diff --git a/extensions/copilot/src/extension.spec.ts b/extensions/copilot/src/extension.spec.ts
index 1e6aed5e89..1445209781 100644
--- a/extensions/copilot/src/extension.spec.ts
+++ b/extensions/copilot/src/extension.spec.ts
@@ -16,11 +16,17 @@
  * SPDX-License-Identifier: Apache-2.0
  ***********************************************************************/
 
-import type { Disposable, ExtensionContext } from '@openkaiden/api';
+import type {
+  Agent,
+  AgentConfigurationFile,
+  AgentWorkspaceContext,
+  Disposable,
+  ExtensionContext,
+} from '@openkaiden/api';
 import { agents } from '@openkaiden/api';
 import { beforeEach, describe, expect, test, vi } from 'vitest';
 
-import { activate } from './extension';
+import { activate, COPILOT_SETTINGS_PATH } from './extension';
 
 const AGENT_DISPOSABLE_MOCK: Disposable = { dispose: vi.fn() };
 
@@ -36,6 +42,30 @@ beforeEach(() => {
   vi.mocked(agents.registerAgent).mockReturnValue(AGENT_DISPOSABLE_MOCK);
 });
 
+function getRegisteredAgent(): Agent {
+  return vi.mocked(agents.registerAgent).mock.calls[0]![0];
+}
+
+function createContext(configFiles: AgentConfigurationFile[], modelLabel = 'gpt-4o'): AgentWorkspaceContext {
+  return {
+    model: {
+      model: { label: modelLabel },
+    },
+    configurationFiles: configFiles,
+    workspace: {},
+  };
+}
+
+function createConfigFile(content = '{}'): AgentConfigurationFile & { updateMock: ReturnType<typeof vi.fn> } {
+  const updateMock = vi.fn();
+  const file: AgentConfigurationFile = {
+    path: COPILOT_SETTINGS_PATH,
+    read: vi.fn().mockResolvedValue(content),
+    update: updateMock,
+  };
+  return Object.assign(file, { updateMock });
+}
+
 describe('activate', () => {
   test('registers copilot agent', async () => {
     await activate(extensionContextMock);
@@ -61,9 +91,93 @@ describe('activate', () => {
   test('registered agent supports all model types except vertexai', async () => {
     await activate(extensionContextMock);
 
-    const agent = vi.mocked(agents.registerAgent).mock.calls[0]![0];
+    const agent = getRegisteredAgent();
     expect(agent.isSupportedModelType!({ name: 'openai' })).toBe(true);
     expect(agent.isSupportedModelType!({ name: 'gemini' })).toBe(true);
     expect(agent.isSupportedModelType!({ name: 'vertexai' })).toBe(false);
   });
+
+  test('registers agent with settings.json configuration file', async () => {
+    await activate(extensionContextMock);
+
+    const agent = getRegisteredAgent();
+    expect(agent.configurationFiles).toHaveLength(1);
+    expect(agent.configurationFiles[0]!.path).toBe(COPILOT_SETTINGS_PATH);
+  });
+
+  describe('preWorkspaceStart', () => {
+    test('writes model configuration into settings.json', async () => {
+      await activate(extensionContextMock);
+      const agent = getRegisteredAgent();
+
+      const configFile = createConfigFile();
+      await agent.preWorkspaceStart(createContext([configFile]));
+
+      expect(configFile.updateMock).toHaveBeenCalledOnce();
+      const written = JSON.parse(configFile.updateMock.mock.calls[0]![0] as string);
+      expect(written).toEqual({
+        chat: { model: 'gpt-4o' },
+      });
+    });
+
+    test('preserves existing configuration fields', async () => {
+      await activate(extensionContextMock);
+      const agent = getRegisteredAgent();
+
+      const existingConfig = JSON.stringify({ chat: { model: 'old-model', temperature: 0.7 }, other: true });
+      const configFile = createConfigFile(existingConfig);
+      await agent.preWorkspaceStart(createContext([configFile], 'claude-sonnet'));
+
+      const written = JSON.parse(configFile.updateMock.mock.calls[0]![0] as string);
+      expect(written.chat.model).toBe('claude-sonnet');
+      expect(written.chat.temperature).toBe(0.7);
+      expect(written.other).toBe(true);
+    });
+
+    test.each([
+      'null',
+      '"a string"',
+      '123',
+      'true',
+      '[1, 2]',
+    ])('falls back to empty config when parsed JSON is non-object: %s', async (payload: string) => {
+      await activate(extensionContextMock);
+      const agent = getRegisteredAgent();
+
+      const configFile = createConfigFile(payload);
+      await agent.preWorkspaceStart(createContext([configFile]));
+
+      expect(configFile.updateMock).toHaveBeenCalledOnce();
+      const written = JSON.parse(configFile.updateMock.mock.calls[0]![0] as string);
+      expect(written.chat.model).toBe('gpt-4o');
+    });
+
+    test('handles invalid JSON by starting with empty config', async () => {
+      await activate(extensionContextMock);
+      const agent = getRegisteredAgent();
+
+      const configFile = createConfigFile('not valid json');
+      await agent.preWorkspaceStart(createContext([configFile]));
+
+      expect(configFile.updateMock).toHaveBeenCalledOnce();
+      const written = JSON.parse(configFile.updateMock.mock.calls[0]![0] as string);
+      expect(written.chat.model).toBe('gpt-4o');
+    });
+
+    test('does nothing when config file is not in context', async () => {
+      await activate(extensionContextMock);
+      const agent = getRegisteredAgent();
+
+      const updateMock = vi.fn();
+      const otherFile: AgentConfigurationFile = {
+        path: 'some/other/path.json',
+        read: vi.fn(),
+        update: updateMock,
+      };
+
+      await agent.preWorkspaceStart(createContext([otherFile]));
+
+      expect(updateMock).not.toHaveBeenCalled();
+    });
+  });
 });
diff --git a/extensions/copilot/src/extension.ts b/extensions/copilot/src/extension.ts
index 0997ea360f..1d57528f72 100644
--- a/extensions/copilot/src/extension.ts
+++ b/extensions/copilot/src/extension.ts
@@ -16,9 +16,11 @@
  * SPDX-License-Identifier: Apache-2.0
  ***********************************************************************/
 
-import type { ExtensionContext } from '@openkaiden/api';
+import type { AgentWorkspaceContext, ExtensionContext } from '@openkaiden/api';
 import { agents } from '@openkaiden/api';
 
+export const COPILOT_SETTINGS_PATH = '.copilot/settings.json';
+
 export async function activate(extensionContext: ExtensionContext): Promise<void> {
   const disposable = agents.registerAgent({
     id: 'copilot',
@@ -39,13 +41,41 @@ export async function activate(extensionContext: ExtensionContext): Promise<void
     command: 'copilot',
     acp: { args: ['--acp'] },
     tags: [],
-    configurationFiles: [],
     destinationSkillsFolder: '${HOME}/.copilot/skills',
+    configurationFiles: [
+      {
+        path: COPILOT_SETTINGS_PATH,
+        async read(): Promise<string> {
+          return '{}';
+        },
+      },
+    ],
     isSupportedModelType(type): boolean {
       return type.name !== 'vertexai';
     },
-    async preWorkspaceStart(): Promise<void> {
-      throw new Error('not implemented');
+    async preWorkspaceStart(context: AgentWorkspaceContext): Promise<void> {
+      const configFile = context.configurationFiles.find(f => f.path === COPILOT_SETTINGS_PATH);
+      if (!configFile) {
+        return;
+      }
+
+      const content = await configFile.read();
+      let config: Record<string, unknown>;
+      try {
+        const parsed: unknown = JSON.parse(content);
+        config =
+          typeof parsed === 'object' && parsed !== null && !Array.isArray(parsed)
+            ? (parsed as Record<string, unknown>)
+            : {};
+      } catch {
+        config = {};
+      }
+
+      const chat = (config.chat as Record<string, unknown> | undefined) ?? {};
+      chat.model = context.model.model.label;
+      config.chat = chat;
+
+      await configFile.update(JSON.stringify(config, undefined, 2));
     },
   });
   extensionContext.subscriptions.push(disposable);