Feature 08 task 3: Anthropic Messages adapter

Add internal/ai/llm/anthropic, an llm.Provider over plain net/http:

- POST {base}/v1/messages and GET {base}/v1/models with x-api-key and
  anthropic-version 2023-06-01. An empty key, a non-positive
  Config.MaxTokens and a base URL that is not absolute http(s), or that
  carries userinfo, a query or a fragment, are construction errors.
- System text goes in the top-level system field. A Request with a
  Schema becomes one tool whose input_schema is the schema, forced by
  tool_choice; Response.JSON is that tool's input. Otherwise the text
  blocks are Response.Text. Usage and stop_reason are parsed.
- ListModels follows has_more/after_id pages; Verify fetches one page
  with limit=1.
- 401/403 map to ErrAuth, 429 and 529 to ErrRateLimited with
  Retry-After, 408 and deadlines to ErrTimeout, other statuses to
  ErrProvider with a redacted excerpt.
- Errors carry the OpenAI adapter's redaction chain, so no Error(),
  Unwrap layer, errors.As target or %#v output holds the key.

Contract tests replay sanitized fixtures adapted from Anthropic's public
Messages API docs over httptest, covering tool-forced JSON, text, each
error mapping and key redaction with the keys x-api-key, anthropic,
HTTP, rate, a one-letter key and a URL-escaped key.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/anthropic/anthropic.go

@@ -0,0 +1,534 @@
+// Package anthropic is the llm.Provider adapter for Anthropic's Messages
+// API. It calls POST {base}/v1/messages and GET {base}/v1/models over plain
+// net/http.
+//
+// The adapter never logs, and no error it returns contains the API key:
+// errors do not quote caller- or vendor-supplied text, and every exported
+// method passes its error through redact as a backstop.
+package anthropic
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"reflect"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/ai/llm"
+)
+
+// APIVersion is the Messages API version sent as the anthropic-version
+// header.
+const APIVersion = "2023-06-01"
+
+// Config configures a Provider.
+type Config struct {
+	// BaseURL is the API root the /v1 endpoint paths are appended to, e.g.
+	// "https://api.anthropic.com".
+	BaseURL string
+	// APIKey is sent as the x-api-key header. It must not be empty.
+	APIKey string
+	// Model is the model Generate requests. ListModels and Verify do not
+	// need it.
+	Model string
+	// MaxTokens is the max_tokens Generate sends when a Request sets none:
+	// the Messages API requires one. It must be positive.
+	MaxTokens int
+}
+
+// Provider is an Anthropic Messages API llm.Provider.
+type Provider struct {
+	base      string
+	apiKey    string
+	model     string
+	maxTokens int
+	client    *http.Client
+}
+
+var _ llm.Provider = (*Provider)(nil)
+
+// New validates cfg and returns a Provider.
+func New(cfg Config) (*Provider, error) {
+	p, err := newFromConfig(cfg)
+	if err != nil {
+		return nil, redact(err, cfg.APIKey)
+	}
+	return p, nil
+}
+
+func newFromConfig(cfg Config) (*Provider, error) {
+	// No error from New quotes caller-supplied text: any of it may equal or
+	// contain the API key. Base URL errors never echo the URL.
+	u, err := url.Parse(cfg.BaseURL)
+	if err != nil {
+		return nil, errors.New("anthropic: base URL does not parse")
+	}
+	if (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
+		return nil, errors.New("anthropic: base URL must be an absolute http or https URL")
+	}
+	// A trailing "?" or "#" parses to an empty query or fragment, yet the
+	// endpoint path appended to it would land in the query or fragment.
+	if u.User != nil || strings.ContainsAny(cfg.BaseURL, "?#") {
+		return nil, errors.New("anthropic: base URL must not carry credentials, a query or a fragment")
+	}
+	if cfg.APIKey == "" {
+		return nil, errors.New("anthropic: no API key configured")
+	}
+	if !validHeaderValue(cfg.APIKey) {
+		return nil, errors.New("anthropic: API key has a control character")
+	}
+	if cfg.MaxTokens <= 0 {
+		return nil, errors.New("anthropic: max tokens must be positive")
+	}
+	return &Provider{
+		base:      strings.TrimRight(cfg.BaseURL, "/"),
+		apiKey:    cfg.APIKey,
+		model:     cfg.Model,
+		maxTokens: cfg.MaxTokens,
+		client:    &http.Client{},
+	}, nil
+}
+
+// validHeaderValue reports whether value is an RFC 9110 field-value: no
+// control characters other than horizontal tab, so no CR, LF or DEL.
+func validHeaderValue(value string) bool {
+	for i := 0; i < len(value); i++ {
+		if c := value[i]; (c < 0x20 && c != '\t') || c == 0x7f {
+			return false
+		}
+	}
+	return true
+}
+
+// toolName is the pattern the Messages API requires of a tool name.
+var toolName = regexp.MustCompile(`^[a-zA-Z0-9_-]{1,64}$`)
+
+// maxResponseBody caps a successful response body; a larger one is an error.
+const maxResponseBody = 16 << 20
+
+// maxErrorBody caps how much of an error response body is read. It is far
+// above llm.MaxBodyExcerpt, so redaction still sees every secret that could
+// reach the excerpt.
+const maxErrorBody = 64 << 10
+
+type message struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type tool struct {
+	Name        string          `json:"name"`
+	InputSchema json.RawMessage `json:"input_schema"`
+}
+
+type toolChoice struct {
+	Type string `json:"type"`
+	Name string `json:"name"`
+}
+
+type messagesRequest struct {
+	Model       string      `json:"model"`
+	MaxTokens   int         `json:"max_tokens"`
+	System      string      `json:"system,omitempty"`
+	Messages    []message   `json:"messages"`
+	Temperature *float64    `json:"temperature,omitempty"`
+	Tools       []tool      `json:"tools,omitempty"`
+	ToolChoice  *toolChoice `json:"tool_choice,omitempty"`
+}
+
+type messagesResponse struct {
+	Type    string `json:"type"`
+	Model   string `json:"model"`
+	Content []struct {
+		Type  string          `json:"type"`
+		Text  string          `json:"text"`
+		Name  string          `json:"name"`
+		Input json.RawMessage `json:"input"`
+	} `json:"content"`
+	StopReason string `json:"stop_reason"`
+	Usage      struct {
+		InputTokens  int `json:"input_tokens"`
+		OutputTokens int `json:"output_tokens"`
+	} `json:"usage"`
+}
+
+// Generate sends req to {base}/v1/messages. A Request with a Schema is sent
+// as one tool whose input_schema is the schema, with tool_choice forcing
+// it; Response.JSON is then that tool's input.
+func (p *Provider) Generate(ctx context.Context, req llm.Request) (llm.Response, error) {
+	resp, err := p.generate(ctx, req)
+	return resp, redact(err, p.apiKey)
+}
+
+func (p *Provider) generate(ctx context.Context, req llm.Request) (llm.Response, error) {
+	if err := req.Validate(); err != nil {
+		return llm.Response{}, fmt.Errorf("anthropic: generate: %w", err)
+	}
+	if p.model == "" {
+		return llm.Response{}, errors.New("anthropic: generate: no model configured")
+	}
+	body := messagesRequest{
+		Model:       p.model,
+		MaxTokens:   p.maxTokens,
+		System:      req.System,
+		Temperature: req.Temperature,
+	}
+	if req.MaxTokens > 0 {
+		body.MaxTokens = req.MaxTokens
+	}
+	if req.Schema != nil {
+		if !toolName.MatchString(req.Schema.Name) {
+			return llm.Response{}, errors.New("anthropic: generate: schema name is not a valid tool name")
+		}
+		body.Tools = []tool{{Name: req.Schema.Name, InputSchema: req.Schema.Schema}}
+		body.ToolChoice = &toolChoice{Type: "tool", Name: req.Schema.Name}
+	}
+	for _, m := range req.Messages {
+		body.Messages = append(body.Messages, message{Role: string(m.Role), Content: m.Content})
+	}
+	payload, err := json.Marshal(body)
+	if err != nil {
+		return llm.Response{}, fmt.Errorf("anthropic: generate: encode request: %w", err)
+	}
+
+	if req.Timeout > 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, req.Timeout)
+		defer cancel()
+	}
+	raw, err := p.do(ctx, http.MethodPost, "/v1/messages", payload)
+	if err != nil {
+		return llm.Response{}, fmt.Errorf("anthropic: generate: %w", err)
+	}
+
+	var mr messagesResponse
+	if err := json.Unmarshal(raw, &mr); err != nil {
+		return llm.Response{}, fmt.Errorf("anthropic: generate: decode response: %w", err)
+	}
+	if mr.Type != "message" {
+		return llm.Response{}, errors.New("anthropic: generate: response is not a message")
+	}
+	resp := llm.Response{
+		Usage:        llm.Usage{InputTokens: mr.Usage.InputTokens, OutputTokens: mr.Usage.OutputTokens},
+		Model:        mr.Model,
+		FinishReason: mr.StopReason,
+	}
+	var text strings.Builder
+	for _, block := range mr.Content {
+		switch block.Type {
+		case "text":
+			text.WriteString(block.Text)
+		case "tool_use":
+			if req.Schema != nil && block.Name == req.Schema.Name && resp.JSON == nil {
+				resp.JSON = block.Input
+			}
+		}
+	}
+	resp.Text = text.String()
+	if req.Schema == nil {
+		return resp, nil
+	}
+	if !isJSONObject(resp.JSON) {
+		resp.JSON = nil
+		return resp, fmt.Errorf("anthropic: generate: reply holds no input for the forced tool: %w", llm.ErrBadOutput)
+	}
+	return resp, nil
+}
+
+// isJSONObject reports whether raw is exactly one JSON object.
+func isJSONObject(raw json.RawMessage) bool {
+	s := bytes.TrimSpace(raw)
+	return bytes.HasPrefix(s, []byte("{")) && json.Valid(s)
+}
+
+type modelsResponse struct {
+	Data []struct {
+		ID          string `json:"id"`
+		DisplayName string `json:"display_name"`
+	} `json:"data"`
+	HasMore bool   `json:"has_more"`
+	LastID  string `json:"last_id"`
+}
+
+// modelsPageLimit is the largest page GET /v1/models serves.
+const modelsPageLimit = 1000
+
+// ListModels returns every model {base}/v1/models lists, following pages.
+func (p *Provider) ListModels(ctx context.Context) ([]llm.Model, error) {
+	models, err := p.listModels(ctx)
+	return models, redact(err, p.apiKey)
+}
+
+func (p *Provider) listModels(ctx context.Context) ([]llm.Model, error) {
+	models := []llm.Model{}
+	after := ""
+	for {
+		page, err := p.modelsPage(ctx, modelsPageLimit, after)
+		if err != nil {
+			return nil, fmt.Errorf("anthropic: list models: %w", err)
+		}
+		for _, m := range page.Data {
+			if m.ID == "" {
+				return nil, fmt.Errorf("anthropic: list models: model %d has no id", len(models))
+			}
+			name := m.DisplayName
+			if name == "" {
+				name = m.ID
+			}
+			models = append(models, llm.Model{ID: m.ID, DisplayName: name})
+		}
+		if !page.HasMore {
+			return models, nil
+		}
+		if page.LastID == "" || page.LastID == after {
+			return nil, errors.New("anthropic: list models: response has more pages but no new cursor")
+		}
+		after = page.LastID
+	}
+}
+
+// modelsPage fetches one page of GET {base}/v1/models.
+func (p *Provider) modelsPage(ctx context.Context, limit int, after string) (modelsResponse, error) {
+	q := url.Values{"limit": {strconv.Itoa(limit)}}
+	if after != "" {
+		q.Set("after_id", after)
+	}
+	raw, err := p.do(ctx, http.MethodGet, "/v1/models?"+q.Encode(), nil)
+	if err != nil {
+		return modelsResponse{}, err
+	}
+	var mr modelsResponse
+	if err := json.Unmarshal(raw, &mr); err != nil {
+		return modelsResponse{}, fmt.Errorf("decode response: %w", err)
+	}
+	if mr.Data == nil {
+		return modelsResponse{}, errors.New("response has no data list")
+	}
+	return mr, nil
+}
+
+// Verify checks the credentials with the cheapest authenticated call: one
+// page of GET {base}/v1/models holding at most one model.
+func (p *Provider) Verify(ctx context.Context) error {
+	_, err := p.modelsPage(ctx, 1, "")
+	if err != nil {
+		err = fmt.Errorf("anthropic: verify: %w", err)
+	}
+	return redact(err, p.apiKey)
+}
+
+// do sends one request and returns the body of a 2xx response. Any other
+// status becomes the matching llm typed error.
+func (p *Provider) do(ctx context.Context, method, path string, payload []byte) ([]byte, error) {
+	var reqBody io.Reader
+	if payload != nil {
+		reqBody = bytes.NewReader(payload)
+	}
+	httpReq, err := http.NewRequestWithContext(ctx, method, p.base+path, reqBody)
+	if err != nil {
+		// The parse error names the URL, whose path may hold the API key.
+		return nil, errors.New("build request: request URL is invalid")
+	}
+	httpReq.Header.Set("x-api-key", p.apiKey)
+	httpReq.Header.Set("anthropic-version", APIVersion)
+	httpReq.Header.Set("Accept", "application/json")
+	if payload != nil {
+		httpReq.Header.Set("Content-Type", "application/json")
+	}
+
+	httpResp, err := p.client.Do(httpReq)
+	if err != nil {
+		return nil, p.transportError(err)
+	}
+	defer httpResp.Body.Close()
+
+	if httpResp.StatusCode < 200 || httpResp.StatusCode > 299 {
+		body, err := io.ReadAll(io.LimitReader(httpResp.Body, maxErrorBody))
+		if err != nil {
+			return nil, p.transportError(err)
+		}
+		return nil, p.statusError(httpResp, body)
+	}
+	body, err := io.ReadAll(io.LimitReader(httpResp.Body, maxResponseBody+1))
+	if err != nil {
+		return nil, p.transportError(err)
+	}
+	if len(body) > maxResponseBody {
+		return nil, fmt.Errorf("response body exceeds %d bytes", maxResponseBody)
+	}
+	return body, nil
+}
+
+// statusOverloaded is the status the Messages API returns when it is
+// temporarily overloaded.
+const statusOverloaded = 529
+
+// statusError maps a non-2xx response to an llm typed error.
+func (p *Provider) statusError(resp *http.Response, body []byte) error {
+	switch resp.StatusCode {
+	case http.StatusUnauthorized, http.StatusForbidden:
+		return fmt.Errorf("HTTP %d: %w", resp.StatusCode, llm.ErrAuth)
+	case http.StatusTooManyRequests, statusOverloaded:
+		return llm.NewRateLimitedError(retryAfter(resp.Header.Get("Retry-After"), time.Now()), p.apiKey)
+	case http.StatusRequestTimeout:
+		return fmt.Errorf("HTTP %d: %w", resp.StatusCode, llm.ErrTimeout)
+	default:
+		return llm.NewProviderError(resp.StatusCode, body, p.apiKey)
+	}
+}
+
+// retryAfter parses a Retry-After header, in delay-seconds or HTTP-date
+// form. A missing, malformed or past value means the vendor gave no usable
+// wait, reported as 0.
+func retryAfter(value string, now time.Time) time.Duration {
+	value = strings.TrimSpace(value)
+	if value == "" {
+		return 0
+	}
+	if secs, err := strconv.Atoi(value); err == nil {
+		if secs < 0 {
+			return 0
+		}
+		return time.Duration(secs) * time.Second
+	}
+	if at, err := http.ParseTime(value); err == nil && at.After(now) {
+		return at.Sub(now).Round(time.Second)
+	}
+	return 0
+}
+
+func isTimeout(err error) bool {
+	var ne net.Error
+	return errors.Is(err, context.DeadlineExceeded) || (errors.As(err, &ne) && ne.Timeout())
+}
+
+// transportFailure is a failed HTTP exchange. It carries the cause's text
+// with the request URL removed and the API key redacted, and unwraps only to
+// the cause's category: llm.ErrTimeout, context.Canceled or nothing.
+type transportFailure struct {
+	detail string
+	kind   error
+}
+
+func (e *transportFailure) Error() string {
+	if e.kind == llm.ErrTimeout {
+		return fmt.Sprintf("%s: transport: %s", llm.ErrTimeout, e.detail)
+	}
+	return "transport: " + e.detail
+}
+
+func (e *transportFailure) Unwrap() error { return e.kind }
+
+// transportError reports a failure to complete the HTTP exchange. A
+// *url.Error names the request URL, and a base URL path may hold the API
+// key, so only the wrapped cause is kept, rendered with the key redacted.
+func (p *Provider) transportError(err error) error {
+	var kind error
+	switch {
+	case isTimeout(err):
+		kind = llm.ErrTimeout
+	case errors.Is(err, context.Canceled):
+		kind = context.Canceled
+	}
+	var ue *url.Error
+	if errors.As(err, &ue) {
+		err = ue.Err
+	}
+	return &transportFailure{detail: llm.Redact(err.Error(), p.apiKey), kind: kind}
+}
+
+// redactedError stands in for one layer of an error chain that holds the
+// API key. Its text is the layer's text with the key redacted, and Unwrap
+// returns the layer's causes passed through redact, so no error reached by
+// unwrapping has text holding the key. Is consults the original layer and
+// its chain, so sentinels still match. As does too. The llm typed errors
+// remove the key from their own complete text, fixed words included, so
+// they always match; As refuses only a match from outside llm whose text
+// holds the key, so it never hands back an unredacted error.
+type redactedError struct {
+	text string
+	err  error
+	key  string
+}
+
+func (e *redactedError) Error() string { return e.text }
+
+// GoString keeps %#v from printing the fields, which hold the key.
+func (e *redactedError) GoString() string { return e.text }
+
+func (e *redactedError) Unwrap() error { return redact(errors.Unwrap(e.err), e.key) }
+
+func (e *redactedError) Is(target error) bool { return errors.Is(e.err, target) }
+
+func (e *redactedError) As(target any) bool {
+	if !errors.As(e.err, target) {
+		return false
+	}
+	found := reflect.ValueOf(target).Elem()
+	if matched, ok := found.Interface().(error); ok && holdsKey(matched, e.key) {
+		found.SetZero()
+		return false
+	}
+	return true
+}
+
+// redactedJoin is redactedError for a layer with several causes, such as
+// one built by errors.Join or by fmt.Errorf with more than one %w.
+type redactedJoin struct {
+	redactedError
+}
+
+func (e *redactedJoin) Unwrap() []error {
+	causes := e.err.(interface{ Unwrap() []error }).Unwrap()
+	safe := make([]error, len(causes))
+	for i, c := range causes {
+		safe[i] = redact(c, e.key)
+	}
+	return safe
+}
+
+// redact is the backstop every exported method applies to its error: when
+// the text of err or of any error in its chain holds a non-empty key, it
+// returns a stand-in whose text, and the text of every error it unwraps
+// to, has the key redacted. Any other err is returned unchanged.
+func redact(err error, key string) error {
+	if err == nil || key == "" || !holdsKey(err, key) {
+		return err
+	}
+	re := redactedError{text: llm.Redact(err.Error(), key), err: err, key: key}
+	if _, ok := err.(interface{ Unwrap() []error }); ok {
+		return &redactedJoin{re}
+	}
+	return &re
+}
+
+// holdsKey reports whether the text of err or of any error in its chain
+// contains key.
+func holdsKey(err error, key string) bool {
+	if err == nil {
+		return false
+	}
+	if strings.Contains(err.Error(), key) {
+		return true
+	}
+	switch u := err.(type) {
+	case interface{ Unwrap() error }:
+		return holdsKey(u.Unwrap(), key)
+	case interface{ Unwrap() []error }:
+		for _, c := range u.Unwrap() {
+			if holdsKey(c, key) {
+				return true
+			}
+		}
+	}
+	return false
+}

internal/ai/llm/anthropic/anthropic_test.go

@@ -0,0 +1,889 @@
+package anthropic
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/ai/llm"
+)
+
+const testKey = "sk-ant-test-9f8e7d6c5b4a39281706f5e4d3c2b1a0"
+
+// recorded is one request the test server received.
+type recorded struct {
+	method string
+	path   string
+	query  url.Values
+	header http.Header
+	body   []byte
+}
+
+// reply is a response the test server replays.
+type reply struct {
+	status  int
+	header  map[string]string
+	fixture string // testdata file; {{KEY}} is replaced with the key
+	body    string // used when fixture is empty
+	delay   time.Duration
+}
+
+// vendorServer replays its replies in order, repeating the last, and
+// records every request.
+type vendorServer struct {
+	*httptest.Server
+	mu  sync.Mutex
+	got []recorded
+}
+
+func newVendorServer(t *testing.T, replies ...reply) *vendorServer {
+	return newVendorServerWithKey(t, testKey, replies...)
+}
+
+func newVendorServerWithKey(t *testing.T, key string, replies ...reply) *vendorServer {
+	t.Helper()
+	payloads := make([][]byte, len(replies))
+	for i, r := range replies {
+		payloads[i] = []byte(r.body)
+		if r.fixture != "" {
+			raw, err := os.ReadFile(filepath.Join("testdata", r.fixture))
+			if err != nil {
+				t.Fatal(err)
+			}
+			payloads[i] = bytes.ReplaceAll(raw, []byte("{{KEY}}"), []byte(key))
+		}
+	}
+	vs := &vendorServer{}
+	vs.Server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		body, _ := io.ReadAll(req.Body)
+		vs.mu.Lock()
+		n := len(vs.got)
+		vs.got = append(vs.got, recorded{req.Method, req.URL.Path, req.URL.Query(), req.Header.Clone(), body})
+		vs.mu.Unlock()
+		i := min(n, len(replies)-1)
+		r := replies[i]
+		if r.delay > 0 {
+			select {
+			case <-time.After(r.delay):
+			case <-req.Context().Done():
+				return
+			}
+		}
+		for k, v := range r.header {
+			w.Header().Set(k, v)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(r.status)
+		w.Write(payloads[i])
+	}))
+	t.Cleanup(vs.Close)
+	return vs
+}
+
+func (vs *vendorServer) requests() []recorded {
+	vs.mu.Lock()
+	defer vs.mu.Unlock()
+	return append([]recorded(nil), vs.got...)
+}
+
+func (vs *vendorServer) only(t *testing.T) recorded {
+	t.Helper()
+	got := vs.requests()
+	if len(got) != 1 {
+		t.Fatalf("server got %d requests, want 1", len(got))
+	}
+	return got[0]
+}
+
+func newProvider(t *testing.T, base string) *Provider {
+	return newProviderWithKey(t, base, testKey)
+}
+
+func newProviderWithKey(t *testing.T, base, key string) *Provider {
+	t.Helper()
+	p, err := New(Config{BaseURL: base, APIKey: key, Model: "test-model", MaxTokens: 1024})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return p
+}
+
+var dinnersSchema = json.RawMessage(`{"type":"object","properties":{"dinners":{"type":"array"}},"required":["dinners"]}`)
+
+func schemaRequest() llm.Request {
+	temp := 0.0
+	return llm.Request{
+		System: "You plan dinners for Our kitchen.",
+		Messages: []llm.Message{
+			{Role: llm.RoleUser, Content: "Suggest a Side quest."},
+			{Role: llm.RoleAssistant, Content: "Something quick?"},
+			{Role: llm.RoleUser, Content: "Yes, a Quick dinner."},
+		},
+		Temperature: &temp,
+		MaxTokens:   400,
+		Schema:      &llm.JSONSchema{Name: "side_quests", Schema: dinnersSchema},
+	}
+}
+
+func textRequest() llm.Request {
+	return llm.Request{Messages: []llm.Message{{Role: llm.RoleUser, Content: "What should we cook tonight?"}}}
+}
+
+// sentBody decodes a request body into a generic map.
+func sentBody(t *testing.T, r recorded) map[string]any {
+	t.Helper()
+	var m map[string]any
+	if err := json.Unmarshal(r.body, &m); err != nil {
+		t.Fatalf("request body %q: %v", r.body, err)
+	}
+	return m
+}
+
+// canonical re-encodes JSON so two encodings of one value compare equal.
+func canonical(t *testing.T, raw []byte) string {
+	t.Helper()
+	var v any
+	if err := json.Unmarshal(raw, &v); err != nil {
+		t.Fatalf("JSON %q: %v", raw, err)
+	}
+	out, _ := json.Marshal(v)
+	return string(out)
+}
+
+func assertHeaders(t *testing.T, r recorded, key string) {
+	t.Helper()
+	if got := r.header.Values("X-Api-Key"); len(got) != 1 || got[0] != key {
+		t.Errorf("x-api-key = %q", got)
+	}
+	if got := r.header.Values("Anthropic-Version"); len(got) != 1 || got[0] != "2023-06-01" {
+		t.Errorf("anthropic-version = %q", got)
+	}
+	if got := r.header.Get("Authorization"); got != "" {
+		t.Errorf("Authorization = %q, want none", got)
+	}
+}
+
+func TestGenerateToolForcedJSON(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_tool_use.json"})
+	p := newProvider(t, vs.URL+"/")
+
+	resp, err := p.Generate(context.Background(), schemaRequest())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := canonical(t, resp.JSON), `{"dinners":[{"name":"Sheet-pan gnocchi","why":"Like your Regular tray bake, but quicker."}]}`; got != want {
+		t.Errorf("Response.JSON = %s, want %s", got, want)
+	}
+	if resp.Text != "" {
+		t.Errorf("Text = %q, want none", resp.Text)
+	}
+	if resp.Usage != (llm.Usage{InputTokens: 472, OutputTokens: 61}) || resp.Model != "claude-sonnet-5" || resp.FinishReason != "tool_use" {
+		t.Errorf("Usage, Model, FinishReason = %+v, %q, %q", resp.Usage, resp.Model, resp.FinishReason)
+	}
+
+	got := vs.only(t)
+	if got.method != http.MethodPost || got.path != "/v1/messages" || len(got.query) != 0 {
+		t.Errorf("request = %s %s?%s", got.method, got.path, got.query.Encode())
+	}
+	assertHeaders(t, got, testKey)
+	if h := got.header.Get("Content-Type"); h != "application/json" {
+		t.Errorf("Content-Type = %q", h)
+	}
+	body := sentBody(t, got)
+	if body["model"] != "test-model" || body["temperature"] != 0.0 || body["max_tokens"] != 400.0 {
+		t.Errorf("model, temperature, max_tokens = %v, %v, %v", body["model"], body["temperature"], body["max_tokens"])
+	}
+	if body["system"] != "You plan dinners for Our kitchen." {
+		t.Errorf("system = %v", body["system"])
+	}
+	msgs := body["messages"].([]any)
+	wantMsgs := []string{"user:Suggest a Side quest.", "assistant:Something quick?", "user:Yes, a Quick dinner."}
+	if len(msgs) != len(wantMsgs) {
+		t.Fatalf("messages = %v", msgs)
+	}
+	for i, m := range msgs {
+		mm := m.(map[string]any)
+		if got := fmt.Sprintf("%v:%v", mm["role"], mm["content"]); got != wantMsgs[i] {
+			t.Errorf("message %d = %s, want %s", i, got, wantMsgs[i])
+		}
+	}
+	tools := body["tools"].([]any)
+	if len(tools) != 1 {
+		t.Fatalf("tools = %v", tools)
+	}
+	tool := tools[0].(map[string]any)
+	schema, _ := json.Marshal(tool["input_schema"])
+	if tool["name"] != "side_quests" || string(schema) != canonical(t, dinnersSchema) {
+		t.Errorf("tool = %v", tool)
+	}
+	choice := body["tool_choice"].(map[string]any)
+	if len(choice) != 2 || choice["type"] != "tool" || choice["name"] != "side_quests" {
+		t.Errorf("tool_choice = %v", choice)
+	}
+}
+
+func TestGenerateText(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_text.json"})
+	p := newProvider(t, vs.URL)
+
+	resp, err := p.Generate(context.Background(), textRequest())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.Text != "Try a crock pot chili: it is a Quick dinner." || resp.JSON != nil {
+		t.Errorf("Text, JSON = %q, %s", resp.Text, resp.JSON)
+	}
+	if resp.Usage != (llm.Usage{InputTokens: 2095, OutputTokens: 503}) || resp.Model != "claude-sonnet-5" || resp.FinishReason != "end_turn" {
+		t.Errorf("Usage, Model, FinishReason = %+v, %q, %q", resp.Usage, resp.Model, resp.FinishReason)
+	}
+
+	got := vs.only(t)
+	if got.path != "/v1/messages" {
+		t.Errorf("path = %s", got.path)
+	}
+	assertHeaders(t, got, testKey)
+	body := sentBody(t, got)
+	if body["max_tokens"] != 1024.0 {
+		t.Errorf("max_tokens = %v, want the configured 1024", body["max_tokens"])
+	}
+	for _, field := range []string{"system", "temperature", "tools", "tool_choice"} {
+		if v, ok := body[field]; ok {
+			t.Errorf("%s = %v, want it omitted", field, v)
+		}
+	}
+}
+
+func TestGenerateTextKeepsStopReason(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_max_tokens.json"})
+	resp, err := newProvider(t, vs.URL).Generate(context.Background(), textRequest())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.FinishReason != "max_tokens" || resp.Usage.OutputTokens != 400 {
+		t.Errorf("FinishReason, Usage = %q, %+v", resp.FinishReason, resp.Usage)
+	}
+}
+
+func TestGenerateBadOutput(t *testing.T) {
+	cases := map[string]string{
+		"text instead of tool": `{"type":"message","model":"m","content":[{"type":"text","text":"Sure!"}],"stop_reason":"end_turn","usage":{"input_tokens":1,"output_tokens":2}}`,
+		"other tool":           `{"type":"message","model":"m","content":[{"type":"tool_use","id":"t","name":"other","input":{"a":1}}],"stop_reason":"tool_use","usage":{}}`,
+		"input not an object":  `{"type":"message","model":"m","content":[{"type":"tool_use","id":"t","name":"side_quests","input":[1]}],"stop_reason":"tool_use","usage":{}}`,
+		"no content":           `{"type":"message","model":"m","content":[],"stop_reason":"max_tokens","usage":{}}`,
+	}
+	for name, body := range cases {
+		t.Run(name, func(t *testing.T) {
+			vs := newVendorServer(t, reply{status: 200, body: body})
+			resp, err := newProvider(t, vs.URL).Generate(context.Background(), schemaRequest())
+			if !errors.Is(err, llm.ErrBadOutput) {
+				t.Fatalf("err = %v, want ErrBadOutput", err)
+			}
+			if resp.JSON != nil || resp.Model != "m" {
+				t.Fatalf("Response = %+v, want no JSON and the model kept", resp)
+			}
+		})
+	}
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_max_tokens.json"})
+	resp, err := newProvider(t, vs.URL).Generate(context.Background(), schemaRequest())
+	if !errors.Is(err, llm.ErrBadOutput) || resp.FinishReason != "max_tokens" {
+		t.Fatalf("truncated: resp, err = %+v, %v", resp, err)
+	}
+}
+
+func TestGenerateRejectsNonMessage(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "error_overloaded.json"})
+	_, err := newProvider(t, vs.URL).Generate(context.Background(), textRequest())
+	if err == nil || errors.Is(err, llm.ErrBadOutput) {
+		t.Fatalf("err = %v, want an untyped malformed-response error", err)
+	}
+}
+
+func TestGenerateRejectsInvalidRequests(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_text.json"})
+	p := newProvider(t, vs.URL)
+	if _, err := p.Generate(context.Background(), llm.Request{}); err == nil {
+		t.Error("Generate accepted a request with no messages")
+	}
+	for _, name := range []string{"side quests", "", strings.Repeat("a", 65), "plan.v2"} {
+		req := schemaRequest()
+		req.Schema.Name = name
+		if _, err := p.Generate(context.Background(), req); err == nil {
+			t.Errorf("Generate accepted tool name %q", name)
+		}
+	}
+	noModel, err := New(Config{BaseURL: vs.URL, APIKey: testKey, MaxTokens: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := noModel.Generate(context.Background(), textRequest()); err == nil {
+		t.Error("Generate succeeded with no model configured")
+	}
+	if n := len(vs.requests()); n != 0 {
+		t.Errorf("invalid requests reached the vendor %d times", n)
+	}
+}
+
+func TestListModelsFollowsPages(t *testing.T) {
+	vs := newVendorServer(t,
+		reply{status: 200, fixture: "models_page1.json"},
+		reply{status: 200, fixture: "models_page2.json"},
+	)
+	models, err := newProvider(t, vs.URL).ListModels(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []llm.Model{
+		{ID: "claude-sonnet-5", DisplayName: "Claude Sonnet 5"},
+		{ID: "claude-haiku-4-5-20251001", DisplayName: "Claude Haiku 4.5"},
+		{ID: "claude-opus-5-5", DisplayName: "claude-opus-5-5"},
+	}
+	if len(models) != len(want) {
+		t.Fatalf("models = %+v", models)
+	}
+	for i := range want {
+		if models[i] != want[i] {
+			t.Errorf("model %d = %+v, want %+v", i, models[i], want[i])
+		}
+	}
+	got := vs.requests()
+	if len(got) != 2 {
+		t.Fatalf("server got %d requests, want 2", len(got))
+	}
+	for i, wantAfter := range []string{"", "claude-sonnet-5"} {
+		r := got[i]
+		if r.method != http.MethodGet || r.path != "/v1/models" || r.query.Get("limit") != "1000" || r.query.Get("after_id") != wantAfter {
+			t.Errorf("request %d = %s %s?%s", i, r.method, r.path, r.query.Encode())
+		}
+		assertHeaders(t, r, testKey)
+		if len(r.body) != 0 || r.header.Get("Content-Type") != "" {
+			t.Errorf("request %d sent a body", i)
+		}
+	}
+}
+
+func TestListModelsRejectsBadPages(t *testing.T) {
+	cases := map[string]string{
+		"no data":      `{"has_more":false}`,
+		"no id":        `{"data":[{"type":"model","display_name":"x"}],"has_more":false}`,
+		"no cursor":    `{"data":[{"id":"a"}],"has_more":true}`,
+		"stuck cursor": `{"data":[{"id":"a"}],"has_more":true,"last_id":"a"}`,
+		"invalid":      `{"data":`,
+	}
+	for name, body := range cases {
+		vs := newVendorServer(t, reply{status: 200, body: body})
+		if models, err := newProvider(t, vs.URL).ListModels(context.Background()); err == nil {
+			t.Errorf("%s: ListModels() = %+v, nil error", name, models)
+		}
+	}
+}
+
+func TestListModelsEmpty(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, body: `{"data":[],"has_more":false}`})
+	models, err := newProvider(t, vs.URL).ListModels(context.Background())
+	if err != nil || models == nil || len(models) != 0 {
+		t.Fatalf("ListModels() = %#v, %v", models, err)
+	}
+}
+
+func TestVerify(t *testing.T) {
+	ok := newVendorServer(t, reply{status: 200, fixture: "models_page1.json"})
+	if err := newProvider(t, ok.URL).Verify(context.Background()); err != nil {
+		t.Fatalf("Verify() = %v", err)
+	}
+	got := ok.only(t)
+	if got.method != http.MethodGet || got.path != "/v1/models" || got.query.Get("limit") != "1" || got.query.Has("after_id") {
+		t.Errorf("Verify called %s %s?%s", got.method, got.path, got.query.Encode())
+	}
+	assertHeaders(t, got, testKey)
+
+	for status, want := range map[int]error{401: llm.ErrAuth, 403: llm.ErrAuth, 529: llm.ErrRateLimited, 500: llm.ErrProvider} {
+		bad := newVendorServer(t, reply{status: status, fixture: "error_api.json"})
+		err := newProvider(t, bad.URL).Verify(context.Background())
+		if !errors.Is(err, want) {
+			t.Errorf("HTTP %d: Verify() = %v, want %v", status, err, want)
+		}
+		assertNoSecret(t, err)
+	}
+	malformed := newVendorServer(t, reply{status: 200, body: `{}`})
+	if err := newProvider(t, malformed.URL).Verify(context.Background()); err == nil {
+		t.Error("Verify accepted a body with no data list")
+	}
+}
+
+// assertNoSecret fails if err's text holds the key or a prefix of it.
+func assertNoSecret(t *testing.T, err error) {
+	t.Helper()
+	for _, secret := range []string{testKey, testKey[:16]} {
+		if strings.Contains(strings.ToLower(err.Error()), strings.ToLower(secret)) {
+			t.Fatalf("error leaks %q: %v", secret, err)
+		}
+	}
+}
+
+func TestErrorMapping(t *testing.T) {
+	rateLimited := func(want time.Duration) func(*testing.T, error) {
+		return func(t *testing.T, err error) {
+			var rl *llm.RateLimitedError
+			if !errors.As(err, &rl) || rl.RetryAfter != want {
+				t.Fatalf("RetryAfter from %v, want %v", err, want)
+			}
+		}
+	}
+	providerError := func(status int, excerpt string) func(*testing.T, error) {
+		return func(t *testing.T, err error) {
+			var pe *llm.ProviderError
+			if !errors.As(err, &pe) || pe.Status() != status || !strings.Contains(pe.Body(), excerpt) {
+				t.Fatalf("ProviderError from %v, want HTTP %d with %q", err, status, excerpt)
+			}
+			if !strings.Contains(err.Error(), pe.Body()) {
+				t.Fatalf("Error() = %q, want it to carry the excerpt", err)
+			}
+		}
+	}
+	cases := []struct {
+		name  string
+		reply reply
+		want  error
+		check func(*testing.T, error)
+	}{
+		{name: "401", reply: reply{status: 401, fixture: "error_authentication.json"}, want: llm.ErrAuth},
+		{name: "403", reply: reply{status: 403, fixture: "error_permission.json"}, want: llm.ErrAuth},
+		{name: "429 seconds", reply: reply{status: 429, header: map[string]string{"Retry-After": "17"}, fixture: "error_rate_limit.json"}, want: llm.ErrRateLimited, check: rateLimited(17 * time.Second)},
+		{name: "429 no Retry-After", reply: reply{status: 429, fixture: "error_rate_limit.json"}, want: llm.ErrRateLimited, check: rateLimited(0)},
+		{name: "429 HTTP-date", reply: reply{status: 429, header: map[string]string{"Retry-After": time.Now().Add(90 * time.Second).UTC().Format(http.TimeFormat)}, fixture: "error_rate_limit.json"}, want: llm.ErrRateLimited,
+			check: func(t *testing.T, err error) {
+				var rl *llm.RateLimitedError
+				if !errors.As(err, &rl) || rl.RetryAfter < 85*time.Second || rl.RetryAfter > 91*time.Second {
+					t.Fatalf("RetryAfter from %v", err)
+				}
+			}},
+		{name: "529 seconds", reply: reply{status: 529, header: map[string]string{"Retry-After": "4"}, fixture: "error_overloaded.json"}, want: llm.ErrRateLimited, check: rateLimited(4 * time.Second)},
+		{name: "529 no Retry-After", reply: reply{status: 529, fixture: "error_overloaded.json"}, want: llm.ErrRateLimited, check: rateLimited(0)},
+		{name: "408", reply: reply{status: 408}, want: llm.ErrTimeout},
+		{name: "400", reply: reply{status: 400, fixture: "error_invalid_request.json"}, want: llm.ErrProvider, check: providerError(400, "max_tokens: Field required")},
+		{name: "404", reply: reply{status: 404, fixture: "error_not_found.json"}, want: llm.ErrProvider, check: providerError(404, "not_found_error")},
+		{name: "500 echoing the key", reply: reply{status: 500, fixture: "error_api.json"}, want: llm.ErrProvider, check: providerError(500, "x-api-key: [REDACTED]")},
+		{name: "503", reply: reply{status: 503, body: "upstream connect error"}, want: llm.ErrProvider, check: providerError(503, "upstream connect error")},
+	}
+	all := []error{llm.ErrAuth, llm.ErrRateLimited, llm.ErrTimeout, llm.ErrBadOutput, llm.ErrProvider}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			vs := newVendorServer(t, c.reply)
+			p := newProvider(t, vs.URL)
+			_, genErr := p.Generate(context.Background(), schemaRequest())
+			_, listErr := p.ListModels(context.Background())
+			for _, err := range []error{genErr, listErr} {
+				for _, sentinel := range all {
+					if errors.Is(err, sentinel) != (sentinel == c.want) {
+						t.Fatalf("errors.Is(%v, %v) = %v", err, sentinel, sentinel != c.want)
+					}
+				}
+				assertNoSecret(t, err)
+				assertChainNoExactKey(t, err, testKey)
+				if c.check != nil {
+					c.check(t, err)
+				}
+			}
+		})
+	}
+}
+
+func TestTimeouts(t *testing.T) {
+	vs := newVendorServer(t, reply{status: 200, fixture: "messages_tool_use.json", delay: 5 * time.Second})
+	p := newProvider(t, vs.URL)
+
+	req := schemaRequest()
+	req.Timeout = 50 * time.Millisecond
+	start := time.Now()
+	_, err := p.Generate(context.Background(), req)
+	if !errors.Is(err, llm.ErrTimeout) {
+		t.Fatalf("Request.Timeout: err = %v, want ErrTimeout", err)
+	}
+	if elapsed := time.Since(start); elapsed > 2*time.Second {
+		t.Fatalf("Request.Timeout not honoured: took %v", elapsed)
+	}
+	assertNoSecret(t, err)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	if _, err := p.ListModels(ctx); !errors.Is(err, llm.ErrTimeout) {
+		t.Fatalf("context deadline: err = %v, want ErrTimeout", err)
+	}
+	ctx, cancel = context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	if err := p.Verify(ctx); !errors.Is(err, llm.ErrTimeout) {
+		t.Fatalf("Verify context deadline: err = %v, want ErrTimeout", err)
+	}
+
+	ctx, cancel = context.WithCancel(context.Background())
+	cancel()
+	if _, err := p.Generate(ctx, schemaRequest()); !errors.Is(err, context.Canceled) || errors.Is(err, llm.ErrTimeout) {
+		t.Fatalf("cancelled context: err = %v, want context.Canceled only", err)
+	}
+}
+
+// TestTransportErrorWithKeyInBaseURLPath fails the exchange against a base
+// URL whose path holds the key, which New accepts; net/http names the full
+// request URL in its transport errors.
+func TestTransportErrorWithKeyInBaseURLPath(t *testing.T) {
+	refused := newVendorServer(t, reply{status: 200})
+	refusedBase := refused.URL
+	refused.Close()
+
+	dropped := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		conn, _, err := w.(http.Hijacker).Hijack()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		conn.Close()
+	}))
+	t.Cleanup(dropped.Close)
+
+	for name, base := range map[string]string{
+		"connection refused": refusedBase + "/" + testKey,
+		"connection dropped": dropped.URL + "/" + testKey,
+	} {
+		p := newProvider(t, base)
+		_, genErr := p.Generate(context.Background(), schemaRequest())
+		_, listErr := p.ListModels(context.Background())
+		verifyErr := p.Verify(context.Background())
+		for _, err := range []error{genErr, listErr, verifyErr} {
+			if err == nil {
+				t.Fatalf("%s: nil error", name)
+			}
+			assertNoSecret(t, err)
+			assertChainNoExactKey(t, err, testKey)
+			var ue *url.Error
+			if errors.As(err, &ue) {
+				t.Fatalf("%s: error chain exposes the request URL: %v", name, ue)
+			}
+			if errors.Is(err, llm.ErrTimeout) || errors.Is(err, llm.ErrProvider) {
+				t.Fatalf("%s: err = %v, want an untyped transport failure", name, err)
+			}
+		}
+	}
+
+	// The request URL escapes a key with spaces, so redacting the key
+	// itself would miss that copy.
+	const spacedKey = "sk ant key"
+	p := newProviderWithKey(t, refusedBase+"/"+spacedKey, spacedKey)
+	_, genErr := p.Generate(context.Background(), textRequest())
+	_, listErr := p.ListModels(context.Background())
+	verifyErr := p.Verify(context.Background())
+	for _, err := range []error{genErr, listErr, verifyErr} {
+		if err == nil {
+			t.Fatal("escaped key: nil error")
+		}
+		assertChainNoExactKey(t, err, spacedKey)
+		assertChainNoExactKey(t, err, url.PathEscape(spacedKey))
+	}
+}
+
+func TestNewRejectsBadConfig(t *testing.T) {
+	cases := map[string]Config{
+		"empty base":         {},
+		"relative base":      {BaseURL: "api.anthropic.com"},
+		"ftp base":           {BaseURL: "ftp://example.com"},
+		"credentials in URL": {BaseURL: "https://user:" + testKey + "@example.com"},
+		"key in query":       {BaseURL: "https://example.com/?key=" + testKey},
+		"key in fragment":    {BaseURL: "https://example.com/#" + testKey},
+		"key, relative base": {BaseURL: "example.com?key=" + testKey},
+		"key, ftp base":      {BaseURL: "ftp://example.com/?key=" + testKey},
+		"key, unparseable":   {BaseURL: "https://example.com/%zz?key=" + testKey},
+		"empty query":        {BaseURL: "https://example.com?"},
+		"empty fragment":     {BaseURL: "https://example.com#"},
+		"empty both":         {BaseURL: "https://example.com/?#"},
+		"empty key":          {BaseURL: "https://example.com", APIKey: "-"},
+		"key with newline":   {BaseURL: "https://example.com", APIKey: testKey + "\n"},
+		"zero max tokens":    {BaseURL: "https://example.com", MaxTokens: -1},
+		"negative max":       {BaseURL: "https://example.com", MaxTokens: -5},
+	}
+	for name, cfg := range cases {
+		switch cfg.APIKey {
+		case "":
+			cfg.APIKey = testKey
+		case "-":
+			cfg.APIKey = ""
+		}
+		switch cfg.MaxTokens {
+		case 0:
+			cfg.MaxTokens = 1024
+		case -1:
+			cfg.MaxTokens = 0
+		}
+		_, err := New(cfg)
+		if err == nil {
+			t.Errorf("%s: New() = nil error", name)
+			continue
+		}
+		assertNoSecret(t, err)
+		assertChainNoExactKey(t, err, testKey)
+	}
+	if _, err := New(Config{BaseURL: "http://anthropic:8080/proxy/", APIKey: testKey, MaxTokens: 1}); err != nil {
+		t.Errorf("New() rejected a valid config: %v", err)
+	}
+}
+
+func TestRetryAfter(t *testing.T) {
+	now := time.Date(2026, 10, 3, 12, 0, 0, 0, time.UTC)
+	cases := map[string]time.Duration{
+		"":                              0,
+		"30":                            30 * time.Second,
+		" 5 ":                           5 * time.Second,
+		"-3":                            0,
+		"soon":                          0,
+		"Sat, 03 Oct 2026 12:02:00 GMT": 2 * time.Minute,
+		"Sat, 03 Oct 2026 11:59:00 GMT": 0,
+	}
+	for in, want := range cases {
+		if got := retryAfter(in, now); got != want {
+			t.Errorf("retryAfter(%q) = %v, want %v", in, got, want)
+		}
+	}
+}
+
+// assertChainNoExactKey fails if key appears, case-sensitively, in the text
+// of err, of any error reached from it by unwrapping, or of a typed error
+// errors.As finds in it, including a ProviderError's Body.
+func assertChainNoExactKey(t *testing.T, err error, key string) {
+	t.Helper()
+	var walk func(error)
+	walk = func(e error) {
+		if e == nil {
+			return
+		}
+		if strings.Contains(e.Error(), key) {
+			t.Fatalf("layer leaks %q: %q", key, e.Error())
+		}
+		switch u := e.(type) {
+		case interface{ Unwrap() error }:
+			walk(u.Unwrap())
+		case interface{ Unwrap() []error }:
+			for _, c := range u.Unwrap() {
+				walk(c)
+			}
+		}
+	}
+	walk(err)
+	for layer := err; layer != nil; layer = errors.Unwrap(layer) {
+		if strings.Contains(layer.Error(), key) {
+			t.Fatalf("errors.Unwrap layer leaks %q: %q", key, layer.Error())
+		}
+	}
+	var pe *llm.ProviderError
+	if errors.As(err, &pe) && (strings.Contains(pe.Body(), key) || strings.Contains(pe.Error(), key)) {
+		t.Fatalf("ProviderError leaks %q: Error %q, Body %q", key, pe.Error(), pe.Body())
+	}
+	var rl *llm.RateLimitedError
+	if errors.As(err, &rl) && strings.Contains(rl.Error(), key) {
+		t.Fatalf("RateLimitedError leaks %q: %q", key, rl.Error())
+	}
+	var syntax *json.SyntaxError
+	if errors.As(err, &syntax) && strings.Contains(syntax.Error(), key) {
+		t.Fatalf("errors.As reached a *json.SyntaxError leaking %q: %q", key, syntax.Error())
+	}
+	if got := fmt.Sprintf("%v %+v %#v", err, err, err); strings.Contains(got, key) {
+		t.Fatalf("formatting leaks %q: %q", key, got)
+	}
+}
+
+// TestHeaderNameKeyNeverLeaks uses the header name "x-api-key" as the API
+// key, a word an error naming the header would print, and drives every
+// construction and request error path with it.
+func TestHeaderNameKeyNeverLeaks(t *testing.T) {
+	const key = "x-api-key"
+	construction := map[string]Config{
+		"unparseable base": {BaseURL: "https://example.com/%zz/" + key, APIKey: key, MaxTokens: 1},
+		"relative base":    {BaseURL: key, APIKey: key, MaxTokens: 1},
+		"query in base":    {BaseURL: "https://example.com/?" + key, APIKey: key, MaxTokens: 1},
+		"userinfo in base": {BaseURL: "https://" + key + "@example.com", APIKey: key, MaxTokens: 1},
+		"no max tokens":    {BaseURL: "https://example.com", APIKey: key},
+		"key with newline": {BaseURL: "https://example.com", APIKey: key + "\r\n", MaxTokens: 1},
+	}
+	for name, cfg := range construction {
+		_, err := New(cfg)
+		if err == nil {
+			t.Fatalf("%s: New() = nil error", name)
+		}
+		assertChainNoExactKey(t, err, key)
+	}
+
+	body := `{"type":"error","error":{"type":"x","message":"bad x-api-key header: x-api-key"}}`
+	statuses := []int{400, 401, 403, 404, 408, 429, 500, 529}
+	for _, status := range statuses {
+		vs := newVendorServerWithKey(t, key, reply{status: status, body: body, header: map[string]string{"Retry-After": "3"}})
+		p := newProviderWithKey(t, vs.URL, key)
+		_, genErr := p.Generate(context.Background(), schemaRequest())
+		_, listErr := p.ListModels(context.Background())
+		verifyErr := p.Verify(context.Background())
+		for _, err := range []error{genErr, listErr, verifyErr} {
+			if err == nil {
+				t.Fatalf("HTTP %d: nil error", status)
+			}
+			assertChainNoExactKey(t, err, key)
+		}
+	}
+
+	ok := newVendorServerWithKey(t, key, reply{status: 200, fixture: "messages_text.json"})
+	p := newProviderWithKey(t, ok.URL, key)
+	requests := map[string]llm.Request{
+		"unknown role":  {Messages: []llm.Message{{Role: llm.Role(key), Content: "hi"}}},
+		"bad tool name": {Messages: textRequest().Messages, Schema: &llm.JSONSchema{Name: key + "!", Schema: dinnersSchema}},
+		"schema array":  {Messages: textRequest().Messages, Schema: &llm.JSONSchema{Name: key, Schema: json.RawMessage(`[]`)}},
+		"bad output":    {Messages: textRequest().Messages, Schema: &llm.JSONSchema{Name: key, Schema: dinnersSchema}},
+	}
+	for name, req := range requests {
+		_, err := p.Generate(context.Background(), req)
+		if err == nil {
+			t.Fatalf("%s: nil error", name)
+		}
+		assertChainNoExactKey(t, err, key)
+	}
+	noModel, err := New(Config{BaseURL: ok.URL, APIKey: key, MaxTokens: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = noModel.Generate(context.Background(), textRequest())
+	if err == nil {
+		t.Fatal("Generate succeeded with no model")
+	}
+	assertChainNoExactKey(t, err, key)
+
+	refused := newVendorServer(t, reply{status: 200})
+	refusedBase := refused.URL
+	refused.Close()
+	_, err = newProviderWithKey(t, refusedBase+"/"+key, key).Generate(context.Background(), textRequest())
+	if err == nil {
+		t.Fatal("Generate against a closed server succeeded")
+	}
+	assertChainNoExactKey(t, err, key)
+}
+
+// TestAdapterNameKeyOn401 uses the adapter's own error prefix "anthropic" as
+// the API key.
+func TestAdapterNameKeyOn401(t *testing.T) {
+	const key = "anthropic"
+	vs := newVendorServerWithKey(t, key, reply{status: 401, fixture: "error_authentication.json"})
+	p := newProviderWithKey(t, vs.URL, key)
+	_, genErr := p.Generate(context.Background(), schemaRequest())
+	_, listErr := p.ListModels(context.Background())
+	verifyErr := p.Verify(context.Background())
+	for _, err := range []error{genErr, listErr, verifyErr} {
+		if !errors.Is(err, llm.ErrAuth) {
+			t.Fatalf("err = %v, want ErrAuth", err)
+		}
+		assertChainNoExactKey(t, err, key)
+	}
+}
+
+// TestFixedWordKeyOn500 uses the fixed word "HTTP" of ProviderError's text
+// as the API key: errors.As must still reach the ProviderError, whose Body
+// and Error must drop it.
+func TestFixedWordKeyOn500(t *testing.T) {
+	const key = "HTTP"
+	vs := newVendorServerWithKey(t, key, reply{status: 500, body: `{"type":"error","error":{"type":"api_error","message":"HTTP backend down"}}`})
+	_, err := newProviderWithKey(t, vs.URL, key).Generate(context.Background(), schemaRequest())
+	var pe *llm.ProviderError
+	if !errors.Is(err, llm.ErrProvider) || !errors.As(err, &pe) || pe.Status() != 500 {
+		t.Fatalf("err = %v, want a 500 ProviderError", err)
+	}
+	if strings.Contains(pe.Error(), key) || strings.Contains(pe.Body(), key) {
+		t.Fatalf("ProviderError leaks the key: Error %q, Body %q", pe.Error(), pe.Body())
+	}
+	if !strings.Contains(pe.Body(), "backend down") {
+		t.Fatalf("Body() = %q, want the redacted excerpt", pe.Body())
+	}
+	assertChainNoExactKey(t, err, key)
+}
+
+// TestRateLimitedFixedWordKey uses the fixed word "rate" of
+// RateLimitedError's text as the API key, on both rate-limit statuses.
+func TestRateLimitedFixedWordKey(t *testing.T) {
+	const key = "rate"
+	for _, status := range []int{429, 529} {
+		vs := newVendorServerWithKey(t, key, reply{status: status, header: map[string]string{"Retry-After": "17"}, fixture: "error_rate_limit.json"})
+		_, err := newProviderWithKey(t, vs.URL, key).Generate(context.Background(), schemaRequest())
+		var rl *llm.RateLimitedError
+		if !errors.Is(err, llm.ErrRateLimited) || !errors.As(err, &rl) || rl.RetryAfter != 17*time.Second {
+			t.Fatalf("HTTP %d: err = %v, want a RateLimitedError with RetryAfter 17s", status, err)
+		}
+		assertChainNoExactKey(t, err, key)
+	}
+}
+
+// TestResponseBodyWithKeyNeverLeaks has the vendor echo the key in error
+// and success bodies, including ones that fail to decode, and checks
+// Error(), every errors.Unwrap layer and every errors.As target.
+func TestResponseBodyWithKeyNeverLeaks(t *testing.T) {
+	bodies := []reply{
+		{status: 500, fixture: "error_api.json"},
+		{status: 400, body: `{"type":"error","error":{"type":"invalid_request_error","message":"key ` + testKey + ` rejected"}}`},
+		{status: 401, fixture: "error_authentication.json"},
+		{status: 429, fixture: "error_rate_limit.json"},
+		{status: 200, body: `{"type": ` + testKey + `}`},
+		{status: 200, body: testKey},
+		{status: 200, body: `{"type":"message","content":[{"type":"text","text":"` + testKey + `"}],"stop_reason":"` + testKey + `"}`},
+		{status: 200, body: `{"data": [` + testKey + `]}`},
+	}
+	for i, r := range bodies {
+		vs := newVendorServer(t, r)
+		p := newProvider(t, vs.URL)
+		_, genErr := p.Generate(context.Background(), schemaRequest())
+		_, listErr := p.ListModels(context.Background())
+		verifyErr := p.Verify(context.Background())
+		for _, err := range []error{genErr, listErr, verifyErr} {
+			if err == nil {
+				t.Fatalf("body %d: nil error", i)
+			}
+			assertNoSecret(t, err)
+			assertChainNoExactKey(t, err, testKey)
+		}
+	}
+	// A short key that decode errors quote from a malformed body.
+	const key = "Z"
+	vs := newVendorServerWithKey(t, key, reply{status: 200, body: `{"type": Z}`})
+	_, err := newProviderWithKey(t, vs.URL, key).Generate(context.Background(), textRequest())
+	var syntax *json.SyntaxError
+	if err == nil || errors.As(err, &syntax) {
+		t.Fatalf("err = %v, want no reachable *json.SyntaxError quoting the key", err)
+	}
+	assertChainNoExactKey(t, err, key)
+}
+
+// TestRedactKeepsChainMatches checks that the redaction backstop keeps
+// errors.Is and errors.As working through a chain that holds the key.
+func TestRedactKeepsChainMatches(t *testing.T) {
+	pe := llm.NewProviderError(500, []byte(`{"error":"down"}`), testKey)
+	rl := llm.NewRateLimitedError(time.Second, testKey)
+	inner := fmt.Errorf("decode %s: %w", testKey, llm.ErrBadOutput)
+	joined := errors.Join(inner, pe, rl, context.Canceled, &transportFailure{detail: "slow", kind: llm.ErrTimeout}, fmt.Errorf("HTTP 401: %w", llm.ErrAuth))
+	err := redact(fmt.Errorf("anthropic %s: %w", testKey, joined), testKey)
+	assertChainNoExactKey(t, err, testKey)
+	for _, target := range []error{llm.ErrBadOutput, llm.ErrProvider, llm.ErrRateLimited, llm.ErrTimeout, llm.ErrAuth, context.Canceled} {
+		if !errors.Is(err, target) {
+			t.Fatalf("errors.Is(%v, %v) = false", err, target)
+		}
+	}
+	var gotPE *llm.ProviderError
+	var gotRL *llm.RateLimitedError
+	if !errors.As(err, &gotPE) || gotPE != pe || !errors.As(err, &gotRL) || gotRL != rl {
+		t.Fatalf("errors.As lost a typed error in %v", err)
+	}
+	clean := errors.New("anthropic: generate: no model configured")
+	if redact(clean, testKey) != clean {
+		t.Fatal("redact wrapped an error that holds no key")
+	}
+}

internal/ai/llm/anthropic/testdata/README.md

@@ -0,0 +1,41 @@
+# Anthropic adapter fixtures
+
+These are the Messages API response bodies the contract tests in
+`anthropic_test.go` replay. They are **not live captures**. Product AI is
+tested with fakes, and live vendor calls are opt-in and never use
+credentials, so no request was sent to Anthropic to make them. Each fixture
+is a sanitized adaptation of the response shapes in Anthropic's public API
+documentation:
+
+- Messages: <https://docs.anthropic.com/en/api/messages>
+- Tool use, including `tool_choice` `{type: tool}`:
+  <https://docs.anthropic.com/en/docs/build-with-claude/tool-use>
+- List models: <https://docs.anthropic.com/en/api/models-list>
+- Errors: <https://docs.anthropic.com/en/api/errors>
+
+Sanitizing:
+
+- IDs, request IDs, timestamps and token counts are illustrative values in
+  the documented format, not values from a real account.
+- Message content and tool input are replaced with MealQuest dinner text.
+- No real credential appears anywhere. `{{KEY}}` is a placeholder the tests
+  replace with a fake key, to prove the adapter redacts a key echoed back in
+  a vendor body.
+
+Below, *not in the public example* marks fields or values that were added
+for a test case and do not come from the documented example.
+
+| Fixture | Shape | Not in the public example |
+| --- | --- | --- |
+| `messages_tool_use.json` | Message with one `tool_use` block, `stop_reason: tool_use`, `usage` with cache fields. | The tool name and input. |
+| `messages_text.json` | Message with `text` blocks, `stop_reason: end_turn`. | A second text block, to test joining. |
+| `messages_max_tokens.json` | Message with a `text` block, `stop_reason: max_tokens`. | — |
+| `models_page1.json` | Models list page: `data`, `has_more: true`, `first_id`, `last_id`. | — |
+| `models_page2.json` | Last models list page, `has_more: false`. | The second model has no `display_name`, to test the fallback to `id`. |
+| `error_authentication.json` | Error, `authentication_error` (401). | The `{{KEY}}` text; added to test redaction. |
+| `error_permission.json` | Error, `permission_error` (403). | — |
+| `error_rate_limit.json` | Error, `rate_limit_error` (429). | The `{{KEY}}` text; added to test redaction. |
+| `error_overloaded.json` | Error, `overloaded_error` (529). | — |
+| `error_not_found.json` | Error, `not_found_error` (404). | — |
+| `error_invalid_request.json` | Error, `invalid_request_error` (400). | — |
+| `error_api.json` | Error, `api_error` (500). | The `x-api-key: {{KEY}}` text; added to test redaction. |

internal/ai/llm/anthropic/testdata/error_api.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "api_error", "message": "An unexpected error has occurred internal to Anthropic's systems. x-api-key: {{KEY}}"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7G4"}

internal/ai/llm/anthropic/testdata/error_authentication.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "authentication_error", "message": "invalid x-api-key: {{KEY}}"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7Fy"}

internal/ai/llm/anthropic/testdata/error_invalid_request.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "invalid_request_error", "message": "max_tokens: Field required"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7G3"}

internal/ai/llm/anthropic/testdata/error_not_found.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "not_found_error", "message": "model: claude-nonexistent"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7G2"}

internal/ai/llm/anthropic/testdata/error_overloaded.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "overloaded_error", "message": "Overloaded"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7G1"}

internal/ai/llm/anthropic/testdata/error_permission.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "permission_error", "message": "Your API key does not have permission to use the specified resource."}, "request_id": "req_011CSHoEeqs5C35K2UUqR7Fz"}

internal/ai/llm/anthropic/testdata/error_rate_limit.json

@@ -0,0 +1 @@
+{"type": "error", "error": {"type": "rate_limit_error", "message": "Number of request tokens has exceeded your per-minute rate limit for key {{KEY}}"}, "request_id": "req_011CSHoEeqs5C35K2UUqR7G0"}

internal/ai/llm/anthropic/testdata/messages_max_tokens.json

@@ -0,0 +1,12 @@
+{
+  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
+  "type": "message",
+  "role": "assistant",
+  "model": "claude-sonnet-5",
+  "content": [
+    {"type": "text", "text": "Here are some Side quests for Our kitchen"}
+  ],
+  "stop_reason": "max_tokens",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 472, "output_tokens": 400}
+}

internal/ai/llm/anthropic/testdata/messages_text.json

@@ -0,0 +1,18 @@
+{
+  "id": "msg_013Zva2CMHLNnXjNJJKqJ2EF",
+  "type": "message",
+  "role": "assistant",
+  "model": "claude-sonnet-5",
+  "content": [
+    {"type": "text", "text": "Try a crock pot chili: "},
+    {"type": "text", "text": "it is a Quick dinner."}
+  ],
+  "stop_reason": "end_turn",
+  "stop_sequence": null,
+  "usage": {
+    "input_tokens": 2095,
+    "cache_creation_input_tokens": 0,
+    "cache_read_input_tokens": 0,
+    "output_tokens": 503
+  }
+}

internal/ai/llm/anthropic/testdata/messages_tool_use.json

@@ -0,0 +1,22 @@
+{
+  "id": "msg_01Aq9w938a90dw8q",
+  "type": "message",
+  "role": "assistant",
+  "model": "claude-sonnet-5",
+  "content": [
+    {
+      "type": "tool_use",
+      "id": "toolu_01A09q90qw90lq917835lq9",
+      "name": "side_quests",
+      "input": {"dinners": [{"name": "Sheet-pan gnocchi", "why": "Like your Regular tray bake, but quicker."}]}
+    }
+  ],
+  "stop_reason": "tool_use",
+  "stop_sequence": null,
+  "usage": {
+    "input_tokens": 472,
+    "cache_creation_input_tokens": 0,
+    "cache_read_input_tokens": 0,
+    "output_tokens": 61
+  }
+}

internal/ai/llm/anthropic/testdata/models_page1.json

@@ -0,0 +1,13 @@
+{
+  "data": [
+    {
+      "type": "model",
+      "id": "claude-sonnet-5",
+      "display_name": "Claude Sonnet 5",
+      "created_at": "2026-02-24T00:00:00Z"
+    }
+  ],
+  "has_more": true,
+  "first_id": "claude-sonnet-5",
+  "last_id": "claude-sonnet-5"
+}

internal/ai/llm/anthropic/testdata/models_page2.json

@@ -0,0 +1,18 @@
+{
+  "data": [
+    {
+      "type": "model",
+      "id": "claude-haiku-4-5-20251001",
+      "display_name": "Claude Haiku 4.5",
+      "created_at": "2025-10-01T00:00:00Z"
+    },
+    {
+      "type": "model",
+      "id": "claude-opus-5-5",
+      "created_at": "2026-05-01T00:00:00Z"
+    }
+  ],
+  "has_more": false,
+  "first_id": "claude-haiku-4-5-20251001",
+  "last_id": "claude-opus-5-5"
+}