1. [OPEN] OpenCode `promptAsync` can resolve before the prompt is durably accepted, then a later `session.error` leaves OMO believing dispatch succeeded while no live parent turn will complete. Distinguishing evidence: OpenCode handler response path returns before durable session acceptance, and OMO currently releases or retains reservations in a way that allows an orphaned dispatch.
2. [OPEN] OMO has at least one raw `session.prompt` or `session.promptAsync` route outside `src/shared/prompt-async-gate.ts`, allowing concurrent idle/error/completion hooks to inject multiple internal prompts or hang one behind another. Distinguishing evidence: raw call sites outside the shared gate or a static invariant test gap.
3. [OPEN] The shared prompt gate does not bound the full dispatch lifecycle correctly when the underlying SDK fetch never resolves, leaving duplicate-injection state or optimistic loop state stuck forever. Distinguishing evidence: a failing test where unresolved `promptAsync` blocks or leaks reservation/task state past the expected timeout path.
4. [OPEN] Sibling `opencode` changed event/session semantics in a way that makes previous OMO idle-settle assumptions too weak. Distinguishing evidence: event/prompt implementation or tests in `../opencode` showing an accepted response can be followed by async prompt rejection/error edge.
## Artifacts To Revert Or Preserve
- [ ] `/Users/yeongyu/local-workspaces/gpt 5.5 xhigh` — temporary worktree. Remove after merged PR with `git worktree remove`.
- [ ] Branch `code-yeongyu/fix-prompt-hang-race` — temporary PR branch. Delete via PR squash merge or `git branch -D` only after safe cleanup.
- [ ] `.debugging` — journal update requested by user. Preserve unless final cleanup requires restoring debug-only notes.
- [ ] `src/tools/call-omo-agent/completion-poller.test.ts` — failing-first regression for 204-without-durable-message prompt acceptance. Preserve as product test.
- Value: loop treats idle with `currentMsgCount === 0` as not complete and only exits at `MAX_POLL_TIME_MS` with `Agent task timed out after 5 minutes.`
- Interpretation: when promptAsync returns 204 but OpenCode fails before persisting the user message, OMO waits for the generic five-minute poll timeout instead of surfacing prompt acceptance failure promptly.
- Confirms: H1; refutes H2 for this hang because production prompt routes are gate-routed.
## Root Cause (confirmed 2026-05-17T13:56:00+09:00)
- Mechanism: OpenCode `session.promptAsync` returns 204 after forking the real prompt, so OMO's sync child prompt path can begin polling before the user message is durably written. If the forked OpenCode prompt fails before `sessions.updateMessage(info)`, status remains idle or absent and `session.messages()` stays empty. `call_omo_agent` then waits for the generic five-minute poll timeout because zero messages never satisfy the stable-message completion condition.
- Evidence: `/Users/yeongyu/local-workspaces/opencode/packages/opencode/src/server/routes/instance/httpapi/handlers/session.ts:295-314`, `/Users/yeongyu/local-workspaces/opencode/packages/opencode/src/session/prompt.ts:1092-1101`, `src/tools/call-omo-agent/completion-poller.ts:25-63`, and red test output in `evidence/task-2-red.txt`.
- Toggle proof: with the pre-fix poller, the red test receives `Agent task timed out after 5 minutes.` With the acceptance-timeout guard, the same simulated OpenCode 204-without-message sequence receives `Prompt was not durably accepted by OpenCode for session ses-undurable.`
- Fix scope: `src/tools/call-omo-agent/completion-poller.ts` and `src/tools/call-omo-agent/completion-poller.test.ts`.
- Command: `bun test src/tools/call-omo-agent/completion-poller.test.ts --bail`
- Output: `Expected substring: "Prompt was not durably accepted by OpenCode"; Received message: "Agent task timed out after 5 minutes."`
### Green phase (2026-05-17T13:55:00+09:00)
- Fix: `src/tools/call-omo-agent/completion-poller.ts` tracks whether any active status was observed and fails after 30s of idle zero-message polling before the generic five-minute timeout.
- Test: `bun test src/tools/call-omo-agent/completion-poller.test.ts --bail` passes with 2 tests.
- Adjacent checks:
- `bun test src/tools/call-omo-agent/sync-executor.test.ts src/tools/call-omo-agent/sync-executor-leak.test.ts src/tools/call-omo-agent/completion-poller.test.ts --bail` passes with 23 tests.
- `bun test src/hooks/shared/prompt-async-gate.test.ts src/shared/prompt-async-route-audit.test.ts --bail` passes with 20 tests.
- Scenario: run the real `waitForCompletion` implementation in a tmux session with an OpenCode-like client that reports idle status and zero messages after promptAsync acceptance.
- Command: `tmux new-session -d -s ulw-qa-call-omo-agent ... bun /tmp/ulw-call-omo-agent-qa.ts`
- Observed output: `Prompt was not durably accepted by OpenCode for session ses_qa.`
- Result: fail with 99 failures caused by Bun/test harness dynamic imports from an encoded path such as `/Users/yeongyu/local-workspaces/gpt%205.5%20xhigh/...`.
- Interpretation: path-space-specific validation failure, not a product regression from this patch. Single-file reproduction: `bun test src/shared/load-opencode-plugins.test.ts --bail` fails before test logic with `Cannot find module '/Users/yeongyu/local-workspaces/gpt%205.5%20xhigh/src/shared/load-opencode-plugins.ts?...'`.
- Full suite in no-space validation worktree with the same patch:
- Product behavior change: only `call_omo_agent` sync polling now fails fast when OpenCode never durably accepts a prompt after returning from `promptAsync`.
- Behavior preserved:
- Durable message completion still requires stable idle message count.
- Busy/non-idle child sessions continue polling as before.
- Existing shared `promptAsync` gate and raw-prompt audit are unchanged and green.
