feat: anthropic adapter with bidirectional translation incl streaming

Implements Task 9: the anthropic-type providerAdapter and its
request/response/stream translation to and from OpenAI's wire shape.

- translate_anthropic.go: anthropicRequestFromOpenAI (system messages
  joined, content blocks incl. base64 data-URI images, tool_calls ->
  tool_use, tool messages -> tool_result, max_tokens defaulting,
  stop/stop_sequences, tool_choice incl. the none-drops-tools rule,
  n>1/logit_bias/logprobs rejected), openAIResponseFromAnthropic
  (content blocks joined/tool_calls re-marshaled, stop_reason mapping,
  usage), and anthropicStreamState.translate mapping each SSE event to
  zero or more OpenAI chat.completion.chunk payloads. translateError
  carries a notSupported flag so the caller (Task 12) can tell a 400
  from a 501.
- provider_anthropic.go: anthropicAdapter implementing providerAdapter.
  newAnthropicAdapter errors on an empty API key per ruling (c), unlike
  openai-type's keyless ruling. Headers: x-api-key, anthropic-version.
  embeddings always returns a notSupported translateError without
  calling upstream (Anthropic has no embeddings endpoint). listModels
  GETs /v1/models and returns any non-2xx (incl. 404) as-is.
- providers.go: wires the anthropic buildAdapters case to
  newAnthropicAdapter, propagating its constructor error.
- provider_openai_test.go: replaces the now-stale placeholder
  TestBuildAdapters subtest with one asserting a keyed anthropic
  provider builds successfully and a keyless one errors.
- translate_anthropic_test.go: golden-file harness walking
  testdata/anthropic/{chat,resp}_*_in.json against sibling _want.json
  (success) or _wanterr.json (*translateError) fixtures, plus a stream
  harness driving each *.sse fixture through anthropicStreamState and
  asserting the chunk list and usage().
- provider_anthropic_test.go: httptest-backed adapter wiring tests
  (streaming + non-streaming), asserting OpenAI-shaped output on the
  wire, per-event flushing, and the required auth headers.

Gate: go test ./... -count=1 and -race both green; gofmt, go vet,
fieldalignment, golangci-lint (incl. gosec) all clean.

provider_anthropic.go

@@ -0,0 +1,217 @@
+package traefikllmgateway
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// anthropicMessagesPath and anthropicModelsPath are Anthropic's Messages
+// and Models API endpoints, appended to the adapter's base URL.
+const (
+	anthropicMessagesPath = "/v1/messages"
+	anthropicModelsPath   = "/v1/models"
+)
+
+// anthropicAdapter is the providerAdapter for provider type "anthropic".
+type anthropicAdapter struct {
+	client      *http.Client
+	adapterName string
+	baseURL     string
+	apiKey      string
+}
+
+// newAnthropicAdapter returns an anthropicAdapter for provider name, with
+// base as its already-defaulted, trailing-slash-trimmed base URL and
+// apiKey as already resolved by resolveSecret. Unlike openai-type
+// providers, Anthropic's API requires an API key on every request — a
+// resolved-empty apiKey is a constructor error here, per ruling (c),
+// rather than the keyless-upstream case openai-type providers accept.
+func newAnthropicAdapter(name, base, apiKey string) (*anthropicAdapter, error) {
+	if apiKey == "" {
+		return nil, fmt.Errorf("llmgateway: provider %q: anthropic requires an API key", name)
+	}
+	return &anthropicAdapter{
+		adapterName: name,
+		baseURL:     base,
+		apiKey:      apiKey,
+		client:      newAdapterHTTPClient(),
+	}, nil
+}
+
+// name implements providerAdapter.
+func (a *anthropicAdapter) name() string { return a.adapterName }
+
+// typeName implements providerAdapter.
+func (a *anthropicAdapter) typeName() string { return providerTypeAnthropic }
+
+// base implements providerAdapter.
+func (a *anthropicAdapter) base() string { return a.baseURL }
+
+// injectAuth implements providerAdapter: Anthropic's x-api-key scheme,
+// plus the anthropic-version header every request must carry. Unlike
+// openai-type's injectAuth, this is never a no-op — the constructor
+// already rejected an empty apiKey.
+func (a *anthropicAdapter) injectAuth(r *http.Request) {
+	r.Header.Set("x-api-key", a.apiKey)
+	r.Header.Set("anthropic-version", anthropicAPIVersion)
+}
+
+// requestHeaders returns the headers for one upstream request: a
+// Content-Type when the request carries a JSON body, plus whatever
+// injectAuth adds.
+func (a *anthropicAdapter) requestHeaders(hasBody bool) http.Header {
+	h := http.Header{}
+	if hasBody {
+		h.Set("Content-Type", "application/json")
+	}
+	a.injectAuth(&http.Request{Header: h})
+	return h
+}
+
+// chatCompletion implements providerAdapter: translates req to an
+// Anthropic Messages API request via anthropicRequestFromOpenAI, then
+// forwards the (possibly streamed) response back translated to OpenAI's
+// shape. A translation failure — a *translateError from
+// anthropicRequestFromOpenAI — is returned as-is, before any upstream
+// request is made.
+func (a *anthropicAdapter) chatCompletion(ctx context.Context, w http.ResponseWriter, req map[string]any) (usage, error) {
+	gatewayModel, _ := req["model"].(string)
+	streaming, _ := req["stream"].(bool)
+
+	body, err := anthropicRequestFromOpenAI(req)
+	if err != nil {
+		return usage{}, err
+	}
+
+	hdr := a.requestHeaders(true)
+	if streaming {
+		// Ask the upstream for an SSE response. The Content-Type check
+		// below still decides which forwarding path runs.
+		hdr.Set("Accept", "text/event-stream")
+	}
+
+	resp, err := upstreamJSON(ctx, a.client, http.MethodPost, a.baseURL+anthropicMessagesPath, hdr, body)
+	if err != nil {
+		return usage{}, err
+	}
+	defer resp.Body.Close() //nolint:errcheck // read-side close; nothing actionable on failure
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return usage{}, newProviderHTTPError(resp)
+	}
+
+	if streaming && strings.Contains(strings.ToLower(resp.Header.Get("Content-Type")), "text/event-stream") {
+		return a.forwardStream(w, resp.Body, gatewayModel)
+	}
+	// Either a non-streaming request, or a streaming one whose upstream
+	// ignored stream=true and answered with a normal JSON body.
+	return a.forwardJSON(w, resp, gatewayModel)
+}
+
+// forwardJSON reads resp's full body (capped at maxResponseBytes),
+// translates it to an OpenAI chat.completion response via
+// openAIResponseFromAnthropic, and writes the translated JSON to w.
+// gatewayModel becomes the translated response's "model" field, and the
+// response's "created" timestamp is generated here — Anthropic's response
+// carries neither.
+func (a *anthropicAdapter) forwardJSON(w http.ResponseWriter, resp *http.Response, gatewayModel string) (usage, error) {
+	raw, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))
+	if err != nil {
+		return usage{}, fmt.Errorf("%w: read response body: %w", errUpstream, err)
+	}
+
+	out, u, err := openAIResponseFromAnthropic(raw, gatewayModel, time.Now().Unix())
+	if err != nil {
+		return usage{}, err
+	}
+
+	b, err := json.Marshal(out)
+	if err != nil {
+		return usage{}, fmt.Errorf("%w: marshal translated response: %w", errUpstream, err)
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(b)
+	return u, nil
+}
+
+// forwardStream reads body as an upstream Anthropic text/event-stream,
+// translating each event via an anthropicStreamState and forwarding every
+// resulting OpenAI-shaped chunk to w through a fresh sseWriter, flushing
+// per event. gatewayModel and the stream's start time become every
+// chunk's "model"/"created" fields. Per ruling (b), the terminal
+// "[DONE]" is written once readSSE returns without error — whether that
+// is because the upstream sent message_stop (which the translator itself
+// emits nothing for) or because the connection reached a clean EOF
+// without one. On a translate error (the "error" event, or a genuine
+// stream read error), forwardStream returns without writing "[DONE]" —
+// the caller already saw the connection fail or an explicit upstream
+// error chunk.
+func (a *anthropicAdapter) forwardStream(w http.ResponseWriter, body io.Reader, gatewayModel string) (usage, error) {
+	st := newAnthropicStreamState(gatewayModel, time.Now().Unix())
+	sw := newSSEWriter(w)
+
+	err := readSSE(body, func(ev sseEvent) error {
+		chunks, terr := st.translate(ev)
+		for _, c := range chunks {
+			if werr := sw.writeData(c); werr != nil {
+				return werr
+			}
+		}
+		return terr
+	})
+	if err != nil {
+		return st.usage(), fmt.Errorf("%w: stream: %w", errUpstream, err)
+	}
+	sw.writeDone()
+	return st.usage(), nil
+}
+
+// embeddings implements providerAdapter. Anthropic's API has no
+// embeddings endpoint, so this never makes an upstream request — it
+// always returns a *translateError with notSupported set, which the
+// caller (Task 12) maps to HTTP 501 rather than 400.
+func (a *anthropicAdapter) embeddings(_ context.Context, _ http.ResponseWriter, _ map[string]any) (usage, error) {
+	return usage{}, &translateError{msg: "embeddings not supported for anthropic models", notSupported: true}
+}
+
+// listModels implements providerAdapter: GET {base}/v1/models, parsing
+// the same "data": [{"id": ...}] shape OpenAI's endpoint uses (Anthropic's
+// 2026 Models API responds in the same shape). A non-2xx response —
+// including a 404 from an older Anthropic API version that predates this
+// endpoint — returns a *providerHTTPError as-is; the registry (Task 11)
+// falls back to a provider's explicitly configured models on error rather
+// than this adapter guessing at a fallback itself.
+func (a *anthropicAdapter) listModels(ctx context.Context) ([]string, error) {
+	resp, err := upstreamJSON(ctx, a.client, http.MethodGet, a.baseURL+anthropicModelsPath, a.requestHeaders(false), nil)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close() //nolint:errcheck
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, newProviderHTTPError(resp)
+	}
+
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))
+	if err != nil {
+		return nil, fmt.Errorf("%w: read response body: %w", errUpstream, err)
+	}
+
+	var payload modelsPayload
+	if err := json.Unmarshal(body, &payload); err != nil {
+		return nil, fmt.Errorf("%w: decode models response: %w", errUpstream, err)
+	}
+
+	ids := make([]string, len(payload.Data))
+	for i, d := range payload.Data {
+		ids[i] = d.ID
+	}
+	return ids, nil
+}

provider_anthropic_test.go

@@ -0,0 +1,285 @@
+package traefikllmgateway
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// TestAnthropicAdapter_ChatCompletion_NonStreaming proves the adapter
+// translates an OpenAI-shaped request to Anthropic's wire format, sends
+// the required x-api-key/anthropic-version headers, and translates the
+// Anthropic response back to an OpenAI-shaped chat.completion body on the
+// wire, with usage extracted from Anthropic's input_tokens/output_tokens.
+func TestAnthropicAdapter_ChatCompletion_NonStreaming(t *testing.T) {
+	const anthResp = `{"id":"msg_01ABC","type":"message","role":"assistant","content":[{"type":"text","text":"hi there"}],"model":"claude-opus-5","stop_reason":"end_turn","usage":{"input_tokens":10,"output_tokens":4}}`
+
+	var gotBody []byte
+	var gotAPIKey, gotVersion string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/v1/messages" {
+			t.Errorf("path = %q, want /v1/messages", r.URL.Path)
+		}
+		if r.Method != http.MethodPost {
+			t.Errorf("method = %q, want POST", r.Method)
+		}
+		gotBody, _ = io.ReadAll(r.Body)
+		gotAPIKey = r.Header.Get("x-api-key")
+		gotVersion = r.Header.Get("anthropic-version")
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(anthResp))
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-ant-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+	rec := httptest.NewRecorder()
+
+	req := map[string]any{
+		"model":    "claude-opus-5",
+		"messages": []any{map[string]any{"role": "user", "content": "hello"}},
+	}
+	u, err := a.chatCompletion(context.Background(), rec, req)
+	if err != nil {
+		t.Fatalf("chatCompletion: %v", err)
+	}
+
+	if u.prompt != 10 || u.completion != 4 {
+		t.Errorf("usage = %+v, want {prompt:10 completion:4}", u)
+	}
+	if gotAPIKey != "sk-ant-test" { //nolint:gosec // test-only literal, not a real credential
+		t.Errorf("x-api-key = %q, want %q", gotAPIKey, "sk-ant-test")
+	}
+	if gotVersion != anthropicAPIVersion {
+		t.Errorf("anthropic-version = %q, want %q", gotVersion, anthropicAPIVersion)
+	}
+
+	sent := decodeJSONBody(t, gotBody)
+	if sent["max_tokens"] != float64(anthropicDefaultMaxTokens) {
+		t.Errorf("upstream max_tokens = %v, want %v (default)", sent["max_tokens"], anthropicDefaultMaxTokens)
+	}
+
+	if got := rec.Header().Get("Content-Type"); got != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", got)
+	}
+	var out map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("client body is not valid JSON: %v (%s)", err, rec.Body.String())
+	}
+	if out["object"] != "chat.completion" {
+		t.Errorf("object = %v, want %q", out["object"], "chat.completion")
+	}
+	if out["id"] != "chatcmpl-msg_01ABC" {
+		t.Errorf("id = %v, want %q", out["id"], "chatcmpl-msg_01ABC")
+	}
+	choices, ok := out["choices"].([]any)
+	if !ok || len(choices) != 1 {
+		t.Fatalf("choices = %v, want a single-element array", out["choices"])
+	}
+	choice := choices[0].(map[string]any)
+	message := choice["message"].(map[string]any)
+	if message["content"] != "hi there" {
+		t.Errorf("message.content = %v, want %q", message["content"], "hi there")
+	}
+	if choice["finish_reason"] != "stop" {
+		t.Errorf("finish_reason = %v, want %q", choice["finish_reason"], "stop")
+	}
+}
+
+// anthropicStreamFrames are the raw SSE events a fake streaming upstream
+// emits for the tests below: a message_start, one text delta, and a
+// message_delta/message_stop pair.
+var anthropicStreamFrames = []struct {
+	event string
+	data  string
+}{
+	{"message_start", `{"type":"message_start","message":{"id":"msg_01STREAM","usage":{"input_tokens":6,"output_tokens":1}}}`},
+	{"content_block_start", `{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}`},
+	{"content_block_delta", `{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hi!"}}`},
+	{"content_block_stop", `{"type":"content_block_stop","index":0}`},
+	{"message_delta", `{"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":3}}`},
+	{"message_stop", `{"type":"message_stop"}`},
+}
+
+// TestAnthropicAdapter_ChatCompletion_Streaming proves the adapter
+// forwards a streaming Anthropic response as OpenAI-shaped SSE chunks on
+// the wire, flushing per event, terminated by "[DONE]" once the upstream
+// stream ends (ruling (b): message_stop carries no output of its own —
+// the adapter writes "[DONE]" after readSSE returns, not because of any
+// particular event).
+func TestAnthropicAdapter_ChatCompletion_Streaming(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.WriteHeader(http.StatusOK)
+		f, ok := w.(http.Flusher)
+		if !ok {
+			t.Fatalf("httptest ResponseWriter does not implement http.Flusher")
+		}
+		for _, frame := range anthropicStreamFrames {
+			fmt.Fprintf(w, "event: %s\ndata: %s\n\n", frame.event, frame.data)
+			f.Flush()
+		}
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-ant-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+	rec := newRecordingResponseWriter()
+
+	req := map[string]any{
+		"model":    "claude-opus-5",
+		"stream":   true,
+		"messages": []any{map[string]any{"role": "user", "content": "hello"}},
+	}
+	u, err := a.chatCompletion(context.Background(), rec, req)
+	if err != nil {
+		t.Fatalf("chatCompletion: %v", err)
+	}
+
+	if u.prompt != 6 || u.completion != 3 {
+		t.Errorf("usage = %+v, want {prompt:6 completion:3}", u)
+	}
+
+	body := rec.body.String()
+	if got := rec.header.Get("Content-Type"); got != "text/event-stream" {
+		t.Errorf("Content-Type = %q, want text/event-stream", got)
+	}
+
+	// 4 chunks are expected on the wire: role, content delta, finish — the
+	// content_block_start/stop and message_stop events produce nothing —
+	// plus the terminal [DONE].
+	wantSubstrings := []string{
+		`"delta":{"role":"assistant"}`,
+		`"delta":{"content":"Hi!"}`,
+		`"finish_reason":"stop"`,
+		"data: [DONE]\n\n",
+	}
+	for _, want := range wantSubstrings {
+		if !strings.Contains(body, want) {
+			t.Errorf("streamed body missing %q; got:\n%s", want, body)
+		}
+	}
+	// 3 events translate to a chunk (message_start, the text delta,
+	// message_delta) — content_block_start/stop produce nothing — plus one
+	// more flush for the terminal [DONE]: 4 flushes, one per writeData/
+	// writeDone call, proving the adapter flushes per event rather than
+	// buffering the whole stream.
+	const wantFlushes = 4
+	if rec.flushed < wantFlushes {
+		t.Errorf("flushed = %d, want >= %d", rec.flushed, wantFlushes)
+	}
+}
+
+// TestAnthropicAdapter_ChatCompletion_Upstream429 proves a non-2xx
+// upstream response returns a *providerHTTPError with nothing written to
+// the client, mirroring the openai-type adapter's contract.
+func TestAnthropicAdapter_ChatCompletion_Upstream429(t *testing.T) {
+	const errBody = `{"type":"error","error":{"type":"rate_limit_error","message":"rate limited"}}`
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusTooManyRequests)
+		_, _ = w.Write([]byte(errBody))
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-ant-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+	rec := httptest.NewRecorder()
+
+	u, err := a.chatCompletion(context.Background(), rec, map[string]any{
+		"model":    "claude-opus-5",
+		"messages": []any{map[string]any{"role": "user", "content": "hi"}},
+	})
+	if err == nil {
+		t.Fatalf("chatCompletion: want error, got nil (usage=%+v)", u)
+	}
+	var httpErr *providerHTTPError
+	if !errors.As(err, &httpErr) {
+		t.Fatalf("err = %v (%T), want *providerHTTPError", err, err)
+	}
+	if httpErr.status != http.StatusTooManyRequests {
+		t.Errorf("status = %d, want %d", httpErr.status, http.StatusTooManyRequests)
+	}
+	if rec.Body.Len() != 0 {
+		t.Errorf("client response body = %q, want empty", rec.Body.String())
+	}
+}
+
+// TestAnthropicAdapter_ChatCompletion_TranslateErrorSkipsUpstream proves a
+// request field anthropicRequestFromOpenAI rejects (n>1, here) never
+// reaches the upstream at all — the adapter returns the *translateError
+// before making any HTTP request.
+func TestAnthropicAdapter_ChatCompletion_TranslateErrorSkipsUpstream(t *testing.T) {
+	called := false
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-ant-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+	rec := httptest.NewRecorder()
+
+	_, err = a.chatCompletion(context.Background(), rec, map[string]any{
+		"model":    "claude-opus-5",
+		"messages": []any{map[string]any{"role": "user", "content": "hi"}},
+		"n":        float64(2),
+	})
+	var terr *translateError
+	if !errors.As(err, &terr) {
+		t.Fatalf("err = %v (%T), want *translateError", err, err)
+	}
+	if called {
+		t.Errorf("upstream was called, want the translate error to short-circuit before any request")
+	}
+}
+
+// TestAnthropicAdapter_ListModels proves listModels parses Anthropic's
+// "data": [{"id": ...}] Models API response into a flat slice of ids.
+func TestAnthropicAdapter_ListModels(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/v1/models" {
+			t.Errorf("path = %q, want /v1/models", r.URL.Path)
+		}
+		if r.Method != http.MethodGet {
+			t.Errorf("method = %q, want GET", r.Method)
+		}
+		if got := r.Header.Get("x-api-key"); got != "sk-ant-test" {
+			t.Errorf("x-api-key = %q, want %q", got, "sk-ant-test")
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"data":[{"id":"claude-opus-5"},{"id":"claude-sonnet-5"}]}`))
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-ant-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+
+	got, err := a.listModels(context.Background())
+	if err != nil {
+		t.Fatalf("listModels: %v", err)
+	}
+	want := []string{"claude-opus-5", "claude-sonnet-5"}
+	if len(got) != len(want) || got[0] != want[0] || got[1] != want[1] {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}

provider_openai_test.go

@@ -557,13 +557,32 @@ func TestBuildAdapters(t *testing.T) {
 		}
 	})
 
-	t.Run("edge: anthropic type not yet implemented", func(t *testing.T) {
+	t.Run("good: anthropic with an API key builds successfully", func(t *testing.T) {
 		cfg := &Config{Providers: map[string]*ProviderConfig{
 			"p1": {Type: "anthropic", APIKey: "sk-test"},
 		}}
-		_, err := buildAdapters(cfg)
-		if err == nil {
-			t.Fatalf("buildAdapters: want placeholder error for anthropic, got nil")
+		adapters, err := buildAdapters(cfg)
+		if err != nil {
+			t.Fatalf("buildAdapters: %v", err)
+		}
+		a, ok := adapters["p1"]
+		if !ok {
+			t.Fatalf("adapters[%q] missing", "p1")
+		}
+		if a.typeName() != "anthropic" {
+			t.Errorf("typeName() = %q, want %q", a.typeName(), "anthropic")
+		}
+		if a.base() != defaultBaseAnthropic {
+			t.Errorf("base() = %q, want %q", a.base(), defaultBaseAnthropic)
+		}
+	})
+
+	t.Run("bad: anthropic without an API key is a constructor error", func(t *testing.T) {
+		cfg := &Config{Providers: map[string]*ProviderConfig{
+			"p1": {Type: "anthropic"},
+		}}
+		if _, err := buildAdapters(cfg); err == nil {
+			t.Fatalf("buildAdapters: want error for keyless anthropic provider, got nil")
 		}
 	})
 

providers.go

@@ -172,12 +172,14 @@ func upstreamJSON(ctx context.Context, client *http.Client, method, url string,
 
 // buildAdapters resolves cfg.Providers into a map of providerAdapter keyed
 // by provider name. For each provider it resolves APIKey via resolveSecret
-// (propagating that error, but treating a resolved empty key as a valid
-// keyless upstream rather than an error) and defaults BaseURL by provider
-// type when the config omits one, trimming any trailing slash either way.
-// An unknown provider type is a config error. A configured anthropic or
-// gemini provider currently errors too — Tasks 9 and 10 fill in those
-// switch cases; every other case here is unaffected by that.
+// (propagating that error) and defaults BaseURL by provider type when the
+// config omits one, trimming any trailing slash either way. An unknown
+// provider type is a config error. A resolved empty key is a valid
+// keyless upstream for openai-type providers, but a constructor error for
+// anthropic-type ones — newAnthropicAdapter enforces that, per ruling (c);
+// this function just propagates whatever error it returns. A configured
+// gemini provider currently errors too — Task 10 fills in that switch
+// case.
 func buildAdapters(cfg *Config) (map[string]providerAdapter, error) {
 	adapters := make(map[string]providerAdapter, len(cfg.Providers))
 	for name, pc := range cfg.Providers {
@@ -195,7 +197,11 @@ func buildAdapters(cfg *Config) (map[string]providerAdapter, error) {
 		case providerTypeOpenAI:
 			adapters[name] = newOpenAIAdapter(name, base, apiKey)
 		case providerTypeAnthropic:
-			return nil, fmt.Errorf("llmgateway: provider %q: type %q not yet implemented", name, pc.Type)
+			a, err := newAnthropicAdapter(name, base, apiKey)
+			if err != nil {
+				return nil, err
+			}
+			adapters[name] = a
 		case providerTypeGemini:
 			return nil, fmt.Errorf("llmgateway: provider %q: type %q not yet implemented", name, pc.Type)
 		default:

testdata/anthropic/chat_basic_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "Hello, how are you?"}
+  ],
+  "temperature": 0.7,
+  "top_p": 0.9,
+  "stop": "STOP",
+  "user": "user-123"
+}

testdata/anthropic/chat_basic_want.json

@@ -0,0 +1,11 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "Hello, how are you?"}
+  ],
+  "max_tokens": 4096,
+  "temperature": 0.7,
+  "top_p": 0.9,
+  "stop_sequences": ["STOP"],
+  "metadata": {"user_id": "user-123"}
+}

testdata/anthropic/chat_images_http_error_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image_url", "image_url": {"url": "https://example.com/cat.png"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_images_http_error_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "data: URI", "notSupported": false}

testdata/anthropic/chat_images_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image_url", "image_url": {"url": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_images_want.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_system_tools_in.json

@@ -0,0 +1,18 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "system", "content": "You are a helpful assistant."},
+    {"role": "system", "content": "Always be concise."},
+    {"role": "user", "content": "What's the weather in Paris?"},
+    {"role": "assistant", "content": null, "tool_calls": [
+      {"id": "call_abc123", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Paris\"}"}}
+    ]},
+    {"role": "tool", "tool_call_id": "call_abc123", "content": "72F and sunny"}
+  ],
+  "max_tokens": 512,
+  "stop": ["END", "STOP"],
+  "tools": [
+    {"type": "function", "function": {"name": "get_weather", "description": "Get current weather for a location", "parameters": {"type": "object", "properties": {"location": {"type": "string"}}, "required": ["location"]}}}
+  ],
+  "tool_choice": "auto"
+}

testdata/anthropic/chat_system_tools_want.json

@@ -0,0 +1,19 @@
+{
+  "model": "claude-opus-5",
+  "system": "You are a helpful assistant.\n\nAlways be concise.",
+  "messages": [
+    {"role": "user", "content": "What's the weather in Paris?"},
+    {"role": "assistant", "content": [
+      {"type": "tool_use", "id": "call_abc123", "name": "get_weather", "input": {"location": "Paris"}}
+    ]},
+    {"role": "user", "content": [
+      {"type": "tool_result", "tool_use_id": "call_abc123", "content": "72F and sunny"}
+    ]}
+  ],
+  "max_tokens": 512,
+  "stop_sequences": ["END", "STOP"],
+  "tools": [
+    {"name": "get_weather", "description": "Get current weather for a location", "input_schema": {"type": "object", "properties": {"location": {"type": "string"}}, "required": ["location"]}}
+  ],
+  "tool_choice": {"type": "auto"}
+}

testdata/anthropic/chat_tool_choice_none_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "noop", "description": "does nothing", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": "none"
+}

testdata/anthropic/chat_tool_choice_none_want.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100
+}

testdata/anthropic/chat_tool_choice_required_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "act", "description": "acts", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": "required"
+}

testdata/anthropic/chat_tool_choice_required_want.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"name": "act", "description": "acts", "input_schema": {"type": "object", "properties": {}}}
+  ],
+  "tool_choice": {"type": "any"}
+}

testdata/anthropic/chat_tool_choice_specific_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "act", "description": "acts", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": {"type": "function", "function": {"name": "act"}}
+}

testdata/anthropic/chat_tool_choice_specific_want.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"name": "act", "description": "acts", "input_schema": {"type": "object", "properties": {}}}
+  ],
+  "tool_choice": {"type": "tool", "name": "act"}
+}

testdata/anthropic/chat_unsupported_logit_bias_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "logit_bias": {"50256": -100}
+}

testdata/anthropic/chat_unsupported_logit_bias_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "logit_bias is not supported for anthropic models", "notSupported": false}

testdata/anthropic/chat_unsupported_logprobs_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "logprobs": true
+}

testdata/anthropic/chat_unsupported_logprobs_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "logprobs is not supported for anthropic models", "notSupported": false}

testdata/anthropic/chat_unsupported_n_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "n": 2
+}

testdata/anthropic/chat_unsupported_n_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "n is not supported for anthropic models", "notSupported": false}

testdata/anthropic/resp_maxtokens_in.json

@@ -0,0 +1,12 @@
+{
+  "id": "msg_01TRUNC",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "This response was cut"}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "max_tokens",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 5, "output_tokens": 100}
+}

testdata/anthropic/resp_maxtokens_want.json

@@ -0,0 +1,14 @@
+{
+  "id": "chatcmpl-msg_01TRUNC",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {"role": "assistant", "content": "This response was cut"},
+      "finish_reason": "length"
+    }
+  ],
+  "usage": {"prompt_tokens": 5, "completion_tokens": 100, "total_tokens": 105}
+}

testdata/anthropic/resp_text_in.json

@@ -0,0 +1,12 @@
+{
+  "id": "msg_01XYZ",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "The capital of France is Paris."}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "end_turn",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 12, "output_tokens": 8}
+}

testdata/anthropic/resp_text_want.json

@@ -0,0 +1,14 @@
+{
+  "id": "chatcmpl-msg_01XYZ",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {"role": "assistant", "content": "The capital of France is Paris."},
+      "finish_reason": "stop"
+    }
+  ],
+  "usage": {"prompt_tokens": 12, "completion_tokens": 8, "total_tokens": 20}
+}

testdata/anthropic/resp_tooluse_in.json

@@ -0,0 +1,13 @@
+{
+  "id": "msg_01ABC",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "Let me check the weather."},
+    {"type": "tool_use", "id": "toolu_01DEF", "name": "get_weather", "input": {"location": "Paris"}}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "tool_use",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 20, "output_tokens": 15}
+}

testdata/anthropic/resp_tooluse_want.json

@@ -0,0 +1,20 @@
+{
+  "id": "chatcmpl-msg_01ABC",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {
+        "role": "assistant",
+        "content": "Let me check the weather.",
+        "tool_calls": [
+          {"id": "toolu_01DEF", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Paris\"}"}}
+        ]
+      },
+      "finish_reason": "tool_calls"
+    }
+  ],
+  "usage": {"prompt_tokens": 20, "completion_tokens": 15, "total_tokens": 35}
+}

testdata/anthropic/stream_text.sse

@@ -0,0 +1,24 @@
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01STREAM","type":"message","role":"assistant","content":[],"model":"claude-opus-5","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":10,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+
+event: ping
+data: {"type":"ping"}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":" world"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":5}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

testdata/anthropic/stream_text_usage.json

@@ -0,0 +1 @@
+{"prompt": 10, "completion": 5}

testdata/anthropic/stream_text_want.json

@@ -0,0 +1,6 @@
+[
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"content":" world"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}
+]

testdata/anthropic/stream_tooluse.sse

@@ -0,0 +1,21 @@
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01TOOL","type":"message","role":"assistant","content":[],"model":"claude-opus-5","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":15,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"tool_use","id":"toolu_01XYZ","name":"get_weather","input":{}}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"{\"location\": \""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"Paris\"}"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":7}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

testdata/anthropic/stream_tooluse_usage.json

@@ -0,0 +1 @@
+{"prompt": 15, "completion": 7}

testdata/anthropic/stream_tooluse_want.json

@@ -0,0 +1,7 @@
+[
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"toolu_01XYZ","type":"function","function":{"name":"get_weather","arguments":""}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\"location\": \""}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"Paris\"}"}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}
+]

translate_anthropic.go

@@ -0,0 +1,716 @@
+package traefikllmgateway
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// anthropicAPIVersion is the "anthropic-version" header value every request
+// to Anthropic's Messages API must carry.
+const anthropicAPIVersion = "2023-06-01"
+
+// anthropicDefaultMaxTokens is the value anthropicRequestFromOpenAI sends
+// for "max_tokens" when the OpenAI request supplies neither "max_tokens"
+// nor "max_completion_tokens" — Anthropic requires the field, OpenAI does
+// not.
+const anthropicDefaultMaxTokens = 4096
+
+// chatCompletionIDPrefix is prepended to Anthropic's message id to form the
+// OpenAI-shaped response "id" field, both for a non-streaming response and
+// for every streaming chunk's "id".
+const chatCompletionIDPrefix = "chatcmpl-"
+
+// translateError is a client-facing translation failure: an OpenAI-shaped
+// request or Anthropic-shaped response could not be mapped to the other
+// side's wire format. It implements error. The caller (Task 12's unified
+// route) maps a *translateError to HTTP 400, or 501 when notSupported is
+// set — embeddings has no Anthropic equivalent at all, which is a
+// different failure than a single unsupported field on an otherwise valid
+// request.
+type translateError struct {
+	msg          string
+	notSupported bool
+}
+
+// Error implements the error interface.
+func (e *translateError) Error() string { return e.msg }
+
+// unsupportedFieldError returns a *translateError for an OpenAI request
+// field Anthropic's API has no equivalent for, per the request mapping
+// table's "n>1, logit_bias, logprobs" row.
+func unsupportedFieldError(field string) *translateError {
+	return &translateError{msg: fmt.Sprintf("%s is not supported for anthropic models", field)}
+}
+
+// toFloat64 extracts a numeric value from v, which may be float64 (the
+// normal shape after json.Unmarshal into map[string]any) or a Go-native
+// int/int64/float32 (a test, or a caller that builds req by hand).
+func toFloat64(v any) (float64, bool) {
+	switch n := v.(type) {
+	case float64:
+		return n, true
+	case float32:
+		return float64(n), true
+	case int:
+		return float64(n), true
+	case int64:
+		return float64(n), true
+	}
+	return 0, false
+}
+
+// toInt64 is toFloat64 truncated to an integer, for fields (max_tokens)
+// Anthropic's API requires as a JSON integer.
+func toInt64(v any) (int64, bool) {
+	f, ok := toFloat64(v)
+	if !ok {
+		return 0, false
+	}
+	return int64(f), true
+}
+
+// stopSequencesFrom maps OpenAI's "stop" field (a string or an array of
+// strings) to Anthropic's "stop_sequences" array. A present-but-empty
+// string returns a nil slice — anthropicRequestFromOpenAI omits the field
+// entirely rather than sending an empty array.
+func stopSequencesFrom(v any) ([]string, error) {
+	switch s := v.(type) {
+	case string:
+		if s == "" {
+			return nil, nil
+		}
+		return []string{s}, nil
+	case []any:
+		seqs := make([]string, 0, len(s))
+		for _, e := range s {
+			str, ok := e.(string)
+			if !ok {
+				return nil, &translateError{msg: "stop[] entries must be strings"}
+			}
+			seqs = append(seqs, str)
+		}
+		return seqs, nil
+	default:
+		return nil, nil
+	}
+}
+
+// anthropicImageSourceFromDataURI parses an OpenAI image_url.url as a
+// "data:<media-type>;base64,<data>" URI and returns Anthropic's image
+// source object. Any URL that is not a base64 data URI (an http(s) image
+// URL, which Anthropic's Messages API does not fetch) returns a
+// *translateError, per the request mapping table's image row.
+func anthropicImageSourceFromDataURI(uri string) (map[string]any, error) {
+	rest, ok := strings.CutPrefix(uri, "data:")
+	if !ok {
+		return nil, &translateError{msg: "image_url must be a data: URI (http(s) image URLs are not supported for anthropic models)"}
+	}
+	header, data, ok := strings.Cut(rest, ",")
+	if !ok {
+		return nil, &translateError{msg: "malformed data: URI in image_url"}
+	}
+	mediaType, ok := strings.CutSuffix(header, ";base64")
+	if !ok {
+		return nil, &translateError{msg: "data: URI in image_url must be base64-encoded"}
+	}
+	return map[string]any{"type": "base64", "media_type": mediaType, "data": data}, nil
+}
+
+// anthropicContentBlockFromOpenAI maps one OpenAI content-array entry
+// ({"type":"text",...} or {"type":"image_url",...}) to an Anthropic
+// content block. An entry of any other type is dropped (nil, nil) rather
+// than erroring — an unrecognized part type is forwarded content this
+// translator does not understand, not necessarily an invalid request.
+func anthropicContentBlockFromOpenAI(pm map[string]any) (map[string]any, error) {
+	switch ptype, _ := pm["type"].(string); ptype {
+	case "text":
+		text, _ := pm["text"].(string)
+		return map[string]any{"type": "text", "text": text}, nil
+	case "image_url":
+		iu, _ := pm["image_url"].(map[string]any)
+		url, _ := iu["url"].(string)
+		src, err := anthropicImageSourceFromDataURI(url)
+		if err != nil {
+			return nil, err
+		}
+		return map[string]any{"type": "image", "source": src}, nil
+	default:
+		return nil, nil
+	}
+}
+
+// anthropicToolUseBlockFromOpenAI maps one assistant tool_calls[] entry
+// ({"id","type":"function","function":{"name","arguments"}}) to an
+// Anthropic {"type":"tool_use","id","name","input"} content block,
+// parsing the OpenAI arguments JSON string into Anthropic's structured
+// "input" object.
+func anthropicToolUseBlockFromOpenAI(tc any) (map[string]any, error) {
+	tm, ok := tc.(map[string]any)
+	if !ok {
+		return nil, &translateError{msg: "assistant tool_calls[] entry is malformed"}
+	}
+	id, _ := tm["id"].(string)
+	fn, _ := tm["function"].(map[string]any)
+	name, _ := fn["name"].(string)
+	argsStr, _ := fn["arguments"].(string)
+
+	input := any(map[string]any{})
+	if argsStr != "" {
+		if err := json.Unmarshal([]byte(argsStr), &input); err != nil {
+			return nil, &translateError{msg: "assistant tool_calls[].function.arguments is not valid JSON"}
+		}
+	}
+	return map[string]any{"type": "tool_use", "id": id, "name": name, "input": input}, nil
+}
+
+// anthropicMessageFromOpenAI maps one OpenAI user/assistant message to an
+// Anthropic message with the same role. A plain string content with no
+// tool_calls passes through as a string (the mapping table's "content
+// string" row); any other combination — a content array, an assistant
+// message carrying tool_calls, or both — produces a content block array,
+// per the mapping table's remaining rows.
+func anthropicMessageFromOpenAI(msg map[string]any, role string) (map[string]any, error) {
+	contentVal := msg["content"]
+
+	var toolCalls []any
+	if role == "assistant" {
+		toolCalls, _ = msg["tool_calls"].([]any)
+	}
+
+	if s, ok := contentVal.(string); ok && len(toolCalls) == 0 {
+		return map[string]any{"role": role, "content": s}, nil
+	}
+
+	var blocks []any
+	switch c := contentVal.(type) {
+	case string:
+		if c != "" {
+			blocks = append(blocks, map[string]any{"type": "text", "text": c})
+		}
+	case []any:
+		for _, part := range c {
+			pm, ok := part.(map[string]any)
+			if !ok {
+				continue
+			}
+			block, err := anthropicContentBlockFromOpenAI(pm)
+			if err != nil {
+				return nil, err
+			}
+			if block != nil {
+				blocks = append(blocks, block)
+			}
+		}
+	}
+
+	for _, tc := range toolCalls {
+		block, err := anthropicToolUseBlockFromOpenAI(tc)
+		if err != nil {
+			return nil, err
+		}
+		blocks = append(blocks, block)
+	}
+
+	if blocks == nil {
+		blocks = []any{}
+	}
+	return map[string]any{"role": role, "content": blocks}, nil
+}
+
+// anthropicToolResultMessage maps one OpenAI role:"tool" message to an
+// Anthropic user message carrying a single tool_result content block, per
+// the mapping table's "messages[role=tool]" row. content passes through
+// unchanged — Anthropic's tool_result.content accepts a string, matching
+// the common case, and this translator does not need to further interpret
+// it.
+func anthropicToolResultMessage(msg map[string]any) map[string]any {
+	toolCallID, _ := msg["tool_call_id"].(string)
+	return map[string]any{
+		"role": "user",
+		"content": []any{map[string]any{
+			"type":        "tool_result",
+			"tool_use_id": toolCallID,
+			"content":     msg["content"],
+		}},
+	}
+}
+
+// anthropicToolChoiceFromOpenAI maps OpenAI's "tool_choice" to Anthropic's
+// tool_choice object, and reports whether "tools" must be dropped from the
+// request. Per the mapping table, tool_choice:"none" has no valid
+// Anthropic {"type":"none"} equivalent for this translator to use, so
+// "none" is expressed by omitting "tools" (and "tool_choice") entirely
+// instead.
+func anthropicToolChoiceFromOpenAI(tc any) (choice map[string]any, dropTools bool) {
+	switch t := tc.(type) {
+	case string:
+		switch t {
+		case "none":
+			return nil, true
+		case "auto":
+			return map[string]any{"type": "auto"}, false
+		case "required":
+			return map[string]any{"type": "any"}, false
+		}
+	case map[string]any:
+		if fn, ok := t["function"].(map[string]any); ok {
+			if name, ok := fn["name"].(string); ok {
+				return map[string]any{"type": "tool", "name": name}, false
+			}
+		}
+	}
+	return nil, false
+}
+
+// anthropicToolsFromOpenAI maps OpenAI's tools[].function{name,
+// description, parameters} to Anthropic's tools[]{name, description,
+// input_schema}, per the mapping table's tools row.
+func anthropicToolsFromOpenAI(toolsRaw []any) []any {
+	out := make([]any, 0, len(toolsRaw))
+	for _, t := range toolsRaw {
+		tm, ok := t.(map[string]any)
+		if !ok {
+			continue
+		}
+		fn, _ := tm["function"].(map[string]any)
+		entry := map[string]any{
+			"name":         fn["name"],
+			"input_schema": fn["parameters"],
+		}
+		if desc, ok := fn["description"]; ok {
+			entry["description"] = desc
+		}
+		out = append(out, entry)
+	}
+	return out
+}
+
+// anthropicRequestFromOpenAI maps an OpenAI chat-completion request body
+// to an Anthropic Messages API request body, per this file's request
+// mapping table. It returns a *translateError — never wrapped — for a
+// field Anthropic has no equivalent for (n>1, logit_bias, logprobs) or a
+// content part this translator cannot map (a non-data-URI image URL, a
+// malformed tool_calls argument string).
+func anthropicRequestFromOpenAI(req map[string]any) (map[string]any, error) {
+	if v, ok := req["n"]; ok {
+		if n, ok2 := toFloat64(v); ok2 && n > 1 {
+			return nil, unsupportedFieldError("n")
+		}
+	}
+	if _, ok := req["logit_bias"]; ok {
+		return nil, unsupportedFieldError("logit_bias")
+	}
+	if _, ok := req["logprobs"]; ok {
+		return nil, unsupportedFieldError("logprobs")
+	}
+
+	out := map[string]any{}
+	if model, ok := req["model"].(string); ok {
+		out["model"] = model
+	}
+
+	msgsRaw, _ := req["messages"].([]any)
+	var systemParts []string
+	anthMsgs := make([]any, 0, len(msgsRaw))
+	for _, m := range msgsRaw {
+		msg, ok := m.(map[string]any)
+		if !ok {
+			continue
+		}
+		switch role, _ := msg["role"].(string); role {
+		case "system":
+			if s, ok := msg["content"].(string); ok {
+				systemParts = append(systemParts, s)
+			}
+		case "user", "assistant":
+			am, err := anthropicMessageFromOpenAI(msg, role)
+			if err != nil {
+				return nil, err
+			}
+			anthMsgs = append(anthMsgs, am)
+		case "tool":
+			anthMsgs = append(anthMsgs, anthropicToolResultMessage(msg))
+		}
+	}
+	if len(systemParts) > 0 {
+		out["system"] = strings.Join(systemParts, "\n\n")
+	}
+	out["messages"] = anthMsgs
+
+	maxTokens := int64(anthropicDefaultMaxTokens)
+	if v, ok := req["max_completion_tokens"]; ok {
+		if n, ok2 := toInt64(v); ok2 {
+			maxTokens = n
+		}
+	} else if v, ok := req["max_tokens"]; ok {
+		if n, ok2 := toInt64(v); ok2 {
+			maxTokens = n
+		}
+	}
+	out["max_tokens"] = maxTokens
+
+	if v, ok := req["temperature"]; ok {
+		out["temperature"] = v
+	}
+	if v, ok := req["top_p"]; ok {
+		out["top_p"] = v
+	}
+	if v, ok := req["stop"]; ok {
+		seqs, err := stopSequencesFrom(v)
+		if err != nil {
+			return nil, err
+		}
+		if len(seqs) > 0 {
+			out["stop_sequences"] = seqs
+		}
+	}
+	if v, ok := req["stream"]; ok {
+		out["stream"] = v
+	}
+
+	toolsRaw, hasTools := req["tools"].([]any)
+	dropTools := false
+	if tc, ok := req["tool_choice"]; ok {
+		var choice map[string]any
+		choice, dropTools = anthropicToolChoiceFromOpenAI(tc)
+		if choice != nil {
+			out["tool_choice"] = choice
+		}
+	}
+	if hasTools && !dropTools {
+		out["tools"] = anthropicToolsFromOpenAI(toolsRaw)
+	}
+
+	if v, ok := req["user"]; ok {
+		out["metadata"] = map[string]any{"user_id": v}
+	}
+
+	return out, nil
+}
+
+// anthropicContentBlock is the subset of one Anthropic response content
+// block's JSON this translator reads: "text" blocks carry Text, "tool_use"
+// blocks carry ID/Name/Input.
+type anthropicContentBlock struct {
+	Type  string          `json:"type"`
+	Text  string          `json:"text,omitempty"`
+	ID    string          `json:"id,omitempty"`
+	Name  string          `json:"name,omitempty"`
+	Input json.RawMessage `json:"input,omitempty"`
+}
+
+// anthropicUsagePayload is the "usage" object on both a non-streaming
+// Anthropic response and a streaming message_start/message_delta event.
+type anthropicUsagePayload struct {
+	InputTokens  int64 `json:"input_tokens"`
+	OutputTokens int64 `json:"output_tokens"`
+}
+
+// anthropicResponseBody is the subset of a non-streaming Anthropic
+// Messages API response this translator reads.
+type anthropicResponseBody struct {
+	ID         string                  `json:"id"`
+	StopReason string                  `json:"stop_reason"`
+	Content    []anthropicContentBlock `json:"content"`
+	Usage      anthropicUsagePayload   `json:"usage"`
+}
+
+// anthropicFinishReason maps an Anthropic stop_reason to an OpenAI
+// finish_reason: max_tokens -> "length", tool_use -> "tool_calls",
+// end_turn and stop_sequence both -> "stop". An unrecognized value
+// defaults to "stop", the OpenAI-safe default for a completed turn.
+func anthropicFinishReason(stopReason string) string {
+	switch stopReason {
+	case "max_tokens":
+		return "length"
+	case "tool_use":
+		return "tool_calls"
+	default:
+		return "stop"
+	}
+}
+
+// openAIResponseFromAnthropic maps a non-streaming Anthropic Messages API
+// response body to an OpenAI chat.completion response, per this file's
+// response mapping table. model and created are supplied by the caller —
+// Anthropic's response carries neither the gateway-facing model id nor a
+// response timestamp, so this function has no other source for them.
+func openAIResponseFromAnthropic(body []byte, model string, created int64) (map[string]any, usage, error) {
+	var resp anthropicResponseBody
+	if err := json.Unmarshal(body, &resp); err != nil {
+		return nil, usage{}, fmt.Errorf("%w: decode anthropic response: %w", errUpstream, err)
+	}
+
+	var textParts []string
+	var toolCalls []any
+	for _, b := range resp.Content {
+		switch b.Type {
+		case "text":
+			textParts = append(textParts, b.Text)
+		case "tool_use":
+			args, err := reMarshalToolInput(b.Input)
+			if err != nil {
+				return nil, usage{}, err
+			}
+			toolCalls = append(toolCalls, map[string]any{
+				"id":   b.ID,
+				"type": "function",
+				"function": map[string]any{
+					"name":      b.Name,
+					"arguments": args,
+				},
+			})
+		}
+	}
+
+	message := map[string]any{"role": "assistant"}
+	if len(textParts) > 0 {
+		message["content"] = strings.Join(textParts, "")
+	} else {
+		message["content"] = nil
+	}
+	if len(toolCalls) > 0 {
+		message["tool_calls"] = toolCalls
+	}
+
+	u := usage{prompt: resp.Usage.InputTokens, completion: resp.Usage.OutputTokens}
+
+	out := map[string]any{
+		"id":      chatCompletionIDPrefix + resp.ID,
+		"object":  "chat.completion",
+		"created": created,
+		"model":   model,
+		"choices": []any{map[string]any{
+			"index":         0,
+			"message":       message,
+			"finish_reason": anthropicFinishReason(resp.StopReason),
+		}},
+		"usage": map[string]any{
+			"prompt_tokens":     u.prompt,
+			"completion_tokens": u.completion,
+			"total_tokens":      u.total(),
+		},
+	}
+	return out, u, nil
+}
+
+// reMarshalToolInput decodes a tool_use block's raw "input" JSON and
+// re-marshals it to a compact JSON string, the shape OpenAI's
+// tool_calls[].function.arguments field expects. An empty input decodes
+// to "{}", matching a tool call with no arguments.
+func reMarshalToolInput(raw json.RawMessage) (string, error) {
+	parsed := any(map[string]any{})
+	if len(raw) > 0 {
+		if err := json.Unmarshal(raw, &parsed); err != nil {
+			return "", fmt.Errorf("%w: decode tool_use input: %w", errUpstream, err)
+		}
+	}
+	b, err := json.Marshal(parsed)
+	if err != nil {
+		return "", fmt.Errorf("%w: re-marshal tool_use input: %w", errUpstream, err)
+	}
+	return string(b), nil
+}
+
+// anthropicStreamState is one streaming chat completion's translation
+// state: the OpenAI-shaped chunk envelope fields (id, captured from the
+// message_start event; model and created, fixed at construction) and the
+// running usage total. A caller constructs one per stream and calls
+// translate once per upstream sseEvent, in order.
+type anthropicStreamState struct {
+	id      string
+	model   string
+	created int64
+	u       usage
+}
+
+// newAnthropicStreamState returns an anthropicStreamState for one
+// streaming chat completion. model is the gateway-facing model id and
+// created the response timestamp every emitted chunk carries — the
+// adapter fixes both once, before the first upstream event arrives.
+func newAnthropicStreamState(model string, created int64) *anthropicStreamState {
+	return &anthropicStreamState{model: model, created: created}
+}
+
+// usage returns the prompt/completion token counts captured so far, from
+// the message_start and message_delta events already translated.
+func (st *anthropicStreamState) usage() usage {
+	return st.u
+}
+
+// chunk builds one OpenAI-shaped "chat.completion.chunk" payload carrying
+// delta, and finishReason as its choices[0].finish_reason — nil for the
+// JSON null every non-terminal chunk carries.
+func (st *anthropicStreamState) chunk(delta map[string]any, finishReason *string) []byte {
+	var fr any
+	if finishReason != nil {
+		fr = *finishReason
+	}
+	b, err := json.Marshal(map[string]any{
+		"id":      chatCompletionIDPrefix + st.id,
+		"object":  "chat.completion.chunk",
+		"created": st.created,
+		"model":   st.model,
+		"choices": []any{map[string]any{
+			"index":         0,
+			"delta":         delta,
+			"finish_reason": fr,
+		}},
+	})
+	if err != nil {
+		// delta is always built from this file's own map[string]any/string/
+		// int64 literals — never a value that can fail to marshal.
+		panic(fmt.Sprintf("llmgateway: anthropic stream chunk failed to marshal: %v", err))
+	}
+	return b
+}
+
+// anthropicMessageStartPayload is the "message_start" event's data shape.
+type anthropicMessageStartPayload struct {
+	Message struct {
+		ID    string                `json:"id"`
+		Usage anthropicUsagePayload `json:"usage"`
+	} `json:"message"`
+}
+
+// anthropicContentBlockStartPayload is the "content_block_start" event's
+// data shape.
+type anthropicContentBlockStartPayload struct {
+	ContentBlock struct {
+		Type string `json:"type"`
+		ID   string `json:"id"`
+		Name string `json:"name"`
+	} `json:"content_block"`
+	Index int `json:"index"`
+}
+
+// anthropicContentBlockDeltaPayload is the "content_block_delta" event's
+// data shape: Delta.Type distinguishes "text_delta" (Delta.Text) from
+// "input_json_delta" (Delta.PartialJSON).
+type anthropicContentBlockDeltaPayload struct {
+	Delta struct {
+		Type        string `json:"type"`
+		Text        string `json:"text"`
+		PartialJSON string `json:"partial_json"`
+	} `json:"delta"`
+	Index int `json:"index"`
+}
+
+// anthropicMessageDeltaPayload is the "message_delta" event's data shape.
+type anthropicMessageDeltaPayload struct {
+	Delta struct {
+		StopReason string `json:"stop_reason"`
+	} `json:"delta"`
+	Usage anthropicUsagePayload `json:"usage"`
+}
+
+// anthropicErrorEventPayload is the "error" event's data shape.
+type anthropicErrorEventPayload struct {
+	Error struct {
+		Type    string `json:"type"`
+		Message string `json:"message"`
+	} `json:"error"`
+}
+
+// translate maps one upstream Anthropic SSE event to zero or more
+// OpenAI-shaped "chat.completion.chunk" payloads, per this file's stream
+// mapping table:
+//
+//   - message_start: a {role:"assistant"} chunk, and captures input_tokens.
+//   - content_block_start (tool_use): a delta.tool_calls[0] chunk carrying
+//     id, type, and function.name with an empty arguments string. A
+//     content_block_start for any other block type produces nothing.
+//   - content_block_delta (text_delta): a {content: text} chunk.
+//   - content_block_delta (input_json_delta): a delta.tool_calls[0] chunk
+//     carrying only index and function.arguments, the incremental JSON
+//     fragment.
+//   - message_delta: captures output_tokens, and a finish_reason chunk
+//     mapped per anthropicFinishReason.
+//   - message_stop, ping, content_block_stop: nothing — the caller (the
+//     adapter) writes the terminal "[DONE]" itself once the upstream
+//     stream ends, on message_stop or on a clean EOF alike, per ruling
+//     (b): a data-less "event:" frame is never relied on to signal the
+//     end of the stream.
+//   - error: one OpenAI-shaped {"error":{...}} payload, and a non-nil
+//     error — translate's caller must stop reading the stream after this
+//     return, but forwards the returned chunk to the client first.
+//
+// Any other or malformed event is ignored (nil, nil) rather than erroring
+// — an event this translator does not recognize is not necessarily a
+// broken stream.
+func (st *anthropicStreamState) translate(ev sseEvent) ([][]byte, error) {
+	switch ev.event {
+	case "message_start":
+		var p anthropicMessageStartPayload
+		if err := json.Unmarshal(ev.data, &p); err != nil {
+			return nil, nil
+		}
+		st.id = p.Message.ID
+		st.u.prompt = p.Message.Usage.InputTokens
+		return [][]byte{st.chunk(map[string]any{"role": "assistant"}, nil)}, nil
+
+	case "content_block_start":
+		var p anthropicContentBlockStartPayload
+		if err := json.Unmarshal(ev.data, &p); err != nil {
+			return nil, nil
+		}
+		if p.ContentBlock.Type != "tool_use" {
+			return nil, nil
+		}
+		delta := map[string]any{"tool_calls": []any{map[string]any{
+			"index": p.Index,
+			"id":    p.ContentBlock.ID,
+			"type":  "function",
+			"function": map[string]any{
+				"name":      p.ContentBlock.Name,
+				"arguments": "",
+			},
+		}}}
+		return [][]byte{st.chunk(delta, nil)}, nil
+
+	case "content_block_delta":
+		var p anthropicContentBlockDeltaPayload
+		if err := json.Unmarshal(ev.data, &p); err != nil {
+			return nil, nil
+		}
+		switch p.Delta.Type {
+		case "text_delta":
+			return [][]byte{st.chunk(map[string]any{"content": p.Delta.Text}, nil)}, nil
+		case "input_json_delta":
+			delta := map[string]any{"tool_calls": []any{map[string]any{
+				"index":    p.Index,
+				"function": map[string]any{"arguments": p.Delta.PartialJSON},
+			}}}
+			return [][]byte{st.chunk(delta, nil)}, nil
+		default:
+			return nil, nil
+		}
+
+	case "message_delta":
+		var p anthropicMessageDeltaPayload
+		if err := json.Unmarshal(ev.data, &p); err != nil {
+			return nil, nil
+		}
+		st.u.completion = p.Usage.OutputTokens
+		reason := anthropicFinishReason(p.Delta.StopReason)
+		return [][]byte{st.chunk(map[string]any{}, &reason)}, nil
+
+	case "error":
+		var p anthropicErrorEventPayload
+		if err := json.Unmarshal(ev.data, &p); err != nil {
+			return nil, fmt.Errorf("%w: decode anthropic error event: %w", errUpstream, err)
+		}
+		chunk, err := json.Marshal(map[string]any{
+			"error": map[string]any{"message": p.Error.Message, "type": p.Error.Type},
+		})
+		if err != nil {
+			return nil, fmt.Errorf("%w: marshal anthropic error chunk: %w", errUpstream, err)
+		}
+		return [][]byte{chunk}, fmt.Errorf("llmgateway: anthropic stream error: %s", p.Error.Message)
+
+	default:
+		// message_stop, ping, content_block_stop, and anything unrecognized.
+		return nil, nil
+	}
+}

translate_anthropic_test.go

@@ -0,0 +1,413 @@
+package traefikllmgateway
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sort"
+	"strings"
+	"testing"
+)
+
+// goldenTestdataDir is where every fixture this file walks lives.
+const goldenTestdataDir = "testdata/anthropic"
+
+// goldenGatewayModel and goldenCreated are the fixed model/created values
+// every response- and stream-translation golden fixture's "_want.json"
+// bakes in — openAIResponseFromAnthropic and newAnthropicStreamState both
+// take these as caller-supplied parameters (Anthropic's own wire formats
+// carry neither), so the golden tests must supply the same fixed values
+// the "_want.json" files were written against.
+const (
+	goldenGatewayModel = "gw-anthropic-model"
+	goldenCreated      = int64(1734000000)
+)
+
+// normalizeJSON round-trips b through json.Unmarshal into a generic
+// map[string]any (or []any), so a comparison via reflect.DeepEqual is not
+// tripped up by Go-native numeric types (int64, e.g.) that differ from
+// the float64 every JSON number decodes to — both v and w.json.Unmarshal
+// output use the same generic decode, so both land on float64 either way.
+func normalizeJSON(t *testing.T, v any) any {
+	t.Helper()
+	b, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("normalizeJSON: marshal: %v", err)
+	}
+	var out any
+	if err := json.Unmarshal(b, &out); err != nil {
+		t.Fatalf("normalizeJSON: unmarshal: %v", err)
+	}
+	return out
+}
+
+// readGoldenJSON reads and json.Unmarshals name (relative to
+// testdata/anthropic) into a fresh map[string]any.
+func readGoldenJSON(t *testing.T, name string) map[string]any {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join(goldenTestdataDir, name))
+	if err != nil {
+		t.Fatalf("read %s: %v", name, err)
+	}
+	var m map[string]any
+	if err := json.Unmarshal(b, &m); err != nil {
+		t.Fatalf("decode %s: %v", name, err)
+	}
+	return m
+}
+
+// wantErrFixture is the shape a "{name}_wanterr.json" fixture decodes
+// into: a substring the produced *translateError.Error() must contain,
+// and whether it must set notSupported.
+type wantErrFixture struct {
+	ErrContains  string `json:"errContains"`
+	NotSupported bool   `json:"notSupported"`
+}
+
+// assertTranslateError asserts err is a *translateError matching want.
+func assertTranslateError(t *testing.T, err error, want wantErrFixture) {
+	t.Helper()
+	if err == nil {
+		t.Fatalf("want a *translateError containing %q, got nil", want.ErrContains)
+	}
+	var terr *translateError
+	if !errors.As(err, &terr) {
+		t.Fatalf("err = %v (%T), want *translateError", err, err)
+	}
+	if !strings.Contains(terr.Error(), want.ErrContains) {
+		t.Errorf("error message = %q, want it to contain %q", terr.Error(), want.ErrContains)
+	}
+	if terr.notSupported != want.NotSupported {
+		t.Errorf("notSupported = %v, want %v", terr.notSupported, want.NotSupported)
+	}
+}
+
+// TestAnthropicRequestGolden walks every "chat_*_in.json" fixture under
+// testdata/anthropic, translating it via anthropicRequestFromOpenAI, and
+// asserts the result against the sibling "_want.json" fixture (a
+// successful translation) or "_wanterr.json" fixture (a *translateError),
+// whichever is present — exactly one of the two exists per fixture.
+func TestAnthropicRequestGolden(t *testing.T) {
+	entries, err := os.ReadDir(goldenTestdataDir)
+	if err != nil {
+		t.Fatalf("read testdata dir: %v", err)
+	}
+
+	var names []string
+	for _, e := range entries {
+		if n, ok := strings.CutSuffix(e.Name(), "_in.json"); ok && strings.HasPrefix(n, "chat_") {
+			names = append(names, n)
+		}
+	}
+	sort.Strings(names)
+	if len(names) == 0 {
+		t.Fatalf("no chat_*_in.json fixtures found under %s", goldenTestdataDir)
+	}
+
+	for _, name := range names {
+		t.Run(name, func(t *testing.T) {
+			in := readGoldenJSON(t, name+"_in.json")
+
+			got, err := anthropicRequestFromOpenAI(in)
+
+			wantErrPath := filepath.Join(goldenTestdataDir, name+"_wanterr.json")
+			if _, statErr := os.Stat(wantErrPath); statErr == nil {
+				var want wantErrFixture
+				b, rerr := os.ReadFile(wantErrPath)
+				if rerr != nil {
+					t.Fatalf("read %s: %v", wantErrPath, rerr)
+				}
+				if uerr := json.Unmarshal(b, &want); uerr != nil {
+					t.Fatalf("decode %s: %v", wantErrPath, uerr)
+				}
+				assertTranslateError(t, err, want)
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("anthropicRequestFromOpenAI: %v", err)
+			}
+			want := readGoldenJSON(t, name+"_want.json")
+			gotN := normalizeJSON(t, got)
+			wantN := normalizeJSON(t, want)
+			if !reflect.DeepEqual(gotN, wantN) {
+				gotB, _ := json.MarshalIndent(gotN, "", "  ")
+				wantB, _ := json.MarshalIndent(wantN, "", "  ")
+				t.Errorf("anthropicRequestFromOpenAI(%s) mismatch:\ngot:\n%s\nwant:\n%s", name, gotB, wantB)
+			}
+		})
+	}
+}
+
+// TestAnthropicResponseGolden walks every "resp_*_in.json" fixture,
+// translating it via openAIResponseFromAnthropic with the fixed
+// goldenGatewayModel/goldenCreated, and asserts the result against the
+// sibling "_want.json" fixture.
+func TestAnthropicResponseGolden(t *testing.T) {
+	entries, err := os.ReadDir(goldenTestdataDir)
+	if err != nil {
+		t.Fatalf("read testdata dir: %v", err)
+	}
+
+	var names []string
+	for _, e := range entries {
+		if n, ok := strings.CutSuffix(e.Name(), "_in.json"); ok && strings.HasPrefix(n, "resp_") {
+			names = append(names, n)
+		}
+	}
+	sort.Strings(names)
+	if len(names) == 0 {
+		t.Fatalf("no resp_*_in.json fixtures found under %s", goldenTestdataDir)
+	}
+
+	for _, name := range names {
+		t.Run(name, func(t *testing.T) {
+			body, err := os.ReadFile(filepath.Join(goldenTestdataDir, name+"_in.json"))
+			if err != nil {
+				t.Fatalf("read %s_in.json: %v", name, err)
+			}
+
+			got, _, err := openAIResponseFromAnthropic(body, goldenGatewayModel, goldenCreated)
+			if err != nil {
+				t.Fatalf("openAIResponseFromAnthropic: %v", err)
+			}
+
+			want := readGoldenJSON(t, name+"_want.json")
+			gotN := normalizeJSON(t, got)
+			wantN := normalizeJSON(t, want)
+			if !reflect.DeepEqual(gotN, wantN) {
+				gotB, _ := json.MarshalIndent(gotN, "", "  ")
+				wantB, _ := json.MarshalIndent(wantN, "", "  ")
+				t.Errorf("openAIResponseFromAnthropic(%s) mismatch:\ngot:\n%s\nwant:\n%s", name, gotB, wantB)
+			}
+		})
+	}
+}
+
+// streamUsageFixture is the shape a "{name}_usage.json" fixture decodes
+// into: the prompt/completion totals anthropicStreamState.usage() must
+// report once every event in the sibling ".sse" fixture has been
+// translated.
+type streamUsageFixture struct {
+	Prompt     int64 `json:"prompt"`
+	Completion int64 `json:"completion"`
+}
+
+// runStreamGolden translates every event in testdata/anthropic/{name}.sse
+// through a fresh anthropicStreamState, in order, and asserts the
+// flattened list of emitted chunks against "{name}_want.json" and the
+// final usage() against "{name}_usage.json".
+func runStreamGolden(t *testing.T, name string) {
+	t.Helper()
+
+	f, err := os.Open(filepath.Join(goldenTestdataDir, name+".sse"))
+	if err != nil {
+		t.Fatalf("open %s.sse: %v", name, err)
+	}
+	defer f.Close() //nolint:errcheck
+
+	st := newAnthropicStreamState(goldenGatewayModel, goldenCreated)
+	var got []any
+	terminalErr := readSSE(f, func(ev sseEvent) error {
+		chunks, terr := st.translate(ev)
+		for _, c := range chunks {
+			var v any
+			if uerr := json.Unmarshal(c, &v); uerr != nil {
+				t.Fatalf("translate(%s) produced invalid JSON: %v (%s)", ev.event, uerr, c)
+			}
+			got = append(got, v)
+		}
+		return terr
+	})
+	if terminalErr != nil {
+		t.Fatalf("readSSE/translate(%s): %v", name, terminalErr)
+	}
+
+	wantBytes, err := os.ReadFile(filepath.Join(goldenTestdataDir, name+"_want.json"))
+	if err != nil {
+		t.Fatalf("read %s_want.json: %v", name, err)
+	}
+	var want []any
+	if uerr := json.Unmarshal(wantBytes, &want); uerr != nil {
+		t.Fatalf("decode %s_want.json: %v", name, uerr)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		gotB, _ := json.MarshalIndent(got, "", "  ")
+		wantB, _ := json.MarshalIndent(want, "", "  ")
+		t.Errorf("translate(%s) chunk list mismatch:\ngot:\n%s\nwant:\n%s", name, gotB, wantB)
+	}
+
+	usageBytes, err := os.ReadFile(filepath.Join(goldenTestdataDir, name+"_usage.json"))
+	if err != nil {
+		t.Fatalf("read %s_usage.json: %v", name, err)
+	}
+	var wantUsage streamUsageFixture
+	if err := json.Unmarshal(usageBytes, &wantUsage); err != nil {
+		t.Fatalf("decode %s_usage.json: %v", name, err)
+	}
+	if gotUsage := st.usage(); gotUsage.prompt != wantUsage.Prompt || gotUsage.completion != wantUsage.Completion {
+		t.Errorf("usage() = %+v, want {prompt:%d completion:%d}", gotUsage, wantUsage.Prompt, wantUsage.Completion)
+	}
+}
+
+// TestAnthropicStreamGolden_Text covers a text-only stream: role chunk,
+// two content deltas, a finish chunk mapped from end_turn, with a ping
+// and a content_block_start/stop interspersed that must produce nothing.
+func TestAnthropicStreamGolden_Text(t *testing.T) {
+	runStreamGolden(t, "stream_text")
+}
+
+// TestAnthropicStreamGolden_ToolUse covers a tool_use stream: role chunk,
+// a tool_calls-start chunk, two input_json_delta argument fragments, and
+// a finish chunk mapped from tool_use.
+func TestAnthropicStreamGolden_ToolUse(t *testing.T) {
+	runStreamGolden(t, "stream_tooluse")
+}
+
+// TestAnthropicStreamState_ErrorEvent proves an "error" SSE event
+// translates to one OpenAI-shaped {"error":{...}} chunk and a non-nil
+// terminal error, per the stream mapping table's error row — built
+// inline rather than as a golden fixture since it is the one case
+// runStreamGolden's readSSE-drives-to-completion harness cannot express
+// (the whole point is that translation stops here).
+func TestAnthropicStreamState_ErrorEvent(t *testing.T) {
+	st := newAnthropicStreamState(goldenGatewayModel, goldenCreated)
+	ev := sseEvent{
+		event: "error",
+		data:  []byte(`{"type":"error","error":{"type":"overloaded_error","message":"Overloaded"}}`),
+	}
+
+	chunks, err := st.translate(ev)
+	if err == nil {
+		t.Fatalf("translate(error event): want a terminal error, got nil")
+	}
+	if len(chunks) != 1 {
+		t.Fatalf("translate(error event) returned %d chunks, want 1", len(chunks))
+	}
+
+	var payload map[string]any
+	if uerr := json.Unmarshal(chunks[0], &payload); uerr != nil {
+		t.Fatalf("emitted error chunk is not valid JSON: %v (%s)", uerr, chunks[0])
+	}
+	errObj, ok := payload["error"].(map[string]any)
+	if !ok {
+		t.Fatalf("emitted error chunk missing \"error\" object: %s", chunks[0])
+	}
+	if errObj["message"] != "Overloaded" {
+		t.Errorf("error.message = %v, want %q", errObj["message"], "Overloaded")
+	}
+	if errObj["type"] != "overloaded_error" {
+		t.Errorf("error.type = %v, want %q", errObj["type"], "overloaded_error")
+	}
+}
+
+// TestAnthropicListModels_404NotSpeciallyHandled proves listModels
+// returns a plain *providerHTTPError on a 404 (an older Anthropic API
+// version that predates the Models endpoint) rather than any special
+// fallback — the registry, not this adapter, decides what to do with a
+// provider's explicitly configured models on a listModels failure.
+func TestAnthropicListModels_404NotSpeciallyHandled(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"error":{"message":"not found"}}`))
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+
+	_, err = a.listModels(context.Background())
+	if err == nil {
+		t.Fatalf("listModels: want error on 404, got nil")
+	}
+	var httpErr *providerHTTPError
+	if !errors.As(err, &httpErr) {
+		t.Fatalf("err = %v (%T), want *providerHTTPError", err, err)
+	}
+	if httpErr.status != http.StatusNotFound {
+		t.Errorf("status = %d, want 404", httpErr.status)
+	}
+}
+
+// TestAnthropicAdapter_Embeddings_NotSupported proves embeddings never
+// makes an upstream request and always returns a *translateError with
+// notSupported set — the caller (Task 12) maps that to HTTP 501.
+func TestAnthropicAdapter_Embeddings_NotSupported(t *testing.T) {
+	called := false
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	a, err := newAnthropicAdapter("p1", srv.URL, "sk-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+
+	_, err = a.embeddings(context.Background(), httptest.NewRecorder(), map[string]any{"model": "claude-opus-5", "input": "hi"})
+	var terr *translateError
+	if !errors.As(err, &terr) {
+		t.Fatalf("err = %v (%T), want *translateError", err, err)
+	}
+	if !terr.notSupported {
+		t.Errorf("notSupported = false, want true")
+	}
+	if called {
+		t.Errorf("embeddings made an upstream request, want none")
+	}
+}
+
+// TestNewAnthropicAdapter_KeylessIsAConstructorError proves ruling (c):
+// an empty resolved API key is a constructor error for anthropic-type
+// providers, unlike the openai-type keyless ruling.
+func TestNewAnthropicAdapter_KeylessIsAConstructorError(t *testing.T) {
+	if _, err := newAnthropicAdapter("p1", "https://api.anthropic.com", ""); err == nil {
+		t.Fatalf("newAnthropicAdapter: want error for empty API key, got nil")
+	}
+}
+
+// TestAnthropicAdapter_InjectAuth proves injectAuth sets both the
+// x-api-key and anthropic-version headers unconditionally.
+func TestAnthropicAdapter_InjectAuth(t *testing.T) {
+	a, err := newAnthropicAdapter("p1", "https://api.anthropic.com", "sk-test")
+	if err != nil {
+		t.Fatalf("newAnthropicAdapter: %v", err)
+	}
+	req := httptest.NewRequest(http.MethodPost, "/", nil)
+	a.injectAuth(req)
+
+	if got := req.Header.Get("x-api-key"); got != "sk-test" {
+		t.Errorf("x-api-key = %q, want %q", got, "sk-test")
+	}
+	if got := req.Header.Get("anthropic-version"); got != anthropicAPIVersion {
+		t.Errorf("anthropic-version = %q, want %q", got, anthropicAPIVersion)
+	}
+}
+
+// TestAnthropicRequestFromOpenAI_MaxCompletionTokensPrecedence proves
+// max_completion_tokens takes precedence over max_tokens when an OpenAI
+// request sends both — the newer field wins.
+func TestAnthropicRequestFromOpenAI_MaxCompletionTokensPrecedence(t *testing.T) {
+	req := map[string]any{
+		"model":                 "claude-opus-5",
+		"messages":              []any{map[string]any{"role": "user", "content": "hi"}},
+		"max_tokens":            float64(100),
+		"max_completion_tokens": float64(200),
+	}
+	got, err := anthropicRequestFromOpenAI(req)
+	if err != nil {
+		t.Fatalf("anthropicRequestFromOpenAI: %v", err)
+	}
+	if got["max_tokens"] != int64(200) {
+		t.Errorf("max_tokens = %v, want 200 (max_completion_tokens takes precedence)", got["max_tokens"])
+	}
+}