diff --git a/docs/plans/state.json b/docs/plans/state.json
index d8ebe16..8ba6887 100644
--- a/docs/plans/state.json
+++ b/docs/plans/state.json
@@ -3684,6 +3684,17 @@
         "docs/plans/state.json"
       ],
       "test_status": "pnpm test:run src/daemon/routing.test.ts + pnpm typecheck passing"
+    },
+    "external-backend-runner-test-coverage": {
+      "status": "completed",
+      "date": "2026-02-17",
+      "updated": "2026-02-17",
+      "summary": "Added unit coverage for external CLI backend execution contract (argument inference, `{prompt}` substitution, empty-output failure path, and backend constructor defaults) across codex/claude_code/opencode/gemini adapters.",
+      "files_modified": [
+        "src/backends/external.test.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/backends/external.test.ts + pnpm typecheck passing"
     }
   },
   "overall_progress": {
diff --git a/src/backends/external.test.ts b/src/backends/external.test.ts
new file mode 100644
index 0000000..f46897c
--- /dev/null
+++ b/src/backends/external.test.ts
@@ -0,0 +1,107 @@
+import { describe, expect, it, vi, beforeEach } from 'vitest';
+import { execFile } from 'child_process';
+
+import {
+  ExternalCliBackend,
+  ClaudeCodeBackend,
+  OpenCodeBackend,
+  CodexBackend,
+  GeminiBackend,
+} from './external.js';
+
+vi.mock('child_process', () => ({
+  execFile: vi.fn(),
+}));
+
+const mockExecFile = vi.mocked(execFile);
+
+describe('ExternalCliBackend', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('uses inferred -p args for codex backend', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(null, 'ok', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new CodexBackend('/usr/bin/codex', ['run']);
+    const result = await backend.process({ prompt: 'hello', history: [] });
+
+    expect(result).toBe('ok');
+    expect(mockExecFile).toHaveBeenCalledWith(
+      '/usr/bin/codex',
+      ['run', '-p', 'USER: hello'],
+      expect.any(Object),
+      expect.any(Function),
+    );
+  });
+
+  it('uses inferred --print args for claude_code backend', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(null, 'ok', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new ClaudeCodeBackend('/usr/bin/claude', []);
+    await backend.process({ prompt: 'hello', history: [] });
+
+    expect(mockExecFile).toHaveBeenCalledWith(
+      '/usr/bin/claude',
+      ['--print', 'USER: hello'],
+      expect.any(Object),
+      expect.any(Function),
+    );
+  });
+
+  it('supports {prompt} substitution in configured args', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(null, 'ok', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new ExternalCliBackend({
+      name: 'opencode',
+      command: 'opencode',
+      args: ['--message', '{prompt}'],
+    });
+    await backend.process({
+      prompt: 'new question',
+      history: [{ role: 'assistant', content: 'previous reply' }],
+    });
+
+    expect(mockExecFile).toHaveBeenCalledWith(
+      'opencode',
+      ['--message', 'ASSISTANT: previous reply\n\nUSER: new question'],
+      expect.any(Object),
+      expect.any(Function),
+    );
+  });
+
+  it('throws when backend returns no output', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(null, '   ', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new GeminiBackend('/usr/bin/gemini', []);
+    await expect(backend.process({ prompt: 'hello', history: [] }))
+      .rejects.toThrow('returned no output');
+  });
+
+  it('constructs default commands for opencode and gemini backends', () => {
+    const opencode = new OpenCodeBackend();
+    const gemini = new GeminiBackend();
+    expect(opencode.name).toBe('opencode');
+    expect(gemini.name).toBe('gemini');
+  });
+});