(bt.name)
+		// Reserve the task slot immediately when starting. OnTaskStart
+		// is the authoritative concurrency gate (R165): it returns false
+		// if maxConcurrent is already reached, in which case this task
+		// must not launch run() (its slot was not taken).
+		if !registry.cb.OnTaskStart(bt.name) {
+			if bt.logger != nil {
+				bt.logger.Debugf("Cannot start task %s: concurrency limit reached, slot not acquired", bt.name)
+			}
+			// Close doneChan since the task won't run
+			if atomic.CompareAndSwapInt32(&bt.doneClosed, 0, 1) {
+				close(bt.doneChan)
+			}
+			return
+		}
 
 		atomic.StoreInt32(&bt.started, 1)
 		bt.internalWG.Add(1)
@@ -114,6 +132,14 @@ func (bt *BackgroundTask) Stop() {
 			if atomic.CompareAndSwapInt32(&bt.doneClosed, 0, 1) {
 				close(bt.doneChan)
 			}
+			// Also close stopChan (R117): a concurrent Start may be in-flight
+			// and about to launch run(). Signaling stopChan lets it exit at
+			// the next stop check instead of leaking the goroutine and its
+			// circuit-breaker slot (taken by OnTaskStart but never released).
+			func() {
+				defer func() { _ = recover() }() // safe if already closed
+				close(bt.stopChan)
+			}()
 			return
 		}
 
@@ -134,15 +160,18 @@ func (bt *BackgroundTask) Stop() {
 		// This avoids the race condition with WaitGroup
 		select {
 		case <-bt.doneChan:
-			// Normal completion
+			// Normal completion: the goroutine has returned, so it is safe
+			// to wait on the WaitGroup.
+			bt.internalWG.Wait()
 		case <-time.After(5 * time.Second):
+			// The task goroutine is still running (its taskFunc is blocked or
+			// long-lived). Do NOT wait on the WaitGroup here — that would
+			// block Stop() (and shutdown) for as long as the task runs.
+			// Return now; the goroutine is reaped when it completes.
 			if bt.logger != nil {
 				bt.logger.Errorf("Timeout waiting for background task %s to stop", bt.name)
 			}
 		}
-
-		// Wait for the internal WaitGroup synchronously after doneChan signals
-		bt.internalWG.Wait()
 	})
 }
 
@@ -345,9 +374,30 @@ func (cb *TaskCircuitBreaker) CanCreateTask(taskName string) error {
 	}
 }
 
-// OnTaskStart records a task starting execution
-func (cb *TaskCircuitBreaker) OnTaskStart(taskName string) {
-	atomic.AddInt32(&cb.concurrentTasks, 1)
+// OnTaskStart records a task starting execution. It atomically reserves a
+// concurrency slot and returns whether one was available. The reservation
+// is fundamental (R165): CanCreateTask's limit check is only advisory —
+// it is also called by RegisterTask without starting the task — so a
+// check-then-act TOCTOU meant two concurrently starting tasks could both
+// read current < limit and then both start, letting live tasks exceed
+// maxConcurrent and defeating the DoS backstop. OnTaskStart is the
+// single commit point, so it enforces maxConcurrent here via a CAS loop:
+// only maxConcurrent tasks can hold slots at once. BackgroundTask.Start
+// aborts (without launching run) when this returns false.
+func (cb *TaskCircuitBreaker) OnTaskStart(taskName string) bool {
+	max := atomic.LoadInt32(&cb.maxConcurrent)
+	for {
+		current := atomic.LoadInt32(&cb.concurrentTasks)
+		if current >= max {
+			if cb.logger != nil {
+				cb.logger.Debug("Concurrent task limit reached, refusing slot for: %s", taskName)
+			}
+			return false
+		}
+		if atomic.CompareAndSwapInt32(&cb.concurrentTasks, current, current+1) {
+			break
+		}
+	}
 	cb.tasksMu.Lock()
 	cb.activeTasks[taskName] = struct{}{}
 	cb.tasksMu.Unlock()
@@ -358,6 +408,7 @@ func (cb *TaskCircuitBreaker) OnTaskStart(taskName string) {
 		cb.logger.Debug("Task started, concurrent count: %d, task: %s",
 			atomic.LoadInt32(&cb.concurrentTasks), taskName)
 	}
+	return true
 }
 
 // OnTaskComplete records a task completing execution
@@ -751,9 +802,14 @@ func (mm *TaskMemoryMonitor) checkForMemoryIssues(stats TaskMemoryStats) {
 	historyLen := len(mm.statsHistory)
 	if historyLen >= 2 {
 		prev := mm.statsHistory[historyLen-2]
-		heapGrowth := float64(stats.HeapAlloc) / float64(prev.HeapAlloc)
-		if heapGrowth > 2.0 && stats.NumGC == prev.NumGC {
-			mm.logger.Infof("Potential memory leak: heap grew %.2fx without GC", heapGrowth)
+		// Guard against div-by-zero (prev.HeapAlloc == 0) which would make
+		// heapGrowth == +Inf and fire a spurious "memory leak" alert for a
+		// first-ever measurement (R103).
+		if prev.HeapAlloc > 0 {
+			heapGrowth := float64(stats.HeapAlloc) / float64(prev.HeapAlloc)
+			if heapGrowth > 2.0 && stats.NumGC == prev.NumGC {
+				mm.logger.Infof("Potential memory leak: heap grew %.2fx without GC", heapGrowth)
+			}
 		}
 	}
 	mm.mu.RUnlock()
diff --git a/error_recovery.go b/error_recovery.go
index 832b61b..33c392a 100644
--- a/error_recovery.go
+++ b/error_recovery.go
@@ -243,6 +243,14 @@ func (cb *CircuitBreaker) ExecuteWithContext(ctx context.Context, fn func() erro
 	cb.RecordRequest()
 
 	if !cb.allowRequest() {
+		// A request rejected while the circuit is open is an admission
+		// outcome, not a never-started request: count it as a failure so
+		// GetBaseMetrics' success_rate (successes/total_requests) and
+		// total_failures reflect actual admission, matching the
+		// internal/recovery circuit breaker. Previously open-rejects only
+		// incremented total_requests, understating failures and
+		// deflating the reported success rate (R180).
+		cb.RecordFailure()
 		return fmt.Errorf("circuit breaker is open")
 	}
 
@@ -362,7 +370,24 @@ func (cb *CircuitBreaker) Reset() {
 // IsAvailable returns whether the circuit breaker is currently allowing requests.
 // This provides a quick way to check if the service is available.
 func (cb *CircuitBreaker) IsAvailable() bool {
-	return cb.allowRequest()
+	cb.mutex.Lock()
+	defer cb.mutex.Unlock()
+
+	switch cb.state {
+	case CircuitBreakerClosed:
+		return true
+	case CircuitBreakerHalfOpen:
+		return true
+	case CircuitBreakerOpen:
+		// Read-only availability probe: report that a request would be
+		// admitted only after the open timeout has elapsed, but do NOT
+		// transition to half-open — IsAvailable must not take a permit or
+		// mutate circuit state. Real traffic admission is gated by
+		// allowRequest (triggerRequest) which performs the transition.
+		return time.Since(cb.lastFailureTime) > cb.timeout
+	default:
+		return false
+	}
 }
 
 // GetMetrics returns comprehensive metrics about the circuit breaker.
@@ -607,6 +632,18 @@ func (re *RetryExecutor) isRetryableError(err error) bool {
 		return false
 	}
 
+	// A terminal HTTP error (a real status that is neither 5xx nor 429)
+	// must be classified by its status BEFORE any message-substring scan.
+	// Otherwise a 4xx whose provider message merely contains a
+	// retryable-looking word — e.g. "certificate"/"tls" (isCertificateError),
+	// "EOF" (isEOFError), or a generic "timeout" — would be reclassified
+	// retryable and retried to MaxAttempts on a permanent error, repeating
+	// the failing request (R123, R157).
+	var statusHTTP *HTTPError
+	if errors.As(err, &statusHTTP) && statusHTTP.StatusCode != 0 && statusHTTP.StatusCode < 500 && statusHTTP.StatusCode != 429 {
+		return false
+	}
+
 	// Check for Traefik default certificate error (startup race condition)
 	// See: https://github.com/lukaszraczylo/traefikoidc/issues/90
 	if isTraefikDefaultCertError(err) {
@@ -631,7 +668,8 @@ func (re *RetryExecutor) isRetryableError(err error) bool {
 		}
 	}
 
-	if netErr, ok := err.(net.Error); ok {
+	var netErr net.Error
+	if errors.As(err, &netErr) {
 		if netErr.Timeout() {
 			return true
 		}
@@ -652,8 +690,18 @@ func (re *RetryExecutor) isRetryableError(err error) bool {
 		}
 	}
 
-	if httpErr, ok := err.(*HTTPError); ok {
-		return httpErr.StatusCode >= 500 || httpErr.StatusCode == 429
+	// errors.As so retry classification survives error wrapping: a transient
+	// or 5xx/429 error returned as fmt.Errorf("...: %w", err) from an
+	// upstream layer must still be retried. Direct type assertions on the
+	// top-level error silently misclassified wrapped transient errors as
+	// permanent, giving up before any retry (R111).
+	var httpErr *HTTPError
+	if errors.As(err, &httpErr) {
+		status := httpErr.StatusCode
+		// A 0 StatusCode means the producer used HTTPError purely as a
+		// message carrier (no real HTTP status). Treat it as unknown
+		// (non-retryable) rather than spuriously matching 5xx (R118).
+		return status != 0 && (status >= 500 || status == 429)
 	}
 
 	return false
@@ -880,11 +928,23 @@ var gdInstances = struct {
 	set map[*GracefulDegradation]struct{}
 }{set: make(map[*GracefulDegradation]struct{})}
 
+// globalHealthCheckMu serializes the global pass. globalPerformHealthChecks
+// runs from more than one goroutine — the shared singleton task's run loop
+// AND the once-guarded StartBackgroundTask path both drive it — so without
+// this, any health check registered via the public RegisterHealthCheck
+// API would be invoked concurrently (a latent production race, and the
+// source of an intermittent full-suite race in tests). Serializing the
+// whole pass keeps registered health checks single-threaded (R189).
+var globalHealthCheckMu sync.Mutex
+
 // globalPerformHealthChecks runs the health check pass for every live
 // GracefulDegradation instance. The shared singleton task calls this, so a
 // health check registered on ANY instance is exercised, and closing one
 // instance does not stall recovery for the others.
 func globalPerformHealthChecks() {
+	globalHealthCheckMu.Lock()
+	defer globalHealthCheckMu.Unlock()
+
 	gdInstances.RLock()
 	instances := make([]*GracefulDegradation, 0, len(gdInstances.set))
 	for gd := range gdInstances.set {
@@ -1201,8 +1261,14 @@ func (erm *ErrorRecoveryManager) GetCircuitBreaker(serviceName string) *CircuitB
 func (erm *ErrorRecoveryManager) ExecuteWithRecovery(ctx context.Context, serviceName string, fn func() error) error {
 	cb := erm.GetCircuitBreaker(serviceName)
 
-	return erm.retryExecutor.Execute(ctx, func() error {
-		return cb.Execute(fn)
+	// Retry INNER, circuit OUTER (matching TokenResilienceManager and the
+	// documented R106 compositing rule). Wrapping cb around the whole retry
+	// run means one logical call counts as a single circuit failure even
+	// when it was retried internally; the previous nesting (cb inside the
+	// retry loop) recorded each retry attempt as a separate failure and
+	// could open the circuit after only one flaky-but-retryable call (R144).
+	return cb.ExecuteWithContext(ctx, func() error {
+		return erm.retryExecutor.Execute(ctx, fn)
 	})
 }
 
diff --git a/goroutine_manager.go b/goroutine_manager.go
index 25ef81a..8322917 100644
--- a/goroutine_manager.go
+++ b/goroutine_manager.go
@@ -63,6 +63,14 @@ func (m *GoroutineManager) StartGoroutine(name string, fn func(context.Context))
 			m.wg.Done()
 			m.mu.Lock()
 			managedGoroutine.running = false
+			// Prune the map entry once this goroutine has finished so that
+			// finished goroutines don't accumulate unboundedly (R157). Only
+			// delete when the current entry still IS the finishing
+			// goroutine: a concurrent Start with the same name may have
+			// already replaced it with a live one.
+			if cur, ok := m.goroutines[goroutineName]; ok && cur == managedGoroutine {
+				delete(m.goroutines, goroutineName)
+			}
 			m.mu.Unlock()
 
 			// Recover from panics
diff --git a/http_client_pool.go b/http_client_pool.go
index 972c4da..dee333c 100644
--- a/http_client_pool.go
+++ b/http_client_pool.go
@@ -81,10 +81,21 @@ func (p *SharedTransportPool) GetOrCreateTransport(config HTTPClientConfig) *htt
 				return shared.transport
 			}
 		}
-		// No TLS-compatible transport available; return nil so the caller falls
-		// back to a default, certificate-verifying transport rather than one
-		// with a different (possibly verification-disabled) trust store.
-		return nil
+		// No TLS-compatible transport available. Returning nil would make the
+		// caller fall back to http.DefaultTransport, silently dropping the
+		// configured RootCAs / InsecureSkipVerify — custom-CA TLS would
+		// break and self-signed skip-verify would be lost (R95). Build and
+		// return the correctly-configured transport instead; honoring TLS
+		// settings takes precedence over the soft client cap.
+		t := newSharedTransport(config)
+		p.transports[p.configKey(config)] = &sharedTransport{
+			transport: t,
+			refCount:  1,
+			lastUsed:  time.Now(),
+			tlsKey:    want,
+		}
+		atomic.AddInt32(&p.clientCount, 1)
+		return t
 	}
 
 	p.mu.Lock()
@@ -101,8 +112,26 @@ func (p *SharedTransportPool) GetOrCreateTransport(config HTTPClientConfig) *htt
 	// Increment client count
 	atomic.AddInt32(&p.clientCount, 1)
 
-	// Create new transport with conservative limits
-	transport := &http.Transport{
+	transport := newSharedTransport(config)
+
+	p.transports[key] = &sharedTransport{
+		transport: transport,
+		refCount:  1,
+		lastUsed:  time.Now(),
+		tlsKey:    tlsConfigKey(config),
+	}
+
+	return transport
+}
+
+// newSharedTransport builds a properly-configured *http.Transport from the
+// given config, enforcing TLS 1.2+ and secure cipher suites. Shared by the
+// normal acquisition path and the at-cap fallback so the TLS settings are
+// NEVER silently dropped (the at-cap fallback previously returned nil,
+// making the caller fall back to http.DefaultTransport and lose the
+// configured RootCAs / InsecureSkipVerify — R95).
+func newSharedTransport(config HTTPClientConfig) *http.Transport {
+	return &http.Transport{
 		Proxy: http.ProxyFromEnvironment,
 		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
 			dialer := &net.Dialer{
@@ -138,15 +167,6 @@ func (p *SharedTransportPool) GetOrCreateTransport(config HTTPClientConfig) *htt
 		WriteBufferSize:       config.WriteBufferSize,
 		ReadBufferSize:        config.ReadBufferSize,
 	}
-
-	p.transports[key] = &sharedTransport{
-		transport: transport,
-		refCount:  1,
-		lastUsed:  time.Now(),
-		tlsKey:    tlsConfigKey(config),
-	}
-
-	return transport
 }
 
 // ReleaseTransport decrements the reference count for a transport
@@ -270,20 +290,31 @@ func tlsConfigKey(config HTTPClientConfig) string {
 	return fmt.Sprintf("%p|%s", config.RootCAs, skip)
 }
 
-// Cleanup closes all transports and stops the cleanup goroutine
+// Cleanup closes all transports, resets the pool, and restarts the cleanup
+// goroutine so the (singleton) pool remains fully usable afterwards. Prior
+// to R125, Cleanup canceled the cleanup goroutine but never restarted it
+// and left clientCount unreset: a reused singleton then had a permanently
+// dead cleaner (idle conns never pruned) and a stale count (the soft
+// maxClients cap keyed off it was wrong after reuse).
 func (p *SharedTransportPool) Cleanup() {
 	p.mu.Lock()
-	defer p.mu.Unlock()
-
-	// Stop the cleanup goroutine
-	if p.cancel != nil {
-		p.cancel()
-	}
-
 	for _, shared := range p.transports {
-		shared.transport.CloseIdleConnections()
+		if shared != nil && shared.transport != nil {
+			shared.transport.CloseIdleConnections()
+		}
 	}
 	p.transports = make(map[string]*sharedTransport)
+	atomic.StoreInt32(&p.clientCount, 0)
+	oldCancel := p.cancel
+	ctx, cancel := context.WithCancel(context.Background())
+	p.ctx = ctx
+	p.cancel = cancel
+	p.mu.Unlock()
+
+	if oldCancel != nil {
+		oldCancel()
+	}
+	go p.cleanupIdleTransports(ctx)
 }
 
 // CreatePooledHTTPClient creates an HTTP client using the shared transport pool
diff --git a/internal/cache/backends/hybrid.go b/internal/cache/backends/hybrid.go
index eccd998..865a9ee 100644
--- a/internal/cache/backends/hybrid.go
+++ b/internal/cache/backends/hybrid.go
@@ -37,7 +37,6 @@ type HybridBackend struct {
 
 // asyncWriteItem represents an async write operation
 type asyncWriteItem struct {
-	ctx   context.Context
 	key   string
 	value []byte
 	ttl   time.Duration
@@ -195,7 +194,6 @@ func (h *HybridBackend) Set(ctx context.Context, key string, value []byte, ttl t
 			key:   key,
 			value: value,
 			ttl:   ttl,
-			ctx:   ctx,
 		}:
 			h.logger.Debugf("Queued async write to L2 for key: %s", redactKey(key))
 		default:
@@ -265,14 +263,23 @@ func (h *HybridBackend) Delete(ctx context.Context, key string) (bool, error) {
 		deleted = true
 	}
 
-	// Delete from L2 if not in fallback mode
-	if !h.fallbackMode.Load() {
-		if d, err := h.secondary.Delete(ctx, key); err != nil {
-			h.logger.Debugf("Failed to delete from L2 cache: %v", err)
-			h.recordL2Error()
-		} else if d {
-			deleted = true
-		}
+	if h.fallbackMode.Load() {
+		return deleted, nil
+	}
+
+	// Drain in-flight async L2 writes before deleting L2. A Set's async
+	// write could otherwise land AFTER this DEL, resurrecting the just-
+	// deleted key with its stale value (L1/L2 divergence). Waiting for
+	// the async worker (which tracks queued + in-flight non-critical
+	// writes) ensures no pre-existing async Set lands post-DEL (R124).
+	h.asyncWg.Wait()
+
+	// Delete from L2
+	if d, err := h.secondary.Delete(ctx, key); err != nil {
+		h.logger.Debugf("Failed to delete from L2 cache: %v", err)
+		h.recordL2Error()
+	} else if d {
+		deleted = true
 	}
 
 	return deleted, nil
@@ -390,12 +397,14 @@ func (h *HybridBackend) Ping(ctx context.Context) error {
 
 // Close shuts down the hybrid backend
 func (h *HybridBackend) Close() error {
-	// Cancel context to stop workers
+	// Cancel context to stop workers.
 	h.cancel()
 
-	// Close async write channel
-	close(h.asyncWriteBuffer)
-	close(h.l1BackfillBuffer)
+	// Do NOT close asyncWriteBuffer / l1BackfillBuffer: the workers key off
+	// h.ctx.Done() and exit on cancel, and closing these channels creates a
+	// race where a concurrent Set's select-send (line ~193) hits a closed
+	// channel and panics ("send on closed channel"). Buffered items a late
+	// Set queues after the workers have exited are harmless at shutdown.
 
 	// Wait for workers to finish with timeout
 	done := make(chan struct{})
@@ -539,7 +548,6 @@ func (h *HybridBackend) SetMany(ctx context.Context, items map[string][]byte, tt
 					key:   key,
 					value: value,
 					ttl:   ttl,
-					ctx:   ctx,
 				}:
 					// Queued
 				default:
@@ -593,7 +601,7 @@ func (h *HybridBackend) l1BackfillWorker() {
 			if !ok {
 				return
 			}
-			if h.keyStillInSecondary(item.key) {
+			if h.keyStillInSecondary(item.key) && !h.keyAlreadyInPrimary(item.key) {
 				writeCtx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
 				if err := h.primary.Set(writeCtx, item.key, item.value, item.ttl); err != nil {
 					h.logger.Debugf("Failed to populate L1 cache from L2 for key %s: %v", redactKey(item.key), err)
@@ -620,6 +628,19 @@ func (h *HybridBackend) keyStillInSecondary(key string) bool {
 	return err != nil || exists
 }
 
+// keyAlreadyInPrimary reports whether the key already exists in the L1
+// cache. The backfill worker writes an L2-resolved value into L1; if a
+// concurrent Set already placed a fresher value there, writing over it
+// would clobber the newer value with the stale L2 snapshot (R124). Skip
+// the backfill when L1 already has the key — L1 is only a cache and the
+// next L2 hit re-arms the write-through.
+func (h *HybridBackend) keyAlreadyInPrimary(key string) bool {
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	exists, err := h.primary.Exists(ctx, key)
+	return err == nil && exists
+}
+
 // asyncWriteWorker processes asynchronous writes to L2
 func (h *HybridBackend) asyncWriteWorker() {
 	defer h.wg.Done()
@@ -658,7 +679,11 @@ func (h *HybridBackend) asyncWriteWorker() {
 			// Clear stall us out so no buffered write can land after a
 			// clear-then-wipe.
 			h.asyncFlushMu.Lock()
-			writeCtx, cancel := context.WithTimeout(item.ctx, 500*time.Millisecond)
+			// Use a detached context, never the originating request's ctx:
+			// this is a background write that typically runs after the
+			// request has returned, so the request ctx would be canceled
+			// and the L2 write silently dropped (R94).
+			writeCtx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
 			if err := h.secondary.Set(writeCtx, item.key, item.value, item.ttl); err != nil {
 				h.errors.Add(1)
 				h.logger.Debugf("Async write to L2 failed for key %s: %v", redactKey(item.key), err)
diff --git a/internal/cache/backends/hybrid_r94_test.go b/internal/cache/backends/hybrid_r94_test.go
new file mode 100644
index 0000000..1b5f1a2
--- /dev/null
+++ b/internal/cache/backends/hybrid_r94_test.go
@@ -0,0 +1,75 @@
+package backends
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// r94CtxProbeBackend records whether any operation was handed a canceled
+// context. Set is the only method the async L2 write path exercises.
+type r94CtxProbeBackend struct {
+	setSeen   *atomic.Bool
+	setHadErr *atomic.Bool
+}
+
+func (b *r94CtxProbeBackend) Set(ctx context.Context, key string, value []byte, ttl time.Duration) error {
+	b.setSeen.Store(true)
+	if ctx.Err() != nil {
+		b.setHadErr.Store(true)
+	}
+	return nil
+}
+func (b *r94CtxProbeBackend) Get(ctx context.Context, key string) ([]byte, time.Duration, bool, error) {
+	return nil, 0, false, nil
+}
+func (b *r94CtxProbeBackend) Delete(ctx context.Context, key string) (bool, error) { return false, nil }
+func (b *r94CtxProbeBackend) Exists(ctx context.Context, key string) (bool, error) { return false, nil }
+func (b *r94CtxProbeBackend) Clear(ctx context.Context) error                      { return nil }
+func (b *r94CtxProbeBackend) GetStats() map[string]interface{}                     { return nil }
+func (b *r94CtxProbeBackend) Close() error                                         { return nil }
+func (b *r94CtxProbeBackend) Ping(ctx context.Context) error                       { return nil }
+
+// TestHybridBackend_AsyncWriteUsesDetachedContext is an R94 regression: the
+// async L2 write-through must be executed with a detached context, never the
+// originating request's context. The L2 write runs after the request has
+// returned; if it carried the (now-canceled) request context the write
+// would be silently dropped, so non-critical keys never reach L2. Before
+// the fix the worker used the request ctx and recorded a canceled context.
+func TestHybridBackend_AsyncWriteUsesDetachedContext(t *testing.T) {
+	primary := &r94CtxProbeBackend{setSeen: &atomic.Bool{}, setHadErr: &atomic.Bool{}}
+	secondary := &r94CtxProbeBackend{setSeen: &atomic.Bool{}, setHadErr: &atomic.Bool{}}
+
+	hybrid, err := NewHybridBackend(&HybridConfig{
+		Primary:             primary,
+		Secondary:           secondary,
+		SyncWriteCacheTypes: map[string]bool{
+			// empty set: no type is sync, so writes are async
+		},
+	})
+	require.NoError(t, err)
+	defer hybrid.Close()
+
+	reqCtx, cancel := context.WithCancel(context.Background())
+	err = hybrid.Set(reqCtx, "general:key1", []byte("test-value"), time.Minute)
+	require.NoError(t, err)
+
+	// The request has returned: cancel its context before the async worker
+	// gets a chance to run the L2 write.
+	cancel()
+
+	// Wait for the async L2 write to be attempted.
+	deadline := time.Now().Add(2 * time.Second)
+	for !secondary.setSeen.Load() {
+		if time.Now().After(deadline) {
+			t.Fatal("async L2 write was never attempted")
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+
+	require.False(t, secondary.setHadErr.Load(),
+		"async L2 write must use a detached (non-canceled) context")
+}
diff --git a/internal/cache/backends/memory.go b/internal/cache/backends/memory.go
index 61c0547..57f8a46 100644
--- a/internal/cache/backends/memory.go
+++ b/internal/cache/backends/memory.go
@@ -202,6 +202,14 @@ func (m *MemoryCacheBackend) Set(ctx context.Context, key string, value interfac
 		return ErrBackendUnavailable
 	}
 
+	// A NEGATIVE TTL means "already expired" (the stack's convention for
+	// a value whose validity has passed — see redis.go). Previously ttl<=0
+	// fell through to a permanent entry, diverging from redis and
+	// universal_cache. Zero remains the documented "no expiry" contract.
+	if ttl < 0 {
+		return nil
+	}
+
 	start := time.Now()
 	defer func() {
 		duration := time.Since(start).Nanoseconds()
@@ -212,15 +220,19 @@ func (m *MemoryCacheBackend) Set(ctx context.Context, key string, value interfac
 	// Calculate item size
 	itemSize := int64(len(key)) + estimateValueSize(value)
 
-	// Enforce global limits before adding new item
-	m.enforceGlobalLimits(itemSize)
+	// Snapshot replacement state so the global limit enforcer accounts for
+	// key replacement: refreshing an existing key neither grows the count
+	// nor adds its full size, so at global capacity it must not evict an
+	// unrelated live entry (mirrors cacheShard.set, R177).
+	shard := m.getShard(key)
+	oldSize, exists := shard.peekSize(key)
+	m.enforceGlobalLimits(itemSize, oldSize, exists)
 
 	var expiresAt time.Time
 	if ttl > 0 {
 		expiresAt = time.Now().Add(ttl)
 	}
 
-	shard := m.getShard(key)
 	shard.set(key, value, expiresAt, itemSize)
 
 	m.sets.Add(1)
@@ -228,14 +240,30 @@ func (m *MemoryCacheBackend) Set(ctx context.Context, key string, value interfac
 }
 
 // enforceGlobalLimits ensures global size and memory limits are respected
-// by evicting from shards when necessary
-func (m *MemoryCacheBackend) enforceGlobalLimits(newItemSize int64) {
-	// Check and enforce size limit
+// by evicting from shards when necessary.
+//
+// It is replacement-aware: an existing key being rewritten (exists=oldSize)
+// is count-neutral (global cap is on the number of entries) and grows
+// memory by only the net delta, mirroring cacheShard.set. At global
+// capacity a pure key replacement must therefore not evict an unrelated
+// live entry (R177).
+func (m *MemoryCacheBackend) enforceGlobalLimits(newItemSize, oldSize int64, exists bool) {
+	// Count (entry-size) budget only grows for genuinely new keys.
+	sizeTrigger := m.maxSize > 0 && !exists
+	// Memory budget grows by the net delta for a replacement.
+	netMemory := newItemSize
+	if exists {
+		netMemory = newItemSize - oldSize
+		if netMemory < 0 {
+			netMemory = 0
+		}
+	}
+
 	for {
 		totalSize, totalMemory := m.getGlobalStats()
 
-		needsSizeEviction := m.maxSize > 0 && totalSize >= m.maxSize
-		needsMemoryEviction := m.maxMemory > 0 && totalMemory+newItemSize > m.maxMemory
+		needsSizeEviction := sizeTrigger && totalSize >= m.maxSize
+		needsMemoryEviction := m.maxMemory > 0 && totalMemory+netMemory > m.maxMemory
 
 		if !needsSizeEviction && !needsMemoryEviction {
 			break
diff --git a/internal/cache/backends/memory_shard.go b/internal/cache/backends/memory_shard.go
index a1e3181..b791ae6 100644
--- a/internal/cache/backends/memory_shard.go
+++ b/internal/cache/backends/memory_shard.go
@@ -30,6 +30,19 @@ func newCacheShard(maxSize, maxMemory int64) *cacheShard {
 
 // get retrieves a value from this shard
 // Returns: value, exists, expired
+// peekSize returns the size of the existing entry for key and whether it
+// exists, without touching LRU state. Used by the global limit enforcer
+// to decide whether a Set is a replacement (count-neutral) or a new item.
+func (s *cacheShard) peekSize(key string) (int64, bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	item, exists := s.items[key]
+	if !exists {
+		return 0, false
+	}
+	return item.size, true
+}
+
 func (s *cacheShard) get(key string) (interface{}, bool, bool) {
 	s.mu.RLock()
 	item, exists := s.items[key]
@@ -72,15 +85,27 @@ func (s *cacheShard) set(key string, value interface{}, expiresAt time.Time, siz
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
-	// Check if we need to evict items
-	if s.maxSize > 0 && s.size >= s.maxSize {
-		s.evictLRULocked()
-	}
-	if s.maxMemory > 0 && s.memoryUsed+size > s.maxMemory {
-		s.evictLRULocked()
+	// Snapshot the existing entry so the eviction decisions account for
+	// replacement: refreshing an existing key neither grows the count nor
+	// adds its full size, so at capacity it must not evict an unrelated
+	// LRU entry (R139 F2).
+	oldItem, exists := s.items[key]
+	var oldSize int64
+	if exists {
+		oldSize = oldItem.size
 	}
 
-	// Remove old item if exists
+	if s.maxSize > 0 && !exists && s.size >= s.maxSize {
+		s.evictLRULocked()
+	}
+	if s.maxMemory > 0 {
+		if net := size - oldSize; net > 0 && s.memoryUsed+net > s.maxMemory {
+			s.evictLRULocked()
+		}
+	}
+
+	// Remove old item if it still exists (it may have been the LRU victim
+	// of an eviction above; re-check so we don't double-account).
 	if oldItem, exists := s.items[key]; exists {
 		s.memoryUsed -= oldItem.size
 		if elem, ok := oldItem.element.(*list.Element); ok && elem != nil {
diff --git a/internal/cache/backends/redis.go b/internal/cache/backends/redis.go
index e906efc..420884e 100644
--- a/internal/cache/backends/redis.go
+++ b/internal/cache/backends/redis.go
@@ -95,7 +95,15 @@ func (r *RedisBackend) Set(ctx context.Context, key string, value []byte, ttl ti
 		return ErrBackendClosed
 	}
 
+	// A NEGATIVE TTL means "already expired" (the stack's convention for a
+	// value whose validity has passed). Previously ttl<=0 fell through to a
+	// bare SET with no expiry, silently making the entry permanent. Zero here
+	// is the documented "no expiry" contract (see SetManyNoTTL), so only
+	// strictly-negative TTLs are skipped.
 	prefixedKey := r.prefixKey(key)
+	if ttl < 0 {
+		return nil
+	}
 
 	// Execute with retry logic
 	return r.executeWithRetry(ctx, func(conn *RedisConn) error {
@@ -450,6 +458,13 @@ func (r *RedisBackend) SetMany(ctx context.Context, items map[string][]byte, ttl
 		return nil
 	}
 
+	if ttl < 0 {
+		// Already-expired TTL: nothing to persist (see Set's guard). Avoids
+		// creating permanent entries for past-dated values via the multi path.
+		// Zero retains the "no expiry" contract (see SetManyNoTTL).
+		return nil
+	}
+
 	// For single items, use regular Set
 	if len(items) == 1 {
 		for key, value := range items {
diff --git a/internal/cache/backends/redis_pool.go b/internal/cache/backends/redis_pool.go
index 18ba06e..da89316 100644
--- a/internal/cache/backends/redis_pool.go
+++ b/internal/cache/backends/redis_pool.go
@@ -67,6 +67,22 @@ func NewConnectionPool(config *PoolConfig) (*ConnectionPool, error) {
 	return pool, nil
 }
 
+// reserveConnectionSlot atomically reserves capacity for one new connection,
+// returning false when the pool is already at MaxConnections. The
+// reservation is released via totalConns.Add(-1) if the dial fails.
+func (p *ConnectionPool) reserveConnectionSlot() bool {
+	max := int32(p.config.MaxConnections) // nolint:gosec // MaxConnections is operator-bounded well below int32 max
+	for {
+		cur := p.totalConns.Load()
+		if cur >= max {
+			return false
+		}
+		if p.totalConns.CompareAndSwap(cur, cur+1) {
+			return true
+		}
+	}
+}
+
 // Get retrieves a connection from the pool or creates a new one
 func (p *ConnectionPool) Get(ctx context.Context) (*RedisConn, error) {
 	if p.closed.Load() {
@@ -102,11 +118,15 @@ func (p *ConnectionPool) Get(ctx context.Context) (*RedisConn, error) {
 			return nil, ctx.Err()
 
 		default:
-			// No available connection, create new one if under limit
-			// #nosec G115 -- MaxConnections is a small config value that fits in int32
-			if p.totalConns.Load() < int32(p.config.MaxConnections) {
+			// No available connection, create new one if under limit.
+			// Reserve capacity atomically before dialing: checking
+			// totalConns then dialing is not atomic, so concurrent Gets
+			// could each pass the check and spike the live socket count
+			// far above MaxConnections (FD / Redis maxclients exhaustion).
+			if p.reserveConnectionSlot() {
 				conn, err = p.createConnection(ctx)
 				if err != nil {
+					p.totalConns.Add(-1) // release the reserved slot
 					// If this is the last attempt, return error
 					if attempt == maxAttempts-1 {
 						return nil, err
@@ -116,7 +136,6 @@ func (p *ConnectionPool) Get(ctx context.Context) (*RedisConn, error) {
 					continue
 				}
 				p.activeConns.Add(1)
-				p.totalConns.Add(1)
 				return conn, nil
 			}
 
@@ -335,7 +354,9 @@ func (c *RedisConn) Do(command string, args ...string) (interface{}, error) {
 	resp, err := reader.ReadResponse()
 	reader.Release() // Return to pool immediately after use
 	if err != nil {
-		if !errors.Is(err, ErrNilResponse) {
+		// A nil response or a Redis command error reply leaves the
+		// connection healthy — only an IO/parse error closes it.
+		if !errors.Is(err, ErrNilResponse) && !errors.Is(err, ErrCommandReply) {
 			c.closed.Store(true)
 		}
 		return nil, err
@@ -470,6 +491,14 @@ func (p *Pipeline) Execute() ([]interface{}, error) {
 				responses[i] = nil
 				continue
 			}
+			if errors.Is(err, ErrCommandReply) {
+				// A Redis command error reply: the connection is healthy
+				// and the remaining replies are still readable. Store the
+				// error value so per-command validation upstream can
+				// report it without aborting the batch.
+				responses[i] = err
+				continue
+			}
 			p.conn.closed.Store(true)
 			return responses[:i], fmt.Errorf("pipeline read error at command %d: %w", i, err)
 		}
diff --git a/internal/cache/backends/resp.go b/internal/cache/backends/resp.go
index 3b7fde7..85bc6be 100644
--- a/internal/cache/backends/resp.go
+++ b/internal/cache/backends/resp.go
@@ -20,6 +20,11 @@ import (
 var (
 	ErrInvalidRESP = errors.New("invalid RESP response")
 	ErrNilResponse = errors.New("nil response")
+	// ErrCommandReply marks a RESP '-' (Redis command error) reply, e.g.
+	// WRONGTYPE or OOM. It is a valid protocol response — the connection
+	// is healthy and any remaining pipelined replies are still readable —
+	// as opposed to an IO/parse error which means the connection is dead.
+	ErrCommandReply = errors.New("redis command error reply")
 )
 
 // RESPWriter writes RESP protocol messages
@@ -110,7 +115,9 @@ func (r *RESPReader) readError() error {
 	if err != nil {
 		return err
 	}
-	return errors.New(line)
+	// Wrap so callers can distinguish a Redis command error reply (healthy
+	// connection) from an IO/parse error (dead connection).
+	return fmt.Errorf("%w: %s", ErrCommandReply, line)
 }
 
 // readInteger reads an integer (:1000\r\n)
diff --git a/internal/cache/backends/resp_test.go b/internal/cache/backends/resp_test.go
index 3b4ae6b..88a5573 100644
--- a/internal/cache/backends/resp_test.go
+++ b/internal/cache/backends/resp_test.go
@@ -142,7 +142,12 @@ func TestRESPReader_ReadError(t *testing.T) {
 			_, err := reader.ReadResponse()
 
 			require.Error(t, err)
-			assert.Equal(t, tt.expectedError, err.Error())
+			// R177: a '-' reply is now wrapped with ErrCommandReply so
+			// callers can tell a Redis command error (healthy connection)
+			// apart from an IO/parse error. The command's message text is
+			// preserved verbatim inside the wrapped error.
+			assert.True(t, errors.Is(err, ErrCommandReply), "want ErrCommandReply, got %v", err)
+			assert.Contains(t, err.Error(), tt.expectedError)
 		})
 	}
 }
diff --git a/internal/cache/backends/review_r110_test.go b/internal/cache/backends/review_r110_test.go
new file mode 100644
index 0000000..81c9e23
--- /dev/null
+++ b/internal/cache/backends/review_r110_test.go
@@ -0,0 +1,54 @@
+package backends
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestConnectionPool_CapHeldUnderBurst regresses the connection-pool cap
+// being non-atomic: ConnectionPool.Get checked `totalConns < MaxConnections`
+// and THEN dialed a new socket, so concurrent Gets could each pass the check
+// and spike the live connection count far above MaxConnections (FD / Redis
+// maxclients exhaustion under burst load). The cap must now be held
+// atomically so at most MaxConnections sockets are ever created at once.
+func TestConnectionPool_CapHeldUnderBurst(t *testing.T) {
+	mr := NewMiniredisServer(t)
+
+	maxConns := 3
+	pool, err := NewConnectionPool(&PoolConfig{
+		Address:        mr.GetAddr(),
+		MaxConnections: maxConns,
+		ConnectTimeout: 80 * time.Millisecond,
+		ReadTimeout:    50 * time.Millisecond,
+		WriteTimeout:   50 * time.Millisecond,
+	})
+	require.NoError(t, err)
+	defer pool.Close()
+
+	ctx := context.Background()
+	const workers = 50
+
+	var wg sync.WaitGroup
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			conn, _ := pool.Get(ctx)
+			if conn != nil {
+				// Hold the connection (simulate an in-flight request) so the
+				// pool stays under pressure and the count is stable.
+				_ = conn
+			}
+		}()
+	}
+	wg.Wait()
+
+	stats := pool.Stats()
+	total := stats["total_connections"].(int32)
+	require.LessOrEqual(t, total, int32(maxConns),
+		"burst Gets must not create more than MaxConnections sockets (got %d)", total)
+}
diff --git a/internal/cache/backends/review_r115_test.go b/internal/cache/backends/review_r115_test.go
new file mode 100644
index 0000000..f63f2ec
--- /dev/null
+++ b/internal/cache/backends/review_r115_test.go
@@ -0,0 +1,36 @@
+package backends
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestRedisBackend_SetNegativeTTL_NotPersistent guards the R115 negative-TTL
+// fix. The cache stack's convention is that a non-positive TTL means "already
+// expired". Previously RedisBackend.Set/SetMany fell through to a bare SET
+// when ttl<=0, writing an entry with NO expiry — i.e. an expired-dated value
+// became permanent. Now expired TTLs write nothing.
+func TestRedisBackend_SetNegativeTTL_NotPersistent(t *testing.T) {
+	_, backend := setupTestRedis(t)
+	ctx := context.Background()
+
+	// Single Set with an expired TTL must not create an entry.
+	require.NoError(t, backend.Set(ctx, "neg", []byte("v"), -time.Second))
+	_, _, exists, err := backend.Get(ctx, "neg")
+	require.NoError(t, err)
+	require.False(t, exists, "negative-TTL Set must not create a persistent entry")
+
+	// SetMany with multiple items and an expired TTL must not create entries.
+	require.NoError(t, backend.SetMany(ctx, map[string][]byte{
+		"a": []byte("1"),
+		"b": []byte("2"),
+	}, -time.Second))
+	for _, k := range []string{"a", "b"} {
+		_, _, exists, err := backend.Get(ctx, k)
+		require.NoError(t, err)
+		require.False(t, exists, "negative-TTL SetMany must not create a persistent entry for %q", k)
+	}
+}
diff --git a/internal/cache/backends/review_r124_test.go b/internal/cache/backends/review_r124_test.go
new file mode 100644
index 0000000..ffb70d0
--- /dev/null
+++ b/internal/cache/backends/review_r124_test.go
@@ -0,0 +1,241 @@
+package backends
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// blockingWriteBackend is a cache backend whose Set blocks until released,
+// used to hold an async L2 write in-flight while Delete runs.
+type blockingWriteBackend struct {
+	mu         sync.Mutex
+	data       map[string]string
+	setStarted chan struct{}
+	release    chan struct{}
+	written    chan struct{}
+	once       sync.Once
+}
+
+func (b *blockingWriteBackend) Set(ctx context.Context, key string, value []byte, ttl time.Duration) error {
+	select {
+	case <-b.setStarted:
+	default:
+		close(b.setStarted)
+	}
+	select {
+	case <-b.release:
+	case <-b.written: // never
+	case <-ctx.Done():
+		return nil
+	}
+	b.mu.Lock()
+	b.data[key] = string(value)
+	b.mu.Unlock()
+	b.once.Do(func() { close(b.written) })
+	return nil
+}
+
+func (b *blockingWriteBackend) Get(ctx context.Context, key string) ([]byte, time.Duration, bool, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	v, ok := b.data[key]
+	if !ok {
+		return nil, 0, false, nil
+	}
+	return []byte(v), time.Minute, true, nil
+}
+
+func (b *blockingWriteBackend) Delete(ctx context.Context, key string) (bool, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	_, ok := b.data[key]
+	delete(b.data, key)
+	return ok, nil
+}
+
+func (b *blockingWriteBackend) Exists(ctx context.Context, key string) (bool, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	_, ok := b.data[key]
+	return ok, nil
+}
+
+func (b *blockingWriteBackend) Clear(ctx context.Context) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.data = map[string]string{}
+	return nil
+}
+
+func (b *blockingWriteBackend) GetStats() map[string]interface{} { return nil }
+func (b *blockingWriteBackend) Close() error                     { return nil }
+func (b *blockingWriteBackend) Ping(ctx context.Context) error   { return nil }
+
+// TestHybrid_DeleteWaitsForAsyncWrite guards the R124 fix to hybrid.go
+// Delete: an in-flight async L2 write (from a non-critical Set) could land
+// after Delete's L2 DEL, resurrecting the just-deleted key with a stale
+// value. Delete must wait for in-flight async writes before the L2 delete.
+func TestHybrid_DeleteWaitsForAsyncWrite(t *testing.T) {
+	sec := &blockingWriteBackend{
+		data:       map[string]string{},
+		setStarted: make(chan struct{}),
+		release:    make(chan struct{}),
+		written:    make(chan struct{}),
+	}
+	primary := newMockBackend()
+
+	hybrid, err := NewHybridBackend(&HybridConfig{
+		Primary:         primary,
+		Secondary:       sec,
+		AsyncBufferSize: 16,
+	})
+	if err != nil {
+		t.Fatalf("hybrid: %v", err)
+	}
+	defer hybrid.Close()
+
+	ctx := context.Background()
+
+	// Non-critical Set -> async L2 write that blocks in-flight.
+	if err := hybrid.Set(ctx, "k", []byte("stale"), time.Minute); err != nil {
+		t.Fatalf("set: %v", err)
+	}
+	select {
+	case <-sec.setStarted:
+	case <-time.After(2 * time.Second):
+		t.Fatal("async L2 write never started")
+	}
+
+	// Issue Delete while the async write is in-flight.
+	done := make(chan struct{})
+	go func() {
+		_, _ = hybrid.Delete(ctx, "k")
+		close(done)
+	}()
+
+	// OLD behavior: Delete returns immediately (no wait for async writes);
+	// it performs its L2 delete and closes done. NEW behavior: Delete
+	// blocks in asyncWg.Wait until the in-flight write finishes, so done
+	// does not fire until released. Distinguish the two before releasing.
+	oldImmediate := false
+	select {
+	case <-done:
+		oldImmediate = true
+	case <-time.After(50 * time.Millisecond):
+		// NEW: Delete is blocked waiting for the async write. 50ms is
+		// far longer than OLD's immediate (non-waiting) return needs,
+		// and far shorter than the worker's 500ms writeCtx, so the
+		// in-flight write reliably survives to be released.
+	}
+	// Release the in-flight L2 write (well within the worker's writeCtx
+	// timeout) so it actually lands.
+	close(sec.release)
+
+	if oldImmediate {
+		// OLD: the worker write now lands AFTER Delete's L2 delete,
+		// resurrecting the key.
+		select {
+		case <-sec.written:
+		case <-time.After(2 * time.Second):
+			t.Fatal("async L2 write never landed")
+		}
+	} else {
+		// NEW: Delete waited for the write, then removed it from L2.
+		select {
+		case <-sec.written:
+		case <-time.After(2 * time.Second):
+			t.Fatal("async L2 write never landed")
+		}
+		select {
+		case <-done:
+		case <-time.After(2 * time.Second):
+			t.Fatal("Delete did not return")
+		}
+	}
+
+	if _, _, exists, _ := sec.Get(ctx, "k"); exists {
+		t.Fatal("L2 must not contain key after Delete: the in-flight async Set resurrected it")
+	}
+}
+
+// existsAlwaysBackend reports the key always present in L1 (so a backfill
+// must be skipped) and counts Set calls; setCalled is closed once Set runs,
+// letting the test observe a (OLD) backfill write deterministically.
+type existsAlwaysBackend struct {
+	setCalls  int32
+	setCalled chan struct{}
+	once      sync.Once
+}
+
+func newExistsAlwaysBackend() *existsAlwaysBackend {
+	return &existsAlwaysBackend{setCalled: make(chan struct{})}
+}
+
+func (b *existsAlwaysBackend) Set(ctx context.Context, key string, value []byte, ttl time.Duration) error {
+	atomic.AddInt32(&b.setCalls, 1)
+	b.once.Do(func() { close(b.setCalled) })
+	return nil
+}
+func (b *existsAlwaysBackend) Get(ctx context.Context, key string) ([]byte, time.Duration, bool, error) {
+	return nil, 0, false, nil
+}
+func (b *existsAlwaysBackend) Delete(ctx context.Context, key string) (bool, error) {
+	return false, nil
+}
+func (b *existsAlwaysBackend) Exists(ctx context.Context, key string) (bool, error) { return true, nil }
+func (b *existsAlwaysBackend) Clear(ctx context.Context) error                      { return nil }
+func (b *existsAlwaysBackend) GetStats() map[string]interface{}                     { return nil }
+func (b *existsAlwaysBackend) Close() error                                         { return nil }
+func (b *existsAlwaysBackend) Ping(ctx context.Context) error                       { return nil }
+
+// TestHybrid_BackfillDoesNotClobberFreshL1 guards the R124 fix to
+// hybrid.go l1BackfillWorker: the backfill only checked L2 key
+// existence, so a stale L2-captured value would overwrite a fresher L1
+// value written concurrently by Set. When L1 already holds the key, the
+// backfill must be skipped.
+func TestHybrid_BackfillDoesNotClobberFreshL1(t *testing.T) {
+	primary := newExistsAlwaysBackend()
+	secondary := newMockBackend()
+	ctx := context.Background()
+	if err := secondary.Set(ctx, "k", []byte("stale"), time.Minute); err != nil {
+		t.Fatalf("seed L2: %v", err)
+	}
+
+	hybrid, err := NewHybridBackend(&HybridConfig{
+		Primary:         primary,
+		Secondary:       secondary,
+		AsyncBufferSize: 16,
+	})
+	if err != nil {
+		t.Fatalf("hybrid: %v", err)
+	}
+	defer hybrid.Close()
+
+	// Queue a stale backfill for a key L1 already holds (fresher).
+	hybrid.queueL1Backfill("k", []byte("stale"), time.Minute)
+
+	// Wait for the worker to drain the backfill buffer.
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if len(hybrid.l1BackfillBuffer) == 0 {
+			time.Sleep(20 * time.Millisecond) // let the worker act (write or skip)
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+
+	// Deterministically observe an OLD backfill write: if the worker
+	// writes, setCalled closes and setCalls becomes 1. On NEW the
+	// worker skips, so nothing fires.
+	select {
+	case <-primary.setCalled:
+	case <-time.After(500 * time.Millisecond):
+	}
+
+	if atomic.LoadInt32(&primary.setCalls) != 0 {
+		t.Fatal("backfill must not overwrite the fresher L1 value (primary already had the key)")
+	}
+}
diff --git a/internal/cache/backends/review_r125_test.go b/internal/cache/backends/review_r125_test.go
new file mode 100644
index 0000000..8ee0ba5
--- /dev/null
+++ b/internal/cache/backends/review_r125_test.go
@@ -0,0 +1,28 @@
+package backends
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+// TestMemoryBackend_NegativeTTLNotStored guards the R125 fix to memory.go:
+// a negative TTL must mean "already expired" (the stack's convention, per
+// redis.go) and NOT store a permanent entry. Previously ttl<=0 fell
+// through to a permanent entry, diverging from redis and universal_cache,
+// so a value whose validity had passed was kept forever in the memory
+// tier.
+func TestMemoryBackend_NegativeTTLNotStored(t *testing.T) {
+	b := NewMemoryCacheBackend(1000, 0, 0)
+	defer b.Close()
+
+	ctx := context.Background()
+	if err := b.Set(ctx, "k", []byte("v"), -1*time.Second); err != nil {
+		t.Fatalf("set with negative TTL: %v", err)
+	}
+
+	if _, err := b.Get(ctx, "k"); !errors.Is(err, ErrCacheMiss) {
+		t.Fatalf("Get after negative-TTL Set = %v, want %v (negative TTL must be treated as already-expired, not stored)", err, ErrCacheMiss)
+	}
+}
diff --git a/internal/cache/backends/review_r139_f2_test.go b/internal/cache/backends/review_r139_f2_test.go
new file mode 100644
index 0000000..2f88767
--- /dev/null
+++ b/internal/cache/backends/review_r139_f2_test.go
@@ -0,0 +1,28 @@
+package backends
+
+import (
+	"testing"
+	"time"
+)
+
+// TestCacheShard_SetRefreshExistingDoesNotEvictAtCapacity verifies the shard
+// set path does not evict an unrelated LRU entry when replacing an existing
+// key at capacity (count stays flat) (R139 F2). Mirrors the Cache.Set fix.
+func TestCacheShard_SetRefreshExistingDoesNotEvictAtCapacity(t *testing.T) {
+	s := newCacheShard(2, 0) // maxSize=2, no memory bound
+	exp := time.Now().Add(time.Minute)
+
+	s.set("a", "va", exp, 1) // becomes LRU
+	s.set("b", "vb", exp, 1) // MRU
+
+	// Refresh the most-recent key while at capacity. On the buggy path the
+	// LRU sibling `a` is evicted; on the fixed path the replacement is free.
+	s.set("b", "vb2", exp, 1)
+
+	if _, exists, _ := s.get("a"); !exists {
+		t.Fatal("refreshing an existing key must not evict an unrelated live entry `a`")
+	}
+	if v, exists, _ := s.get("b"); !exists || v != "vb2" {
+		t.Fatalf("refreshed key `b` should hold the new value, got %v exists=%v", v, exists)
+	}
+}
diff --git a/internal/cache/backends/review_r141_test.go b/internal/cache/backends/review_r141_test.go
new file mode 100644
index 0000000..852b4b7
--- /dev/null
+++ b/internal/cache/backends/review_r141_test.go
@@ -0,0 +1,81 @@
+package backends
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestSingleflightCache_PanickingFetcherDoesNotDeadlock verifies that a
+// fetcher which panics (routines in this codebase treat panics as a normal
+// failure mode) does not wedge the singleflight entry: the leading caller
+// yields an error and waiting callers are released instead of blocking on
+// call.wg forever (R141).
+func TestSingleflightCache_PanickingFetcherDoesNotDeadlock(t *testing.T) {
+	backend, err := NewMemoryBackend(DefaultConfig())
+	requireNoErrorFatal(t, err)
+	defer backend.Close()
+
+	cache := NewSingleflightCache(backend)
+	ctx := context.Background()
+	key := "panic-key"
+
+	panicFetcher := func(ctx context.Context) ([]byte, time.Duration, error) {
+		panic("boom")
+	}
+
+	var wg sync.WaitGroup
+	var doneA, doneB = make(chan struct{}), make(chan struct{})
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		defer func() { _ = recover() }() // tolerate old code's out-of-band panic
+		_, _ = cache.GetOrFetch(ctx, key, panicFetcher)
+		close(doneA)
+	}()
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_, _ = cache.GetOrFetch(ctx, key, panicFetcher)
+		close(doneB)
+	}()
+
+	select {
+	case <-doneB:
+	case <-time.After(3 * time.Second):
+		t.Fatal("concurrent waiter hung on a panicking singleflight fetcher")
+	}
+	select {
+	case <-doneA:
+	case <-time.After(3 * time.Second):
+		t.Fatal("leading caller hung on its own panicking fetcher")
+	}
+	wg.Wait()
+
+	// After the entry is cleaned up, a well-behaved fetcher for the same
+	// key must execute normally (proves the entry was not permanently
+	// wedged by the panic).
+	ran := false
+	good := func(ctx context.Context) ([]byte, time.Duration, error) {
+		ran = true
+		return []byte("ok"), time.Minute, nil
+	}
+	// Give the deferred cleanup a moment to release the key.
+	time.Sleep(200 * time.Millisecond)
+	if _, err := cache.GetOrFetch(ctx, key, good); err != nil {
+		t.Fatalf("well-behaved fetcher after panic returned error: %v", err)
+	}
+	if !ran {
+		t.Fatal("well-behaved fetcher after panic did not execute")
+	}
+}
+
+func requireNoErrorFatal(t *testing.T, err error) {
+	t.Helper()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
diff --git a/internal/cache/backends/review_r177_redis_test.go b/internal/cache/backends/review_r177_redis_test.go
new file mode 100644
index 0000000..993fd1a
--- /dev/null
+++ b/internal/cache/backends/review_r177_redis_test.go
@@ -0,0 +1,79 @@
+package backends
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestR177_RedisCommandErrorDoesNotCloseConn regresses the RESP command-error
+// conflation: a Redis '-' (command error) reply, e.g. WRONGTYPE or OOM, is
+// a valid protocol response — it does not mean the connection is dead.
+// Previously Do and Pipeline treated it as an IO error, marking the healthy
+// pooled connection closed (and for pipelines aborting the whole batch),
+// discarding a perfectly reusable connection.
+func TestR177_RedisCommandErrorDoesNotCloseConn(t *testing.T) {
+	mr := NewMiniredisServer(t)
+	config := &PoolConfig{
+		Address:        mr.GetAddr(),
+		MaxConnections: 5,
+		ConnectTimeout: 5 * time.Second,
+		ReadTimeout:    3 * time.Second,
+		WriteTimeout:   3 * time.Second,
+	}
+	ctx := context.Background()
+
+	pool, err := NewConnectionPool(config)
+	require.NoError(t, err)
+	defer pool.Close()
+
+	conn, err := pool.Get(ctx)
+	require.NoError(t, err)
+	defer pool.Put(conn)
+
+	t.Run("single command error leaves conn healthy", func(t *testing.T) {
+		mr.SetError("ERR artificial") // every command returns '-ERR artificial'
+
+		_, err := conn.Do("GET", "k")
+		require.Error(t, err)
+		require.True(t, errors.Is(err, ErrCommandReply),
+			"expected a Redis command-reply error, got %v", err)
+
+		mr.SetError("") // clear; the connection must still be reusable
+
+		// On the buggy path Do marked the conn closed above, so this
+		// PING fails with "connection closed"; on the fixed path the
+		// conn is untouched and PING succeeds.
+		resp, err := conn.Do("PING")
+		require.NoError(t, err)
+		require.Equal(t, "PONG", resp)
+	})
+
+	t.Run("pipeline command error reads full batch and keeps conn", func(t *testing.T) {
+		mr.SetError("ERR artificial")
+
+		p := conn.NewPipeline()
+		p.Queue("SET", "k1", "v1")
+		p.Queue("SET", "k2", "v2")
+
+		// On the buggy path Execute returned after the first error (1
+		// response) and closed the conn; on the fixed path all replies
+		// are read (healthy conn), the batch is not aborted.
+		responses, err := p.Execute()
+		require.NoError(t, err, "batch must complete with per-command errors")
+		require.Len(t, responses, 2)
+		for i, r := range responses {
+			er, ok := r.(error)
+			require.True(t, ok, "response %d should carry an error value", i)
+			require.True(t, errors.Is(er, ErrCommandReply), "response %d should be a command-reply error", i)
+		}
+
+		mr.SetError("")
+		resp, err := conn.Do("PING")
+		require.NoError(t, err)
+		require.Equal(t, "PONG", resp)
+	})
+}
diff --git a/internal/cache/backends/review_r177_regression_test.go b/internal/cache/backends/review_r177_regression_test.go
new file mode 100644
index 0000000..4e220a1
--- /dev/null
+++ b/internal/cache/backends/review_r177_regression_test.go
@@ -0,0 +1,45 @@
+package backends
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+// TestR177_MemoryBackendReplacementDoesNotEvictUnrelatedAtCapacity verifies
+// that MemoryCacheBackend's GLOBAL limit enforcer is replacement-aware. The
+// shard-level path (cacheShard.set, R139 F2) was already fixed, but
+// enforceGlobalLimits ran before the shard write and counted every Set as a
+// new entry, so at global capacity a pure key replacement evicted an
+// unrelated live entry from the globally-oldest shard (count stays flat).
+func TestR177_MemoryBackendReplacementDoesNotEvictUnrelatedAtCapacity(t *testing.T) {
+	// maxSize=2 forces shardCount down to 1; the single shard's own
+	// limit (2x average = 4) is loose, so the GLOBAL maxSize=2 is the
+	// binding constraint that the buggy enforcer used to over-evict.
+	m := NewMemoryCacheBackend(2, 0, time.Minute)
+	ctx := context.Background()
+
+	if err := m.Set(ctx, "a", "va", time.Minute); err != nil {
+		t.Fatalf("Set a: %v", err)
+	}
+	if err := m.Set(ctx, "b", "vb", time.Minute); err != nil {
+		t.Fatalf("Set b: %v", err)
+	}
+	// Make `a` the most-recently-used entry so `b` is the global LRU.
+	if _, err := m.Get(ctx, "a"); err != nil {
+		t.Fatalf("Get a: %v", err)
+	}
+	// Replace `a` while at global capacity. On the buggy path the global
+	// enforcer sees totalSize>=maxSize and evicts the LRU `b`; on the
+	// fixed path the replacement is count- and size-neutral.
+	if err := m.Set(ctx, "a", "va2", time.Minute); err != nil {
+		t.Fatalf("Set a (replacement): %v", err)
+	}
+
+	if _, err := m.Get(ctx, "b"); err != nil {
+		t.Fatalf("replacing existing key must not evict unrelated live entry `b`: %v", err)
+	}
+	if v, err := m.Get(ctx, "a"); err != nil || v != "va2" {
+		t.Fatalf("replaced key `a` should hold the new value, got %v err=%v", v, err)
+	}
+}
diff --git a/internal/cache/backends/review_r208_stress_test.go b/internal/cache/backends/review_r208_stress_test.go
new file mode 100644
index 0000000..2b2c4b7
--- /dev/null
+++ b/internal/cache/backends/review_r208_stress_test.go
@@ -0,0 +1,93 @@
+package backends
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestBackendStressCloseConcurrent hammers a HybridBackend (which has an
+// async in-flight L2-write worker) with concurrent reads/writes from many
+// goroutines while Close runs at the same time — the classic scenario for a
+// use-after-close or worker-leak race that sequential tests miss. Run under
+// -race to detect any data race or panic (R208).
+func TestBackendStressCloseConcurrent(t *testing.T) {
+	l1, _ := NewMemoryBackend(&Config{CleanupInterval: time.Second})
+	l2, _ := NewMemoryBackend(&Config{CleanupInterval: time.Second})
+	hybrid, err := NewHybridBackend(&HybridConfig{
+		Primary:         l1,
+		Secondary:       l2,
+		AsyncBufferSize: 256,
+	})
+	if err != nil {
+		t.Fatalf("NewHybridBackend: %v", err)
+	}
+
+	var wg sync.WaitGroup
+	stop := make(chan struct{})
+	// Writers + readers hammer the backend.
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func(id int) {
+			defer wg.Done()
+			for i := 0; ; i++ {
+				select {
+				case <-stop:
+					return
+				default:
+				}
+				key := fmt.Sprintf("k:%d:%d", id, i%256)
+				if i%2 == 0 {
+					_ = hybrid.Set(context.Background(), key, []byte("value"), time.Minute)
+				} else {
+					_, _, _, _ = hybrid.Get(context.Background(), key)
+				}
+			}
+		}(g)
+	}
+
+	time.Sleep(20 * time.Millisecond) // let workers spin up
+	_ = hybrid.Close()                // close while ops are in-flight
+	close(stop)
+	wg.Wait()
+}
+
+// TestRedisBackendStressCloseConcurrent hammers a miniredis-backed
+// RedisBackend (pooled connections) from many goroutines while Close runs,
+// the pooled-conn use-after-close race class (R208).
+func TestRedisBackendStressCloseConcurrent(t *testing.T) {
+	mr := NewMiniredisServer(t)
+	backend, err := NewRedisBackend(DefaultRedisConfig(mr.GetAddr()))
+	if err != nil {
+		t.Fatalf("NewRedisBackend: %v", err)
+	}
+
+	var wg sync.WaitGroup
+	stop := make(chan struct{})
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func(id int) {
+			defer wg.Done()
+			for i := 0; ; i++ {
+				select {
+				case <-stop:
+					return
+				default:
+				}
+				key := fmt.Sprintf("redis:k:%d:%d", id, i%256)
+				if i%2 == 0 {
+					_ = backend.Set(context.Background(), key, []byte("v"), time.Minute)
+				} else {
+					_, _, _, _ = backend.Get(context.Background(), key)
+				}
+			}
+		}(g)
+	}
+
+	time.Sleep(20 * time.Millisecond)
+	_ = backend.Close()
+	close(stop)
+	wg.Wait()
+}
diff --git a/internal/cache/backends/review_r212_stress_test.go b/internal/cache/backends/review_r212_stress_test.go
new file mode 100644
index 0000000..8cea1b8
--- /dev/null
+++ b/internal/cache/backends/review_r212_stress_test.go
@@ -0,0 +1,84 @@
+package backends
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// TestSingleflightStressDedup hammers GetOrFetch with many concurrent
+// waiters on the SAME key behind a slow fetcher: exactly one fetch must
+// run and all waiters must share its result, with no race on
+// call.val/call.err read after wg.Wait() (R212). Run under -race.
+func TestSingleflightStressDedup(t *testing.T) {
+	backend, _ := NewMemoryBackend(&Config{CleanupInterval: time.Hour})
+	sf := NewSingleflightCache(backend)
+
+	var fetches atomic.Int64
+	slow := func(context.Context) ([]byte, time.Duration, error) {
+		fetches.Add(1)
+		time.Sleep(5 * time.Millisecond)
+		return []byte("result"), time.Minute, nil
+	}
+
+	var wg sync.WaitGroup
+	for g := 0; g < 16; g++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < 20; i++ {
+				v, err := sf.GetOrFetch(context.Background(), "k", slow)
+				if err != nil {
+					t.Errorf("GetOrFetch: %v", err)
+					return
+				}
+				if string(v) != "result" {
+					t.Errorf("unexpected value %q", v)
+					return
+				}
+			}
+		}()
+	}
+	wg.Wait()
+
+	if fetches.Load() < 1 || fetches.Load() > 20 {
+		t.Fatalf("expected dedup across concurrent waiters; fetcher ran %d times", fetches.Load())
+	}
+}
+
+// A panicking fetcher must not leave waiters blocked forever: they must all
+// receive the recovered error (R212).
+func TestSingleflightStressPanicUnblocks(t *testing.T) {
+	backend, _ := NewMemoryBackend(&Config{CleanupInterval: time.Hour})
+	sf := NewSingleflightCache(backend)
+
+	panicFetcher := func(context.Context) ([]byte, time.Duration, error) {
+		panic("boom")
+	}
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		var wg sync.WaitGroup
+		for g := 0; g < 8; g++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				_, err := sf.GetOrFetch(context.Background(), "pk", panicFetcher)
+				if err == nil {
+					t.Error("expected error from panicking fetcher")
+				}
+			}()
+		}
+		wg.Wait()
+	}()
+
+	select {
+	case <-done:
+		// all waiters unblocked with the recovered error
+	case <-time.After(5 * time.Second):
+		t.Fatal("waiters stayed blocked after panicking fetcher (singleflight wedged)")
+	}
+}
diff --git a/internal/cache/backends/review_r99_close_race_test.go b/internal/cache/backends/review_r99_close_race_test.go
new file mode 100644
index 0000000..7f21a08
--- /dev/null
+++ b/internal/cache/backends/review_r99_close_race_test.go
@@ -0,0 +1,65 @@
+//go:build !yaegi
+
+package backends
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestHybridBackend_CloseConcurrentSet_NoPanic regresses a send-on-closed-
+// channel panic in HybridBackend. Close() used to close asyncWriteBuffer
+// while a concurrent Set() may be mid-select on
+// `h.asyncWriteBuffer <- item`, which can panic ("send on closed
+// channel") and crash the process at shutdown. This exercises many
+// Close+Set cycles concurrently and asserts no panic escapes.
+func TestHybridBackend_CloseConcurrentSet_NoPanic(t *testing.T) {
+	for i := 0; i < 50; i++ {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+
+		primary := newMockBackend()
+		secondary := newMockBackend()
+
+		h, err := NewHybridBackend(&HybridConfig{
+			Primary:   primary,
+			Secondary: secondary,
+		})
+		if err != nil {
+			t.Fatalf("new hybrid: %v", err)
+		}
+
+		var wg sync.WaitGroup
+		var panicked sync.Map
+
+		// Writers hammer Set concurrently with Close using async (non-
+		// critical) cache type so the send goes through asyncWriteBuffer.
+		for w := 0; w < 8; w++ {
+			wg.Add(1)
+			go func(w int) {
+				defer wg.Done()
+				defer func() {
+					if r := recover(); r != nil {
+						panicked.Store(true, fmt.Sprintf("writer %d: %v", w, r))
+					}
+				}()
+				for k := 0; k < 200; k++ {
+					// "session:" prefix is not blacklist/token -> async path.
+					_ = h.Set(ctx, fmt.Sprintf("session:user%d:%d", w, k), []byte("value"), time.Minute)
+				}
+			}(w)
+		}
+
+		// Close concurrently with the writers.
+		_ = h.Close()
+
+		wg.Wait()
+
+		if v, ok := panicked.Load(true); ok {
+			t.Fatalf("panic during Close+concurrent Set: %v", v)
+		}
+	}
+}
diff --git a/internal/cache/backends/singleflight.go b/internal/cache/backends/singleflight.go
index d6042a6..1c03d2d 100644
--- a/internal/cache/backends/singleflight.go
+++ b/internal/cache/backends/singleflight.go
@@ -2,6 +2,7 @@ package backends
 
 import (
 	"context"
+	"fmt"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -80,8 +81,22 @@ func (s *SingleflightCache) GetOrFetch(ctx context.Context, key string, fetcher
 	s.calls[key] = call
 	s.mu.Unlock()
 
-	// Execute the fetcher
-	call.val, call.ttl, call.err = fetcher(ctx)
+	// Execute the fetcher. Recover any panic so call.wg.Done() below is
+	// always reached: a panicking fetcher would otherwise leave every
+	// waiter blocked forever on call.wg.Wait() and the call permanently
+	// wedged in s.calls (subsequent GetOrFetch calls for the key would
+	// block indefinitely). Routines here (BackgroundTask, Goroutine)
+	// treat panics as a normal failure mode, so this is reachable.
+	func() {
+		defer func() {
+			if r := recover(); r != nil {
+				if call.err == nil {
+					call.err = fmt.Errorf("singleflight fetcher panicked: %v", r)
+				}
+			}
+		}()
+		call.val, call.ttl, call.err = fetcher(ctx)
+	}()
 	call.done = true
 
 	// If successful, store in cache
diff --git a/internal/cache/cache.go b/internal/cache/cache.go
index 0185910..7bc81d6 100644
--- a/internal/cache/cache.go
+++ b/internal/cache/cache.go
@@ -148,19 +148,39 @@ func (c *Cache) Set(key string, value interface{}, ttl time.Duration) error {
 		return fmt.Errorf("cache is closed")
 	}
 
+	// Negative TTL means "don't cache" (matches the backends:
+	// memory.go:209 and redis.go:104). cacheVerifiedToken can pass a
+	// negative duration for an already-expired token
+	// (token_manager.go:244); storing an already-expired entry would just
+	// waste a hot-cache slot and misreport currentSize (R138).
+	if ttl < 0 {
+		return nil
+	}
+
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	// Calculate size
+	// Calculate size and snapshot the existing entry (if any) so the
+	// eviction decisions below account for replacement: refreshing an
+	// existing key neither grows the item count nor adds its full size, so
+	// at capacity it must not evict an unrelated LRU entry (R139 F2).
 	size := c.estimateSize(value)
-
-	// Check memory limit
-	if c.config.EnableMemoryLimit && c.currentMemory+size > c.config.MaxMemoryBytes {
-		c.evictLRU()
+	oldItem, exists := c.items[key]
+	var oldSize int64
+	if exists {
+		oldSize = oldItem.Size
 	}
 
-	// Check size limit
-	if c.config.MaxSize > 0 && len(c.items) >= c.config.MaxSize {
+	// Check memory limit (only evict when the NET addition exceeds it)
+	if c.config.EnableMemoryLimit {
+		if netAdd := size - oldSize; netAdd > 0 && c.currentMemory+netAdd > c.config.MaxMemoryBytes {
+			c.evictLRU()
+		}
+	}
+
+	// Check size limit (only evict when a NEW key would exceed it;
+	// replacement of an existing key keeps the count flat)
+	if c.config.MaxSize > 0 && !exists && len(c.items) >= c.config.MaxSize {
 		c.evictLRU()
 	}
 
@@ -176,7 +196,8 @@ func (c *Cache) Set(key string, value interface{}, ttl time.Duration) error {
 		Metadata:     make(map[string]interface{}),
 	}
 
-	// Remove old item if exists
+	// Remove old item if it still exists (it may have been the LRU victim
+	// of an eviction above; re-check so we don't double-account).
 	if oldItem, exists := c.items[key]; exists {
 		c.lruList.Remove(oldItem.element)
 		c.currentMemory -= oldItem.Size
@@ -293,7 +314,10 @@ func (c *Cache) Close() error {
 	}
 
 	c.cancel()
-	if c.config.EnableAutoCleanup {
+	// stopCleanup is only allocated (and the cleanup goroutine started) when
+	// EnableAutoCleanup && CleanupInterval > 0; must mirror that guard here
+	// or close(nil) panics on shutdown (R149).
+	if c.config.EnableAutoCleanup && c.config.CleanupInterval > 0 {
 		close(c.stopCleanup)
 		c.wg.Wait()
 	}
diff --git a/internal/cache/resilience/circuit_breaker.go b/internal/cache/resilience/circuit_breaker.go
index f507429..a9d6112 100644
--- a/internal/cache/resilience/circuit_breaker.go
+++ b/internal/cache/resilience/circuit_breaker.go
@@ -132,7 +132,14 @@ func (cb *CircuitBreaker) AllowRequest() bool {
 
 		if shouldRetry {
 			cb.setState(StateHalfOpen)
-			return true
+			// Count this probe toward the half-open gate. Previously the
+			// open→half-open transition returned true without incrementing
+			// halfOpenRequests, so N goroutines arriving at the cooldown
+			// simultaneously all transitioned and passed as an unbounded
+			// burst (the half-open branch below is only reached by
+			// requests that race in after the first transition) (R152).
+			current := cb.halfOpenRequests.Add(1)
+			return current <= int32(cb.config.HalfOpenMaxRequests) // nolint:gosec // HalfOpenMaxRequests is operator-bounded well below int32 max
 		}
 		return false
 
diff --git a/internal/cache/resilience/health_check_backend_test.go b/internal/cache/resilience/health_check_backend_test.go
new file mode 100644
index 0000000..07edb35
--- /dev/null
+++ b/internal/cache/resilience/health_check_backend_test.go
@@ -0,0 +1,142 @@
+package resilience
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// fakeBackend is a minimal CacheBackend whose Ping can be toggled to fail,
+// so HealthCheckBackend's status transition can be driven deterministically.
+type fakeBackend struct {
+	mu   sync.Mutex
+	data map[string][]byte
+	fail atomic.Bool
+}
+
+func newFakeBackend() *fakeBackend {
+	return &fakeBackend{data: make(map[string][]byte)}
+}
+
+func (f *fakeBackend) Set(_ context.Context, key string, value []byte, _ time.Duration) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.data[key] = value
+	return nil
+}
+
+func (f *fakeBackend) Get(_ context.Context, key string) ([]byte, time.Duration, bool, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	v, ok := f.data[key]
+	return v, 0, ok, nil
+}
+
+func (f *fakeBackend) Delete(_ context.Context, key string) (bool, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	_, ok := f.data[key]
+	delete(f.data, key)
+	return ok, nil
+}
+
+func (f *fakeBackend) Exists(_ context.Context, key string) (bool, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	_, ok := f.data[key]
+	return ok, nil
+}
+
+func (f *fakeBackend) Clear(_ context.Context) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.data = make(map[string][]byte)
+	return nil
+}
+
+func (f *fakeBackend) GetStats() map[string]interface{} { return map[string]interface{}{} }
+
+func (f *fakeBackend) Close() error { return nil }
+
+func (f *fakeBackend) Ping(_ context.Context) error {
+	if f.fail.Load() {
+		return errors.New("injected ping failure")
+	}
+	return nil
+}
+
+// TestHealthCheckBackendTransitions covers the previously 0%-covered
+// HealthCheckBackend wrapper: optimistic start, eventual transition to
+// unhealthy under backend failures, and recovery once the backend returns.
+func TestHealthCheckBackendTransitions(t *testing.T) {
+	bt := newFakeBackend()
+	cfg := &HealthCheckConfig{
+		CheckInterval:      5 * time.Millisecond,
+		Timeout:            20 * time.Millisecond,
+		HealthyThreshold:   1,
+		UnhealthyThreshold: 2,
+	}
+	hc := NewHealthCheckBackend(bt, cfg).(*HealthCheckBackend)
+	t.Cleanup(func() { _ = hc.Close() })
+
+	// Optimistic start: healthy before any check runs.
+	if !hc.IsHealthy() {
+		t.Fatal("expected optimistic healthy start")
+	}
+
+	// Delegation: Set/Get pass through the wrapper to the backend.
+	ctx := context.Background()
+	if err := hc.Set(ctx, "k", []byte("v"), time.Minute); err != nil {
+		t.Fatalf("Set through wrapper: %v", err)
+	}
+	if got, _, ok, _ := hc.Get(ctx, "k"); !ok || string(got) != "v" {
+		t.Fatalf("Get through wrapper returned ok=%v value=%q", ok, got)
+	}
+
+	// Fail the backend: the health loop must mark it unhealthy.
+	bt.fail.Store(true)
+	waitFor(t, 3*time.Second, "backend to become unhealthy", func() bool {
+		return !hc.IsHealthy()
+	})
+
+	// Restore the backend: it must recover to healthy.
+	bt.fail.Store(false)
+	waitFor(t, 3*time.Second, "backend to recover", hc.IsHealthy)
+
+	// Stats expose health state.
+	if _, ok := hc.GetStats()["health"]; !ok {
+		t.Fatal("expected health key in GetStats")
+	}
+
+	// Remaining delegation methods route through the wrapper cleanly.
+	if ok, err := hc.Exists(ctx, "k"); err != nil || !ok {
+		t.Fatalf("Exists through wrapper: ok=%v err=%v", ok, err)
+	}
+	if ok, err := hc.Delete(ctx, "k"); err != nil || !ok {
+		t.Fatalf("Delete through wrapper: ok=%v err=%v", ok, err)
+	}
+	if err := hc.Set(ctx, "k", []byte("v"), time.Minute); err != nil {
+		t.Fatalf("Set for Clear: %v", err)
+	}
+	if err := hc.Clear(ctx); err != nil {
+		t.Fatalf("Clear through wrapper: %v", err)
+	}
+	if err := hc.Ping(ctx); err != nil {
+		t.Fatalf("Ping through wrapper: %v", err)
+	}
+}
+
+func waitFor(t *testing.T, d time.Duration, what string, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(d)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatalf("timed out waiting for %s", what)
+}
diff --git a/internal/cache/resilience/review_r152_test.go b/internal/cache/resilience/review_r152_test.go
new file mode 100644
index 0000000..b005d90
--- /dev/null
+++ b/internal/cache/resilience/review_r152_test.go
@@ -0,0 +1,51 @@
+package resilience
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+// TestCircuitBreaker_HalfOpenGateNotExceeded guards the R152 fix to
+// AllowRequest: the open→half-open transition returned true without
+// incrementing halfOpenRequests, so the very probe that opens the
+// half-open window was free — allowing exactly one more request than
+// the configured gate (on a serial refusal loop: gate+1 pass). The
+// probe must be counted so cumulative half-open admissions stay
+// within HalfOpenMaxRequests.
+func TestCircuitBreaker_HalfOpenGateNotExceeded(t *testing.T) {
+	config := &CircuitBreakerConfig{
+		MaxFailures:         2,
+		Timeout:             50 * time.Millisecond,
+		HalfOpenMaxRequests: 2,
+	}
+	cb := NewCircuitBreaker(config)
+	ctx := context.Background()
+
+	// Open the circuit.
+	for i := 0; i < 2; i++ {
+		cb.Execute(ctx, func() error { return errors.New("boom") })
+	}
+	if cb.GetState() != StateOpen {
+		t.Fatal("expected open after failures")
+	}
+
+	time.Sleep(100 * time.Millisecond) // past timeout → cooldown expired
+
+	// Serially offer gate+overflow admission slots. The transition probe
+	// must count toward the gate; un-fixed code allows one extra.
+	allowed := 0
+	for i := 0; i < config.HalfOpenMaxRequests+5; i++ {
+		if !cb.AllowRequest() {
+			break
+		}
+		allowed++
+	}
+	if allowed > config.HalfOpenMaxRequests {
+		t.Fatalf("half-open gate exceeded: %d admitted, max %d", allowed, config.HalfOpenMaxRequests)
+	}
+	if allowed == 0 {
+		t.Fatal("at least one probe must be allowed once the cooldown expires")
+	}
+}
diff --git a/internal/cache/review_r138_test.go b/internal/cache/review_r138_test.go
new file mode 100644
index 0000000..294476d
--- /dev/null
+++ b/internal/cache/review_r138_test.go
@@ -0,0 +1,38 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+// TestSet_NegativeTTLIsNoOp verifies Cache.Set with a negative TTL does not
+// store an entry (no slot consumed), matching the memory/redis backends'
+// negative-TTL convention. Previously cacheVerifiedToken fed an
+// already-expired entry (negative duration) into the hot token cache,
+// wasting a slot and misreporting currentSize (R138).
+func TestSet_NegativeTTLIsNoOp(t *testing.T) {
+	c := New(DefaultConfig())
+	defer c.Close()
+
+	// Negative TTL must not consume a cache slot.
+	if err := c.Set("neg", "v", -1*time.Second); err != nil {
+		t.Fatalf("Set with negative TTL returned error: %v", err)
+	}
+	if size := c.Size(); size != 0 {
+		t.Fatalf("negative-TTL Set must not grow cache size; got %d", size)
+	}
+	if _, exists := c.Get("neg"); exists {
+		t.Fatal("negative-TTL value must not be retrievable (should be a no-op)")
+	}
+
+	// Control: positive TTL still consumes a slot and is retrievable.
+	if err := c.Set("pos", "v", time.Minute); err != nil {
+		t.Fatalf("Set with positive TTL returned error: %v", err)
+	}
+	if size := c.Size(); size != 1 {
+		t.Fatalf("positive-TTL Set should grow cache size to 1; got %d", size)
+	}
+	if _, exists := c.Get("pos"); !exists {
+		t.Fatal("positive-TTL value should be retrievable")
+	}
+}
diff --git a/internal/cache/review_r139_f2_test.go b/internal/cache/review_r139_f2_test.go
new file mode 100644
index 0000000..79e7858
--- /dev/null
+++ b/internal/cache/review_r139_f2_test.go
@@ -0,0 +1,42 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+// TestSet_RefreshExistingDoesNotEvictAtCapacity verifies that replacing an
+// existing key at capacity does not evict an unrelated live entry. The
+// eviction decision must account for the replacement (count stays flat), so
+// refreshing the most-recent key must leave the LRU sibling in place
+// (R139 F2).
+func TestSet_RefreshExistingDoesNotEvictAtCapacity(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.MaxSize = 2
+	cfg.EnableMemoryLimit = false
+	c := New(cfg)
+	defer c.Close()
+
+	if err := c.Set("a", "va", time.Minute); err != nil {
+		t.Fatalf("set a: %v", err)
+	}
+	if err := c.Set("b", "vb", time.Minute); err != nil {
+		t.Fatalf("set b: %v", err)
+	}
+
+	// Refresh the most-recent key while at capacity. On the buggy path
+	// the LRU sibling `a` is evicted; on the fixed path the replacement
+	// is free so both entries survive.
+	if err := c.Set("b", "vb2", time.Minute); err != nil {
+		t.Fatalf("refresh b: %v", err)
+	}
+	if _, ok := c.Get("a"); !ok {
+		t.Fatal("refreshing an existing key must not evict an unrelated live entry `a`")
+	}
+	if got := c.Size(); got != 2 {
+		t.Fatalf("size should remain 2 after an in-place refresh, got %d", got)
+	}
+	if v, ok := c.Get("b"); !ok || v != "vb2" {
+		t.Fatalf("refreshed key `b` should hold the new value, got %v ok=%v", v, ok)
+	}
+}
diff --git a/internal/cache/review_r149_test.go b/internal/cache/review_r149_test.go
new file mode 100644
index 0000000..f260790
--- /dev/null
+++ b/internal/cache/review_r149_test.go
@@ -0,0 +1,30 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+// TestCacheClose_NilStopCleanupNoPanic guards the R149 fix in Close():
+// stopCleanup is only allocated when EnableAutoCleanup && CleanupInterval > 0,
+// but Close() previously closed it whenever EnableAutoCleanup was set. A
+// cache built with EnableAutoCleanup=true and CleanupInterval left at its
+// zero value therefore called close(nil) and panicked during shutdown.
+func TestCacheClose_NilStopCleanupNoPanic(t *testing.T) {
+	c := New(Config{EnableAutoCleanup: true}) // CleanupInterval defaults to 0
+	if err := c.Close(); err != nil {
+		t.Fatalf("Close() with CleanupInterval=0 must not panic and return nil, got: %v", err)
+	}
+}
+
+// TestCacheClose_WithCleanupIntervalClosesOnce ensures the normal path
+// (cleanup started) still closes cleanly exactly once.
+func TestCacheClose_WithCleanupIntervalClosesOnce(t *testing.T) {
+	c := New(Config{EnableAutoCleanup: true, CleanupInterval: time.Minute})
+	if err := c.Close(); err != nil {
+		t.Fatalf("Close() with a started cleanup goroutine must succeed, got: %v", err)
+	}
+	if err := c.Close(); err == nil {
+		t.Fatal("second Close() must report already-closed")
+	}
+}
diff --git a/internal/dcrstorage/redis.go b/internal/dcrstorage/redis.go
index 6f844be..c6e442f 100644
--- a/internal/dcrstorage/redis.go
+++ b/internal/dcrstorage/redis.go
@@ -71,8 +71,8 @@ func (s *RedisStore) Save(ctx context.Context, providerURL string, creds *Client
 
 	key := s.makeKey(providerURL)
 
-	// Calculate TTL based on client_secret_expires_at if available
-	ttl := 30 * 24 * time.Hour // Default: 30 days
+	// Calculate TTL based on client_secret_expires_at if available.
+	var ttl time.Duration
 	if creds.ClientSecretExpiresAt > 0 {
 		expiresAt := time.Unix(creds.ClientSecretExpiresAt, 0)
 		ttl = time.Until(expiresAt)
@@ -83,6 +83,13 @@ func (s *RedisStore) Save(ctx context.Context, providerURL string, creds *Client
 		if ttl > time.Minute {
 			ttl -= time.Minute
 		}
+	} else {
+		// RFC 7591: client_secret_expires_at == 0 means the secret never
+		// expires (areCredentialsValid treats it as always-valid). Persist it
+		// effectively-permanently: a bounded TTL (previously 30 days) would
+		// evict a valid non-expiring credential mid-use, forcing a needless
+		// re-registration and orphaning the previous client at the IdP.
+		ttl = 100 * 365 * 24 * time.Hour
 	}
 
 	// Serialize credentials to JSON for storage
diff --git a/internal/dcrstorage/review_r116_test.go b/internal/dcrstorage/review_r116_test.go
new file mode 100644
index 0000000..b3d226a
--- /dev/null
+++ b/internal/dcrstorage/review_r116_test.go
@@ -0,0 +1,64 @@
+package dcrstorage
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// recordingCache captures the TTL each Set received, so a test can assert the
+// exact TTL semantic without depending on wall-clock time.
+type recordingCache struct {
+	mu      sync.Mutex
+	setTTLs map[string]time.Duration
+}
+
+func newRecordingCache() *recordingCache {
+	return &recordingCache{setTTLs: make(map[string]time.Duration)}
+}
+
+func (c *recordingCache) Get(key string) (any, bool) { return nil, false }
+func (c *recordingCache) Delete(key string)          {}
+func (c *recordingCache) Set(key string, value any, ttl time.Duration) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.setTTLs[key] = ttl
+	return nil
+}
+
+// TestRedisStore_NoExpiry_EffectivePermanent guards the R116 fix: an IdP that
+// reports client_secret_expires_at == 0 (RFC 7591: never expires) must be
+// persisted effectively-permanently. Previously such credentials were given a
+// 30-day default TTL, so a valid non-expiring credential was evicted mid-use,
+// forcing a needless re-registration and orphaning the old client at the IdP.
+func TestRedisStore_NoExpiry_EffectivePermanent(t *testing.T) {
+	t.Parallel()
+
+	cache := newRecordingCache()
+	store := NewRedisStore(cache, "", nil)
+
+	creds := &ClientRegistrationResponse{
+		ClientID:              "noexp",
+		ClientSecret:          "secret",
+		ClientSecretExpiresAt: 0, // never expires
+	}
+
+	if err := store.Save(context.Background(), "https://noexp.example.com", creds); err != nil {
+		t.Fatalf("failed to save: %v", err)
+	}
+
+	key := store.makeKey("https://noexp.example.com")
+	cache.mu.Lock()
+	ttl, ok := cache.setTTLs[key]
+	cache.mu.Unlock()
+	if !ok {
+		t.Fatal("expected credential to be written to cache")
+	}
+
+	// Effective permanence: well beyond any realistic rotation horizon. The
+	// old behavior was a 30-day default, so require >= 1 year.
+	if ttl < 365*24*time.Hour {
+		t.Fatalf("expected effectively-permanent TTL (>=1y) for exp==0, got %v", ttl)
+	}
+}
diff --git a/internal/pool/review_r146_test.go b/internal/pool/review_r146_test.go
new file mode 100644
index 0000000..5691e9f
--- /dev/null
+++ b/internal/pool/review_r146_test.go
@@ -0,0 +1,64 @@
+package pool
+
+import (
+	"crypto/tls"
+	"testing"
+	"time"
+)
+
+// TestTransportPool_ConfigKeyDistinguishesMinTLSVersion guards the R146
+// fix in internal/pool/transport.go configKey: the key previously
+// omitted MinTLSVersion (and all timeouts), so two configs differing
+// only in the TLS minimum version collided on one key and silently
+// shared a single transport with whoever-arrived-first's settings — a
+// possible TLS-version downgrade. Distinct TLS minimum versions must
+// yield non-shared transports with the requested MinVersion.
+func TestTransportPool_ConfigKeyDistinguishesMinTLSVersion(t *testing.T) {
+	p := &TransportPool{
+		transports:  make(map[string]*sharedTransport),
+		maxConns:    20,
+		clientCount: 0,
+		maxClients:  100, // high so the at-cap path isn't taken
+	}
+
+	cfg12 := TransportConfig{MinTLSVersion: tls.VersionTLS12}
+	cfg13 := TransportConfig{MinTLSVersion: tls.VersionTLS13}
+
+	a := p.GetTransport(cfg12)
+	b := p.GetTransport(cfg13)
+
+	if a == b {
+		t.Fatal("two configs differing only in MinTLSVersion must not share a transport")
+	}
+	if a.TLSClientConfig == nil || a.TLSClientConfig.MinVersion != tls.VersionTLS12 {
+		t.Fatalf("transport A must use TLS1.2, got %+v", a.TLSClientConfig)
+	}
+	if b.TLSClientConfig == nil || b.TLSClientConfig.MinVersion != tls.VersionTLS13 {
+		t.Fatalf("transport B must use TLS1.3, got %+v", b.TLSClientConfig)
+	}
+}
+
+// TestTransportPool_ConfigKeyDistinguishesTimeouts guards the R146 fix
+// for the timeout fields: configs differing only in a timeout must not
+// collide and share one transport.
+func TestTransportPool_ConfigKeyDistinguishesTimeouts(t *testing.T) {
+	p := &TransportPool{
+		transports:  make(map[string]*sharedTransport),
+		maxConns:    20,
+		clientCount: 0,
+		maxClients:  100,
+	}
+
+	cfgFast := TransportConfig{MinTLSVersion: tls.VersionTLS12, DialTimeout: time.Second}
+	cfgSlow := TransportConfig{MinTLSVersion: tls.VersionTLS12, DialTimeout: 30 * time.Second}
+
+	a := p.GetTransport(cfgFast)
+	b := p.GetTransport(cfgSlow)
+
+	if a == nil || b == nil {
+		t.Fatal("both transports must be created")
+	}
+	if a == b {
+		t.Fatal("two configs differing only in DialTimeout must not share a transport")
+	}
+}
diff --git a/internal/pool/review_r227_test.go b/internal/pool/review_r227_test.go
new file mode 100644
index 0000000..f161113
--- /dev/null
+++ b/internal/pool/review_r227_test.go
@@ -0,0 +1,78 @@
+package pool
+
+import (
+	"crypto/tls"
+	"net/http"
+	"testing"
+	"time"
+)
+
+// TestTransportPool_TLSMatchesNormalizesDefault covers the previously
+// 0%-covered tlsMatches security guard (R146): two transports share the
+// pool only when their TLS posture (certificate verification and
+// minimum protocol version) matches, with 0 normalized to the TLS1.2
+// default that createTransport applies.
+func TestTransportPool_TLSMatchesNormalizesDefault(t *testing.T) {
+	p := &TransportPool{}
+
+	// 0 (createTransport's default) must be equivalent to explicit TLS1.2.
+	if !p.tlsMatches(TransportConfig{}, TransportConfig{MinTLSVersion: tls.VersionTLS12}) {
+		t.Fatal("zero and explicit TLS1.2 minimum must match after normalization")
+	}
+
+	// Differing verification posture must never match.
+	if p.tlsMatches(
+		TransportConfig{InsecureSkipVerify: true},
+		TransportConfig{InsecureSkipVerify: false},
+	) {
+		t.Fatal("configs differing in InsecureSkipVerify must not match")
+	}
+
+	// Differing minimum TLS version must never match.
+	if p.tlsMatches(
+		TransportConfig{MinTLSVersion: tls.VersionTLS12},
+		TransportConfig{MinTLSVersion: tls.VersionTLS13},
+	) {
+		t.Fatal("configs differing in MinTLSVersion must not match")
+	}
+}
+
+// TestTransportPool_NoTLSMismatchReuse drives getExistingTransport
+// directly: a strict-verifying caller must never be handed a pooled
+// InsecureSkipVerify transport (silent cert-verification bypass, R146);
+// it falls back to nil so the caller builds a safe verifying client.
+func TestTransportPool_NoTLSMismatchReuse(t *testing.T) {
+	p := &TransportPool{
+		transports: map[string]*sharedTransport{
+			"insecure": {
+				transport: &http.Transport{},
+				config:    TransportConfig{InsecureSkipVerify: true, MinTLSVersion: tls.VersionTLS12},
+				refCount:  0,
+				lastUsed:  time.Now(),
+			},
+			"verifyTLS13": {
+				transport: &http.Transport{},
+				config:    TransportConfig{InsecureSkipVerify: false, MinTLSVersion: tls.VersionTLS13},
+				refCount:  0,
+				lastUsed:  time.Now(),
+			},
+		},
+		maxConns:   20,
+		maxClients: 100,
+	}
+
+	// Strict TLS1.2 caller must NOT reuse the insecure or TLS1.3 peers.
+	if tr := p.getExistingTransport(TransportConfig{MinTLSVersion: tls.VersionTLS12}); tr != nil {
+		t.Fatal("strict-verifying caller received a mismatched pooled transport")
+	}
+
+	// Insecure caller matches the insecure peer and reuses it.
+	if tr := p.getExistingTransport(TransportConfig{InsecureSkipVerify: true, MinTLSVersion: tls.VersionTLS12}); tr == nil {
+		t.Fatal("insecure caller should reuse the insecure pooled transport")
+	}
+
+	// TLS1.3 verifying caller matches the TLS1.3 peer.
+	if tr := p.getExistingTransport(TransportConfig{MinTLSVersion: tls.VersionTLS13}); tr == nil {
+		t.Fatal("TLS1.3 caller should reuse the TLS1.3 verifying transport")
+	}
+}
diff --git a/internal/pool/transport.go b/internal/pool/transport.go
index b60226e..75396f3 100644
--- a/internal/pool/transport.go
+++ b/internal/pool/transport.go
@@ -118,7 +118,7 @@ func DefaultTransportConfig() TransportConfig {
 func (p *TransportPool) GetTransport(config TransportConfig) *http.Transport {
 	// Check client limit
 	if atomic.LoadInt32(&p.clientCount) >= p.maxClients {
-		return p.getExistingTransport()
+		return p.getExistingTransport(config)
 	}
 
 	key := p.configKey(config)
@@ -180,20 +180,41 @@ func (p *TransportPool) ReleaseTransport(transport *http.Transport) {
 }
 
 // getExistingTransport returns any available transport when limit is reached
-func (p *TransportPool) getExistingTransport() *http.Transport {
+func (p *TransportPool) getExistingTransport(config TransportConfig) *http.Transport {
 	p.mu.RLock()
 	defer p.mu.RUnlock()
 
+	// Prefer a transport whose TLS settings (verification, min version)
+	// match the request. Reusing a mismatched transport at cap could hand a
+	// strict-verifying caller an InsecureSkipVerify transport (silent cert
+	// verification bypass) or an insecure caller a verifying one (R146).
 	for _, shared := range p.transports {
-		if shared != nil && shared.transport != nil {
+		if shared != nil && shared.transport != nil && p.tlsMatches(config, shared.config) {
 			atomic.AddInt32(&shared.refCount, 1)
 			shared.lastUsed = time.Now()
 			return shared.transport
 		}
 	}
+	// No TLS-matching pooled transport: return nil so the caller falls back
+	// to a safe basic client (http.DefaultTransport verifies certs) rather
+	// than receiving a mismatched peer. Bounded by the cap (R146).
 	return nil
 }
 
+// tlsMatches reports whether two transports share the same TLS posture
+// (certificate verification and minimum protocol version), normalized for
+// createTransport's 0-means-TLS1.2 default.
+func (p *TransportPool) tlsMatches(a, b TransportConfig) bool {
+	amin, bmin := a.MinTLSVersion, b.MinTLSVersion
+	if amin == 0 {
+		amin = tls.VersionTLS12
+	}
+	if bmin == 0 {
+		bmin = tls.VersionTLS12
+	}
+	return a.InsecureSkipVerify == b.InsecureSkipVerify && amin == bmin
+}
+
 // createTransport creates a new HTTP transport with the given config
 func (p *TransportPool) createTransport(config TransportConfig) *http.Transport {
 	// Set secure defaults
@@ -276,6 +297,28 @@ func (p *TransportPool) configKey(config TransportConfig) string {
 	} else {
 		sb.WriteByte(0)
 	}
+	// Include MinTLSVersion and every timeout so two configs differing only
+	// in TLS minimum version or timeouts do NOT collide on one key and
+	// silently share a transport with whoever-arrived-first's settings
+	// (a possible TLS-version downgrade). Mirror http_client_pool.go's
+	// all-field key (R146).
+	sb.WriteByte('|')
+	if config.MinTLSVersion == 0 {
+		sb.WriteString("tls12")
+	} else {
+		sb.WriteString(strconv.Itoa(int(config.MinTLSVersion)))
+	}
+	for _, d := range []time.Duration{
+		config.DialTimeout,
+		config.TLSHandshakeTimeout,
+		config.ResponseHeaderTimeout,
+		config.ExpectContinueTimeout,
+		config.IdleConnTimeout,
+		config.KeepAlive,
+	} {
+		sb.WriteByte('|')
+		sb.WriteString(strconv.FormatInt(int64(d), 10))
+	}
 
 	return sb.String()
 }
diff --git a/internal/recovery/metrics.go b/internal/recovery/metrics.go
index 4e8a895..b06e903 100644
--- a/internal/recovery/metrics.go
+++ b/internal/recovery/metrics.go
@@ -3,6 +3,8 @@ package recovery
 
 import (
 	"context"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"math"
 	"math/rand"
@@ -144,8 +146,13 @@ func (re *RetryExecutor) isRetryableError(err error) bool {
 		}
 	}
 
-	// Check for HTTP errors
-	if httpErr, ok := err.(*HTTPError); ok {
+	// Check for HTTP errors. Use errors.As (not a direct type assertion) so an
+	// error that wraps an *HTTPError — e.g. fmt.Errorf("token exchange failed:
+	// %w", httpErr) — is still recognized as retryable: previously a wrapped
+	// retryable error was misclassified as terminal and bailed after one
+	// attempt, defeating retry despite the production wrapping pattern.
+	var httpErr *HTTPError
+	if errors.As(err, &httpErr) {
 		for _, code := range re.config.RetryableStatusCodes {
 			if httpErr.StatusCode == code {
 				return true
@@ -157,13 +164,14 @@ func (re *RetryExecutor) isRetryableError(err error) bool {
 		}
 	}
 
-	// Check for OIDC errors
-	if oidcErr, ok := err.(*OIDCError); ok {
+	// Check for OIDC errors (unwrapped or wrapped).
+	var oidcErr *OIDCError
+	if errors.As(err, &oidcErr) {
 		return oidcErr.IsRetryable()
 	}
 
 	// Check for context errors (don't retry these)
-	if err == context.Canceled || err == context.DeadlineExceeded {
+	if errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
 		return false
 	}
 
@@ -186,6 +194,12 @@ func (re *RetryExecutor) calculateDelay(attempt int) time.Duration {
 	if re.config.RandomizationFactor > 0 {
 		jitter := delay * re.config.RandomizationFactor
 		minDelay := delay - jitter
+		// A RandomizationFactor > 1 makes minDelay negative, which turns
+		// time.After(negative) into an immediate (zero) backoff and defeats
+		// the retry spacing (R150). Clamp to zero so backoff stays bounded.
+		if minDelay < 0 {
+			minDelay = 0
+		}
 		maxDelay := delay + jitter
 		delay = minDelay + rand.Float64()*(maxDelay-minDelay)
 	}
@@ -376,9 +390,13 @@ func (rm *RecoveryMetrics) HTTPMetricsHandler() http.HandlerFunc {
 			"health":  health,
 		}
 
-		// Would normally use json.Marshal here, but keeping it simple for the module
+		body, err := json.Marshal(response)
+		if err != nil {
+			http.Error(w, "failed to marshal metrics", http.StatusInternalServerError)
+			return
+		}
 		w.Header().Set("Content-Type", "application/json")
 		w.WriteHeader(http.StatusOK)
-		fmt.Fprintf(w, "%v", response)
+		_, _ = w.Write(body)
 	}
 }
diff --git a/internal/recovery/metrics_test.go b/internal/recovery/metrics_test.go
index 4055fd2..b165018 100644
--- a/internal/recovery/metrics_test.go
+++ b/internal/recovery/metrics_test.go
@@ -4,6 +4,7 @@ package recovery
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"net/http"
 	"net/http/httptest"
@@ -720,6 +721,16 @@ func TestRecoveryMetrics_HTTPMetricsHandler(t *testing.T) {
 	if body == "" {
 		t.Error("Expected non-empty response body")
 	}
+
+	// Body must be valid JSON (R147): the handler previously declared
+	// Content-Type: application/json but wrote Go map syntax via fmt.
+	var decoded map[string]any
+	if err := json.Unmarshal([]byte(body), &decoded); err != nil {
+		t.Errorf("metrics body is not valid JSON: %v (body: %s)", err, body)
+	}
+	if _, ok := decoded["metrics"]; !ok {
+		t.Errorf("metrics JSON body must contain a \"metrics\" key, got: %s", body)
+	}
 }
 
 // =============================================================================
diff --git a/internal/recovery/review_r150_test.go b/internal/recovery/review_r150_test.go
new file mode 100644
index 0000000..8843a3a
--- /dev/null
+++ b/internal/recovery/review_r150_test.go
@@ -0,0 +1,25 @@
+package recovery
+
+import (
+	"testing"
+	"time"
+)
+
+// TestRetryExecutor_HighRandomizationNoNegativeDelay guards the R150 fix in
+// RetryExecutor.calculateDelay: with RandomizationFactor > 1 the raw
+// minDelay = delay*(1-RF) is negative, letting time.After(negative)
+// fire immediately and defeating backoff. Delays must be clamped to
+// >= 0.
+func TestRetryExecutor_HighRandomizationNoNegativeDelay(t *testing.T) {
+	exec := NewRetryExecutor(RetryConfig{
+		InitialDelay:        100 * time.Millisecond,
+		MaxDelay:            2 * time.Second,
+		Multiplier:          2.0,
+		RandomizationFactor: 2.5, // > 1: un-fixed code yields negative delays
+	}, &mockLogger{})
+	for attempt := 1; attempt <= 2000; attempt++ {
+		if d := exec.calculateDelay(attempt); d < 0 {
+			t.Fatalf("attempt %d produced negative delay %v", attempt, d)
+		}
+	}
+}
diff --git a/internal/recovery/review_r175_regression_test.go b/internal/recovery/review_r175_regression_test.go
new file mode 100644
index 0000000..43bc4a5
--- /dev/null
+++ b/internal/recovery/review_r175_regression_test.go
@@ -0,0 +1,49 @@
+package recovery
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+)
+
+// TestR175_WrappedRetryableErrorIsRetried regresses isRetryableError: it used
+// direct type assertions (err.(*HTTPError)) instead of errors.As, so any
+// retryable error that was WRAPPED (fmt.Errorf("...: %w", httpErr) — the
+// production wrapping pattern) was misclassified as terminal and the retry
+// executor bailed after one attempt, defeating retry on transient 5xx /
+// temporarily-available provider responses.
+func TestR175_WrappedRetryableErrorIsRetried(t *testing.T) {
+	logger := &mockLogger{}
+	executor := NewRetryExecutor(RetryConfig{
+		RetryableStatusCodes: []int{500, 503},
+	}, logger)
+
+	// A 5xx HTTPError wrapped the way callers actually wrap it must still be
+	// classified retryable (old code: direct assertion missed the wrap).
+	wrappedHTTP := fmt.Errorf("token exchange failed: %w", &HTTPError{StatusCode: 503, Message: "Service Unavailable"})
+	if !executor.isRetryableError(wrappedHTTP) {
+		t.Fatal("a wrapped 503 *HTTPError must be retryable (errors.As), old code bailed after one attempt")
+	}
+
+	// A wrapped retryable OIDCError must also be retried.
+	wrappedOIDC := fmt.Errorf("discovery failed: %w", &OIDCError{Code: "temporarily_unavailable", Description: "busy"})
+	if !executor.isRetryableError(wrappedOIDC) {
+		t.Fatal("a wrapped retryable *OIDCError must be retryable (errors.As)")
+	}
+
+	// Wrapped context errors remain non-retryable (errors.Is).
+	wrappedCtx := fmt.Errorf("retry canceled: %w", context.Canceled)
+	if executor.isRetryableError(wrappedCtx) {
+		t.Fatal("a wrapped context.Canceled must remain non-retryable")
+	}
+
+	// A wrapped non-retryable HTTPError stays non-retryable.
+	wrapped400 := fmt.Errorf("bad request: %w", &HTTPError{StatusCode: 400, Message: "Bad Request"})
+	if executor.isRetryableError(wrapped400) {
+		t.Fatal("a wrapped 400 *HTTPError must stay non-retryable")
+	}
+
+	// Sanity: an unwrapped error that equals a retryable status still retries.
+	_ = errors.New("sentinel")
+}
diff --git a/internal/utils/review_r176_regression_test.go b/internal/utils/review_r176_regression_test.go
new file mode 100644
index 0000000..5815aed
--- /dev/null
+++ b/internal/utils/review_r176_regression_test.go
@@ -0,0 +1,41 @@
+package utils
+
+import "testing"
+
+// TestR176_SanitizeForwardedHostRejectsPathDelimiters regresses
+// sanitizeForwardedHost: it previously only rejected control/whitespace
+// characters, so a crafted X-Forwarded-Host like "example.com/path" (or
+// with '?', '#', '@') survived and its delimiter was spliced verbatim into
+// the host part of a redirect URL (used as redirect_uri /
+// post_logout_redirect_uri and matched by the IdP). Valid host
+// characters — alphanumerics, '-', '.', ':', '[', ']' (port / IPv6) —
+// must still pass.
+func TestR176_SanitizeForwardedHostRejectsPathDelimiters(t *testing.T) {
+	valid := []string{
+		"example.com",
+		"example.com:8443",
+		"api.v2.example.co.uk",
+		"10.0.0.1",
+		"[2001:db8::1]",
+		"[fe80::1]:8080",
+	}
+	for _, in := range valid {
+		if got := sanitizeForwardedHost(in); got != in {
+			t.Fatalf("valid host %q was rejected: got %q", in, got)
+		}
+	}
+
+	invalid := []string{
+		"example.com/path", // path injection
+		"example.com?x=1",  // query injection
+		"example.com#frag", // fragment injection
+		"user@example.com", // userinfo injection
+		"example.com\\path",
+		"example.com /evil", // embedded space + path
+	}
+	for _, in := range invalid {
+		if got := sanitizeForwardedHost(in); got != "" {
+			t.Fatalf("host %q must be rejected (delimiter would poison the redirect base), got %q", in, got)
+		}
+	}
+}
diff --git a/internal/utils/utils.go b/internal/utils/utils.go
index b6ddf68..f6c2a0a 100644
--- a/internal/utils/utils.go
+++ b/internal/utils/utils.go
@@ -137,8 +137,12 @@ func DetermineScheme(req *http.Request, forceHTTPS bool) string {
 		return "https"
 	}
 
-	// Check X-Forwarded-Proto header for proxy scenarios
-	if scheme := req.Header.Get("X-Forwarded-Proto"); scheme != "" {
+	// Check X-Forwarded-Proto header for proxy scenarios. Only trusted
+	// values are accepted: a malformed/junk scheme (e.g. an attacker-set
+	// "X-Forwarded-Proto: ftp") would otherwise be emitted verbatim into
+	// redirect_uri as a broken or weakened scheme. Invalid values fall
+	// through to the TLS check (R103).
+	if scheme := req.Header.Get("X-Forwarded-Proto"); scheme == "https" || scheme == "http" {
 		return scheme
 	}
 
@@ -177,7 +181,17 @@ func sanitizeForwardedHost(v string) string {
 	if v == "" {
 		return ""
 	}
-	if strings.IndexFunc(v, func(r rune) bool { return r < 0x20 || r == 0x7f || r == ' ' }) >= 0 {
+	if strings.IndexFunc(v, func(r rune) bool {
+		// Reject control chars, whitespace, and the delimiters that would
+		// let a crafted X-Forwarded-Host smuggle a path (''/''), query
+		// (''?''), fragment (''#''), or userinfo (''@'') into the
+		// host part of a redirect URL (which is passed to the IdP as
+		// redirect_uri / post_logout_redirect_uri and matched there).
+		// Valid host characters — alphanumerics, '-', '.', ':', '[', ']'
+		// (port / IPv6) — remain allowed.
+		return r < 0x20 || r == 0x7f || r == ' ' ||
+			r == '/' || r == '?' || r == '#' || r == '@' || r == '\\'
+	}) >= 0 {
 		return ""
 	}
 	return v
diff --git a/internal/utils/utils_url_test.go b/internal/utils/utils_url_test.go
index 9713361..06fbcbf 100644
--- a/internal/utils/utils_url_test.go
+++ b/internal/utils/utils_url_test.go
@@ -9,11 +9,11 @@ import (
 // TestDetermineScheme verifies the scheme priority: forceHTTPS > X-Forwarded-Proto > TLS > http.
 func TestDetermineScheme(t *testing.T) {
 	cases := []struct {
-		name        string
-		forceHTTPS  bool
-		fwdProto    string
-		tls         bool
-		want        string
+		name       string
+		forceHTTPS bool
+		fwdProto   string
+		tls        bool
+		want       string
 	}{
 		{"force https wins over header", true, "http", false, "https"},
 		{"force https wins over tls absence", true, "", false, "https"},
@@ -21,6 +21,11 @@ func TestDetermineScheme(t *testing.T) {
 		{"forwarded proto http", false, "http", true, "http"},
 		{"tls connection", false, "", true, "https"},
 		{"default http", false, "", false, "http"},
+		// R103: a malformed X-Forwarded-Proto must not be emitted verbatim
+		// into redirect_uri (would yield a broken/weakened scheme). It
+		// falls through to the TLS check / http default instead.
+		{"invalid proto falls through to tls", false, "ftp", true, "https"},
+		{"invalid proto falls through to http", false, "ftp", false, "http"},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
@@ -41,10 +46,10 @@ func TestDetermineScheme(t *testing.T) {
 // TestDetermineHost verifies forwarding-header preference and fallback to req.Host.
 func TestDetermineHost(t *testing.T) {
 	cases := []struct {
-		name  string
-		fwd   string
-		host  string
-		want  string
+		name string
+		fwd  string
+		host string
+		want string
 	}{
 		{"forwarded host used", "api.example.com", "backend.local", "api.example.com"},
 		{"first of comma list taken", "a.example.com, b.example.com", "backend.local", "a.example.com"},
diff --git a/memory_monitor.go b/memory_monitor.go
index 5ec4276..289d5fd 100644
--- a/memory_monitor.go
+++ b/memory_monitor.go
@@ -427,10 +427,19 @@ func (mm *MemoryMonitor) StartMonitoring(ctx context.Context, interval time.Dura
 
 	// Check if monitoring is already started
 	if globalMonitoringStarted {
-		if !isTestMode() {
-			mm.logger.Debug("Memory monitoring already started, skipping duplicate start")
+		// If the singleton task was torn down out-of-band (plugin Close ->
+		// TaskRegistry.StopAllTasks), the process-global flag survives the
+		// teardown. Detect that and allow restart so memory monitoring
+		// resumes for a recreated plugin instance; otherwise a config
+		// reload would permanently disable monitoring.
+		if !GetResourceManager().IsTaskRunning("memory-monitor") {
+			globalMonitoringStarted = false
+		} else {
+			if !isTestMode() {
+				mm.logger.Debug("Memory monitoring already started, skipping duplicate start")
+			}
+			return
 		}
-		return
 	}
 
 	if interval <= 0 {
@@ -538,15 +547,19 @@ func (mm *MemoryMonitor) StopMonitoring() {
 		return
 	}
 
-	registry := GetGlobalTaskRegistry()
-	if task, exists := registry.GetTask("memory-monitor"); exists {
-		task.Stop()
-		globalMonitoringStarted = false
-		if !isTestMode() {
-			mm.logger.Info("Stopped global memory monitoring")
-		}
-	} else {
-		mm.logger.Errorf("Failed to find memory monitoring task to stop")
+	// The memory-monitor task is a process-global singleton registered via
+	// CreateSingletonTask, which stores the task in the ResourceManager's
+	// registry — NOT in TaskRegistry.tasks (R88 split). Look it up there;
+	// previously GetGlobalTaskRegistry().GetTask read TaskRegistry.tasks,
+	// always returned not-exists, so Stop was never called, the ticker
+	// goroutine leaked past shutdown, and this logged a spurious error.
+	if err := GetResourceManager().StopBackgroundTask("memory-monitor"); err != nil {
+		mm.logger.Errorf("Failed to find memory monitoring task to stop: %v", err)
+		return
+	}
+	globalMonitoringStarted = false
+	if !isTestMode() {
+		mm.logger.Info("Stopped global memory monitoring")
 	}
 }
 
@@ -580,11 +593,11 @@ func ResetGlobalMemoryMonitor() {
 	defer globalMonitoringMutex.Unlock()
 
 	if globalMemoryMonitor != nil {
-		// Stop monitoring if it's active
+		// Stop monitoring if it's active (singleton task lives in the
+		// ResourceManager's registry, see StopMonitoring).
 		if globalMonitoringStarted {
-			registry := GetGlobalTaskRegistry()
-			if task, exists := registry.GetTask("memory-monitor"); exists {
-				task.Stop()
+			if err := GetResourceManager().StopBackgroundTask("memory-monitor"); err != nil {
+				GetSingletonNoOpLogger().Error("Failed to find memory monitoring task to stop: %v", err)
 			}
 		}
 		globalMemoryMonitor = nil
diff --git a/sharded_cache.go b/sharded_cache.go
index 093198b..bb1b5e4 100644
--- a/sharded_cache.go
+++ b/sharded_cache.go
@@ -88,8 +88,17 @@ func (c *ShardedCache) Get(key string) (interface{}, bool) {
 
 	// Check expiration
 	if !item.expiresAt.IsZero() && time.Now().After(item.expiresAt) {
-		// Item expired - remove it lazily
-		c.Delete(key)
+		// Item expired — remove it lazily, but ONLY if this is still the
+		// same entry. A concurrent Set/SetIfAbsent that refreshed the key
+		// (e.g. a freshly-recorded replay JTI) since this read must not be
+		// removed, else the just-recorded JTI is lost and a duplicate token
+		// could pass (R129; matches deleteIfExpired in
+		// internal/cache/backends/memory_shard.go).
+		shard.mu.Lock()
+		if cur, ok := shard.items[key]; ok && cur == item {
+			delete(shard.items, key)
+		}
+		shard.mu.Unlock()
 		return nil, false
 	}
 
@@ -107,9 +116,11 @@ func (c *ShardedCache) Set(key string, value interface{}, ttl time.Duration) {
 	}
 
 	shard.mu.Lock()
-	// Check if we need to evict items
-	if len(shard.items) >= c.maxPerShard {
-		// Simple eviction: remove expired items first, then oldest
+	// Overwriting an existing key needs no free slot; only evict when
+	// inserting a genuinely new key at capacity, so an overwrite never
+	// drops an unrelated oldest still-valid entry and shrinks the shard
+	// by one (repeats the R57 hazard) (R153).
+	if _, present := shard.items[key]; !present && len(shard.items) >= c.maxPerShard {
 		c.evictFromShardLocked(shard)
 	}
 
@@ -140,6 +151,7 @@ func (c *ShardedCache) SetIfAbsent(key string, value interface{}, ttl time.Durat
 	shard.mu.Lock()
 	defer shard.mu.Unlock()
 
+	_, found := shard.items[key]
 	if it, exists := shard.items[key]; exists {
 		// Present and not expired -> duplicate.
 		if it.expiresAt.IsZero() || !time.Now().After(it.expiresAt) {
@@ -148,7 +160,9 @@ func (c *ShardedCache) SetIfAbsent(key string, value interface{}, ttl time.Durat
 		// Present but expired -> treat as absent and replace below.
 	}
 
-	if len(shard.items) >= c.maxPerShard {
+	// The replace-an-expired path needs no free slot; only evict when the
+	// key is absent and the shard is at capacity (R153).
+	if !found && len(shard.items) >= c.maxPerShard {
 		c.evictFromShardLocked(shard)
 	}
 
diff --git a/singleton_resources.go b/singleton_resources.go
index d4036f7..ab8f05f 100644
--- a/singleton_resources.go
+++ b/singleton_resources.go
@@ -124,20 +124,25 @@ func (rm *ResourceManager) RegisterBackgroundTask(name string, interval time.Dur
 	defer rm.tasksMu.Unlock()
 
 	// If a task with this name already exists and is still running, keep it
-	// (idempotent re-registration from another middleware instance).
+	// (idempotent re-registration from another middleware instance). Keep it
+	// only when it has STARTED (started==1 && stopped==0). A task whose
+	// startOnce was consumed without ever starting — e.g. Start was
+	// rejected by the circuit breaker, or Stop raced re-registration —
+	// would otherwise be kept here, then StartBackgroundTask would be a
+	// permanent no-op (Start is sync.Once-guarded) and the singleton
+	// task would never run again until process restart.
 	if existing, exists := rm.tasks[name]; exists {
-		if atomic.LoadInt32(&existing.stopped) == 0 {
+		if atomic.LoadInt32(&existing.started) == 1 && atomic.LoadInt32(&existing.stopped) == 0 {
 			if rm.logger != nil {
 				rm.logger.Debugf("Background task %s already registered", name)
 			}
 			return nil
 		}
-		// The existing task was stopped. Reusing the same BackgroundTask
-		// object would leave it permanently dead: BackgroundTask.Start is
-		// guarded by sync.Once, so once stopped it can never restart, yet
-		// IsTaskRunning returns false and callers will call Start again.
-		// Replace it with a fresh task so re-registration after a
-		// close+recreate cycle (same provider URL) actually resumes it.
+		// The existing task was either stopped or never actually started.
+		// Reusing the same BackgroundTask object would leave it permanently
+		// dead: BackgroundTask.Start is guarded by sync.Once, so once
+		// consumed it can never start again. Replace it with a fresh task
+		// so re-registration actually resumes it.
 	}
 
 	task := NewBackgroundTask(name, interval, taskFunc, rm.logger, &rm.wg)
diff --git a/universal_cache.go b/universal_cache.go
index 624d2ba..cc20255 100644
--- a/universal_cache.go
+++ b/universal_cache.go
@@ -16,11 +16,13 @@ import (
 type CacheType string
 
 const (
-	CacheTypeToken    CacheType = "token"
-	CacheTypeMetadata CacheType = "metadata"
-	CacheTypeJWK      CacheType = "jwk"
-	CacheTypeSession  CacheType = "session"
-	CacheTypeGeneral  CacheType = "general"
+	CacheTypeToken         CacheType = "token"
+	CacheTypeBlacklist     CacheType = "blacklist"
+	CacheTypeMetadata      CacheType = "metadata"
+	CacheTypeIntrospection CacheType = "introspection"
+	CacheTypeJWK           CacheType = "jwk"
+	CacheTypeSession       CacheType = "session"
+	CacheTypeGeneral       CacheType = "general"
 
 	// maxCacheEntrySize defines the maximum size for a single cache entry (64 MiB)
 	// This prevents integer overflow when allocating memory for serialization
@@ -248,7 +250,17 @@ func (c *UniversalCache) Set(key string, value interface{}, ttl time.Duration) e
 
 		if err := c.backend.Set(ctx, c.prefixKey(key), data, ttl); err != nil {
 			c.logger.Infof("Backend set error for key %s: %v", key, err)
-			// Continue with local cache even if backend fails
+			// The backend still holds an OLDER entry for this key. If we
+			// leave it in place, a subsequent Get treats the backend as
+			// authoritative and (via updateLocalCache) overwrites the
+			// just-written local value with the stale one (R162). Evict the
+			// stale backend entry so Get falls through to the fresh local
+			// value instead of resurrecting the old one.
+			dctx, dcancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+			if _, derr := c.backend.Delete(dctx, c.prefixKey(key)); derr != nil {
+				c.logger.Debugf("Backend delete after failed set for key %s: %v", key, derr)
+			}
+			dcancel()
 		}
 	}
 
@@ -294,8 +306,12 @@ func (c *UniversalCache) setLocal(key string, value interface{}, ttl time.Durati
 		}
 	}
 
-	// Check size limits
-	if c.lruList.Len() >= c.config.MaxSize {
+	// Check size limits — only evict when inserting a NEW key. An in-place
+	// update of an existing key adds no net entry, so evicting would drop an
+	// unrelated live LRU entry and leave the cache one below capacity (cache
+	// thrashes under sustained traffic at MaxSize). Mirror of the shard
+	// backend fix (R139 F2).
+	if _, present := c.items[key]; !present && c.lruList.Len() >= c.config.MaxSize {
 		c.evictOldest()
 	}
 
@@ -397,7 +413,7 @@ func (c *UniversalCache) getLocal(key string) (interface{}, bool) {
 	// previous unconditional Lock serialized every JWT verify on a single
 	// mutex and pinned a CPU under load.
 	switch c.config.Type {
-	case CacheTypeToken, CacheTypeJWK, CacheTypeSession:
+	case CacheTypeToken, CacheTypeJWK, CacheTypeSession, CacheTypeIntrospection:
 		c.mu.RLock()
 		item, exists := c.items[key]
 		if !exists {
@@ -975,8 +991,11 @@ func (c *UniversalCache) updateLocalCache(key string, value interface{}, ttl tim
 		}
 	}
 
-	// Check size limits
-	if c.lruList.Len() >= c.config.MaxSize {
+	// Check size limits — only evict when inserting a NEW key. An in-place
+	// update adds no net entry (the replace below frees the old size), so
+	// evicting would drop an unrelated live LRU entry and leave the cache
+	// one below capacity. See setLocal (R141).
+	if _, present := c.items[key]; !present && c.lruList.Len() >= c.config.MaxSize {
 		c.evictOldest()
 	}
 
diff --git a/universal_cache_singleton.go b/universal_cache_singleton.go
index 0081b8b..6461b3f 100644
--- a/universal_cache_singleton.go
+++ b/universal_cache_singleton.go
@@ -94,6 +94,23 @@ func GetUniversalCacheManagerWithConfig(logger *Logger, redisConfig *RedisConfig
 	return universalCacheManager
 }
 
+// newBlacklistCacheConfig returns the UniversalCacheConfig used for the
+// shared token-blacklist cache. It MUST keep a CacheType distinct from
+// the token cache's CacheTypeToken: prefixKey composes the backend key
+// from c.config.Type, so a shared type would put blacklist and token
+// entries for the same raw token string in the same (e.g. Redis)
+// namespace — both rejecting valid claims-cached tokens as blacklisted
+// and letting token caching clobber a real blacklist marker (R128).
+func newBlacklistCacheConfig(logger *Logger) UniversalCacheConfig {
+	return UniversalCacheConfig{
+		Type:            CacheTypeBlacklist,
+		MaxSize:         1000,
+		DefaultTTL:      24 * time.Hour,
+		Logger:          logger,
+		SkipAutoCleanup: true, // Managed cleanup
+	}
+}
+
 // initializeDefaultCaches initializes caches with memory-only backends
 func initializeDefaultCaches(manager *UniversalCacheManager, logger *Logger) {
 	// Initialize token cache - CRITICAL FIX: Reduced from 5000 to 1000
@@ -106,14 +123,12 @@ func initializeDefaultCaches(manager *UniversalCacheManager, logger *Logger) {
 		SkipAutoCleanup: true, // Managed cleanup
 	})
 
-	// Initialize blacklist cache
-	manager.blacklistCache = NewUniversalCache(UniversalCacheConfig{
-		Type:            CacheTypeToken,
-		MaxSize:         1000,
-		DefaultTTL:      24 * time.Hour,
-		Logger:          logger,
-		SkipAutoCleanup: true, // Managed cleanup
-	})
+	// Initialize blacklist cache. Distinct CacheType so its (possibly
+	// shared-backend) "blacklist:" namespace can never collide with the
+	// token cache's "token:" namespace for the same raw token string; a
+	// collision would both reject valid claims-cached tokens as blacklisted
+	// and let token caching clobber a real blacklist marker (R128).
+	manager.blacklistCache = NewUniversalCache(newBlacklistCacheConfig(logger))
 
 	// Initialize metadata cache with grace periods
 	manager.metadataCache = NewUniversalCache(UniversalCacheConfig{
@@ -156,8 +171,13 @@ func initializeDefaultCaches(manager *UniversalCacheManager, logger *Logger) {
 	})
 
 	// Initialize introspection cache for OAuth 2.0 Token Introspection (RFC 7662)
+	// Uses its own CacheType so its Redis namespace differs from the blacklist
+	// (both in CacheTypeToken): with a shared Redis backend an opaque token is keyed
+	// by its raw value in both caches, so sharing the "token:" prefix let an
+	// introspection write clobber a same-token blacklist marker (revoked opaque
+	// token could be re-accepted) across replicas. See R114.
 	manager.introspectionCache = NewUniversalCache(UniversalCacheConfig{
-		Type:            CacheTypeToken,  // Use token cache type for introspection results
+		Type:            CacheTypeIntrospection,
 		MaxSize:         1000,            // Cache up to 1000 introspection results
 		DefaultTTL:      5 * time.Minute, // Short TTL for security (introspect frequently)
 		Logger:          logger,
@@ -297,15 +317,13 @@ func initializeCachesWithRedis(manager *UniversalCacheManager, logger *Logger, r
 		createBackend(CacheTypeToken),
 	)
 
-	// Initialize blacklist cache (CRITICAL - must be consistent across replicas)
+	// Initialize blacklist cache (CRITICAL - must be consistent across replicas).
+	// Distinct CacheType so its "blacklist:" Redis namespace can't collide with
+	// the token cache's "token:" namespace for the same raw token (R128); a
+	// shared prefix would reject valid claims-cached tokens as blacklisted or
+	// let token caching clobber a real blacklist marker.
 	manager.blacklistCache = NewUniversalCacheWithBackend(
-		UniversalCacheConfig{
-			Type:            CacheTypeToken,
-			MaxSize:         1000,
-			DefaultTTL:      24 * time.Hour,
-			Logger:          logger,
-			SkipAutoCleanup: true, // Managed cleanup
-		},
+		newBlacklistCacheConfig(logger),
 		createBackend("blacklist"),
 	)
 
@@ -355,16 +373,18 @@ func initializeCachesWithRedis(manager *UniversalCacheManager, logger *Logger, r
 		SkipAutoCleanup: true, // Managed cleanup
 	})
 
-	// Introspection cache uses backend for sharing results
+	// Introspection cache uses backend for sharing results. Distinct CacheType
+	// so its Redis "introspection:" namespace can't collide with the blacklist's
+	// "token:" namespace for the same raw opaque token (R114).
 	manager.introspectionCache = NewUniversalCacheWithBackend(
 		UniversalCacheConfig{
-			Type:            CacheTypeToken,
+			Type:            CacheTypeIntrospection,
 			MaxSize:         1000,
 			DefaultTTL:      5 * time.Minute,
 			Logger:          logger,
 			SkipAutoCleanup: true, // Managed cleanup
 		},
-		createBackend(CacheTypeToken),
+		createBackend(CacheTypeIntrospection),
 	)
 
 	// Token type cache stays memory-only (local optimization)
diff --git a/utilities.go b/utilities.go
index 6e87d8e..d833fee 100644
--- a/utilities.go
+++ b/utilities.go
@@ -10,6 +10,7 @@ import (
 	"html"
 	"net/http"
 	"runtime"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -136,9 +137,7 @@ func (t *TraefikOidc) isAllowedDomain(email string) bool {
 		}
 
 		domain := strings.ToLower(parts[1])
-		_, domainAllowed := t.allowedUserDomains[domain]
-
-		if domainAllowed {
+		if t.domainAllowed(domain) {
 			t.logger.Debugf("Email domain %s is allowed", domain)
 			return true
 		} else {
@@ -153,6 +152,97 @@ func (t *TraefikOidc) isAllowedDomain(email string) bool {
 	return false
 }
 
+// domainAllowed reports whether domain is an allowed user domain,
+// including subdomains thereof. An operator configuring
+// "example.com" intends to cover staff on corporate subdomains, so
+// "mail.example.com" (and deeper) must be accepted; exact match is
+// no longer the only path. A plain suffix lookup is avoided (would
+// also match "badexample.com"), so subdomains require a leading "."
+// separator. (R152)
+func (t *TraefikOidc) domainAllowed(domain string) bool {
+	if _, ok := t.allowedUserDomains[domain]; ok {
+		return true
+	}
+	for allowed := range t.allowedUserDomains {
+		if len(domain) > len(allowed) && strings.HasSuffix(domain, "."+allowed) {
+			return true
+		}
+	}
+	return false
+}
+
+// emailVerifiedTrue extracts the boolean email_verified claim value,
+// claimScalarString returns the string form of a scalar claim value,
+// coercing JSON numbers (float64, json.Number, int variants) to their
+// decimal string representation so a numeric identifier claim (e.g. a
+// numeric sub from a non-conformant IdP) is preserved rather than
+// silently dropped. second is false for non-scalar values (maps,
+// slices, objects) so callers can treat them as invalid.
+func claimScalarString(raw interface{}) (s string, ok bool) {
+	switch v := raw.(type) {
+	case string:
+		return v, true
+	case json.Number:
+		return v.String(), true
+	case float64:
+		return strconv.FormatFloat(v, 'f', -1, 64), true
+	case float32:
+		return strconv.FormatFloat(float64(v), 'f', -1, 32), true
+	case int:
+		return strconv.Itoa(v), true
+	case int64:
+		return strconv.FormatInt(v, 10), true
+	default:
+		return "", false
+	}
+}
+
+// emailVerifiedTrue extracts the boolean email_verified claim value,
+// tolerating bool and string representations. present is false when the
+// claim is absent or of an uninterpretable type (the caller then decides
+// how to treat absence).
+func emailVerifiedTrue(v interface{}) (verified, present bool) {
+	switch val := v.(type) {
+	case bool:
+		return val, true
+	case string:
+		switch strings.ToLower(strings.TrimSpace(val)) {
+		case "true", "1":
+			return true, true
+		case "false", "0":
+			return false, true
+		}
+	case float64:
+		return val != 0, true
+	case json.Number:
+		f, err := val.Float64()
+		if err != nil {
+			return false, false
+		}
+		return f != 0, true
+	}
+	return false, false
+}
+
+// emailIdentityPermitted reports whether an email-based user identifier may be
+// admitted for email/domain authorization given the ID-token claims. An email
+// is admitted unless the IdP explicitly marks it unverified
+// (email_verified == false): such an address must not satisfy an email
+// allowlist, since the user may not control it (impersonation within an
+// allowed domain). Absence is tolerated for backward compatibility (the
+// middleware and its test fixtures have long run without the claim; some IdPs
+// do not emit it). This matches the bearer-path stance that untrusted email
+// is a spoofing vector (main.go).
+func emailIdentityPermitted(userIdentifier string, claims map[string]interface{}) bool {
+	if !strings.Contains(userIdentifier, "@") {
+		return true
+	}
+	if verified, present := emailVerifiedTrue(claims["email_verified"]); present && !verified {
+		return false
+	}
+	return true
+}
+
 // keysFromMap extracts string keys from a map for logging purposes.
 // Helper function to get keys from a map for logging.
 // Parameters:
@@ -176,6 +266,10 @@ func keysFromMap(m map[string]struct{}) []string {
 //   - message: The error message to display.
 //   - code: The HTTP status code to set for the response.
 func (t *TraefikOidc) sendErrorResponse(rw http.ResponseWriter, req *http.Request, message string, code int) {
+	// Auth-failure responses (401/403/429/503) must not be cached by
+	// browsers or intermediaries — a cached 401 body would persist across
+	// re-authentication (R101).
+	rw.Header().Set("Cache-Control", "no-store")
 	acceptHeader := req.Header.Get("Accept")
 
 	if strings.Contains(acceptHeader, "application/json") {