feat(lsp): capture server stderr to diagnose init failures

drainStderr discarded all server stderr, so an LSP init failure (e.g. 'command not found', missing module) surfaced as an opaque 'initialization failed'. Retain a bounded tail (StderrCaptureMax=4096B, oldest dropped) under a mutex and attach it to the init-failure error context. Adds TestClientStderrCapture.

internal/lsp/client.go

@@ -22,6 +22,10 @@ const ProcessKillTimeout = 5 * time.Second
 // StderrBufferSize is the buffer size for draining stderr.
 const StderrBufferSize = 1024
 
+// StderrCaptureMax is the maximum number of recent stderr bytes retained for
+// diagnostics (e.g. to explain an init failure). Older bytes are dropped.
+const StderrCaptureMax = 4096
+
 // Client represents an LSP client connection.
 type Client struct {
 	stdin         io.WriteCloser
@@ -35,6 +39,8 @@ type Client struct {
 	runningMu     sync.RWMutex
 	stopOnce      sync.Once
 	mu            sync.Mutex
+	stderrMu      sync.Mutex
+	stderrTail    []byte // most recent stderr, capped at StderrCaptureMax (diagnostics)
 	running       bool
 }
 
@@ -135,14 +141,35 @@ func (c *Client) drainStderr() {
 			return
 		default:
 		}
-		// Read and discard stderr output
-		_, err := c.stderr.Read(buf)
+		n, err := c.stderr.Read(buf)
+		if n > 0 {
+			c.captureStderr(buf[:n])
+		}
 		if err != nil {
 			return
 		}
 	}
 }
 
+// captureStderr appends p to the retained stderr tail, keeping at most
+// StderrCaptureMax bytes (dropping the oldest).
+func (c *Client) captureStderr(p []byte) {
+	c.stderrMu.Lock()
+	defer c.stderrMu.Unlock()
+	c.stderrTail = append(c.stderrTail, p...)
+	if len(c.stderrTail) > StderrCaptureMax {
+		c.stderrTail = c.stderrTail[len(c.stderrTail)-StderrCaptureMax:]
+	}
+}
+
+// RecentStderr returns the retained tail of the server's stderr, trimmed. Useful
+// for surfacing why an LSP server failed to start.
+func (c *Client) RecentStderr() string {
+	c.stderrMu.Lock()
+	defer c.stderrMu.Unlock()
+	return strings.TrimSpace(string(c.stderrTail))
+}
+
 // Call sends a request and waits for a response.
 func (c *Client) Call(ctx context.Context, method string, params interface{}) (*Response, error) {
 	c.runningMu.RLock()

internal/lsp/client_test.go

@@ -0,0 +1,28 @@
+package lsp
+
+import (
+	"strings"
+	"testing"
+)
+
+// drainStderr retains a bounded tail of server stderr for diagnostics; capture
+// must keep recent bytes and never grow past StderrCaptureMax.
+func TestClientStderrCapture(t *testing.T) {
+	c := &Client{}
+
+	if c.RecentStderr() != "" {
+		t.Errorf("expected empty stderr initially, got %q", c.RecentStderr())
+	}
+
+	c.captureStderr([]byte("gopls: command not found\n"))
+	if !strings.Contains(c.RecentStderr(), "command not found") {
+		t.Errorf("expected captured stderr, got %q", c.RecentStderr())
+	}
+
+	// Exceed the cap: only the most recent StderrCaptureMax bytes are kept.
+	big := strings.Repeat("x", StderrCaptureMax*2)
+	c.captureStderr([]byte(big))
+	if got := len(c.RecentStderr()); got > StderrCaptureMax {
+		t.Errorf("stderr tail not bounded: len=%d, want <= %d", got, StderrCaptureMax)
+	}
+}

internal/lsp/manager.go

@@ -212,10 +212,16 @@ func (m *Manager) GetServer(ctx context.Context, lang protocol.Language) (*Manag
 	// Initialize server
 	if err := m.initializeServer(ctx, newSrv); err != nil {
 		newSrv.initErr = err
-		return nil, errors.Wrap(errors.ErrLSPInitFailed, "LSP server initialization failed", err).
+		wrapped := errors.Wrap(errors.ErrLSPInitFailed, "LSP server initialization failed", err).
 			WithContext("language", string(lang)).
 			WithContext("command", config.Command[0]).
 			WithRemediation("Check LSP server logs for initialization errors")
+		// Surface the server's own stderr (e.g. "command not found", missing
+		// module) so the failure is diagnosable instead of opaque.
+		if stderr := client.RecentStderr(); stderr != "" {
+			wrapped = wrapped.WithContext("stderr", stderr)
+		}
+		return nil, wrapped
 	}
 
 	// Mark as successfully initialized to prevent cleanup