Fix OpenCode backend invocation and diagnostics

src/backends/external.test.ts

@@ -59,6 +59,25 @@ describe('ExternalCliBackend', () => {
     );
   });
 
+  it('uses inferred run args for opencode backend', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(null, 'ok', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new OpenCodeBackend('/usr/bin/opencode', []);
+    await backend.process({ prompt: 'hello', history: [] });
+
+    expect(mockExecFile).toHaveBeenCalledWith(
+      '/usr/bin/opencode',
+      ['run', '--format', 'default', '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') {
@@ -98,6 +117,19 @@ describe('ExternalCliBackend', () => {
       .rejects.toThrow('returned no output');
   });
 
+  it('includes stdout details when backend process fails', async () => {
+    mockExecFile.mockImplementation((_cmd, _args, _opts, callback) => {
+      if (typeof callback === 'function') {
+        callback(new Error('Command failed: opencode'), 'default agent "code-planner" is a subagent', '');
+      }
+      return {} as never;
+    });
+
+    const backend = new OpenCodeBackend('/usr/bin/opencode', []);
+    await expect(backend.process({ prompt: 'hello', history: [] }))
+      .rejects.toThrow('default agent "code-planner" is a subagent');
+  });
+
   it('constructs default commands for opencode and gemini backends', () => {
     const opencode = new OpenCodeBackend();
     const gemini = new GeminiBackend();

src/backends/external.ts

@@ -37,6 +37,9 @@ function inferArgs(name: ExternalBackendName, prompt: string): string[] {
   if (name === 'claude_code') {
     return ['--print', prompt];
   }
+  if (name === 'opencode') {
+    return ['run', '--format', 'default', prompt];
+  }
   return ['-p', prompt];
 }
 
@@ -99,7 +102,11 @@ function execFileAsync(command: string, args: string[], timeoutMs: number): Prom
   return new Promise((resolve, reject) => {
     execFile(command, args, { timeout: timeoutMs, maxBuffer: 1024 * 1024 }, (error, stdout, stderr) => {
       if (error) {
-        reject(new Error(`${error.message}${stderr ? `\n${stderr}` : ''}`));
+        const details = [stderr, stdout]
+          .map((entry) => entry.trim())
+          .filter((entry) => entry.length > 0)
+          .join('\n');
+        reject(new Error(details ? `${error.message}\n${details}` : error.message));
         return;
       }
       resolve(stdout || '');