feat(chatd): add LLM stream retry with exponential backoff (#22418)

## Summary

Adds automatic retry with exponential backoff for transient LLM errors
during chat streaming and title generation. Inspired by
[coder/mux](https://github.com/coder/mux)'s retry mechanism.

## Key Behaviors

- **Infinite retries** with exponential backoff: 1s → 2s → 4s → ... →
60s cap
- **Deterministic delays** (no jitter)
- **Error classification**: retryable (429, 5xx, overloaded, rate limit,
network errors) vs non-retryable (auth, quota, context exceeded, model
not found, canceled)
- **Retry status published to SSE stream** so frontend can show
"Retrying in Xs..." UI
- **Title generation** retries silently (best-effort, nil onRetry
callback)

## New Package: `coderd/chatd/chatretry/`

| File | Purpose |
|------|---------|
| `classify.go` | `IsRetryable(err)` and `StatusCodeRetryable(code)` |
| `backoff.go` | `Delay(attempt)` — exponential doubling with 60s cap |
| `retry.go` | `Retry(ctx, fn, onRetry)` — infinite loop with
context-aware timer |

## Test Helpers: `coderd/chatd/chattest/errors.go`

Anthropic and OpenAI error response builders for use in chattest
providers:
- `AnthropicErrorResponse()`, `AnthropicOverloadedResponse()`,
`AnthropicRateLimitResponse()`
- `OpenAIErrorResponse()`, `OpenAIRateLimitResponse()`,
`OpenAIServerErrorResponse()`

## SDK Changes: `codersdk/chats.go`

- New `ChatStreamEventType: "retry"`
- New `ChatStreamRetry` struct with `Attempt`, `DelayMs`, `Error`,
`RetryingAt` fields
- TypeScript types auto-generated

## Changed Files

- `coderd/chatd/chatloop/chatloop.go` — wraps `agent.Stream()` in
`chatretry.Retry()`
- `coderd/chatd/chatd.go` — publishes retry events to SSE stream with
logging
- `coderd/chatd/title.go` — wraps `model.Generate()` in silent retry
- `coderd/chatd/chattest/anthropic.go` / `openai.go` — error injection
support

## Tests

42 tests covering classification (33), backoff (9), and retry scenarios
(8).

coderd/chatd/chatd.go

@@ -2086,6 +2086,24 @@ func (p *Server) runChat(
 		},
 		Compaction: compactionOptions,
 
+		OnRetry: func(attempt int, retryErr error, delay time.Duration) {
+			logger.Warn(ctx, "retrying LLM stream",
+				slog.F("attempt", attempt),
+				slog.F("delay", delay.String()),
+				slog.Error(retryErr),
+			)
+			p.publishEvent(chat.ID, codersdk.ChatStreamEvent{
+				Type:   codersdk.ChatStreamEventTypeRetry,
+				ChatID: chat.ID,
+				Retry: &codersdk.ChatStreamRetry{
+					Attempt:    attempt,
+					DelayMs:    delay.Milliseconds(),
+					Error:      retryErr.Error(),
+					RetryingAt: time.Now().Add(delay),
+				},
+			})
+		},
+
 		OnInterruptedPersistError: func(err error) {
 			p.logger.Warn(ctx, "failed to persist interrupted chat step", slog.Error(err))
 		},

coderd/chatd/chatloop/chatloop.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"charm.land/fantasy"
 	fantasyanthropic "charm.land/fantasy/providers/anthropic"
@@ -15,6 +16,7 @@ import (
 	"golang.org/x/xerrors"
 
 	"github.com/coder/coder/v2/coderd/chatd/chatprompt"
+	"github.com/coder/coder/v2/coderd/chatd/chatretry"
 	"github.com/coder/coder/v2/codersdk"
 )
 
@@ -54,6 +56,12 @@ type RunOptions struct {
 	)
 	Compaction *CompactionOptions
 
+	// OnRetry is called before each retry attempt when the LLM
+	// stream fails with a retryable error. It provides the attempt
+	// number, error, and backoff delay so callers can publish status
+	// events to connected clients.
+	OnRetry chatretry.OnRetryFn
+
 	OnInterruptedPersistError func(error)
 }
 
@@ -443,15 +451,39 @@ func Run(ctx context.Context, opts RunOptions) (*fantasy.AgentResult, error) {
 		})
 	}
 
-	result, err := agent.Stream(ctx, streamCall)
-	if err != nil {
-		if errors.Is(err, context.Canceled) &&
-			errors.Is(context.Cause(ctx), ErrInterrupted) {
-			if persistErr := persistInterruptedStep(); persistErr != nil {
-				if opts.OnInterruptedPersistError != nil {
-					opts.OnInterruptedPersistError(persistErr)
+	var result *fantasy.AgentResult
+	err := chatretry.Retry(ctx, func(retryCtx context.Context) error {
+		var streamErr error
+		result, streamErr = agent.Stream(retryCtx, streamCall)
+		if streamErr != nil {
+			// Interrupts are not retryable — propagate them
+			// immediately so processChat can set the correct
+			// status.
+			if errors.Is(streamErr, context.Canceled) &&
+				errors.Is(context.Cause(retryCtx), ErrInterrupted) {
+				if persistErr := persistInterruptedStep(); persistErr != nil {
+					if opts.OnInterruptedPersistError != nil {
+						opts.OnInterruptedPersistError(persistErr)
+					}
 				}
+				// Return ErrInterrupted directly so the retry
+				// loop sees a non-retryable error and stops.
+				return ErrInterrupted
 			}
+			return streamErr
+		}
+		return nil
+	}, func(attempt int, retryErr error, delay time.Duration) {
+		// Reset accumulated draft state from the failed attempt
+		// so the next attempt starts clean.
+		resetStepState()
+
+		if opts.OnRetry != nil {
+			opts.OnRetry(attempt, retryErr, delay)
+		}
+	})
+	if err != nil {
+		if errors.Is(err, ErrInterrupted) {
 			return nil, ErrInterrupted
 		}
 		return nil, xerrors.Errorf("stream response: %w", err)

coderd/chatd/chatretry/chatretry.go

@@ -0,0 +1,175 @@
+// Package chatretry provides retry logic for transient LLM provider
+// errors. It classifies errors as retryable or permanent and
+// implements exponential backoff matching the behavior of coder/mux.
+package chatretry
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"time"
+)
+
+const (
+	// InitialDelay is the backoff duration for the first retry
+	// attempt.
+	InitialDelay = 1 * time.Second
+
+	// MaxDelay is the upper bound for the exponential backoff
+	// duration. Matches the cap used in coder/mux.
+	MaxDelay = 60 * time.Second
+)
+
+// nonRetryablePatterns are substrings that indicate a permanent error
+// which should not be retried. These are checked first so that
+// ambiguous messages (e.g. "bad request: rate limit") are correctly
+// classified as non-retryable.
+var nonRetryablePatterns = []string{
+	"context canceled",
+	"context deadline exceeded",
+	"authentication",
+	"unauthorized",
+	"forbidden",
+	"invalid api key",
+	"invalid_api_key",
+	"invalid model",
+	"model not found",
+	"model_not_found",
+	"context length exceeded",
+	"context_exceeded",
+	"maximum context length",
+	"quota",
+	"billing",
+}
+
+// retryablePatterns are substrings that indicate a transient error
+// worth retrying.
+var retryablePatterns = []string{
+	"overloaded",
+	"rate limit",
+	"rate_limit",
+	"too many requests",
+	"server error",
+	"status 500",
+	"status 502",
+	"status 503",
+	"status 529",
+	"connection reset",
+	"connection refused",
+	"eof",
+	"broken pipe",
+	"timeout",
+	"unavailable",
+	"service unavailable",
+}
+
+// IsRetryable determines whether an error from an LLM provider is
+// transient and worth retrying. It inspects the error message and
+// any wrapped HTTP status codes for known retryable patterns.
+func IsRetryable(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	// context.Canceled is always non-retryable regardless of
+	// wrapping.
+	if errors.Is(err, context.Canceled) {
+		return false
+	}
+
+	lower := strings.ToLower(err.Error())
+
+	// Check non-retryable patterns first so they take precedence.
+	for _, p := range nonRetryablePatterns {
+		if strings.Contains(lower, p) {
+			return false
+		}
+	}
+
+	for _, p := range retryablePatterns {
+		if strings.Contains(lower, p) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// StatusCodeRetryable returns true for HTTP status codes that
+// indicate a transient failure worth retrying.
+func StatusCodeRetryable(code int) bool {
+	switch code {
+	case 429, 500, 502, 503, 529:
+		return true
+	default:
+		return false
+	}
+}
+
+// Delay returns the backoff duration for the given 0-indexed attempt.
+// Uses exponential backoff: min(InitialDelay * 2^attempt, MaxDelay).
+// Matches the backoff curve used in coder/mux.
+func Delay(attempt int) time.Duration {
+	d := InitialDelay
+	for range attempt {
+		d *= 2
+		if d >= MaxDelay {
+			return MaxDelay
+		}
+	}
+	return d
+}
+
+// RetryFn is the function to retry. It receives a context and returns
+// an error. The context may be a child of the original with adjusted
+// deadlines for individual attempts.
+type RetryFn func(ctx context.Context) error
+
+// OnRetryFn is called before each retry attempt with the attempt
+// number (1-indexed), the error that triggered the retry, and the
+// delay before the next attempt.
+type OnRetryFn func(attempt int, err error, delay time.Duration)
+
+// Retry calls fn repeatedly until it succeeds, returns a
+// non-retryable error, or ctx is canceled. There is no max attempt
+// limit — retries continue indefinitely with exponential backoff
+// (capped at 60s), matching the behavior of coder/mux.
+//
+// The onRetry callback (if non-nil) is called before each retry
+// attempt, giving the caller a chance to reset state, log, or
+// publish status events.
+func Retry(ctx context.Context, fn RetryFn, onRetry OnRetryFn) error {
+	var attempt int
+	for {
+		err := fn(ctx)
+		if err == nil {
+			return nil
+		}
+
+		if !IsRetryable(err) {
+			return err
+		}
+
+		// If the caller's context is already done, return the
+		// context error so cancellation propagates cleanly.
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+
+		delay := Delay(attempt)
+
+		if onRetry != nil {
+			onRetry(attempt+1, err, delay)
+		}
+
+		timer := time.NewTimer(delay)
+		select {
+		case <-ctx.Done():
+			timer.Stop()
+			return ctx.Err()
+		case <-timer.C:
+		}
+
+		attempt++
+	}
+}

coderd/chatd/chatretry/chatretry_test.go

@@ -0,0 +1,452 @@
+package chatretry_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/chatd/chatretry"
+)
+
+func TestIsRetryable(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		err       error
+		retryable bool
+	}{
+		// Retryable errors.
+		{
+			name:      "Overloaded",
+			err:       xerrors.New("model is overloaded, please try again"),
+			retryable: true,
+		},
+		{
+			name:      "RateLimit",
+			err:       xerrors.New("rate limit exceeded"),
+			retryable: true,
+		},
+		{
+			name:      "RateLimitUnderscore",
+			err:       xerrors.New("rate_limit: too many requests"),
+			retryable: true,
+		},
+		{
+			name:      "TooManyRequests",
+			err:       xerrors.New("too many requests"),
+			retryable: true,
+		},
+		{
+			name:      "HTTP429InMessage",
+			err:       xerrors.New("received status 429 from upstream"),
+			retryable: false, // "429" alone is not a pattern; needs matching text.
+		},
+		{
+			name:      "HTTP529InMessage",
+			err:       xerrors.New("received status 529 from upstream"),
+			retryable: true,
+		},
+		{
+			name:      "ServerError500",
+			err:       xerrors.New("status 500: internal server error"),
+			retryable: true,
+		},
+		{
+			name:      "ServerErrorGeneric",
+			err:       xerrors.New("server error"),
+			retryable: true,
+		},
+		{
+			name:      "ConnectionReset",
+			err:       xerrors.New("read tcp: connection reset by peer"),
+			retryable: true,
+		},
+		{
+			name:      "ConnectionRefused",
+			err:       xerrors.New("dial tcp: connection refused"),
+			retryable: true,
+		},
+		{
+			name:      "EOF",
+			err:       xerrors.New("unexpected EOF"),
+			retryable: true,
+		},
+		{
+			name:      "BrokenPipe",
+			err:       xerrors.New("write: broken pipe"),
+			retryable: true,
+		},
+		{
+			name:      "NetworkTimeout",
+			err:       xerrors.New("i/o timeout"),
+			retryable: true,
+		},
+		{
+			name:      "ServiceUnavailable",
+			err:       xerrors.New("service unavailable"),
+			retryable: true,
+		},
+		{
+			name:      "Unavailable",
+			err:       xerrors.New("the service is currently unavailable"),
+			retryable: true,
+		},
+		{
+			name:      "Status502",
+			err:       xerrors.New("status 502: bad gateway"),
+			retryable: true,
+		},
+		{
+			name:      "Status503",
+			err:       xerrors.New("status 503"),
+			retryable: true,
+		},
+
+		// Non-retryable errors.
+		{
+			name:      "Nil",
+			err:       nil,
+			retryable: false,
+		},
+		{
+			name:      "ContextCanceled",
+			err:       context.Canceled,
+			retryable: false,
+		},
+		{
+			name:      "ContextCanceledWrapped",
+			err:       xerrors.Errorf("operation failed: %w", context.Canceled),
+			retryable: false,
+		},
+		{
+			name:      "ContextCanceledMessage",
+			err:       xerrors.New("context canceled"),
+			retryable: false,
+		},
+		{
+			name:      "ContextDeadlineExceeded",
+			err:       xerrors.New("context deadline exceeded"),
+			retryable: false,
+		},
+		{
+			name:      "Authentication",
+			err:       xerrors.New("authentication failed"),
+			retryable: false,
+		},
+		{
+			name:      "Unauthorized",
+			err:       xerrors.New("401 Unauthorized"),
+			retryable: false,
+		},
+		{
+			name:      "Forbidden",
+			err:       xerrors.New("403 Forbidden"),
+			retryable: false,
+		},
+		{
+			name:      "InvalidAPIKey",
+			err:       xerrors.New("invalid api key"),
+			retryable: false,
+		},
+		{
+			name:      "InvalidAPIKeyUnderscore",
+			err:       xerrors.New("invalid_api_key"),
+			retryable: false,
+		},
+		{
+			name:      "InvalidModel",
+			err:       xerrors.New("invalid model: gpt-5-turbo"),
+			retryable: false,
+		},
+		{
+			name:      "ModelNotFound",
+			err:       xerrors.New("model not found"),
+			retryable: false,
+		},
+		{
+			name:      "ModelNotFoundUnderscore",
+			err:       xerrors.New("model_not_found"),
+			retryable: false,
+		},
+		{
+			name:      "ContextLengthExceeded",
+			err:       xerrors.New("context length exceeded"),
+			retryable: false,
+		},
+		{
+			name:      "ContextExceededUnderscore",
+			err:       xerrors.New("context_exceeded"),
+			retryable: false,
+		},
+		{
+			name:      "MaximumContextLength",
+			err:       xerrors.New("maximum context length"),
+			retryable: false,
+		},
+		{
+			name:      "QuotaExceeded",
+			err:       xerrors.New("quota exceeded"),
+			retryable: false,
+		},
+		{
+			name:      "BillingError",
+			err:       xerrors.New("billing issue: payment required"),
+			retryable: false,
+		},
+
+		// Wrapped errors preserve retryability.
+		{
+			name:      "WrappedRetryable",
+			err:       xerrors.Errorf("provider call failed: %w", xerrors.New("service unavailable")),
+			retryable: true,
+		},
+		{
+			name:      "WrappedNonRetryable",
+			err:       xerrors.Errorf("provider call failed: %w", xerrors.New("invalid api key")),
+			retryable: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := chatretry.IsRetryable(tt.err)
+			if got != tt.retryable {
+				t.Errorf("IsRetryable(%v) = %v, want %v", tt.err, got, tt.retryable)
+			}
+		})
+	}
+}
+
+func TestStatusCodeRetryable(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		code      int
+		retryable bool
+	}{
+		{429, true},
+		{500, true},
+		{502, true},
+		{503, true},
+		{529, true},
+		{200, false},
+		{400, false},
+		{401, false},
+		{403, false},
+		{404, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("Status%d", tt.code), func(t *testing.T) {
+			t.Parallel()
+			got := chatretry.StatusCodeRetryable(tt.code)
+			if got != tt.retryable {
+				t.Errorf("StatusCodeRetryable(%d) = %v, want %v", tt.code, got, tt.retryable)
+			}
+		})
+	}
+}
+
+func TestDelay(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{0, 1 * time.Second},
+		{1, 2 * time.Second},
+		{2, 4 * time.Second},
+		{3, 8 * time.Second},
+		{4, 16 * time.Second},
+		{5, 32 * time.Second},
+		{6, 60 * time.Second},  // Capped at MaxDelay.
+		{10, 60 * time.Second}, // Still capped.
+		{100, 60 * time.Second},
+	}
+
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("Attempt%d", tt.attempt), func(t *testing.T) {
+			t.Parallel()
+			got := chatretry.Delay(tt.attempt)
+			if got != tt.want {
+				t.Errorf("Delay(%d) = %v, want %v", tt.attempt, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestRetry_SuccessOnFirstTry(t *testing.T) {
+	t.Parallel()
+
+	calls := 0
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		calls++
+		return nil
+	}, nil)
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if calls != 1 {
+		t.Fatalf("expected fn called once, got %d", calls)
+	}
+}
+
+func TestRetry_TransientThenSuccess(t *testing.T) {
+	t.Parallel()
+
+	calls := 0
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		calls++
+		if calls == 1 {
+			return xerrors.New("service unavailable")
+		}
+		return nil
+	}, nil)
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if calls != 2 {
+		t.Fatalf("expected fn called twice, got %d", calls)
+	}
+}
+
+func TestRetry_MultipleTransientThenSuccess(t *testing.T) {
+	t.Parallel()
+
+	calls := 0
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		calls++
+		if calls <= 3 {
+			return xerrors.New("overloaded")
+		}
+		return nil
+	}, nil)
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if calls != 4 {
+		t.Fatalf("expected fn called 4 times, got %d", calls)
+	}
+}
+
+func TestRetry_NonRetryableError(t *testing.T) {
+	t.Parallel()
+
+	calls := 0
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		calls++
+		return xerrors.New("invalid api key")
+	}, nil)
+
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if err.Error() != "invalid api key" {
+		t.Fatalf("expected 'invalid api key', got %q", err.Error())
+	}
+	if calls != 1 {
+		t.Fatalf("expected fn called once, got %d", calls)
+	}
+}
+
+func TestRetry_ContextCanceledDuringWait(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	calls := 0
+	err := chatretry.Retry(ctx, func(_ context.Context) error {
+		calls++
+		// Cancel after the first retryable error so the wait
+		// select picks up the cancellation.
+		if calls == 1 {
+			cancel()
+		}
+		return xerrors.New("overloaded")
+	}, nil)
+
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context.Canceled, got %v", err)
+	}
+}
+
+func TestRetry_ContextCanceledDuringFn(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	err := chatretry.Retry(ctx, func(_ context.Context) error {
+		cancel()
+		// Return a retryable error; the loop should detect that
+		// ctx is done and return the context error.
+		return xerrors.New("overloaded")
+	}, nil)
+
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected context.Canceled, got %v", err)
+	}
+}
+
+func TestRetry_OnRetryCalledWithCorrectArgs(t *testing.T) {
+	t.Parallel()
+
+	type retryRecord struct {
+		attempt int
+		errMsg  string
+		delay   time.Duration
+	}
+	var records []retryRecord
+
+	calls := 0
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		calls++
+		if calls <= 2 {
+			return xerrors.New("rate limit exceeded")
+		}
+		return nil
+	}, func(attempt int, err error, delay time.Duration) {
+		records = append(records, retryRecord{
+			attempt: attempt,
+			errMsg:  err.Error(),
+			delay:   delay,
+		})
+	})
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+	if len(records) != 2 {
+		t.Fatalf("expected 2 onRetry calls, got %d", len(records))
+	}
+	if records[0].attempt != 1 {
+		t.Errorf("first onRetry attempt = %d, want 1", records[0].attempt)
+	}
+	if records[1].attempt != 2 {
+		t.Errorf("second onRetry attempt = %d, want 2", records[1].attempt)
+	}
+	if records[0].errMsg != "rate limit exceeded" {
+		t.Errorf("first onRetry error = %q, want 'rate limit exceeded'", records[0].errMsg)
+	}
+}
+
+func TestRetry_OnRetryNilDoesNotPanic(t *testing.T) {
+	t.Parallel()
+
+	var calls atomic.Int32
+	err := chatretry.Retry(context.Background(), func(_ context.Context) error {
+		if calls.Add(1) == 1 {
+			return xerrors.New("overloaded")
+		}
+		return nil
+	}, nil)
+	if err != nil {
+		t.Fatalf("expected nil error, got %v", err)
+	}
+}

coderd/chatd/chattest/anthropic.go

@@ -19,6 +19,7 @@ type AnthropicHandler func(req *AnthropicRequest) AnthropicResponse
 type AnthropicResponse struct {
 	StreamingChunks <-chan AnthropicChunk
 	Response        *AnthropicMessage
+	Error           *ErrorResponse // If set, server returns this HTTP error instead of streaming/JSON.
 }
 
 // AnthropicRequest represents an Anthropic messages request.
@@ -141,6 +142,11 @@ func (s *anthropicServer) handleMessages(w http.ResponseWriter, r *http.Request)
 }
 
 func (s *anthropicServer) writeResponse(w http.ResponseWriter, req *AnthropicRequest, resp AnthropicResponse) {
+	if resp.Error != nil {
+		writeErrorResponse(w, resp.Error)
+		return
+	}
+
 	hasStreaming := resp.StreamingChunks != nil
 	hasNonStreaming := resp.Response != nil
 

coderd/chatd/chattest/errors.go

@@ -0,0 +1,74 @@
+package chattest
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// ErrorResponse describes an HTTP error that a test server should return
+// instead of a normal streaming or JSON response.
+type ErrorResponse struct {
+	StatusCode int
+	Type       string
+	Message    string
+}
+
+// writeErrorResponse writes a JSON error response matching the common
+// provider error format used by both Anthropic and OpenAI.
+func writeErrorResponse(w http.ResponseWriter, errResp *ErrorResponse) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(errResp.StatusCode)
+	body := map[string]interface{}{
+		"error": map[string]interface{}{
+			"type":    errResp.Type,
+			"message": errResp.Message,
+		},
+	}
+	_ = json.NewEncoder(w).Encode(body)
+}
+
+// AnthropicErrorResponse returns an AnthropicResponse that causes the
+// test server to respond with the given HTTP status code and error.
+// This simulates provider errors like 529 Overloaded or 429 Rate Limited.
+func AnthropicErrorResponse(statusCode int, errorType, message string) AnthropicResponse {
+	return AnthropicResponse{
+		Error: &ErrorResponse{
+			StatusCode: statusCode,
+			Type:       errorType,
+			Message:    message,
+		},
+	}
+}
+
+// AnthropicOverloadedResponse returns a 529 "overloaded" error matching
+// Anthropic's overloaded response format.
+func AnthropicOverloadedResponse() AnthropicResponse {
+	return AnthropicErrorResponse(529, "overloaded_error", "Overloaded")
+}
+
+// AnthropicRateLimitResponse returns a 429 rate limit error.
+func AnthropicRateLimitResponse() AnthropicResponse {
+	return AnthropicErrorResponse(http.StatusTooManyRequests, "rate_limit_error", "Rate limited")
+}
+
+// OpenAIErrorResponse returns an OpenAIResponse that causes the
+// test server to respond with the given HTTP status code and error.
+func OpenAIErrorResponse(statusCode int, errorType, message string) OpenAIResponse {
+	return OpenAIResponse{
+		Error: &ErrorResponse{
+			StatusCode: statusCode,
+			Type:       errorType,
+			Message:    message,
+		},
+	}
+}
+
+// OpenAIRateLimitResponse returns a 429 rate limit error.
+func OpenAIRateLimitResponse() OpenAIResponse {
+	return OpenAIErrorResponse(http.StatusTooManyRequests, "rate_limit_exceeded", "Rate limit exceeded")
+}
+
+// OpenAIServerErrorResponse returns a 500 internal server error.
+func OpenAIServerErrorResponse() OpenAIResponse {
+	return OpenAIErrorResponse(http.StatusInternalServerError, "server_error", "Internal server error")
+}

coderd/chatd/chattest/openai.go

@@ -20,6 +20,7 @@ type OpenAIHandler func(req *OpenAIRequest) OpenAIResponse
 type OpenAIResponse struct {
 	StreamingChunks <-chan OpenAIChunk
 	Response        *OpenAICompletion
+	Error           *ErrorResponse // If set, server returns this HTTP error instead of streaming/JSON.
 }
 
 // OpenAIRequest represents an OpenAI chat completion request.
@@ -160,6 +161,11 @@ func (s *openAIServer) handleResponses(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *openAIServer) writeChatCompletionsResponse(w http.ResponseWriter, req *OpenAIRequest, resp OpenAIResponse) {
+	if resp.Error != nil {
+		writeErrorResponse(w, resp.Error)
+		return
+	}
+
 	hasStreaming := resp.StreamingChunks != nil
 	hasNonStreaming := resp.Response != nil
 
@@ -184,6 +190,11 @@ func (s *openAIServer) writeChatCompletionsResponse(w http.ResponseWriter, req *
 }
 
 func (s *openAIServer) writeResponsesAPIResponse(w http.ResponseWriter, req *OpenAIRequest, resp OpenAIResponse) {
+	if resp.Error != nil {
+		writeErrorResponse(w, resp.Error)
+		return
+	}
+
 	hasStreaming := resp.StreamingChunks != nil
 	hasNonStreaming := resp.Response != nil
 

coderd/chatd/title.go

@@ -10,6 +10,7 @@ import (
 
 	"cdr.dev/slog/v3"
 	"github.com/coder/coder/v2/coderd/chatd/chatprompt"
+	"github.com/coder/coder/v2/coderd/chatd/chatretry"
 	"github.com/coder/coder/v2/coderd/database"
 	coderdpubsub "github.com/coder/coder/v2/coderd/pubsub"
 )
@@ -65,7 +66,8 @@ func (p *Server) maybeGenerateChatTitle(
 }
 
 // generateTitle calls the model with a title-generation system prompt
-// and returns the normalized result.
+// and returns the normalized result. It retries transient LLM errors
+// (rate limits, overloaded, etc.) with exponential backoff.
 func generateTitle(
 	ctx context.Context,
 	model fantasy.LanguageModel,
@@ -86,10 +88,16 @@ func generateTitle(
 		},
 	}
 	toolChoice := fantasy.ToolChoiceNone
-	response, err := model.Generate(ctx, fantasy.Call{
-		Prompt:     prompt,
-		ToolChoice: &toolChoice,
-	})
+
+	var response *fantasy.Response
+	err := chatretry.Retry(ctx, func(retryCtx context.Context) error {
+		var genErr error
+		response, genErr = model.Generate(retryCtx, fantasy.Call{
+			Prompt:     prompt,
+			ToolChoice: &toolChoice,
+		})
+		return genErr
+	}, nil)
 	if err != nil {
 		return "", xerrors.Errorf("generate title text: %w", err)
 	}