diff --git a/src/models/openai.test.ts b/src/models/openai.test.ts
index 1b37a0d..823f0cd 100644
--- a/src/models/openai.test.ts
+++ b/src/models/openai.test.ts
@@ -1,14 +1,17 @@
 import { describe, it, expect, vi } from 'vitest';
 import { OpenAIClient } from './openai.js';
 
+// Shared mock function so we can override per-test
+const mockCreate = vi.fn().mockResolvedValue({
+  choices: [{ message: { content: 'Hello from GPT!' }, finish_reason: 'stop' }],
+  usage: { prompt_tokens: 10, completion_tokens: 5 },
+});
+
 vi.mock('openai', () => ({
   default: vi.fn().mockImplementation(() => ({
     chat: {
       completions: {
-        create: vi.fn().mockResolvedValue({
-          choices: [{ message: { content: 'Hello from GPT!' }, finish_reason: 'stop' }],
-          usage: { prompt_tokens: 10, completion_tokens: 5 },
-        }),
+        create: mockCreate,
       },
     },
   })),
@@ -31,3 +34,44 @@ describe('OpenAIClient', () => {
     expect(response.usage.outputTokens).toBe(5);
   });
 });
+
+describe('OpenAIClient tool use', () => {
+  it('passes tools to API and parses tool_calls response', async () => {
+    mockCreate.mockResolvedValueOnce({
+      choices: [{
+        message: {
+          content: null,
+          tool_calls: [{
+            id: 'call_1',
+            type: 'function',
+            function: { name: 'shell.exec', arguments: '{"command":"ls"}' },
+          }],
+        },
+        finish_reason: 'tool_calls',
+      }],
+      usage: { prompt_tokens: 20, completion_tokens: 15 },
+    });
+
+    const client = new OpenAIClient({
+      apiKey: 'test-key',
+      model: 'gpt-4o',
+    });
+
+    const response = await client.chat({
+      messages: [{ role: 'user', content: 'list files' }],
+      tools: [{
+        name: 'shell.exec',
+        description: 'Run shell command',
+        input_schema: { type: 'object', properties: { command: { type: 'string' } }, required: ['command'] },
+      }],
+    });
+
+    expect(response.stopReason).toBe('tool_calls');
+    expect(response.toolCalls).toHaveLength(1);
+    expect(response.toolCalls![0]).toEqual({
+      id: 'call_1',
+      name: 'shell.exec',
+      args: { command: 'ls' },
+    });
+  });
+});
diff --git a/src/models/openai.ts b/src/models/openai.ts
index 3b75ca0..51faeac 100644
--- a/src/models/openai.ts
+++ b/src/models/openai.ts
@@ -33,15 +33,36 @@ export class OpenAIClient implements ModelClient {
       messages.push({ role: msg.role, content: msg.content });
     }
 
-    const response = await this.client.chat.completions.create({
+    // Build params, conditionally including tools
+    const params: OpenAI.ChatCompletionCreateParamsNonStreaming = {
       model: this.model,
       max_tokens: request.maxTokens ?? this.defaultMaxTokens,
       messages,
-    });
+    };
+
+    if (request.tools && request.tools.length > 0) {
+      params.tools = request.tools.map(t => ({
+        type: 'function' as const,
+        function: {
+          name: t.name,
+          description: t.description,
+          parameters: t.input_schema as OpenAI.FunctionParameters,
+        },
+      }));
+    }
+
+    const response = await this.client.chat.completions.create(params);
 
     const choice = response.choices[0];
     const content = choice?.message?.content ?? '';
 
+    // Parse tool_calls from the response if present
+    const toolCalls = choice?.message?.tool_calls?.map((tc: OpenAI.ChatCompletionMessageToolCall) => ({
+      id: tc.id,
+      name: tc.function.name,
+      args: JSON.parse(tc.function.arguments),
+    })) ?? [];
+
     return {
       content,
       stopReason: choice?.finish_reason ?? 'stop',
@@ -49,6 +70,7 @@ export class OpenAIClient implements ModelClient {
         inputTokens: response.usage?.prompt_tokens ?? 0,
         outputTokens: response.usage?.completion_tokens ?? 0,
       },
+      ...(toolCalls.length > 0 ? { toolCalls } : {}),
     };
   }
 }