fix(openai_compat): omit reasoning_content for Mistral requests

pkg/providers/openai_compat/provider.go

@@ -100,10 +100,11 @@ func (p *Provider) buildRequestBody(
 	messages []Message, tools []ToolDefinition, model string, options map[string]any,
 ) map[string]any {
 	model = normalizeModel(model, p.apiBase)
+	serializedMessages := common.SerializeMessages(messagesForRequest(messages, p.apiBase))
 
 	requestBody := map[string]any{
 		"model":    model,
-		"messages": common.SerializeMessages(messages),
+		"messages": serializedMessages,
 	}
 
 	// When fallback uses a different provider (e.g. DeepSeek), that provider must not inject web_search_preview.
@@ -156,6 +157,27 @@ func (p *Provider) buildRequestBody(
 	return requestBody
 }
 
+func messagesForRequest(messages []Message, apiBase string) []Message {
+	if !isReasoningContentUnsupportedHost(apiBase) {
+		return messages
+	}
+	out := make([]Message, len(messages))
+	copy(out, messages)
+	for i := range out {
+		out[i].ReasoningContent = ""
+	}
+	return out
+}
+
+func isReasoningContentUnsupportedHost(apiBase string) bool {
+	u, err := url.Parse(apiBase)
+	if err != nil {
+		return false
+	}
+	host := strings.ToLower(u.Hostname())
+	return host == "api.mistral.ai" || strings.HasSuffix(host, ".mistral.ai")
+}
+
 func (p *Provider) Chat(
 	ctx context.Context,
 	messages []Message,

pkg/providers/openai_compat/provider_test.go

@@ -252,6 +252,75 @@ func TestProviderChat_PreservesReasoningContentInHistory(t *testing.T) {
 	}
 }
 
+func TestProviderChat_OmitsReasoningContentForMistral(t *testing.T) {
+	var requestBody map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := json.NewDecoder(r.Body).Decode(&requestBody); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+
+		reqMessages, _ := requestBody["messages"].([]any)
+		for _, rawMsg := range reqMessages {
+			msg, ok := rawMsg.(map[string]any)
+			if !ok {
+				continue
+			}
+			if _, exists := msg["reasoning_content"]; exists {
+				w.WriteHeader(http.StatusUnprocessableEntity)
+				_, _ = w.Write([]byte(
+					`{"object":"error","message":"Extra inputs are not permitted","type":"invalid_request_error"}`,
+				))
+				return
+			}
+		}
+
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "")
+	p.apiBase = "https://api.mistral.ai/v1"
+	p.httpClient = &http.Client{
+		Transport: roundTripperFunc(func(r *http.Request) (*http.Response, error) {
+			r.URL, _ = url.Parse(server.URL + r.URL.Path)
+			return http.DefaultTransport.RoundTrip(r)
+		}),
+	}
+
+	messages := []Message{
+		{Role: "user", Content: "What is 1+1?"},
+		{Role: "assistant", Content: "2", ReasoningContent: "Let me think... 1+1=2"},
+		{Role: "user", Content: "What about 2+2?"},
+	}
+
+	if _, err := p.Chat(t.Context(), messages, nil, "mistral-small-latest", nil); err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+
+	reqMessages, ok := requestBody["messages"].([]any)
+	if !ok {
+		t.Fatalf("messages is not []any: %T", requestBody["messages"])
+	}
+	assistantMsg, ok := reqMessages[1].(map[string]any)
+	if !ok {
+		t.Fatalf("assistant message is not map[string]any: %T", reqMessages[1])
+	}
+	if _, exists := assistantMsg["reasoning_content"]; exists {
+		t.Fatal("reasoning_content should be omitted for Mistral requests")
+	}
+}
+
 func TestProviderChat_HTTPError(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bad request", http.StatusBadRequest)
@@ -929,6 +998,25 @@ func TestSupportsPromptCacheKey(t *testing.T) {
 	}
 }
 
+func TestIsReasoningContentUnsupportedHost(t *testing.T) {
+	tests := []struct {
+		apiBase string
+		want    bool
+	}{
+		{"https://api.mistral.ai/v1", true},
+		{"https://edge.mistral.ai/v1", true},
+		{"https://api.openai.com/v1", false},
+		{"https://api.deepseek.com/v1", false},
+		{"", false},
+		{"not-a-url", false},
+	}
+	for _, tt := range tests {
+		if got := isReasoningContentUnsupportedHost(tt.apiBase); got != tt.want {
+			t.Errorf("isReasoningContentUnsupportedHost(%q) = %v, want %v", tt.apiBase, got, tt.want)
+		}
+	}
+}
+
 func TestBuildToolsList_NativeSearchAddsWebSearchPreview(t *testing.T) {
 	tools := []ToolDefinition{
 		{Type: "function", Function: ToolFunctionDefinition{Name: "read_file", Description: "read"}},