- src/web/settings-writer.ts:110: export assertSettingsWritable(), the writer's own read-for-write rule run on its own
- src/web/routes/case-routes.ts:1860: an admin's rename checks settings.json before it ends any session or moves the folder, and answers 409 CONFLICT with the writer's message (placed before the session-starting check, so nothing is awaited between it and the lock)
- test/routes/case-rename-routes.test.ts:314: the torn-file pin now expects 409 CONFLICT (no stoppedSessions, like the other pre-checks)
- test/routes/case-rename-routes.test.ts:331: new test, a live session with stopSessions and a torn file: 409 CONFLICT, cleanupSession not called
- docs/api-reference.md: the Settings paragraph says an admin's rename answers this refusal as 409 CONFLICT before ending sessions
- src/hooks-config.ts:1161, src/prompt-cache-keepwarm.ts:341: comments describe the atomic writer, not an in-place rewrite
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Quick-start saved lastUsedCase with an un-awaited, non-atomic read-modify-write, and the settings PUT, model config, case order, case rename, Ralph start and the boot tunnel reset each ran their own. Two overlapping truncate-then-write calls left one write's bytes over the tail of the other (the CI flake in test/quick-start.test.ts), and most writers then read the torn file as {} and wrote back only its own keys, erasing the rest of the user's settings.
All of them now go through one writer that runs one change at a time, replaces the file atomically, and refuses (409 CONFLICT) to overwrite a file that does not parse.
Codeman maintainer