fix(gemini): improve tool-calling support and handle function_call response

- Support tool definitions in Gemini requests
- Map tool role to 'function' in Gemini content
- Ensure tool results are wrapped in JSON objects for Gemini compatibility
- Parse FunctionCall from Gemini response and map to OpenAI-compatible ToolCalls
- Correctly map finish_reason for tool calls

internal/providers/gemini.go

@@ -31,9 +31,14 @@ func (p *GeminiProvider) Name() string {
 
 type GeminiRequest struct {
 	Contents         []GeminiContent         `json:"contents"`
+	Tools            []GeminiTool            `json:"tools,omitempty"`
 	GenerationConfig *GeminiGenerationConfig `json:"generationConfig,omitempty"`
 }
 
+type GeminiTool struct {
+	FunctionDeclarations []models.FunctionDef `json:"functionDeclarations"`
+}
+
 type GeminiGenerationConfig struct {
 	Temperature     *float32 `json:"temperature,omitempty"`
 	TopP            *float32 `json:"topP,omitempty"`
@@ -77,7 +82,7 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 		if msg.Role == "assistant" {
 			role = "model"
 		} else if msg.Role == "tool" {
-			role = "user" // Tool results are user-side in Gemini
+			role = "function" // Function results use 'function' role in Gemini contents
 		}
 		
 		var parts []GeminiPart
@@ -95,10 +100,19 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 				name = *msg.Name
 			}
 			
+			// Try to parse text as JSON if it looks like it, Gemini expects an object
+			var responseObj interface{}
+			if err := json.Unmarshal([]byte(text), &responseObj); err != nil {
+				// If not valid JSON, wrap it in an object
+				responseObj = map[string]interface{}{"result": text}
+			}
+
+			respBytes, _ := json.Marshal(responseObj)
+
 			parts = append(parts, GeminiPart{
 				FunctionResponse: &GeminiFunctionResponse{
 					Name:     name,
-					Response: json.RawMessage(text),
+					Response: json.RawMessage(respBytes),
 				},
 			})
 		} else {
@@ -161,6 +175,17 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 		GenerationConfig: genConfig,
 	}
 
+	// Map Tools
+	if len(req.Tools) > 0 {
+		geminiTool := GeminiTool{FunctionDeclarations: []models.FunctionDef{}}
+		for _, t := range req.Tools {
+			if t.Type == "function" {
+				geminiTool.FunctionDeclarations = append(geminiTool.FunctionDeclarations, t.Function)
+			}
+		}
+		body.Tools = []GeminiTool{geminiTool}
+	}
+
 	baseURL := p.config.BaseURL
 	lowerModel := strings.ToLower(req.Model)
 	if strings.Contains(lowerModel, "preview") || strings.Contains(lowerModel, "3.1") || strings.Contains(lowerModel, "2.0") || strings.Contains(lowerModel, "thinking") {
@@ -190,8 +215,10 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 	var geminiResp struct {
 		Candidates []struct {
 			Content struct {
+				Role  string `json:"role"`
 				Parts []struct {
-					Text string `json:"text"`
+					Text         string              `json:"text"`
+					FunctionCall *GeminiFunctionCall `json:"functionCall"`
 				} `json:"parts"`
 			} `json:"content"`
 			FinishReason string `json:"finishReason"`
@@ -212,23 +239,44 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 	}
 
 	content := ""
-	for _, p := range geminiResp.Candidates[0].Content.Parts {
-		content += p.Text
+	var toolCalls []models.ToolCall
+	for _, part := range geminiResp.Candidates[0].Content.Parts {
+		if part.Text != "" {
+			content += part.Text
+		}
+		if part.FunctionCall != nil {
+			toolCalls = append(toolCalls, models.ToolCall{
+				ID:   fmt.Sprintf("call_%s", part.FunctionCall.Name), // Gemini doesn't have call IDs
+				Type: "function",
+				Function: models.FunctionCall{
+					Name:      part.FunctionCall.Name,
+					Arguments: string(part.FunctionCall.Args),
+				},
+			})
+		}
+	}
+
+	finishReason := strings.ToLower(geminiResp.Candidates[0].FinishReason)
+	if finishReason == "stop" {
+		finishReason = "stop"
+	} else if len(toolCalls) > 0 {
+		finishReason = "tool_calls"
 	}
 
 	openAIResp := &models.ChatCompletionResponse{
 		ID:      "gemini-" + req.Model,
 		Object:  "chat.completion",
-		Created: 0, // Should be current timestamp
+		Created: 0, 
 		Model:   req.Model,
 		Choices: []models.ChatChoice{
 			{
 				Index: 0,
 				Message: models.ChatMessage{
-					Role:    "assistant",
-					Content: content,
+					Role:      "assistant",
+					Content:   content,
+					ToolCalls: toolCalls,
 				},
-				FinishReason: &geminiResp.Candidates[0].FinishReason,
+				FinishReason: &finishReason,
 			},
 		},
 		Usage: &models.Usage{
@@ -248,11 +296,48 @@ func (p *GeminiProvider) ChatCompletionStream(ctx context.Context, req *models.U
 		role := "user"
 		if msg.Role == "assistant" {
 			role = "model"
+		} else if msg.Role == "tool" {
+			role = "function"
 		}
 		
 		var parts []GeminiPart
-		for _, p := range msg.Content {
-			parts = append(parts, GeminiPart{Text: p.Text})
+		
+		if msg.Role == "tool" {
+			text := ""
+			if len(msg.Content) > 0 {
+				text = msg.Content[0].Text
+			}
+			name := "unknown"
+			if msg.Name != nil {
+				name = *msg.Name
+			}
+			
+			var responseObj interface{}
+			if err := json.Unmarshal([]byte(text), &responseObj); err != nil {
+				responseObj = map[string]interface{}{"result": text}
+			}
+			respBytes, _ := json.Marshal(responseObj)
+
+			parts = append(parts, GeminiPart{
+				FunctionResponse: &GeminiFunctionResponse{
+					Name:     name,
+					Response: json.RawMessage(respBytes),
+				},
+			})
+		} else {
+			for _, p := range msg.Content {
+				parts = append(parts, GeminiPart{Text: p.Text})
+			}
+			if msg.Role == "assistant" && len(msg.ToolCalls) > 0 {
+				for _, tc := range msg.ToolCalls {
+					parts = append(parts, GeminiPart{
+						FunctionCall: &GeminiFunctionCall{
+							Name: tc.Function.Name,
+							Args: json.RawMessage(tc.Function.Arguments),
+						},
+					})
+				}
+			}
 		}
 		
 		contents = append(contents, GeminiContent{
@@ -287,6 +372,16 @@ func (p *GeminiProvider) ChatCompletionStream(ctx context.Context, req *models.U
 		GenerationConfig: genConfig,
 	}
 
+	if len(req.Tools) > 0 {
+		geminiTool := GeminiTool{FunctionDeclarations: []models.FunctionDef{}}
+		for _, t := range req.Tools {
+			if t.Type == "function" {
+				geminiTool.FunctionDeclarations = append(geminiTool.FunctionDeclarations, t.Function)
+			}
+		}
+		body.Tools = []GeminiTool{geminiTool}
+	}
+
 	baseURL := p.config.BaseURL
 	lowerModel := strings.ToLower(req.Model)
 	if strings.Contains(lowerModel, "preview") || strings.Contains(lowerModel, "3.1") || strings.Contains(lowerModel, "2.0") || strings.Contains(lowerModel, "thinking") {