feat: enhance tool call handling and serialization in chat formats

- Added support for tool calls in NormalizedFinalResponse type.
- Implemented functions to collect and serialize tool calls from OpenAI and Claude responses.
- Updated convertClaudeRequestToOpenAiBody to handle tool use and tool result messages.
- Enhanced normalizeUpstreamFinalResponse to include tool calls in the response structure.
- Introduced utility functions for safe JSON stringification and parsing.
- Improved conversion functions for OpenAI tools to Anthropic format and vice versa.
- Updated input handling in convertOpenAiBodyToMessagesBody and convertOpenAiBodyToResponsesBody to accommodate tool messages.
- Refactored serialization logic in serializeFinalResponse and buildSyntheticOpenAiChunks to include tool calls.

src/server/routes/proxy/chat.stream.test.ts

@@ -931,6 +931,90 @@ describe('chat proxy stream behavior', () => {
     expect(targetUrl).not.toContain('/v1/messages');
   });
 
+  it('preserves claude tool_use/tool_result when claude downstream is routed to openai chat endpoint', async () => {
+    selectChannelMock.mockReturnValue({
+      channel: { id: 11, routeId: 22 },
+      site: { name: 'openai-site', url: 'https://api.openai.com', platform: 'openai' },
+      account: { id: 33, username: 'demo-user' },
+      tokenName: 'default',
+      tokenValue: 'sk-openai',
+      actualModel: 'gpt-4o-mini',
+    });
+
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'chatcmpl-openai-tools',
+      object: 'chat.completion',
+      created: 1_706_000_006,
+      model: 'gpt-4o-mini',
+      choices: [{
+        index: 0,
+        message: { role: 'assistant', content: 'tool payload received' },
+        finish_reason: 'stop',
+      }],
+      usage: { prompt_tokens: 10, completion_tokens: 3, total_tokens: 13 },
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/messages',
+      payload: {
+        model: 'gpt-4o-mini',
+        max_tokens: 256,
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              {
+                type: 'tool_use',
+                id: 'toolu_abc',
+                name: 'Glob',
+                input: { pattern: 'README*' },
+              },
+            ],
+          },
+          {
+            role: 'user',
+            content: [
+              {
+                type: 'tool_result',
+                tool_use_id: 'toolu_abc',
+                content: [{ type: 'text', text: '{\"matches\":1}' }],
+              },
+              {
+                type: 'text',
+                text: 'continue',
+              },
+            ],
+          },
+        ],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+
+    const [_targetUrl, options] = fetchMock.mock.calls[0] as [string, any];
+    const forwardedBody = JSON.parse(options.body);
+    const forwardedMessages = Array.isArray(forwardedBody.messages) ? forwardedBody.messages : [];
+
+    const assistantWithToolCalls = forwardedMessages.find((item: any) =>
+      item?.role === 'assistant'
+      && Array.isArray(item?.tool_calls)
+      && item.tool_calls.length > 0,
+    );
+    expect(assistantWithToolCalls).toBeTruthy();
+    expect(assistantWithToolCalls.tool_calls[0].id).toBe('toolu_abc');
+    expect(assistantWithToolCalls.tool_calls[0].function?.name).toBe('Glob');
+    expect(assistantWithToolCalls.tool_calls[0].function?.arguments).toContain('README*');
+
+    const toolMessage = forwardedMessages.find((item: any) => item?.role === 'tool');
+    expect(toolMessage).toBeTruthy();
+    expect(toolMessage.tool_call_id).toBe('toolu_abc');
+    expect(toolMessage.content).toContain('matches');
+  });
+
   it('forces claude platform to use /v1/messages with x-api-key auth for openai downstream requests', async () => {
     selectChannelMock.mockReturnValue({
       channel: { id: 11, routeId: 22 },
@@ -972,6 +1056,277 @@ describe('chat proxy stream behavior', () => {
     expect(options.headers.Authorization).toBeUndefined();
   });
 
+  it('preserves openai tool context when /v1/chat/completions is routed to /v1/messages upstream', async () => {
+    selectChannelMock.mockReturnValue({
+      channel: { id: 11, routeId: 22 },
+      site: { name: 'claude-site', url: 'https://api.anthropic.com', platform: 'claude' },
+      account: { id: 33, username: 'demo-user' },
+      tokenName: 'default',
+      tokenValue: 'sk-claude',
+      actualModel: 'claude-sonnet-4-5-20250929',
+    });
+
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'msg_claude_tool_context',
+      type: 'message',
+      model: 'claude-sonnet-4-5-20250929',
+      content: [{ type: 'text', text: 'ok' }],
+      stop_reason: 'end_turn',
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/chat/completions',
+      payload: {
+        model: 'claude-sonnet-4-5-20250929',
+        stream: false,
+        tools: [{
+          type: 'function',
+          function: {
+            name: 'Glob',
+            description: 'Search files',
+            parameters: {
+              type: 'object',
+              properties: {
+                pattern: { type: 'string' },
+              },
+              required: ['pattern'],
+            },
+          },
+        }],
+        tool_choice: {
+          type: 'function',
+          function: {
+            name: 'Glob',
+          },
+        },
+        messages: [
+          {
+            role: 'assistant',
+            tool_calls: [{
+              id: 'call_abc',
+              type: 'function',
+              function: {
+                name: 'Glob',
+                arguments: '{"pattern":"README*"}',
+              },
+            }],
+          },
+          {
+            role: 'tool',
+            tool_call_id: 'call_abc',
+            content: '{"matches":1}',
+          },
+          {
+            role: 'user',
+            content: 'continue',
+          },
+        ],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+
+    const [_targetUrl, options] = fetchMock.mock.calls[0] as [string, any];
+    const forwardedBody = JSON.parse(options.body);
+    expect(Array.isArray(forwardedBody.messages)).toBe(true);
+
+    const assistantMessage = forwardedBody.messages.find((item: any) => item?.role === 'assistant');
+    expect(Array.isArray(assistantMessage?.content)).toBe(true);
+    expect(assistantMessage.content.some((part: any) => part?.type === 'tool_use')).toBe(true);
+
+    const userToolResultMessage = forwardedBody.messages.find((item: any) =>
+      item?.role === 'user'
+      && Array.isArray(item?.content)
+      && item.content.some((part: any) => part?.type === 'tool_result'),
+    );
+    expect(userToolResultMessage).toBeTruthy();
+    expect(userToolResultMessage.content[0].tool_use_id).toBe('call_abc');
+
+    expect(forwardedBody.tools?.[0]?.name).toBe('Glob');
+    expect(forwardedBody.tools?.[0]?.input_schema?.properties?.pattern?.type).toBe('string');
+    expect(forwardedBody.tool_choice).toEqual({ type: 'tool', name: 'Glob' });
+  });
+
+  it('converts /v1/responses function_call SSE to OpenAI tool_calls on /v1/chat/completions', async () => {
+    fetchModelPricingCatalogMock.mockResolvedValue({
+      models: [
+        {
+          modelName: 'upstream-gpt',
+          supportedEndpointTypes: ['/v1/responses'],
+        },
+      ],
+      groupRatio: {},
+    });
+
+    const encoder = new TextEncoder();
+    const upstreamBody = new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(encoder.encode('event: response.created\ndata: {"type":"response.created","response":{"id":"resp_123","model":"upstream-gpt","created_at":1706000000,"status":"in_progress","output":[]}}\n\n'));
+        controller.enqueue(encoder.encode('event: response.output_item.added\ndata: {"type":"response.output_item.added","output_index":0,"item":{"type":"function_call","id":"fc_1","call_id":"call_abc","name":"Glob"}}\n\n'));
+        controller.enqueue(encoder.encode('event: response.function_call_arguments.delta\ndata: {"type":"response.function_call_arguments.delta","output_index":0,"call_id":"call_abc","delta":"{\\"pattern\\":\\"README*\\"}"}\n\n'));
+        controller.enqueue(encoder.encode('event: response.completed\ndata: {"type":"response.completed","response":{"id":"resp_123","model":"upstream-gpt","status":"completed","usage":{"input_tokens":5,"output_tokens":3,"total_tokens":8}}}\n\n'));
+        controller.enqueue(encoder.encode('data: [DONE]\n\n'));
+        controller.close();
+      },
+    });
+
+    fetchMock.mockResolvedValue(new Response(upstreamBody, {
+      status: 200,
+      headers: { 'content-type': 'text/event-stream; charset=utf-8' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/chat/completions',
+      payload: {
+        model: 'claude-haiku-4-5-20251001',
+        stream: true,
+        messages: [{ role: 'user', content: 'find readme' }],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    expect(response.body).toContain('"tool_calls"');
+    expect(response.body).toContain('"id":"call_abc"');
+    expect(response.body).toContain('"name":"Glob"');
+    expect(response.body).toContain('\\"pattern\\":\\"README*\\"');
+  });
+
+  it('preserves non-stream function_call output when /v1/chat/completions falls back to /v1/responses', async () => {
+    fetchModelPricingCatalogMock.mockResolvedValue({
+      models: [
+        {
+          modelName: 'upstream-gpt',
+          supportedEndpointTypes: ['/v1/responses'],
+        },
+      ],
+      groupRatio: {},
+    });
+
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'resp_tool_nonstream',
+      object: 'response',
+      model: 'upstream-gpt',
+      status: 'completed',
+      output: [{
+        type: 'function_call',
+        id: 'fc_1',
+        call_id: 'call_abc',
+        name: 'Glob',
+        arguments: '{"pattern":"README*"}',
+      }],
+      output_text: '',
+      usage: { input_tokens: 3, output_tokens: 2, total_tokens: 5 },
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/chat/completions',
+      payload: {
+        model: 'claude-haiku-4-5-20251001',
+        stream: false,
+        messages: [{ role: 'user', content: 'find readme' }],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const body = response.json();
+    expect(body?.choices?.[0]?.message?.tool_calls?.[0]?.id).toBe('call_abc');
+    expect(body?.choices?.[0]?.message?.tool_calls?.[0]?.function?.name).toBe('Glob');
+    expect(body?.choices?.[0]?.message?.tool_calls?.[0]?.function?.arguments).toContain('README*');
+    expect(body?.choices?.[0]?.finish_reason).toBe('tool_calls');
+  });
+
+  it('preserves openai tool context when /v1/chat/completions is routed to /v1/responses upstream', async () => {
+    fetchModelPricingCatalogMock.mockResolvedValue({
+      models: [
+        {
+          modelName: 'upstream-gpt',
+          supportedEndpointTypes: ['/v1/responses'],
+        },
+      ],
+      groupRatio: {},
+    });
+
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'resp_tool_forward',
+      object: 'response',
+      model: 'upstream-gpt',
+      status: 'completed',
+      output: [],
+      output_text: 'ok',
+      usage: { input_tokens: 3, output_tokens: 2, total_tokens: 5 },
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/chat/completions',
+      payload: {
+        model: 'claude-haiku-4-5-20251001',
+        stream: false,
+        tools: [{
+          type: 'function',
+          function: {
+            name: 'Glob',
+            parameters: {
+              type: 'object',
+              properties: { pattern: { type: 'string' } },
+              required: ['pattern'],
+            },
+          },
+        }],
+        tool_choice: {
+          type: 'function',
+          function: { name: 'Glob' },
+        },
+        messages: [
+          {
+            role: 'assistant',
+            tool_calls: [{
+              id: 'call_abc',
+              type: 'function',
+              function: {
+                name: 'Glob',
+                arguments: '{"pattern":"README*"}',
+              },
+            }],
+          },
+          {
+            role: 'tool',
+            tool_call_id: 'call_abc',
+            content: '{"matches":1}',
+          },
+          {
+            role: 'user',
+            content: 'continue',
+          },
+        ],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+
+    const [targetUrl, options] = fetchMock.mock.calls[0] as [string, any];
+    expect(targetUrl).toContain('/v1/responses');
+
+    const forwardedBody = JSON.parse(options.body);
+    expect(Array.isArray(forwardedBody.input)).toBe(true);
+    expect(forwardedBody.input.some((item: any) => item?.type === 'function_call')).toBe(true);
+    expect(forwardedBody.input.some((item: any) => item?.type === 'function_call_output')).toBe(true);
+    expect(forwardedBody.tools?.[0]?.name).toBe('Glob');
+    expect(forwardedBody.tool_choice).toEqual({ type: 'function', name: 'Glob' });
+  });
+
   it('routes gemini platform to OpenAI-compatible upstream endpoint path', async () => {
     selectChannelMock.mockReturnValue({
       channel: { id: 11, routeId: 22 },

src/server/routes/proxy/chatFormats.ts

@@ -49,6 +49,11 @@ export type NormalizedFinalResponse = {
   content: string;
   reasoningContent: string;
   finishReason: string;
+  toolCalls: Array<{
+    id: string;
+    name: string;
+    arguments: string;
+  }>;
 };
 
 export type ParsedDownstreamChatRequest = {
@@ -282,16 +287,132 @@ function parseResponsesOutputText(payload: Record<string, unknown>): string {
   return parts.join('\n\n');
 }
 
+function stringifyUnknownValue(value: unknown): string {
+  if (typeof value === 'string') return value;
+  if (value === undefined || value === null) return '';
+  if (typeof value === 'number' || typeof value === 'boolean') return String(value);
+  if (Array.isArray(value) || isRecord(value)) {
+    try {
+      return JSON.stringify(value);
+    } catch {
+      return '';
+    }
+  }
+  return '';
+}
+
+function parseJsonLike(value: string): unknown {
+  const trimmed = value.trim();
+  if (!trimmed) return {};
+  try {
+    return JSON.parse(trimmed);
+  } catch {
+    return { value };
+  }
+}
+
+function collectToolCallsFromOpenAiChoice(choice: any): Array<{ id: string; name: string; arguments: string }> {
+  const message = isRecord(choice?.message) ? choice.message : {};
+  const rawToolCalls = Array.isArray((message as any).tool_calls)
+    ? (message as any).tool_calls
+    : (Array.isArray(choice?.tool_calls) ? choice.tool_calls : []);
+
+  const toolCalls: Array<{ id: string; name: string; arguments: string }> = [];
+  for (let index = 0; index < rawToolCalls.length; index += 1) {
+    const rawToolCall = rawToolCalls[index];
+    if (!isRecord(rawToolCall)) continue;
+    const fn = isRecord(rawToolCall.function) ? rawToolCall.function : {};
+    const id = (
+      typeof rawToolCall.id === 'string' && rawToolCall.id.trim().length > 0
+        ? rawToolCall.id.trim()
+        : `call_${index}`
+    );
+    const name = (
+      typeof fn.name === 'string' && fn.name.trim().length > 0
+        ? fn.name.trim()
+        : (typeof rawToolCall.name === 'string' ? rawToolCall.name.trim() : '')
+    );
+    const argumentsText = (
+      typeof fn.arguments === 'string'
+        ? fn.arguments
+        : stringifyUnknownValue(fn.arguments ?? rawToolCall.arguments)
+    );
+    toolCalls.push({
+      id,
+      name,
+      arguments: argumentsText,
+    });
+  }
+
+  return toolCalls;
+}
+
+function collectToolCallsFromClaudeContent(content: unknown): Array<{ id: string; name: string; arguments: string }> {
+  const contentItems = Array.isArray(content) ? content : [];
+  const toolCalls: Array<{ id: string; name: string; arguments: string }> = [];
+
+  for (let index = 0; index < contentItems.length; index += 1) {
+    const block = contentItems[index];
+    if (!isRecord(block)) continue;
+    if (block.type !== 'tool_use') continue;
+
+    const id = (
+      typeof block.id === 'string' && block.id.trim().length > 0
+        ? block.id.trim()
+        : `toolu_${index}`
+    );
+    const name = typeof block.name === 'string' ? block.name.trim() : '';
+    const argumentsText = stringifyUnknownValue(block.input);
+    toolCalls.push({
+      id,
+      name,
+      arguments: argumentsText,
+    });
+  }
+
+  return toolCalls;
+}
+
+function collectToolCallsFromResponsesPayload(payload: Record<string, unknown>): Array<{ id: string; name: string; arguments: string }> {
+  const output = Array.isArray(payload.output) ? payload.output : [];
+  const toolCalls: Array<{ id: string; name: string; arguments: string }> = [];
+
+  for (let index = 0; index < output.length; index += 1) {
+    const item = output[index];
+    if (!isRecord(item)) continue;
+    if (item.type !== 'function_call') continue;
+
+    const id = (
+      typeof item.call_id === 'string' && item.call_id.trim().length > 0
+        ? item.call_id.trim()
+        : (typeof item.id === 'string' && item.id.trim().length > 0 ? item.id.trim() : `call_${index}`)
+    );
+    const name = typeof item.name === 'string' ? item.name.trim() : '';
+    const argumentsText = (
+      typeof item.arguments === 'string'
+        ? item.arguments
+        : stringifyUnknownValue(item.arguments)
+    );
+    toolCalls.push({
+      id,
+      name,
+      arguments: argumentsText,
+    });
+  }
+
+  return toolCalls;
+}
+
 function convertClaudeRequestToOpenAiBody(body: Record<string, unknown>): {
   model: string;
   stream: boolean;
-  messages: Array<{ role: string; content: string }>;
+  messages: Array<Record<string, unknown>>;
   payload: Record<string, unknown>;
 } {
   const model = typeof body.model === 'string' ? body.model.trim() : '';
   const stream = body.stream === true;
 
-  const messages: Array<{ role: string; content: string }> = [];
+  const messages: Array<Record<string, unknown>> = [];
 
   const appendMessage = (role: string, content: unknown) => {
     const text = parseClaudeMessageContent(content);
@@ -299,6 +420,23 @@ function convertClaudeRequestToOpenAiBody(body: Record<string, unknown>): {
     messages.push({ role, content: text });
   };
 
+  const appendToolResultMessage = (toolUseId: unknown, content: unknown) => {
+    const toolCallId = typeof toolUseId === 'string' ? toolUseId.trim() : '';
+    if (!toolCallId) return;
+
+    const text = (
+      parseClaudeMessageContent(content)
+      || stringifyUnknownValue(content)
+    ).trim();
+    if (!text) return;
+
+    messages.push({
+      role: 'tool',
+      tool_call_id: toolCallId,
+      content: text,
+    });
+  };
+
   const system = body.system;
   if (typeof system === 'string') {
     appendMessage('system', system);
@@ -312,7 +450,78 @@ function convertClaudeRequestToOpenAiBody(body: Record<string, unknown>): {
     if (!isRecord(message)) continue;
     const role = typeof message.role === 'string' ? message.role : 'user';
     const mappedRole = role === 'assistant' || role === 'system' ? role : 'user';
-    appendMessage(mappedRole, message.content);
+    const content = message.content;
+
+    if (!Array.isArray(content)) {
+      appendMessage(mappedRole, content);
+      continue;
+    }
+
+    // Claude tool blocks need explicit OpenAI mapping:
+    // - assistant.tool_use  -> assistant.tool_calls
+    // - user.tool_result    -> tool messages with tool_call_id
+    const textParts: string[] = [];
+    const toolCalls: Array<Record<string, unknown>> = [];
+
+    const flushTextAsMessage = () => {
+      const merged = textParts.map((item) => item.trim()).filter(Boolean).join('\n\n');
+      textParts.length = 0;
+      if (!merged) return;
+      messages.push({ role: mappedRole, content: merged });
+    };
+
+    for (const block of content) {
+      if (!isRecord(block)) continue;
+      const blockType = typeof block.type === 'string' ? block.type : '';
+
+      if (blockType === 'tool_result') {
+        flushTextAsMessage();
+        appendToolResultMessage(block.tool_use_id, block.content);
+        continue;
+      }
+
+      if (mappedRole === 'assistant' && blockType === 'tool_use') {
+        const id = typeof block.id === 'string' && block.id.trim().length > 0
+          ? block.id.trim()
+          : `call_${toolCalls.length}`;
+        const name = typeof block.name === 'string' ? block.name.trim() : '';
+        const rawInput = block.input;
+        const args = (
+          typeof rawInput === 'string'
+            ? rawInput
+            : stringifyUnknownValue(rawInput) || '{}'
+        );
+
+        const fn: Record<string, unknown> = { arguments: args };
+        if (name) fn.name = name;
+
+        toolCalls.push({
+          id,
+          type: 'function',
+          function: fn,
+        });
+        continue;
+      }
+
+      const text = parseClaudeMessageContent(block);
+      if (text) textParts.push(text);
+    }
+
+    const merged = textParts.map((item) => item.trim()).filter(Boolean).join('\n\n');
+    if (toolCalls.length > 0) {
+      const assistantMessage: Record<string, unknown> = {
+        role: 'assistant',
+        tool_calls: toolCalls,
+      };
+      // Keep textual assistant preface when present.
+      assistantMessage.content = merged || '';
+      messages.push(assistantMessage);
+    } else if (merged) {
+      messages.push({
+        role: mappedRole,
+        content: merged,
+      });
+    }
   }
 
   const payload: Record<string, unknown> = {
@@ -424,35 +633,45 @@ export function normalizeUpstreamFinalResponse(
     const choice = payload.choices[0] ?? {};
     const content = extractAssistantContent(choice) || extractAssistantContent(payload);
     const reasoning = extractAssistantReasoning(choice) || extractAssistantReasoning(payload);
+    const toolCalls = collectToolCallsFromOpenAiChoice(choice);
     return {
       id: isNonEmptyString(payload.id) ? payload.id : fallbackId,
       model: isNonEmptyString(payload.model) ? payload.model : fallbackModel,
       created: ensureIntegerTimestamp(payload.created, now),
-      content: content || fallbackText,
+      content: content || (toolCalls.length > 0 ? '' : fallbackText),
       reasoningContent: reasoning,
-      finishReason: normalizeStopReason(choice?.finish_reason ?? payload.stop_reason) || 'stop',
+      finishReason: toolCalls.length > 0
+        ? 'tool_calls'
+        : (normalizeStopReason(choice?.finish_reason ?? payload.stop_reason) || 'stop'),
+      toolCalls,
     };
   }
 
   if (isRecord(payload) && typeof payload.type === 'string' && payload.type === 'message') {
+    const toolCalls = collectToolCallsFromClaudeContent(payload.content);
     return {
       id: isNonEmptyString(payload.id) ? payload.id : fallbackId,
       model: isNonEmptyString(payload.model) ? payload.model : fallbackModel,
       created: now,
-      content: parseClaudeMessageContent(payload.content) || fallbackText,
+      content: parseClaudeMessageContent(payload.content) || (toolCalls.length > 0 ? '' : fallbackText),
       reasoningContent: extractTextAndReasoning(payload.content).reasoning,
-      finishReason: normalizeStopReason(payload.stop_reason) || 'stop',
+      finishReason: toolCalls.length > 0 ? 'tool_calls' : (normalizeStopReason(payload.stop_reason) || 'stop'),
+      toolCalls,
     };
   }
 
   if (isRecord(payload) && ((payload as any).object === 'response' || Array.isArray((payload as any).output))) {
+    const toolCalls = collectToolCallsFromResponsesPayload(payload);
     return {
       id: isNonEmptyString(payload.id) ? payload.id : fallbackId,
       model: isNonEmptyString(payload.model) ? payload.model : fallbackModel,
       created: ensureIntegerTimestamp(payload.created, now),
-      content: parseResponsesOutputText(payload) || fallbackText,
+      content: parseResponsesOutputText(payload) || (toolCalls.length > 0 ? '' : fallbackText),
       reasoningContent: '',
-      finishReason: normalizeStopReason(payload.finish_reason ?? payload.status) || 'stop',
+      finishReason: toolCalls.length > 0
+        ? 'tool_calls'
+        : (normalizeStopReason(payload.finish_reason ?? payload.status) || 'stop'),
+      toolCalls,
     };
   }
 
@@ -468,6 +687,7 @@ export function normalizeUpstreamFinalResponse(
       content: parsedCandidate.content || fallbackText,
       reasoningContent: parsedCandidate.reasoning,
       finishReason: normalizeStopReason(candidate?.finishReason || (payload as any).finishReason) || 'stop',
+      toolCalls: [],
     };
   }
 
@@ -479,6 +699,7 @@ export function normalizeUpstreamFinalResponse(
       content: payload,
       reasoningContent: '',
       finishReason: 'stop',
+      toolCalls: [],
     };
   }
 
@@ -489,6 +710,7 @@ export function normalizeUpstreamFinalResponse(
     content: fallbackText,
     reasoningContent: '',
     finishReason: 'stop',
+    toolCalls: [],
   };
 }
 
@@ -570,6 +792,64 @@ export function normalizeUpstreamStreamEvent(
     };
   }
 
+  if (type === 'response.reasoning_summary_text.delta') {
+    const deltaText = typeof payload.delta === 'string'
+      ? payload.delta
+      : extractTextAndReasoning(payload.delta).content;
+    return {
+      reasoningDelta: deltaText || undefined,
+    };
+  }
+
+  if (type === 'response.output_item.added' && isRecord((payload as any).item)) {
+    const item = (payload as any).item as Record<string, unknown>;
+    if (item.type === 'function_call') {
+      const outputIndex = (
+        typeof (payload as any).output_index === 'number' && Number.isFinite((payload as any).output_index)
+          ? Math.max(0, Math.trunc((payload as any).output_index))
+          : 0
+      );
+      const toolCallId = (
+        isNonEmptyString(item.call_id) ? item.call_id
+          : (isNonEmptyString(item.id) ? item.id : undefined)
+      );
+      const toolName = isNonEmptyString(item.name) ? item.name : undefined;
+      return {
+        toolCallDeltas: [{
+          index: outputIndex,
+          id: toolCallId,
+          name: toolName,
+        }],
+      };
+    }
+  }
+
+  if (type === 'response.function_call_arguments.delta' || type === 'response.function_call_arguments.done') {
+    const outputIndex = (
+      typeof (payload as any).output_index === 'number' && Number.isFinite((payload as any).output_index)
+        ? Math.max(0, Math.trunc((payload as any).output_index))
+        : 0
+    );
+    const toolCallId = (
+      isNonEmptyString((payload as any).call_id) ? (payload as any).call_id
+        : (isNonEmptyString((payload as any).item_id) ? (payload as any).item_id : undefined)
+    );
+    const toolName = isNonEmptyString((payload as any).name) ? (payload as any).name : undefined;
+    const argumentsDelta = (
+      typeof payload.delta === 'string'
+        ? payload.delta
+        : (typeof (payload as any).arguments === 'string' ? (payload as any).arguments : undefined)
+    );
+    return {
+      toolCallDeltas: [{
+        index: outputIndex,
+        id: toolCallId,
+        name: toolName,
+        argumentsDelta,
+      }],
+    };
+  }
+
   if (type === 'response.completed' && isRecord((payload as any).response)) {
     const responsePayload = (payload as any).response as Record<string, unknown>;
     if (isNonEmptyString(responsePayload.id)) context.id = responsePayload.id;
@@ -1020,21 +1300,61 @@ export function serializeStreamDone(
   return buildClaudeDoneEvents(context, claudeContext, 'stop');
 }
 
+function toOpenAiToolCalls(
+  toolCalls: Array<{ id: string; name: string; arguments: string }>,
+): Array<Record<string, unknown>> {
+  return toolCalls.map((toolCall, index) => ({
+    index,
+    id: toolCall.id || `call_${index}`,
+    type: 'function',
+    function: {
+      name: toolCall.name || '',
+      arguments: toolCall.arguments || '',
+    },
+  }));
+}
+
 export function serializeFinalResponse(
   downstreamFormat: DownstreamFormat,
   normalized: NormalizedFinalResponse,
   usage: { promptTokens: number; completionTokens: number; totalTokens: number },
 ): Record<string, unknown> {
+  const toolCalls = Array.isArray(normalized.toolCalls) ? normalized.toolCalls : [];
+
   if (downstreamFormat === 'claude') {
+    const contentBlocks: Array<Record<string, unknown>> = [];
+    if (normalized.reasoningContent) {
+      contentBlocks.push({
+        type: 'thinking',
+        thinking: normalized.reasoningContent,
+      });
+    }
+    if (normalized.content) {
+      contentBlocks.push({
+        type: 'text',
+        text: normalized.content,
+      });
+    }
+    for (let index = 0; index < toolCalls.length; index += 1) {
+      const toolCall = toolCalls[index];
+      contentBlocks.push({
+        type: 'tool_use',
+        id: toolCall.id || `toolu_${index}`,
+        name: toolCall.name || `tool_${index}`,
+        input: parseJsonLike(toolCall.arguments || ''),
+      });
+    }
+
+    const content = contentBlocks.length > 0
+      ? contentBlocks
+      : [{ type: 'text', text: '' }];
+
     return {
       id: buildClaudeMessageId(normalized.id),
       type: 'message',
       role: 'assistant',
       model: normalized.model,
-      content: [{
-        type: 'text',
-        text: normalized.content,
-      }],
+      content,
       stop_reason: toClaudeStopReason(normalized.finishReason),
       stop_sequence: null,
       usage: {
@@ -1051,6 +1371,16 @@ export function serializeFinalResponse(
   if (normalized.reasoningContent) {
     message.reasoning_content = normalized.reasoningContent;
   }
+  if (toolCalls.length > 0) {
+    message.tool_calls = toOpenAiToolCalls(toolCalls);
+    if (!normalized.content) message.content = '';
+  }
+
+  const finishReason = (
+    toolCalls.length > 0
+      ? 'tool_calls'
+      : (normalizeStopReason(normalized.finishReason) || 'stop')
+  );
 
   return {
     id: normalized.id,
@@ -1060,7 +1390,7 @@ export function serializeFinalResponse(
     choices: [{
       index: 0,
       message,
-      finish_reason: normalizeStopReason(normalized.finishReason) || 'stop',
+      finish_reason: finishReason,
     }],
     usage: {
       prompt_tokens: usage.promptTokens,
@@ -1071,6 +1401,28 @@ export function serializeFinalResponse(
 }
 
 export function buildSyntheticOpenAiChunks(normalized: NormalizedFinalResponse): Array<Record<string, unknown>> {
+  const toolCalls = Array.isArray(normalized.toolCalls) ? normalized.toolCalls : [];
+  const finishReason = (
+    toolCalls.length > 0
+      ? 'tool_calls'
+      : (normalizeStopReason(normalized.finishReason) || 'stop')
+  );
+
+  const startDelta: Record<string, unknown> = {
+    role: 'assistant',
+  };
+  if (normalized.content) {
+    startDelta.content = normalized.content;
+  } else {
+    startDelta.content = '';
+  }
+  if (normalized.reasoningContent) {
+    startDelta.reasoning_content = normalized.reasoningContent;
+  }
+  if (toolCalls.length > 0) {
+    startDelta.tool_calls = toOpenAiToolCalls(toolCalls);
+  }
+
   const startChunk: Record<string, unknown> = {
     id: normalized.id,
     object: 'chat.completion.chunk',
@@ -1078,17 +1430,11 @@ export function buildSyntheticOpenAiChunks(normalized: NormalizedFinalResponse):
     model: normalized.model,
     choices: [{
       index: 0,
-      delta: normalized.content
-        ? { role: 'assistant', content: normalized.content }
-        : { role: 'assistant' },
+      delta: startDelta,
       finish_reason: null,
     }],
   };
 
-  if (normalized.reasoningContent) {
-    (startChunk.choices as any[])[0].delta.reasoning_content = normalized.reasoningContent;
-  }
-
   const endChunk = {
     id: normalized.id,
     object: 'chat.completion.chunk',
@@ -1097,7 +1443,7 @@ export function buildSyntheticOpenAiChunks(normalized: NormalizedFinalResponse):
     choices: [{
       index: 0,
       delta: {},
-      finish_reason: normalizeStopReason(normalized.finishReason) || 'stop',
+      finish_reason: finishReason,
     }],
   };
 

src/server/routes/proxy/upstreamEndpoint.ts

@@ -216,6 +216,166 @@ function normalizeContentText(content: unknown): string {
   return '';
 }
 
+function safeJsonStringify(value: unknown): string {
+  try {
+    return JSON.stringify(value);
+  } catch {
+    return '';
+  }
+}
+
+function parseJsonString(raw: string): unknown {
+  const trimmed = raw.trim();
+  if (!trimmed) return {};
+  try {
+    return JSON.parse(trimmed);
+  } catch {
+    return { value: raw };
+  }
+}
+
+function normalizeOpenAiToolArguments(raw: unknown): string {
+  if (typeof raw === 'string') return raw;
+  if (raw === undefined || raw === null) return '';
+  if (typeof raw === 'number' || typeof raw === 'boolean') return String(raw);
+  if (Array.isArray(raw) || isRecord(raw)) {
+    return safeJsonStringify(raw);
+  }
+  return '';
+}
+
+function normalizeAnthropicToolInput(raw: unknown): unknown {
+  if (raw === undefined || raw === null) return {};
+  if (isRecord(raw) || Array.isArray(raw)) return raw;
+  if (typeof raw === 'string') return parseJsonString(raw);
+  if (typeof raw === 'number' || typeof raw === 'boolean') return raw;
+  return {};
+}
+
+function normalizeToolMessageContent(raw: unknown): string {
+  const contentText = normalizeContentText(raw).trim();
+  if (contentText) return contentText;
+
+  if (raw === undefined || raw === null) return '';
+  if (typeof raw === 'string') return raw;
+  if (typeof raw === 'number' || typeof raw === 'boolean') return String(raw);
+  if (Array.isArray(raw) || isRecord(raw)) return safeJsonStringify(raw);
+  return '';
+}
+
+function convertOpenAiToolsToAnthropic(rawTools: unknown): unknown {
+  if (!Array.isArray(rawTools)) return rawTools;
+
+  const converted = rawTools
+    .map((item) => {
+      if (!isRecord(item)) return null;
+
+      const type = asTrimmedString(item.type).toLowerCase();
+      if (type === 'function' && isRecord(item.function)) {
+        const fn = item.function;
+        const name = asTrimmedString(fn.name);
+        if (!name) return null;
+
+        const mapped: Record<string, unknown> = { name };
+        const description = asTrimmedString(fn.description);
+        if (description) mapped.description = description;
+        if (fn.parameters !== undefined) mapped.input_schema = fn.parameters;
+        if (item.cache_control !== undefined) mapped.cache_control = item.cache_control;
+        return mapped;
+      }
+
+      if (asTrimmedString(item.name) && item.input_schema !== undefined) {
+        return item;
+      }
+
+      return null;
+    })
+    .filter((item): item is Record<string, unknown> => !!item);
+
+  return converted.length > 0 ? converted : rawTools;
+}
+
+function convertOpenAiToolChoiceToAnthropic(rawToolChoice: unknown): unknown {
+  if (rawToolChoice === undefined) return undefined;
+
+  if (typeof rawToolChoice === 'string') {
+    const normalized = rawToolChoice.trim().toLowerCase();
+    if (normalized === 'required') return { type: 'any' };
+    if (normalized === 'none') return { type: 'none' };
+    if (normalized === 'auto') return { type: 'auto' };
+    if (normalized === 'any') return { type: 'any' };
+    return rawToolChoice;
+  }
+
+  if (!isRecord(rawToolChoice)) return rawToolChoice;
+
+  const type = asTrimmedString(rawToolChoice.type).toLowerCase();
+  if (type === 'function' && isRecord(rawToolChoice.function)) {
+    const name = asTrimmedString(rawToolChoice.function.name);
+    return name ? { type: 'tool', name } : { type: 'any' };
+  }
+
+  if (type === 'tool' || type === 'auto' || type === 'any' || type === 'none') {
+    return rawToolChoice;
+  }
+
+  return rawToolChoice;
+}
+
+function convertOpenAiToolsToResponses(rawTools: unknown): unknown {
+  if (!Array.isArray(rawTools)) return rawTools;
+
+  const converted = rawTools
+    .map((item) => {
+      if (!isRecord(item)) return null;
+
+      const type = asTrimmedString(item.type).toLowerCase();
+      if (type === 'function' && isRecord(item.function)) {
+        const fn = item.function;
+        const name = asTrimmedString(fn.name);
+        if (!name) return null;
+
+        const mapped: Record<string, unknown> = {
+          type: 'function',
+          name,
+        };
+        const description = asTrimmedString(fn.description);
+        if (description) mapped.description = description;
+        if (fn.parameters !== undefined) mapped.parameters = fn.parameters;
+        if (fn.strict !== undefined) mapped.strict = fn.strict;
+        return mapped;
+      }
+
+      if (type === 'function' && asTrimmedString(item.name)) {
+        return item;
+      }
+
+      if (type === 'image_generation') {
+        return item;
+      }
+
+      return null;
+    })
+    .filter((item): item is Record<string, unknown> => !!item);
+
+  return converted.length > 0 ? converted : rawTools;
+}
+
+function convertOpenAiToolChoiceToResponses(rawToolChoice: unknown): unknown {
+  if (rawToolChoice === undefined) return undefined;
+  if (typeof rawToolChoice === 'string') return rawToolChoice;
+  if (!isRecord(rawToolChoice)) return rawToolChoice;
+
+  const type = asTrimmedString(rawToolChoice.type).toLowerCase();
+  if (type === 'function' && isRecord(rawToolChoice.function)) {
+    const name = asTrimmedString(rawToolChoice.function.name);
+    if (!name) return 'required';
+    return { type: 'function', name };
+  }
+
+  return rawToolChoice;
+}
+
 function convertOpenAiBodyToMessagesBody(
   openaiBody: Record<string, unknown>,
   modelName: string,
@@ -223,21 +383,98 @@ function convertOpenAiBodyToMessagesBody(
 ): Record<string, unknown> {
   const rawMessages = Array.isArray(openaiBody.messages) ? openaiBody.messages : [];
   const systemContents: string[] = [];
-  const messages: Array<{ role: 'user' | 'assistant'; content: string }> = [];
+  const messages: Array<{
+    role: 'user' | 'assistant';
+    content: string | Array<Record<string, unknown>>;
+  }> = [];
 
   for (const item of rawMessages) {
     if (!isRecord(item)) continue;
-    const role = asTrimmedString(item.role) || 'user';
-    const content = normalizeContentText(item.content);
-    if (!content) continue;
+    const role = asTrimmedString(item.role).toLowerCase() || 'user';
 
-    if (role === 'system') {
+    if (role === 'system' || role === 'developer') {
+      const content = normalizeContentText(item.content);
+      if (!content) continue;
       systemContents.push(content);
       continue;
     }
 
+    if (role === 'tool') {
+      const toolUseId = asTrimmedString(item.tool_call_id) || asTrimmedString(item.id);
+      if (!toolUseId) continue;
+
+      const toolResultContent = normalizeToolMessageContent(item.content).trim();
+      if (!toolResultContent) continue;
+
+      messages.push({
+        role: 'user',
+        content: [{
+          type: 'tool_result',
+          tool_use_id: toolUseId,
+          content: toolResultContent,
+        }],
+      });
+      continue;
+    }
+
+    if (role === 'assistant') {
+      const contentBlocks: Array<Record<string, unknown>> = [];
+
+      const textContent = normalizeContentText(item.content).trim();
+      if (textContent) {
+        contentBlocks.push({
+          type: 'text',
+          text: textContent,
+        });
+      }
+
+      const rawToolCalls = Array.isArray(item.tool_calls) ? item.tool_calls : [];
+      for (let index = 0; index < rawToolCalls.length; index += 1) {
+        const toolCall = rawToolCalls[index];
+        if (!isRecord(toolCall)) continue;
+        const functionPart = isRecord(toolCall.function) ? toolCall.function : {};
+
+        const id = asTrimmedString(toolCall.id) || `toolu_${Date.now()}_${index}`;
+        const name = (
+          asTrimmedString(functionPart.name)
+          || asTrimmedString(toolCall.name)
+          || `tool_${index}`
+        );
+        const input = normalizeAnthropicToolInput(functionPart.arguments ?? toolCall.arguments);
+
+        contentBlocks.push({
+          type: 'tool_use',
+          id,
+          name,
+          input,
+        });
+      }
+
+      if (contentBlocks.length <= 0) continue;
+
+      if (contentBlocks.length === 1 && contentBlocks[0].type === 'text') {
+        const singleText = asTrimmedString(contentBlocks[0].text);
+        if (singleText) {
+          messages.push({
+            role: 'assistant',
+            content: singleText,
+          });
+        }
+        continue;
+      }
+
+      messages.push({
+        role: 'assistant',
+        content: contentBlocks,
+      });
+      continue;
+    }
+
+    const content = normalizeContentText(item.content).trim();
+    if (!content) continue;
+
     messages.push({
-      role: role === 'assistant' ? 'assistant' : 'user',
+      role: 'user',
       content,
     });
   }
@@ -263,8 +500,11 @@ function convertOpenAiBodyToMessagesBody(
     body.stop_sequences = openaiBody.stop;
   }
 
-  if (openaiBody.tools !== undefined) body.tools = openaiBody.tools;
-  if (openaiBody.tool_choice !== undefined) body.tool_choice = openaiBody.tool_choice;
+  if (openaiBody.tools !== undefined) body.tools = convertOpenAiToolsToAnthropic(openaiBody.tools);
+
+  const anthropicToolChoice = convertOpenAiToolChoiceToAnthropic(openaiBody.tool_choice);
+  if (anthropicToolChoice !== undefined) body.tool_choice = anthropicToolChoice;
+
   return body;
 }
 
@@ -275,38 +515,107 @@ function convertOpenAiBodyToResponsesBody(
 ): Record<string, unknown> {
   const rawMessages = Array.isArray(openaiBody.messages) ? openaiBody.messages : [];
   const systemContents: string[] = [];
-  const messages: Array<{ role: 'user' | 'assistant'; content: string }> = [];
+  const inputItems: Array<Record<string, unknown>> = [];
 
   for (const item of rawMessages) {
     if (!isRecord(item)) continue;
-    const role = asTrimmedString(item.role) || 'user';
-    const content = normalizeContentText(item.content);
-    if (!content) continue;
+    const role = asTrimmedString(item.role).toLowerCase() || 'user';
 
-    if (role === 'system') {
+    if (role === 'system' || role === 'developer') {
+      const content = normalizeContentText(item.content).trim();
+      if (!content) continue;
       systemContents.push(content);
       continue;
     }
 
-    messages.push({
-      role: role === 'assistant' ? 'assistant' : 'user',
-      content,
+    if (role === 'assistant') {
+      const textContent = normalizeContentText(item.content).trim();
+      if (textContent) {
+        inputItems.push({
+          type: 'message',
+          role: 'assistant',
+          content: [{ type: 'output_text', text: textContent }],
+        });
+      }
+
+      const rawToolCalls = Array.isArray(item.tool_calls) ? item.tool_calls : [];
+      for (let index = 0; index < rawToolCalls.length; index += 1) {
+        const toolCall = rawToolCalls[index];
+        if (!isRecord(toolCall)) continue;
+        const functionPart = isRecord(toolCall.function) ? toolCall.function : {};
+        const callId = asTrimmedString(toolCall.id) || `call_${Date.now()}_${index}`;
+        const name = (
+          asTrimmedString(functionPart.name)
+          || asTrimmedString(toolCall.name)
+          || `tool_${index}`
+        );
+        const argumentsValue = normalizeOpenAiToolArguments(
+          functionPart.arguments ?? toolCall.arguments,
+        );
+
+        inputItems.push({
+          type: 'function_call',
+          call_id: callId,
+          name,
+          arguments: argumentsValue,
+        });
+      }
+      continue;
+    }
+
+    if (role === 'tool') {
+      const callId = asTrimmedString(item.tool_call_id) || asTrimmedString(item.id);
+      if (!callId) continue;
+
+      inputItems.push({
+        type: 'function_call_output',
+        call_id: callId,
+        output: normalizeToolMessageContent(item.content),
+      });
+      continue;
+    }
+
+    const content = normalizeContentText(item.content).trim();
+    if (!content) continue;
+    inputItems.push({
+      type: 'message',
+      role: 'user',
+      content: [{ type: 'input_text', text: content }],
     });
   }
 
+  let input: unknown = inputItems;
+  if (systemContents.length === 0 && inputItems.length === 1) {
+    const first = inputItems[0];
+    const firstRole = asTrimmedString(first.role).toLowerCase();
+    const firstContent = Array.isArray(first.content) ? first.content : [];
+    if (
+      firstRole === 'user'
+      && firstContent.length === 1
+      && isRecord(firstContent[0])
+      && asTrimmedString(firstContent[0].type).toLowerCase() === 'input_text'
+      && asTrimmedString(firstContent[0].text)
+    ) {
+      input = asTrimmedString(firstContent[0].text);
+    }
+  }
+
+  const maxOutputTokens = (
+    toFiniteNumber(openaiBody.max_output_tokens)
+    ?? toFiniteNumber(openaiBody.max_completion_tokens)
+    ?? toFiniteNumber(openaiBody.max_tokens)
+    ?? 4096
+  );
+
   const body: Record<string, unknown> = {
     model: modelName,
     stream,
-    max_output_tokens: toFiniteNumber(openaiBody.max_tokens) ?? 4096,
+    max_output_tokens: maxOutputTokens,
+    input,
   };
 
-  if (messages.length === 1 && messages[0].role === 'user' && systemContents.length === 0) {
-    body.input = messages[0].content;
-  } else {
-    body.input = messages;
-    if (systemContents.length > 0) {
-      body.instructions = systemContents.join('\n\n');
-    }
+  if (systemContents.length > 0) {
+    body.instructions = systemContents.join('\n\n');
   }
 
   const temperature = toFiniteNumber(openaiBody.temperature);
@@ -315,8 +624,15 @@ function convertOpenAiBodyToResponsesBody(
   const topP = toFiniteNumber(openaiBody.top_p);
   if (topP !== null) body.top_p = topP;
 
-  if (openaiBody.tools !== undefined) body.tools = openaiBody.tools;
-  if (openaiBody.tool_choice !== undefined) body.tool_choice = openaiBody.tool_choice;
+  if (openaiBody.metadata !== undefined) body.metadata = openaiBody.metadata;
+  if (openaiBody.reasoning !== undefined) body.reasoning = openaiBody.reasoning;
+  if (openaiBody.parallel_tool_calls !== undefined) body.parallel_tool_calls = openaiBody.parallel_tool_calls;
+
+  if (openaiBody.tools !== undefined) body.tools = convertOpenAiToolsToResponses(openaiBody.tools);
+
+  const responsesToolChoice = convertOpenAiToolChoiceToResponses(openaiBody.tool_choice);
+  if (responsesToolChoice !== undefined) body.tool_choice = responsesToolChoice;
+
   return body;
 }