diff --git a/internal/ai/llm/errors.go b/internal/ai/llm/errors.go
new file mode 100644
index 0000000..be8e922
--- /dev/null
+++ b/internal/ai/llm/errors.go
@@ -0,0 +1,103 @@
+package llm
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+	"unicode/utf8"
+)
+
+var (
+	// ErrAuth means the vendor rejected the credentials.
+	ErrAuth = errors.New("llm: authentication failed")
+	// ErrRateLimited means the vendor throttled the request. The concrete
+	// error is a *RateLimitedError carrying the requested wait.
+	ErrRateLimited = errors.New("llm: rate limited")
+	// ErrTimeout means the request did not finish within its timeout.
+	ErrTimeout = errors.New("llm: request timed out")
+	// ErrBadOutput means the model replied but the reply is unusable, e.g.
+	// structured output that does not parse.
+	ErrBadOutput = errors.New("llm: unusable model output")
+	// ErrProvider means the vendor returned an unexpected HTTP status. The
+	// concrete error is a *ProviderError carrying the status and a redacted
+	// body excerpt.
+	ErrProvider = errors.New("llm: provider error")
+)
+
+// RateLimitedError is a throttled request. It matches ErrRateLimited.
+type RateLimitedError struct {
+	// RetryAfter is how long the vendor asked callers to wait; 0 means it
+	// did not say.
+	RetryAfter time.Duration
+}
+
+func (e *RateLimitedError) Error() string {
+	if e.RetryAfter == 0 {
+		return ErrRateLimited.Error()
+	}
+	return fmt.Sprintf("%s: retry after %v", ErrRateLimited, e.RetryAfter)
+}
+
+// Is reports whether target is ErrRateLimited.
+func (e *RateLimitedError) Is(target error) bool { return target == ErrRateLimited }
+
+// MaxBodyExcerpt is the longest response-body excerpt, in bytes, a
+// ProviderError keeps.
+const MaxBodyExcerpt = 512
+
+// redacted replaces each secret in a ProviderError body.
+const redacted = "[REDACTED]"
+
+// ProviderError is an unexpected HTTP status from the vendor. It matches
+// ErrProvider. Build it with NewProviderError, which redacts the body.
+type ProviderError struct {
+	status int
+	body   string
+}
+
+// NewProviderError returns the error for an unexpected HTTP status.
+// secrets must hold the API key (and any other credential sent with the
+// request). Every occurrence of each non-empty secret in body is replaced
+// before the body is cut to MaxBodyExcerpt bytes, so no part of a secret
+// survives. If secrets holds no non-empty value the body cannot be
+// redacted, so no excerpt is kept at all.
+func NewProviderError(status int, body []byte, secrets []string) *ProviderError {
+	s := string(body)
+	redactedAny := false
+	for _, secret := range secrets {
+		if secret != "" {
+			s = strings.ReplaceAll(s, secret, redacted)
+			redactedAny = true
+		}
+	}
+	if !redactedAny {
+		return &ProviderError{status: status}
+	}
+	s = strings.TrimSpace(s)
+	if len(s) > MaxBodyExcerpt {
+		cut := MaxBodyExcerpt
+		for cut > 0 && !utf8.RuneStart(s[cut]) {
+			cut--
+		}
+		s = s[:cut] + "…"
+	}
+	return &ProviderError{status: status, body: s}
+}
+
+// Status returns the HTTP status code.
+func (e *ProviderError) Status() int { return e.status }
+
+// Body returns the redacted, truncated response-body excerpt; it is empty
+// when the vendor sent no body or no secret was given to redact it with.
+func (e *ProviderError) Body() string { return e.body }
+
+func (e *ProviderError) Error() string {
+	if e.body == "" {
+		return fmt.Sprintf("%s: HTTP %d", ErrProvider, e.status)
+	}
+	return fmt.Sprintf("%s: HTTP %d: %s", ErrProvider, e.status, e.body)
+}
+
+// Is reports whether target is ErrProvider.
+func (e *ProviderError) Is(target error) bool { return target == ErrProvider }
diff --git a/internal/ai/llm/errors_test.go b/internal/ai/llm/errors_test.go
new file mode 100644
index 0000000..321f887
--- /dev/null
+++ b/internal/ai/llm/errors_test.go
@@ -0,0 +1,121 @@
+package llm
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSentinelErrorsMatchWhenWrapped(t *testing.T) {
+	for _, sentinel := range []error{ErrAuth, ErrTimeout, ErrBadOutput} {
+		wrapped := fmt.Errorf("openai: generate: %w", sentinel)
+		if !errors.Is(wrapped, sentinel) {
+			t.Errorf("errors.Is(%v, %v) = false", wrapped, sentinel)
+		}
+		for _, other := range []error{ErrAuth, ErrTimeout, ErrBadOutput, ErrRateLimited, ErrProvider} {
+			if other != sentinel && errors.Is(wrapped, other) {
+				t.Errorf("errors.Is(%v, %v) = true", wrapped, other)
+			}
+		}
+	}
+}
+
+func TestRateLimitedError(t *testing.T) {
+	err := fmt.Errorf("anthropic: %w", &RateLimitedError{RetryAfter: 30 * time.Second})
+	if !errors.Is(err, ErrRateLimited) {
+		t.Fatal("errors.Is(err, ErrRateLimited) = false")
+	}
+	if errors.Is(err, ErrProvider) || errors.Is(err, ErrAuth) {
+		t.Fatal("rate-limited error matches an unrelated sentinel")
+	}
+	var rl *RateLimitedError
+	if !errors.As(err, &rl) {
+		t.Fatal("errors.As(err, *RateLimitedError) = false")
+	}
+	if rl.RetryAfter != 30*time.Second {
+		t.Fatalf("RetryAfter = %v, want 30s", rl.RetryAfter)
+	}
+	if got := err.Error(); got != "anthropic: llm: rate limited: retry after 30s" {
+		t.Fatalf("Error() = %q", got)
+	}
+	if got := (&RateLimitedError{}).Error(); got != "llm: rate limited" {
+		t.Fatalf("Error() without RetryAfter = %q", got)
+	}
+}
+
+func TestProviderError(t *testing.T) {
+	err := fmt.Errorf("gemini: %w", NewProviderError(503, []byte(` {"error":"overloaded"} `), []string{"sk-test"}))
+	if !errors.Is(err, ErrProvider) {
+		t.Fatal("errors.Is(err, ErrProvider) = false")
+	}
+	if errors.Is(err, ErrRateLimited) || errors.Is(err, ErrAuth) {
+		t.Fatal("provider error matches an unrelated sentinel")
+	}
+	var pe *ProviderError
+	if !errors.As(err, &pe) {
+		t.Fatal("errors.As(err, *ProviderError) = false")
+	}
+	if pe.Status() != 503 || pe.Body() != `{"error":"overloaded"}` {
+		t.Fatalf("Status, Body = %d, %q", pe.Status(), pe.Body())
+	}
+	if got := err.Error(); got != `gemini: llm: provider error: HTTP 503: {"error":"overloaded"}` {
+		t.Fatalf("Error() = %q", got)
+	}
+	if got := NewProviderError(500, nil, []string{"sk-test"}).Error(); got != "llm: provider error: HTTP 500" {
+		t.Fatalf("Error() without body = %q", got)
+	}
+}
+
+func TestProviderErrorRedactsSecrets(t *testing.T) {
+	const key = "sk-live-0123456789abcdef"
+	body := []byte(`{"error":"invalid key sk-live-0123456789abcdef","echo":"Bearer sk-live-0123456789abcdef"}`)
+	pe := NewProviderError(400, body, []string{key, ""})
+	for _, s := range []string{pe.Error(), pe.Body()} {
+		if strings.Contains(s, key) {
+			t.Fatalf("secret leaked: %q", s)
+		}
+	}
+	want := `{"error":"invalid key [REDACTED]","echo":"Bearer [REDACTED]"}`
+	if pe.Body() != want {
+		t.Fatalf("Body() = %q, want %q", pe.Body(), want)
+	}
+}
+
+func TestProviderErrorRedactsBeforeTruncating(t *testing.T) {
+	const key = "sk-secret-straddling-the-cut"
+	// The key starts just before the excerpt limit, so truncating first
+	// would leave its prefix in the excerpt.
+	body := strings.Repeat("x", MaxBodyExcerpt-5) + key
+	pe := NewProviderError(500, []byte(body), []string{key})
+	if strings.Contains(pe.Body(), "sk-se") {
+		t.Fatalf("secret prefix leaked: %q", pe.Body())
+	}
+}
+
+func TestProviderErrorTruncatesOnRuneBoundary(t *testing.T) {
+	body := strings.Repeat("a", MaxBodyExcerpt-1) + "é" + strings.Repeat("b", 100)
+	pe := NewProviderError(500, []byte(body), []string{"sk-test"})
+	want := strings.Repeat("a", MaxBodyExcerpt-1) + "…"
+	if pe.Body() != want {
+		t.Fatalf("Body() has %d bytes, want %d", len(pe.Body()), len(want))
+	}
+}
+
+func TestProviderErrorOmitsBodyWithoutSecret(t *testing.T) {
+	const key = "sk-live-0123456789abcdef"
+	body := []byte(`{"error":"invalid key sk-live-0123456789abcdef"}`)
+	for _, secrets := range [][]string{nil, {}, {""}} {
+		pe := NewProviderError(401, body, secrets)
+		if pe.Body() != "" {
+			t.Fatalf("secrets %q: Body() = %q, want empty", secrets, pe.Body())
+		}
+		if got := pe.Error(); got != "llm: provider error: HTTP 401" {
+			t.Fatalf("secrets %q: Error() = %q", secrets, got)
+		}
+		if strings.Contains(pe.Error(), key) {
+			t.Fatalf("secrets %q: secret leaked: %q", secrets, pe.Error())
+		}
+	}
+}
diff --git a/internal/ai/llm/fake.go b/internal/ai/llm/fake.go
new file mode 100644
index 0000000..b418d82
--- /dev/null
+++ b/internal/ai/llm/fake.go
@@ -0,0 +1,127 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"slices"
+	"sync"
+)
+
+// Fake is a scripted Provider for tests. Each method returns its queued
+// results in order, one per call, and records every Request Generate
+// receives. A call with nothing left in its queue fails, so an unexpected
+// call is never silently answered. It is safe for concurrent use.
+type Fake struct {
+	mu       sync.Mutex
+	generate []GenerateResult
+	models   []ListModelsResult
+	verify   []error
+	requests []Request
+}
+
+var _ Provider = (*Fake)(nil)
+
+// GenerateResult is one scripted Generate outcome.
+type GenerateResult struct {
+	Response Response
+	Err      error
+}
+
+// ListModelsResult is one scripted ListModels outcome.
+type ListModelsResult struct {
+	Models []Model
+	Err    error
+}
+
+// QueueGenerate appends results to the Generate script.
+func (f *Fake) QueueGenerate(results ...GenerateResult) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.generate = append(f.generate, results...)
+}
+
+// QueueListModels appends results to the ListModels script.
+func (f *Fake) QueueListModels(results ...ListModelsResult) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.models = append(f.models, results...)
+}
+
+// QueueVerify appends results to the Verify script; a nil entry is success.
+func (f *Fake) QueueVerify(results ...error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.verify = append(f.verify, results...)
+}
+
+// Generate records req and returns the next scripted result.
+func (f *Fake) Generate(_ context.Context, req Request) (Response, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.requests = append(f.requests, cloneRequest(req))
+	if len(f.generate) == 0 {
+		return Response{}, fmt.Errorf("llm: fake: unscripted Generate call %d", len(f.requests))
+	}
+	r := f.generate[0]
+	f.generate = f.generate[1:]
+	return r.Response, r.Err
+}
+
+// ListModels returns the next scripted result.
+func (f *Fake) ListModels(context.Context) ([]Model, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if len(f.models) == 0 {
+		return nil, fmt.Errorf("llm: fake: unscripted ListModels call")
+	}
+	r := f.models[0]
+	f.models = f.models[1:]
+	return slices.Clone(r.Models), r.Err
+}
+
+// Verify returns the next scripted result.
+func (f *Fake) Verify(context.Context) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if len(f.verify) == 0 {
+		return fmt.Errorf("llm: fake: unscripted Verify call")
+	}
+	err := f.verify[0]
+	f.verify = f.verify[1:]
+	return err
+}
+
+// Requests returns a copy of every Request Generate received, in order.
+func (f *Fake) Requests() []Request {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	out := make([]Request, len(f.requests))
+	for i, r := range f.requests {
+		out[i] = cloneRequest(r)
+	}
+	return out
+}
+
+// Remaining reports how many scripted results of each kind are unused, so a
+// test can assert its script was consumed.
+func (f *Fake) Remaining() (generate, listModels, verify int) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return len(f.generate), len(f.models), len(f.verify)
+}
+
+// cloneRequest deep-copies r so a recorded Request is unaffected by later
+// changes the caller makes to its slices and pointers.
+func cloneRequest(r Request) Request {
+	r.Messages = slices.Clone(r.Messages)
+	if r.Temperature != nil {
+		t := *r.Temperature
+		r.Temperature = &t
+	}
+	if r.Schema != nil {
+		s := JSONSchema{Name: r.Schema.Name, Schema: json.RawMessage(slices.Clone([]byte(r.Schema.Schema)))}
+		r.Schema = &s
+	}
+	return r
+}
diff --git a/internal/ai/llm/fake_test.go b/internal/ai/llm/fake_test.go
new file mode 100644
index 0000000..bba1b3c
--- /dev/null
+++ b/internal/ai/llm/fake_test.go
@@ -0,0 +1,138 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestFakeGenerateReturnsScriptInOrder(t *testing.T) {
+	ctx := context.Background()
+	var f Fake
+	f.QueueGenerate(
+		GenerateResult{Response: Response{Text: "first", Model: "m1"}},
+		GenerateResult{Err: &RateLimitedError{RetryAfter: time.Second}},
+	)
+	f.QueueGenerate(GenerateResult{Response: Response{JSON: json.RawMessage(`{"ok":true}`), FinishReason: "stop"}})
+
+	req := Request{Messages: []Message{{Role: RoleUser, Content: "one"}}}
+	resp, err := f.Generate(ctx, req)
+	if err != nil || resp.Text != "first" || resp.Model != "m1" {
+		t.Fatalf("call 1 = %+v, %v", resp, err)
+	}
+	if _, err := f.Generate(ctx, req); !errors.Is(err, ErrRateLimited) {
+		t.Fatalf("call 2 err = %v, want ErrRateLimited", err)
+	}
+	resp, err = f.Generate(ctx, req)
+	if err != nil || string(resp.JSON) != `{"ok":true}` || resp.FinishReason != "stop" {
+		t.Fatalf("call 3 = %+v, %v", resp, err)
+	}
+	if _, err := f.Generate(ctx, req); err == nil {
+		t.Fatal("unscripted call 4 succeeded")
+	}
+	if g, l, v := f.Remaining(); g != 0 || l != 0 || v != 0 {
+		t.Fatalf("Remaining() = %d, %d, %d", g, l, v)
+	}
+	if n := len(f.Requests()); n != 4 {
+		t.Fatalf("recorded %d requests, want 4 (unscripted calls are recorded too)", n)
+	}
+}
+
+func TestFakeRecordsRequests(t *testing.T) {
+	var f Fake
+	f.QueueGenerate(GenerateResult{}, GenerateResult{})
+	temp := 0.2
+	first := Request{
+		System:      "be brief",
+		Messages:    []Message{{Role: RoleUser, Content: "plan dinners"}},
+		Temperature: &temp,
+		MaxTokens:   100,
+		Schema:      &JSONSchema{Name: "plan", Schema: json.RawMessage(`{"type":"object"}`)},
+		Timeout:     5 * time.Second,
+	}
+	if _, err := f.Generate(context.Background(), first); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.Generate(context.Background(), Request{System: "second"}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Later changes by the caller must not alter what was recorded.
+	first.Messages[0].Content = "changed"
+	temp = 1
+	first.Schema.Schema[2] = 'X'
+
+	got := f.Requests()
+	if len(got) != 2 || got[1].System != "second" {
+		t.Fatalf("Requests() = %+v", got)
+	}
+	r := got[0]
+	if r.System != "be brief" || r.Messages[0].Content != "plan dinners" || *r.Temperature != 0.2 ||
+		r.MaxTokens != 100 || r.Schema.Name != "plan" || string(r.Schema.Schema) != `{"type":"object"}` ||
+		r.Timeout != 5*time.Second {
+		t.Fatalf("recorded request = %+v", r)
+	}
+
+	// Changing a returned copy must not alter the record either.
+	got[0].Messages[0].Content = "mutated"
+	if f.Requests()[0].Messages[0].Content != "plan dinners" {
+		t.Fatal("Requests() returned shared state")
+	}
+}
+
+func TestFakeListModelsAndVerify(t *testing.T) {
+	ctx := context.Background()
+	var f Fake
+	f.QueueListModels(
+		ListModelsResult{Models: []Model{{ID: "a", DisplayName: "A"}, {ID: "b", DisplayName: "b"}}},
+		ListModelsResult{Err: ErrAuth},
+	)
+	f.QueueVerify(nil, fmt.Errorf("wrapped: %w", ErrAuth))
+
+	models, err := f.ListModels(ctx)
+	if err != nil || len(models) != 2 || models[0].ID != "a" {
+		t.Fatalf("ListModels 1 = %+v, %v", models, err)
+	}
+	if _, err := f.ListModels(ctx); !errors.Is(err, ErrAuth) {
+		t.Fatalf("ListModels 2 err = %v, want ErrAuth", err)
+	}
+	if _, err := f.ListModels(ctx); err == nil {
+		t.Fatal("unscripted ListModels succeeded")
+	}
+	if err := f.Verify(ctx); err != nil {
+		t.Fatalf("Verify 1 = %v", err)
+	}
+	if err := f.Verify(ctx); !errors.Is(err, ErrAuth) {
+		t.Fatalf("Verify 2 = %v, want ErrAuth", err)
+	}
+	if err := f.Verify(ctx); err == nil {
+		t.Fatal("unscripted Verify succeeded")
+	}
+}
+
+func TestFakeConcurrentUse(t *testing.T) {
+	const n = 50
+	var f Fake
+	for i := range n {
+		f.QueueGenerate(GenerateResult{Response: Response{Text: fmt.Sprint(i)}})
+	}
+	var wg sync.WaitGroup
+	for range n {
+		wg.Go(func() {
+			if _, err := f.Generate(context.Background(), Request{}); err != nil {
+				t.Error(err)
+			}
+		})
+	}
+	wg.Wait()
+	if len(f.Requests()) != n {
+		t.Fatalf("recorded %d requests, want %d", len(f.Requests()), n)
+	}
+	if g, _, _ := f.Remaining(); g != 0 {
+		t.Fatalf("%d Generate results unused", g)
+	}
+}
diff --git a/internal/ai/llm/llm.go b/internal/ai/llm/llm.go
new file mode 100644
index 0000000..ce2aaad
--- /dev/null
+++ b/internal/ai/llm/llm.go
@@ -0,0 +1,132 @@
+// Package llm is the single interface MealQuest uses to talk to a language
+// model. Every vendor adapter implements Provider over plain net/http (no
+// vendor SDKs) and reports failures with this package's typed errors, so
+// callers decide what to do from errors.Is and errors.As alone.
+//
+// Errors from this package and its adapters never contain an API key.
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"time"
+)
+
+// Provider is a language-model vendor.
+type Provider interface {
+	// Generate sends one request and returns the model's reply.
+	Generate(ctx context.Context, req Request) (Response, error)
+	// ListModels returns the models the configured credentials can use.
+	ListModels(ctx context.Context) ([]Model, error)
+	// Verify checks that the configured credentials are accepted.
+	Verify(ctx context.Context) error
+}
+
+// Role is the author of a Message.
+type Role string
+
+// The roles a Message may have. The system prompt is Request.System, not a
+// message.
+const (
+	RoleUser      Role = "user"
+	RoleAssistant Role = "assistant"
+)
+
+// Message is one turn of a conversation.
+type Message struct {
+	Role    Role
+	Content string
+}
+
+// JSONSchema is a named structured-output contract: when a Request carries
+// one, the model must reply with JSON matching Schema.
+type JSONSchema struct {
+	// Name identifies the contract to the vendor, e.g. "side_quests".
+	Name string
+	// Schema is a JSON Schema object.
+	Schema json.RawMessage
+}
+
+// Request is one generation request.
+type Request struct {
+	// System is the system prompt; empty means none.
+	System string
+	// Messages is the conversation, oldest first. It must not be empty.
+	Messages []Message
+	// Temperature is the sampling temperature; nil means the vendor default.
+	Temperature *float64
+	// MaxTokens caps the reply length; 0 means the vendor default.
+	MaxTokens int
+	// Schema, when set, requests structured output; the reply is then in
+	// Response.JSON.
+	Schema *JSONSchema
+	// Timeout bounds the request; 0 means only ctx bounds it.
+	Timeout time.Duration
+}
+
+// Validate reports whether r is well formed. Adapters call it before
+// contacting the vendor.
+func (r Request) Validate() error {
+	if len(r.Messages) == 0 {
+		return errors.New("llm: request has no messages")
+	}
+	for i, m := range r.Messages {
+		if m.Role != RoleUser && m.Role != RoleAssistant {
+			return fmt.Errorf("llm: message %d has unknown role %q", i, m.Role)
+		}
+	}
+	if r.Temperature != nil && *r.Temperature < 0 {
+		return fmt.Errorf("llm: temperature %v is negative", *r.Temperature)
+	}
+	if r.MaxTokens < 0 {
+		return fmt.Errorf("llm: max tokens %d is negative", r.MaxTokens)
+	}
+	if r.Timeout < 0 {
+		return fmt.Errorf("llm: timeout %v is negative", r.Timeout)
+	}
+	if r.Schema != nil {
+		if r.Schema.Name == "" {
+			return errors.New("llm: schema has no name")
+		}
+		var obj map[string]json.RawMessage
+		if err := json.Unmarshal(r.Schema.Schema, &obj); err != nil || obj == nil {
+			return fmt.Errorf("llm: schema %q is not a JSON object", r.Schema.Name)
+		}
+	}
+	return nil
+}
+
+// Usage is the token count of one Generate call.
+type Usage struct {
+	InputTokens  int
+	OutputTokens int
+}
+
+// Total returns InputTokens + OutputTokens.
+func (u Usage) Total() int { return u.InputTokens + u.OutputTokens }
+
+// Response is the model's reply to a Request.
+type Response struct {
+	// Text is the reply text.
+	Text string
+	// JSON is the structured reply when the Request carried a Schema, and
+	// nil otherwise.
+	JSON json.RawMessage
+	// Usage is the tokens the call consumed.
+	Usage Usage
+	// Model is the model that answered, as the vendor reports it.
+	Model string
+	// FinishReason is why generation stopped, as the vendor reports it.
+	FinishReason string
+}
+
+// Model is a model a Provider offers.
+type Model struct {
+	// ID is the vendor's model identifier, used in requests.
+	ID string
+	// DisplayName is a human-readable name; it equals ID when the vendor
+	// gives none.
+	DisplayName string
+}
diff --git a/internal/ai/llm/llm_test.go b/internal/ai/llm/llm_test.go
new file mode 100644
index 0000000..1b2774a
--- /dev/null
+++ b/internal/ai/llm/llm_test.go
@@ -0,0 +1,44 @@
+package llm
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestRequestValidate(t *testing.T) {
+	neg := -0.1
+	valid := func() Request {
+		return Request{
+			Messages: []Message{{Role: RoleUser, Content: "hi"}, {Role: RoleAssistant, Content: "hello"}},
+			Schema:   &JSONSchema{Name: "plan", Schema: json.RawMessage(`{"type":"object"}`)},
+		}
+	}
+	if err := valid().Validate(); err != nil {
+		t.Fatalf("valid request: %v", err)
+	}
+	cases := map[string]func(*Request){
+		"no messages":    func(r *Request) { r.Messages = nil },
+		"unknown role":   func(r *Request) { r.Messages[0].Role = "system" },
+		"negative temp":  func(r *Request) { r.Temperature = &neg },
+		"negative max":   func(r *Request) { r.MaxTokens = -1 },
+		"negative time":  func(r *Request) { r.Timeout = -1 },
+		"unnamed schema": func(r *Request) { r.Schema.Name = "" },
+		"schema array":   func(r *Request) { r.Schema.Schema = json.RawMessage(`[]`) },
+		"schema null":    func(r *Request) { r.Schema.Schema = json.RawMessage(`null`) },
+		"schema invalid": func(r *Request) { r.Schema.Schema = json.RawMessage(`{`) },
+		"schema empty":   func(r *Request) { r.Schema.Schema = nil },
+	}
+	for name, mutate := range cases {
+		r := valid()
+		mutate(&r)
+		if err := r.Validate(); err == nil {
+			t.Errorf("%s: Validate() = nil", name)
+		}
+	}
+}
+
+func TestUsageTotal(t *testing.T) {
+	if got := (Usage{InputTokens: 12, OutputTokens: 30}).Total(); got != 42 {
+		t.Fatalf("Total() = %d, want 42", got)
+	}
+}