fix(openai_compat): accept object tool call arguments

pkg/providers/openai_compat/provider.go

@@ -283,8 +283,8 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 					ID       string `json:"id"`
 					Type     string `json:"type"`
 					Function *struct {
-						Name      string `json:"name"`
-						Arguments string `json:"arguments"`
+						Name      string          `json:"name"`
+						Arguments json.RawMessage `json:"arguments"`
 					} `json:"function"`
 					ExtraContent *struct {
 						Google *struct {
@@ -323,12 +323,7 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 
 		if tc.Function != nil {
 			name = tc.Function.Name
-			if tc.Function.Arguments != "" {
-				if err := json.Unmarshal([]byte(tc.Function.Arguments), &arguments); err != nil {
-					log.Printf("openai_compat: failed to decode tool call arguments for %q: %v", name, err)
-					arguments["raw"] = tc.Function.Arguments
-				}
-			}
+			arguments = decodeToolCallArguments(tc.Function.Arguments, name)
 		}
 
 		// Build ToolCall with ExtraContent for Gemini 3 thought_signature persistence
@@ -361,6 +356,39 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 	}, nil
 }
 
+func decodeToolCallArguments(raw json.RawMessage, name string) map[string]any {
+	arguments := make(map[string]any)
+	raw = bytes.TrimSpace(raw)
+	if len(raw) == 0 || bytes.Equal(raw, []byte("null")) {
+		return arguments
+	}
+
+	var decoded any
+	if err := json.Unmarshal(raw, &decoded); err != nil {
+		log.Printf("openai_compat: failed to decode tool call arguments payload for %q: %v", name, err)
+		arguments["raw"] = string(raw)
+		return arguments
+	}
+
+	switch v := decoded.(type) {
+	case string:
+		if strings.TrimSpace(v) == "" {
+			return arguments
+		}
+		if err := json.Unmarshal([]byte(v), &arguments); err != nil {
+			log.Printf("openai_compat: failed to decode tool call arguments for %q: %v", name, err)
+			arguments["raw"] = v
+		}
+		return arguments
+	case map[string]any:
+		return v
+	default:
+		log.Printf("openai_compat: unsupported tool call arguments type for %q: %T", name, decoded)
+		arguments["raw"] = string(raw)
+		return arguments
+	}
+}
+
 // openaiMessage is the wire-format message for OpenAI-compatible APIs.
 // It mirrors protocoltypes.Message but omits SystemParts, which is an
 // internal field that would be unknown to third-party endpoints.

pkg/providers/openai_compat/provider_test.go

@@ -108,6 +108,55 @@ func TestProviderChat_ParsesToolCalls(t *testing.T) {
 	}
 }
 
+func TestProviderChat_ParsesToolCallsWithObjectArguments(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message": map[string]any{
+						"content": "",
+						"tool_calls": []map[string]any{
+							{
+								"id":   "call_1",
+								"type": "function",
+								"function": map[string]any{
+									"name": "get_weather",
+									"arguments": map[string]any{
+										"city":   "SF",
+										"metric": true,
+									},
+								},
+							},
+						},
+					},
+					"finish_reason": "tool_calls",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "")
+	out, err := p.Chat(t.Context(), []Message{{Role: "user", Content: "hi"}}, nil, "gpt-4o", nil)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if len(out.ToolCalls) != 1 {
+		t.Fatalf("len(ToolCalls) = %d, want 1", len(out.ToolCalls))
+	}
+	if out.ToolCalls[0].Name != "get_weather" {
+		t.Fatalf("ToolCalls[0].Name = %q, want %q", out.ToolCalls[0].Name, "get_weather")
+	}
+	if out.ToolCalls[0].Arguments["city"] != "SF" {
+		t.Fatalf("ToolCalls[0].Arguments[city] = %v, want SF", out.ToolCalls[0].Arguments["city"])
+	}
+	if out.ToolCalls[0].Arguments["metric"] != true {
+		t.Fatalf("ToolCalls[0].Arguments[metric] = %v, want true", out.ToolCalls[0].Arguments["metric"])
+	}
+}
+
 func TestProviderChat_ParsesReasoningContent(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		resp := map[string]any{