Add OpenAI tool calling support

internal/api/types.go

@@ -96,6 +96,7 @@ type InputItem struct {
 type Message struct {
 	Role    string         `json:"role"`
 	Content []ContentBlock `json:"content"`
+	CallID  string         `json:"call_id,omitempty"` // for tool messages
 }
 
 // ContentBlock is a typed content element.
@@ -138,6 +139,7 @@ func (r *ResponseRequest) NormalizeInput() []Message {
 			msgs = append(msgs, Message{
 				Role:    "tool",
 				Content: []ContentBlock{{Type: "input_text", Text: item.Output}},
+				CallID:  item.CallID,
 			})
 		}
 	}
@@ -193,6 +195,9 @@ type OutputItem struct {
 	Status    string        `json:"status"`
 	Role      string        `json:"role,omitempty"`
 	Content   []ContentPart `json:"content,omitempty"`
+	CallID    string        `json:"call_id,omitempty"`    // for function_call
+	Name      string        `json:"name,omitempty"`       // for function_call
+	Arguments string        `json:"arguments,omitempty"`  // for function_call
 }
 
 // ContentPart is a content block within an output item.
@@ -259,6 +264,7 @@ type StreamEvent struct {
 	Part           *ContentPart `json:"part,omitempty"`
 	Delta          string       `json:"delta,omitempty"`
 	Text           string       `json:"text,omitempty"`
+	Arguments      string       `json:"arguments,omitempty"` // for function_call_arguments.done
 }
 
 // ============================================================
@@ -271,6 +277,7 @@ type ProviderResult struct {
 	Model     string
 	Text      string
 	Usage     Usage
+	ToolCalls []ToolCall
 }
 
 // ProviderStreamDelta is sent through the stream channel.
@@ -280,6 +287,22 @@ type ProviderStreamDelta struct {
 	Text          string
 	Done          bool
 	Usage         *Usage
+	ToolCallDelta *ToolCallDelta
+}
+
+// ToolCall represents a function call from the model.
+type ToolCall struct {
+	ID        string
+	Name      string
+	Arguments string // JSON string
+}
+
+// ToolCallDelta represents a streaming chunk of a tool call.
+type ToolCallDelta struct {
+	Index     int
+	ID        string
+	Name      string
+	Arguments string
 }
 
 // ============================================================

internal/providers/openai/convert.go

@@ -0,0 +1,120 @@
+package openai
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/ajac-zero/latticelm/internal/api"
+	"github.com/openai/openai-go"
+	"github.com/openai/openai-go/shared"
+)
+
+// parseTools converts Open Responses tools to OpenAI format
+func parseTools(req *api.ResponseRequest) ([]openai.ChatCompletionToolParam, error) {
+	if req.Tools == nil || len(req.Tools) == 0 {
+		return nil, nil
+	}
+
+	var toolDefs []map[string]interface{}
+	if err := json.Unmarshal(req.Tools, &toolDefs); err != nil {
+		return nil, fmt.Errorf("unmarshal tools: %w", err)
+	}
+
+	var tools []openai.ChatCompletionToolParam
+	for _, td := range toolDefs {
+		// Convert Open Responses tool to OpenAI ChatCompletionToolParam
+		// Extract: name, description, parameters
+		name, _ := td["name"].(string)
+		desc, _ := td["description"].(string)
+		params, _ := td["parameters"].(map[string]interface{})
+
+		tool := openai.ChatCompletionToolParam{
+			Function: shared.FunctionDefinitionParam{
+				Name: name,
+			},
+		}
+
+		if desc != "" {
+			tool.Function.Description = openai.String(desc)
+		}
+
+		if params != nil {
+			tool.Function.Parameters = shared.FunctionParameters(params)
+		}
+
+		tools = append(tools, tool)
+	}
+
+	return tools, nil
+}
+
+// parseToolChoice converts Open Responses tool_choice to OpenAI format
+func parseToolChoice(req *api.ResponseRequest) (openai.ChatCompletionToolChoiceOptionUnionParam, error) {
+	var result openai.ChatCompletionToolChoiceOptionUnionParam
+
+	if req.ToolChoice == nil || len(req.ToolChoice) == 0 {
+		return result, nil
+	}
+
+	var choice interface{}
+	if err := json.Unmarshal(req.ToolChoice, &choice); err != nil {
+		return result, fmt.Errorf("unmarshal tool_choice: %w", err)
+	}
+
+	// Handle string values: "auto", "none", "required"
+	if str, ok := choice.(string); ok {
+		result.OfAuto = openai.String(str)
+		return result, nil
+	}
+
+	// Handle specific function selection: {"type": "function", "name": "..."}
+	if obj, ok := choice.(map[string]interface{}); ok {
+		funcObj, _ := obj["function"].(map[string]interface{})
+		name, _ := funcObj["name"].(string)
+
+		result.OfChatCompletionNamedToolChoice = &openai.ChatCompletionNamedToolChoiceParam{
+			Function: openai.ChatCompletionNamedToolChoiceFunctionParam{
+				Name: name,
+			},
+		}
+		return result, nil
+	}
+
+	return result, fmt.Errorf("invalid tool_choice format")
+}
+
+// extractToolCalls converts OpenAI tool calls to api.ToolCall
+func extractToolCalls(message openai.ChatCompletionMessage) []api.ToolCall {
+	if len(message.ToolCalls) == 0 {
+		return nil
+	}
+
+	var toolCalls []api.ToolCall
+	for _, tc := range message.ToolCalls {
+		toolCalls = append(toolCalls, api.ToolCall{
+			ID:        tc.ID,
+			Name:      tc.Function.Name,
+			Arguments: tc.Function.Arguments,
+		})
+	}
+	return toolCalls
+}
+
+// extractToolCallDelta extracts tool call delta from streaming chunk choice
+func extractToolCallDelta(choice openai.ChatCompletionChunkChoice) *api.ToolCallDelta {
+	if len(choice.Delta.ToolCalls) == 0 {
+		return nil
+	}
+
+	// OpenAI sends tool calls with index in the delta
+	for _, tc := range choice.Delta.ToolCalls {
+		return &api.ToolCallDelta{
+			Index:     int(tc.Index),
+			ID:        tc.ID,
+			Name:      tc.Function.Name,
+			Arguments: tc.Function.Arguments,
+		}
+	}
+
+	return nil
+}

internal/providers/openai/openai.go

@@ -91,6 +91,8 @@ func (p *Provider) Generate(ctx context.Context, messages []api.Message, req *ap
 			oaiMessages = append(oaiMessages, openai.SystemMessage(content))
 		case "developer":
 			oaiMessages = append(oaiMessages, openai.SystemMessage(content))
+		case "tool":
+			oaiMessages = append(oaiMessages, openai.ToolMessage(content, msg.CallID))
 		}
 	}
 
@@ -108,6 +110,29 @@ func (p *Provider) Generate(ctx context.Context, messages []api.Message, req *ap
 		params.TopP = openai.Float(*req.TopP)
 	}
 
+	// Add tools if present
+	if req.Tools != nil && len(req.Tools) > 0 {
+		tools, err := parseTools(req)
+		if err != nil {
+			return nil, fmt.Errorf("parse tools: %w", err)
+		}
+		params.Tools = tools
+	}
+
+	// Add tool_choice if present
+	if req.ToolChoice != nil && len(req.ToolChoice) > 0 {
+		toolChoice, err := parseToolChoice(req)
+		if err != nil {
+			return nil, fmt.Errorf("parse tool_choice: %w", err)
+		}
+		params.ToolChoice = toolChoice
+	}
+
+	// Add parallel_tool_calls if specified
+	if req.ParallelToolCalls != nil {
+		params.ParallelToolCalls = openai.Bool(*req.ParallelToolCalls)
+	}
+
 	// Call OpenAI API
 	resp, err := p.client.Chat.Completions.New(ctx, params)
 	if err != nil {
@@ -115,14 +140,20 @@ func (p *Provider) Generate(ctx context.Context, messages []api.Message, req *ap
 	}
 
 	var combinedText string
+	var toolCalls []api.ToolCall
+
 	for _, choice := range resp.Choices {
 		combinedText += choice.Message.Content
+		if len(choice.Message.ToolCalls) > 0 {
+			toolCalls = append(toolCalls, extractToolCalls(choice.Message)...)
+		}
 	}
 
 	return &api.ProviderResult{
 		ID:        resp.ID,
 		Model:     resp.Model,
 		Text:      combinedText,
+		ToolCalls: toolCalls,
 		Usage: api.Usage{
 			InputTokens:  int(resp.Usage.PromptTokens),
 			OutputTokens: int(resp.Usage.CompletionTokens),
@@ -168,6 +199,8 @@ func (p *Provider) GenerateStream(ctx context.Context, messages []api.Message, r
 				oaiMessages = append(oaiMessages, openai.SystemMessage(content))
 			case "developer":
 				oaiMessages = append(oaiMessages, openai.SystemMessage(content))
+			case "tool":
+				oaiMessages = append(oaiMessages, openai.ToolMessage(content, msg.CallID))
 			}
 		}
 
@@ -185,6 +218,31 @@ func (p *Provider) GenerateStream(ctx context.Context, messages []api.Message, r
 			params.TopP = openai.Float(*req.TopP)
 		}
 
+		// Add tools if present
+		if req.Tools != nil && len(req.Tools) > 0 {
+			tools, err := parseTools(req)
+			if err != nil {
+				errChan <- fmt.Errorf("parse tools: %w", err)
+				return
+			}
+			params.Tools = tools
+		}
+
+		// Add tool_choice if present
+		if req.ToolChoice != nil && len(req.ToolChoice) > 0 {
+			toolChoice, err := parseToolChoice(req)
+			if err != nil {
+				errChan <- fmt.Errorf("parse tool_choice: %w", err)
+				return
+			}
+			params.ToolChoice = toolChoice
+		}
+
+		// Add parallel_tool_calls if specified
+		if req.ParallelToolCalls != nil {
+			params.ParallelToolCalls = openai.Bool(*req.ParallelToolCalls)
+		}
+
 		// Create streaming request
 		stream := p.client.Chat.Completions.NewStreaming(ctx, params)
 
@@ -193,10 +251,8 @@ func (p *Provider) GenerateStream(ctx context.Context, messages []api.Message, r
 			chunk := stream.Current()
 
 			for _, choice := range chunk.Choices {
-				if choice.Delta.Content == "" {
+				// Handle text content
-					continue
+				if choice.Delta.Content != "" {
-				}
-
 					select {
 					case deltaChan <- &api.ProviderStreamDelta{
 						ID:    chunk.ID,
@@ -208,6 +264,24 @@ func (p *Provider) GenerateStream(ctx context.Context, messages []api.Message, r
 						return
 					}
 				}
+
+				// Handle tool call deltas
+				if len(choice.Delta.ToolCalls) > 0 {
+					delta := extractToolCallDelta(choice)
+					if delta != nil {
+						select {
+						case deltaChan <- &api.ProviderStreamDelta{
+							ID:            chunk.ID,
+							Model:         chunk.Model,
+							ToolCallDelta: delta,
+						}:
+						case <-ctx.Done():
+							errChan <- ctx.Err()
+							return
+						}
+					}
+				}
+			}
 		}
 
 		if err := stream.Err(); err != nil {

internal/server/server.go

@@ -224,6 +224,17 @@ func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.R
 	var streamErr error
 	var providerModel string
 
+	// Track tool calls being built
+	type toolCallBuilder struct {
+		itemID    string
+		id        string
+		name      string
+		arguments string
+	}
+	toolCallsInProgress := make(map[int]*toolCallBuilder)
+	nextOutputIdx := 0
+	textItemAdded := false
+
 loop:
 	for {
 		select {
@@ -234,7 +245,14 @@ loop:
 			if delta.Model != "" && providerModel == "" {
 				providerModel = delta.Model
 			}
+
+			// Handle text content
 			if delta.Text != "" {
+				// Add text item on first text delta
+				if !textItemAdded {
+					textItemAdded = true
+					nextOutputIdx++
+				}
 				fullText += delta.Text
 				s.sendSSE(w, flusher, &seq, "response.output_text.delta", &api.StreamEvent{
 					Type:         "response.output_text.delta",
@@ -244,6 +262,53 @@ loop:
 					Delta:        delta.Text,
 				})
 			}
+
+			// Handle tool call delta
+			if delta.ToolCallDelta != nil {
+				tc := delta.ToolCallDelta
+
+				// First chunk for this tool call index
+				if _, exists := toolCallsInProgress[tc.Index]; !exists {
+					toolItemID := generateID("item_")
+					toolOutputIdx := nextOutputIdx
+					nextOutputIdx++
+
+					// Send response.output_item.added
+					s.sendSSE(w, flusher, &seq, "response.output_item.added", &api.StreamEvent{
+						Type:        "response.output_item.added",
+						OutputIndex: &toolOutputIdx,
+						Item: &api.OutputItem{
+							ID:     toolItemID,
+							Type:   "function_call",
+							Status: "in_progress",
+							CallID: tc.ID,
+							Name:   tc.Name,
+						},
+					})
+
+					toolCallsInProgress[tc.Index] = &toolCallBuilder{
+						itemID:    toolItemID,
+						id:        tc.ID,
+						name:      tc.Name,
+						arguments: "",
+					}
+				}
+
+				// Send function_call_arguments.delta
+				if tc.Arguments != "" {
+					builder := toolCallsInProgress[tc.Index]
+					builder.arguments += tc.Arguments
+					toolOutputIdx := outputIdx + 1 + tc.Index
+
+					s.sendSSE(w, flusher, &seq, "response.function_call_arguments.delta", &api.StreamEvent{
+						Type:        "response.function_call_arguments.delta",
+						ItemID:      builder.itemID,
+						OutputIndex: &toolOutputIdx,
+						Delta:       tc.Arguments,
+					})
+				}
+			}
+
 			if delta.Done {
 				break loop
 			}
@@ -277,6 +342,8 @@ loop:
 		return
 	}
 
+	// Send done events for text output if text was added
+	if textItemAdded && fullText != "" {
 		// response.output_text.done
 		s.sendSSE(w, flusher, &seq, "response.output_text.done", &api.StreamEvent{
 			Type:         "response.output_text.done",
@@ -313,18 +380,70 @@ loop:
 			OutputIndex: &outputIdx,
 			Item:        completedItem,
 		})
+	}
+
+	// Send done events for each tool call
+	for idx, builder := range toolCallsInProgress {
+		toolOutputIdx := outputIdx + 1 + idx
+
+		s.sendSSE(w, flusher, &seq, "response.function_call_arguments.done", &api.StreamEvent{
+			Type:        "response.function_call_arguments.done",
+			ItemID:      builder.itemID,
+			OutputIndex: &toolOutputIdx,
+			Arguments:   builder.arguments,
+		})
+
+		s.sendSSE(w, flusher, &seq, "response.output_item.done", &api.StreamEvent{
+			Type:        "response.output_item.done",
+			OutputIndex: &toolOutputIdx,
+			Item: &api.OutputItem{
+				ID:        builder.itemID,
+				Type:      "function_call",
+				Status:    "completed",
+				CallID:    builder.id,
+				Name:      builder.name,
+				Arguments: builder.arguments,
+			},
+		})
+	}
 
 	// Build final completed response
 	model := origReq.Model
 	if providerModel != "" {
 		model = providerModel
 	}
+
+	// Collect tool calls for result
+	var toolCalls []api.ToolCall
+	for _, builder := range toolCallsInProgress {
+		toolCalls = append(toolCalls, api.ToolCall{
+			ID:        builder.id,
+			Name:      builder.name,
+			Arguments: builder.arguments,
+		})
+	}
+
 	finalResult := &api.ProviderResult{
 		Model:     model,
 		Text:      fullText,
+		ToolCalls: toolCalls,
 	}
 	completedResp := s.buildResponse(origReq, finalResult, provider.Name(), responseID)
+
+	// Update item IDs to match what we sent during streaming
+	if textItemAdded && len(completedResp.Output) > 0 {
 		completedResp.Output[0].ID = itemID
+	}
+	for idx, builder := range toolCallsInProgress {
+		// Find the corresponding output item
+		for i := range completedResp.Output {
+			if completedResp.Output[i].Type == "function_call" && completedResp.Output[i].CallID == builder.id {
+				completedResp.Output[i].ID = builder.itemID
+				break
+			}
+		}
+		_ = idx // unused
+	}
 
 	// response.completed
 	s.sendSSE(w, flusher, &seq, "response.completed", &api.StreamEvent{
@@ -363,10 +482,13 @@ func (s *GatewayServer) buildResponse(req *api.ResponseRequest, result *api.Prov
 		model = req.Model
 	}
 
-	// Build output item
+	// Build output items array
-	itemID := generateID("msg_")
+	outputItems := []api.OutputItem{}
-	outputItem := api.OutputItem{
+
-		ID:     itemID,
+	// Add message item if there's text
+	if result.Text != "" {
+		outputItems = append(outputItems, api.OutputItem{
+			ID:     generateID("msg_"),
 			Type:   "message",
 			Status: "completed",
 			Role:   "assistant",
@@ -375,6 +497,19 @@ func (s *GatewayServer) buildResponse(req *api.ResponseRequest, result *api.Prov
 				Text:        result.Text,
 				Annotations: []api.Annotation{},
 			}},
+		})
+	}
+
+	// Add function_call items
+	for _, tc := range result.ToolCalls {
+		outputItems = append(outputItems, api.OutputItem{
+			ID:        generateID("item_"),
+			Type:      "function_call",
+			Status:    "completed",
+			CallID:    tc.ID,
+			Name:      tc.Name,
+			Arguments: tc.Arguments,
+		})
 	}
 
 	// Echo back request params with defaults
@@ -454,7 +589,7 @@ func (s *GatewayServer) buildResponse(req *api.ResponseRequest, result *api.Prov
 		Model:              model,
 		PreviousResponseID: req.PreviousResponseID,
 		Instructions:       req.Instructions,
-		Output:             []api.OutputItem{outputItem},
+		Output:             outputItems,
 		Error:              nil,
 		Tools:              tools,
 		ToolChoice:         toolChoice,