diff --git a/autorouter.go b/autorouter.go
index 96cb45a..b17395a 100644
--- a/autorouter.go
+++ b/autorouter.go
@@ -961,6 +961,11 @@ func normalizeProviderRequest(raw map[string]any, providerName string) {
 		return
 	}
 
+	if providerName == "anthropic" {
+		normalizeAnthropicRequest(raw)
+		return
+	}
+
 	if providerName != "deepseek" {
 		return
 	}
@@ -993,6 +998,49 @@ func normalizeProviderRequest(raw map[string]any, providerName string) {
 	}
 }
 
+const defaultAnthropicMaxTokens = 1024
+
+func normalizeAnthropicRequest(raw map[string]any) {
+	if hasPositiveNumber(raw["max_tokens"]) {
+		return
+	}
+	raw["max_tokens"] = defaultAnthropicMaxTokens
+}
+
+func hasPositiveNumber(value any) bool {
+	switch v := value.(type) {
+	case int:
+		return v > 0
+	case int8:
+		return v > 0
+	case int16:
+		return v > 0
+	case int32:
+		return v > 0
+	case int64:
+		return v > 0
+	case uint:
+		return v > 0
+	case uint8:
+		return v > 0
+	case uint16:
+		return v > 0
+	case uint32:
+		return v > 0
+	case uint64:
+		return v > 0
+	case float32:
+		return v > 0
+	case float64:
+		return v > 0
+	case json.Number:
+		f, err := v.Float64()
+		return err == nil && f > 0
+	default:
+		return false
+	}
+}
+
 func normalizeGoogleAIRequest(raw map[string]any) {
 	delete(raw, "stream")
 
diff --git a/autorouter_test.go b/autorouter_test.go
index a4f6de4..f32b668 100644
--- a/autorouter_test.go
+++ b/autorouter_test.go
@@ -1411,6 +1411,101 @@ func TestAutoRouter_AnthropicStreamingNoStreamOptions(t *testing.T) {
 	}
 }
 
+func TestAutoRouter_AnthropicDefaultMaxTokens(t *testing.T) {
+	var receivedBody map[string]any
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		json.Unmarshal(body, &receivedBody)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"id":"msg_test","type":"message","model":"claude-3-opus","content":[{"type":"text","text":"Hello"}],"usage":{"input_tokens":8,"output_tokens":1}}`))
+	}))
+	defer upstream.Close()
+
+	provider := &mockProvider{
+		name: "anthropic",
+		parseFn: func(body io.ReadCloser) (BodyMetadata, []byte, error) {
+			data, _ := io.ReadAll(body)
+			var raw map[string]any
+			_ = json.Unmarshal(data, &raw)
+			maxTokens, _ := raw["max_tokens"].(float64)
+			return BodyMetadata{Model: "claude-3-opus", MaxTokens: int(maxTokens)}, data, nil
+		},
+		enrichFn: func(req *http.Request, meta BodyMetadata, body []byte) error { return nil },
+		resolveFn: func(meta BodyMetadata) (*url.URL, error) {
+			return url.Parse(upstream.URL)
+		},
+		extractFn: func(resp *http.Response) (ResponseMetadata, []byte, error) {
+			body, _ := io.ReadAll(resp.Body)
+			return ResponseMetadata{ID: "msg_test"}, body, nil
+		},
+	}
+
+	router := NewAutoRouter(
+		WithAutoRouterDetector(ProviderDetectorFunc(func(hint ProviderHint) string { return "anthropic" })),
+	)
+	router.RegisterProvider(provider)
+
+	req := httptest.NewRequestWithContext(context.Background(), "POST", "/", bytes.NewReader([]byte(`{"model":"claude-3-opus","messages":[{"role":"user","content":"Hello"}]}`)))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("StatusCode = %d, want 200", w.Code)
+	}
+	if got := receivedBody["max_tokens"]; got != float64(defaultAnthropicMaxTokens) {
+		t.Fatalf("max_tokens = %v, want %d", got, defaultAnthropicMaxTokens)
+	}
+}
+
+func TestAutoRouter_AnthropicPreservesMaxTokens(t *testing.T) {
+	var receivedBody map[string]any
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		json.Unmarshal(body, &receivedBody)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"id":"msg_test","type":"message","model":"claude-3-opus","content":[{"type":"text","text":"Hello"}],"usage":{"input_tokens":8,"output_tokens":1}}`))
+	}))
+	defer upstream.Close()
+
+	provider := &mockProvider{
+		name: "anthropic",
+		parseFn: func(body io.ReadCloser) (BodyMetadata, []byte, error) {
+			data, _ := io.ReadAll(body)
+			return BodyMetadata{Model: "claude-3-opus", MaxTokens: 64}, data, nil
+		},
+		enrichFn: func(req *http.Request, meta BodyMetadata, body []byte) error { return nil },
+		resolveFn: func(meta BodyMetadata) (*url.URL, error) {
+			return url.Parse(upstream.URL)
+		},
+		extractFn: func(resp *http.Response) (ResponseMetadata, []byte, error) {
+			body, _ := io.ReadAll(resp.Body)
+			return ResponseMetadata{ID: "msg_test"}, body, nil
+		},
+	}
+
+	router := NewAutoRouter(
+		WithAutoRouterDetector(ProviderDetectorFunc(func(hint ProviderHint) string { return "anthropic" })),
+	)
+	router.RegisterProvider(provider)
+
+	req := httptest.NewRequestWithContext(context.Background(), "POST", "/", bytes.NewReader([]byte(`{"model":"claude-3-opus","max_tokens":64,"messages":[{"role":"user","content":"Hello"}]}`)))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("StatusCode = %d, want 200", w.Code)
+	}
+	if got := receivedBody["max_tokens"]; got != float64(64) {
+		t.Fatalf("max_tokens = %v, want 64", got)
+	}
+}
+
 func TestAutoRouter_StreamingWritesGatewayMetadataEvent(t *testing.T) {
 	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/event-stream")