## Symptom
Users reported Planning Mode regularly failing with **"AI returned no
valid JSON.. Retry this planning session or start a new one."**, and
that leaving and returning to the interface mid-generation **duplicates
the generation — infinitely**. Every app-tab switch unmounts the
Planning view, so the leave/return path is the *normal* path, not an
edge case.
## Root causes
1. **Check-then-act turn admission.** `activeGenerations.has()` was
checked at turn entry, but the record was only created inside
`runGenerationWithTimeout`, after several awaits. Overlapping turn
entries (remounted view re-submitting, racing auto-retries, duplicate
start of an existing session) both passed the guard; the second
displaced the first, and the displaced teardown disposed the
**session-shared agent the surviving turn was actively prompting** —
which then read an empty assistant message and failed parse with "AI
returned no valid JSON".
2. **Per-mount auto-retry budget.** The client reset its 3-attempt
auto-retry budget on every mount, so each return to an errored session
re-ran a full-turn regeneration (agent rebuild + complete history
replay) — forever.
3. **SSE replay appended onto existing output.** Fresh stream
connections replay buffered thinking; the client pre-seeded from
persisted `thinkingOutput` (or kept prior output on silent reconnect)
and then appended the replay — visibly doubling the generation on every
reconnect. The 100-event buffer only held a suffix of a turn, forcing
that pre-seed.
4. **Raw `session.prompt()` at context limits.** A long interview that
overflowed the model's context window errored terminally, and auto-retry
replayed the full history into a fresh agent — overflowing again,
unrecoverably.
## Fix
- Synchronous per-session **turn reservation** shared by
`submitResponse`, `retrySession`, `startExistingSession`, and the
initial turn; losers get `GenerationInProgressError` instead of
displacing the winner. Duplicate starts of a generating session are
no-ops. Rewind aborts an active generation through its own teardown
first.
- Client auto-retry budget is **module-scoped per session** (survives
remounts); exhausted budget shows the error view instead of a stuck
spinner. Retry rejections for "already in progress" rejoin the live run.
- Fresh SSE connections **clear streamed output before the buffered
replay**; buffer deepened to a full turn (2000 events); rejoin paths
reconnect cleanly instead of seeding persisted thinking.
- All six planning prompt sites route through the engine's
**`promptWithFallback`**, recovering context-window overflows via
prompt/memory compaction and `session.compact()`.
- Cosmetic: no more doubled period in the retryable parse error message.
## Symptom Verification
- **Original symptom:** "AI returned no valid JSON" after answering
questions; generations duplicating on leave/return.
- **Exact reproduction:** concurrent turn entries on one session
(submit×2, retry×2, start-while-generating) — previously
displaced/disposed the live agent mid-prompt.
- **Assertion it is gone:** `planning-turn-admission.test.ts` asserts
exactly one turn is admitted per race, the winner completes with a
question and no session error, and the shared agent is never disposed;
`planning-context-compaction.test.ts` asserts every planning prompt
routes through `promptWithFallback` (signal forwarded) and that a
recovered context overflow leaves the turn healthy.
## Verification
- `vitest run` on all 7 planning server test files + the 2 new
regression files: **40/40 pass**.
- `routes-planning*` failures at main tip are pre-existing (identical
103/126 + 3/6 counts with and without this diff; main is mid-refactor on
route wiring). `planning-answered-question-reemit` 3 timeouts also
reproduce on clean main.
- `pnpm verify:fast` green; dashboard `tsconfig.json` +
`tsconfig.app.json` typechecks clean; eslint clean on touched files.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Prevented Planning Mode from duplicating generations and triggering
“AI returned no valid JSON” errors when leaving and re-entering mid-run.
* Made planning turn handling concurrency-safe and idempotent across
submit, retry, rewind, and duplicate start actions.
* Improved SSE reconnect recovery: clearer replay after reconnect, no
duplicated “thinking” output, and preserved auto-retry limits across
remounts.
* Improved long-context recovery via fallback prompting and cleaned up
retry error formatting.
* **Tests**
* Expanded coverage for concurrent Planning actions, reconnect replay,
context compaction, rewind behavior, and retry formatting; improved
parallel test-harness reliability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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