fix: accept object tool arguments in openai compat

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 {
@@ -314,6 +314,7 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 	for _, tc := range choice.Message.ToolCalls {
 		arguments := make(map[string]any)
 		name := ""
+		rawArguments := ""
 
 		// Extract thought_signature from Gemini/Google-specific extra content
 		thoughtSignature := ""
@@ -323,22 +324,25 @@ 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
-				}
-			}
+			rawArguments, arguments = decodeToolCallArguments(tc.Function.Arguments)
 		}
 
 		// Build ToolCall with ExtraContent for Gemini 3 thought_signature persistence
 		toolCall := ToolCall{
 			ID:               tc.ID,
+			Type:             tc.Type,
 			Name:             name,
 			Arguments:        arguments,
 			ThoughtSignature: thoughtSignature,
 		}
 
+		if tc.Function != nil {
+			toolCall.Function = &FunctionCall{
+				Name:      name,
+				Arguments: rawArguments,
+			}
+		}
+
 		if thoughtSignature != "" {
 			toolCall.ExtraContent = &ExtraContent{
 				Google: &GoogleExtra{
@@ -361,6 +365,43 @@ func parseResponse(body io.Reader) (*LLMResponse, error) {
 	}, nil
 }
 
+func decodeToolCallArguments(raw json.RawMessage) (string, map[string]any) {
+	raw = bytes.TrimSpace(raw)
+	if len(raw) == 0 || bytes.Equal(raw, []byte("null")) {
+		return "", map[string]any{}
+	}
+
+	if raw[0] == '"' {
+		var encoded string
+		if err := json.Unmarshal(raw, &encoded); err != nil {
+			log.Printf("openai_compat: failed to decode tool call argument string: %v", err)
+			return string(raw), map[string]any{"raw": string(raw)}
+		}
+
+		arguments := map[string]any{}
+		if encoded == "" {
+			return "", arguments
+		}
+		if err := json.Unmarshal([]byte(encoded), &arguments); err != nil {
+			log.Printf("openai_compat: failed to decode tool call arguments JSON: %v", err)
+			arguments["raw"] = encoded
+		}
+		return encoded, arguments
+	}
+
+	var decoded any
+	if err := json.Unmarshal(raw, &decoded); err != nil {
+		log.Printf("openai_compat: failed to decode tool call arguments payload: %v", err)
+		return string(raw), map[string]any{"raw": string(raw)}
+	}
+
+	if object, ok := decoded.(map[string]any); ok {
+		return string(raw), object
+	}
+
+	return string(raw), map[string]any{"raw": decoded}
+}
+
 // 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

@@ -106,6 +106,58 @@ func TestProviderChat_ParsesToolCalls(t *testing.T) {
 	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].Function == nil {
+		t.Fatal("ToolCalls[0].Function = nil, want populated function call")
+	}
+	if out.ToolCalls[0].Function.Arguments != "{\"city\":\"SF\"}" {
+		t.Fatalf("ToolCalls[0].Function.Arguments = %q, want JSON string", out.ToolCalls[0].Function.Arguments)
+	}
+}
+
+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"},
+								},
+							},
+						},
+					},
+					"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].Arguments["city"] != "SF" {
+		t.Fatalf("ToolCalls[0].Arguments[city] = %v, want SF", out.ToolCalls[0].Arguments["city"])
+	}
+	if out.ToolCalls[0].Function == nil {
+		t.Fatal("ToolCalls[0].Function = nil, want populated function call")
+	}
+	if out.ToolCalls[0].Function.Arguments != "{\"city\":\"SF\"}" {
+		t.Fatalf("ToolCalls[0].Function.Arguments = %q, want JSON object string", out.ToolCalls[0].Function.Arguments)
+	}
 }
 
 func TestProviderChat_ParsesReasoningContent(t *testing.T) {