## Summary
Adds `docs/CLAUDE-AUTH-CONCURRENCY.md` documenting why the external `flock` on `${HOME}/.claude/session.lock` in `lib/agent-sdk.sh` is load-bearing rather than belt-and-suspenders, and provides a decision matrix for adding new containers that run Claude Code.
Pure docs change. No code touched.
## Why
The factory runs N+1 concurrent Claude Code processes across containers (`disinto-agents` plus every transient container spawned by `docker/edge/dispatcher.sh`), all sharing `~/.claude` via bind mount. The historical "agents losing auth, frequent re-logins" issue that motivated the original `session.lock` flock is the OAuth refresh race — and the flock is the only thing currently protecting against it.
A reasonable assumption when looking at Claude Code is that its internal `proper-lockfile.lock(claudeDir)` (in `src/utils/auth.ts:1491` of the leaked TS source) handles the refresh race, making the external flock redundant. **It does not**, in our specific bind-mount layout. Empirically verified:
- `proper-lockfile` defaults to `<target>.lock` as a sibling file when no `lockfilePath` is given
- For `claudeDir = /home/agent/.claude`, the lock lands at `/home/agent/.claude.lock`
- `/home/agent/` is **not** bind-mounted in our setup — it is the container's local overlay filesystem
- Each container creates its own private `.claude.lock`, none shared
- Cross-container OAuth refresh race is therefore unprotected by Claude Code's internal lock
The external flock works because the lock file path `${HOME}/.claude/session.lock` is **inside** the bind-mounted directory, so all containers see the same inode.
This came up during design discussion of the chat container in #623, where the temptation was to mount the existing `~/.claude` and skip the external flock for interactive responsiveness. The doc captures the analysis so future implementers don't take that shortcut.
## Changes
- New file: `docs/CLAUDE-AUTH-CONCURRENCY.md` (~135 lines): rationale, empirical evidence, decision matrix for new containers, pointer to the upstream fix
- `lib/AGENTS.md`: one-line **Concurrency** addendum to the `lib/agent-sdk.sh` row pointing at the new doc
## Test plan
- [ ] Markdown renders correctly in Forgejo
- [ ] Relative link from `lib/AGENTS.md` to `docs/CLAUDE-AUTH-CONCURRENCY.md` resolves (`../docs/CLAUDE-AUTH-CONCURRENCY.md`)
- [ ] Code references in the doc still match the current state of `lib/agent-sdk.sh:139,144` and `docker/agents/entrypoint.sh:119-125`
## Refs
- #623 — chat container, the issue this analysis was driven by; #623 has a comment with the same analysis pointing back here once merged
Co-authored-by: Claude <noreply@anthropic.com>
Reviewed-on: #637
Co-authored-by: dev-bot <dev-bot@disinto.local>
Co-committed-by: dev-bot <dev-bot@disinto.local>
