fix(persona-kit): spawn opencode through a shell exec (#330)

packages/persona-kit/src/interactive-spec.test.ts

@@ -324,7 +324,7 @@ test('opencode configFiles carries a well-formed opencode.json with the agent de
   });
 });
 
-test('opencode non-interactive spec omits cwd/model flags and normalizes the agent model', () => {
+test('opencode non-interactive spec execs through a shell, omits cwd/model flags, and normalizes the agent model', () => {
   const result = buildNonInteractiveSpec({
     harness: 'opencode',
     personaId: 'daily-ship',
@@ -335,8 +335,16 @@ test('opencode non-interactive spec omits cwd/model flags and normalizes the age
     workingDirectory: '/tmp/project'
   });
 
-  assert.equal(result.bin, 'opencode');
+  // workforce#330: opencode is spawned through `sh -c 'exec "$0" "$@"'`.
+  // Spawned directly by the runtime it fails in ~1.2s with an opaque
+  // `UnknownError: Unexpected server error`; with one shell in between the
+  // identical invocation runs normally. `exec` means the shell is replaced, so
+  // the caller still waits on the real opencode process.
+  // Windows has no /bin/sh, so the wrapper is POSIX-only (see the spec builder).
+  const expectShell = process.platform !== 'win32';
+  assert.equal(result.bin, expectShell ? '/bin/sh' : 'opencode');
   assert.deepEqual(result.args, [
+    ...(expectShell ? ['-c', 'exec "$0" "$@"', 'opencode'] : []),
     'run',
     '--agent',
     'daily-ship',

packages/persona-kit/src/interactive-spec.ts

@@ -738,9 +738,42 @@ export function buildNonInteractiveSpec(
       // separately, and `opencode run` does not support `--dir`.
       const args = ['run', ...interactive.args, '--format', 'default'];
       if (input.name) args.push('--title', input.name);
+      // Spawn opencode through `sh -c 'exec "$0" "$@"'` rather than exec'ing the
+      // binary directly (workforce#330). Spawned directly by the runtime,
+      // opencode fails in ~1.2s with an opaque
+      //   Error: {"name":"UnknownError","data":{"message":"Unexpected server
+      //           error. Check server logs for details.","ref":"..."}}
+      // and its own log file is created but stays 0 bytes. With one shell
+      // between Node and the binary — same binary, args, cwd, env, stdin,
+      // model and credential — it runs normally. Verified end to end on
+      // daily-ship: opaque failure without, real digests posted with.
+      //
+      // Ruled out by direct experiment in a live sandbox, so this is narrower
+      // than "wrap it and hope": PATH; prompt size and content (119KB synthetic
+      // and a real 72KB prompt both fine); provider flakiness (6/6 clean);
+      // NODE_OPTIONS; HOME / XDG_DATA_HOME; USER / LOGNAME; cwd; FD limits down
+      // to `ulimit -n 64`; a stdin write race (delayed stdin is read fine);
+      // inherited SIGPIPE disposition (deliberately re-ignoring it still
+      // succeeded); and the process group (`timeout`'s setpgid — bash exec with
+      // no timeout also succeeded). The shell alone is what matters.
+      //
+      // `exec` keeps the process count unchanged, so the caller still waits on
+      // the real opencode process and its exit code is reported verbatim. Only
+      // the opencode branch is routed this way; claude / codex / grok spawn
+      // directly and are unaffected.
+      //
+      // Windows has no `/bin/sh`, and `cmd.exe` has no exec-replacement
+      // semantics — wrapping there would ADD a process rather than replace one,
+      // breaking exit-code and signal passthrough. The failure this works around
+      // is observed in the Linux cloud sandboxes, so win32 keeps the direct
+      // spawn. Same platform split the sandbox exec path already uses
+      // (runtime/src/cloud-defaults.ts).
+      const execThroughShell = process.platform !== 'win32';
       return {
-        bin: interactive.bin,
-        args,
+        bin: execThroughShell ? '/bin/sh' : interactive.bin,
+        args: execThroughShell
+          ? ['-c', 'exec "$0" "$@"', interactive.bin, ...args]
+          : args,
         prompt: { mode: 'stdin', contents: input.task },
         configFiles: interactive.configFiles,
         warnings: interactive.warnings