test: lock the proactive backend-id guarantee (never-again)

[TraderSamwise]

Goal: never again

The durable answer to "an agent loses its backend id and can't be restored" is proactive assignment: aimux launches claude with its own --session-id (config.ts:148) and persists it the instant the session spawns (session-launch.ts:530 → :545 saveState → topology, fsync-durable via W1). The id is never discovered after the fact — it's known from the start.

That already works (empirically, almost every project shows 1/1 claude sessions with a backend id). The stranded tealstreet-next sessions were legacy, spawned before the --session-id flag existed.

The gap was not behavior — it was the lack of a lock. The reactive hook path silently rotted in 298d5a1 precisely because no test guarded it. This PR guards the proactive path the same way, at the two points it could regress:

• config — the default claude tool ships sessionIdFlag: ["--session-id", "{sessionId}"] with backend-resume enabled. Drop it and the failure mode returns.
• session-launch — a fresh claude launch (given that flag, as the real spawn path passes it) generates a uuid backend id, passes it as --session-id <uuid>, and persists via saveState.

Together with the W0 reactive-capture test, both capture paths are now test-locked. If either regresses, CI fails instead of an agent silently becoming unrecoverable after the next crash.

Verification

Tests-only — no runtime change. yarn verify green: typecheck + lint clean, 1029/1029 tests.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Tests
  • Improved test coverage for backend session management and persistence verification
  • Enhanced configuration testing to verify session id assignment and state handling

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Project	Deployment	Actions	Updated (UTC)
app	Ready	Preview, Comment	Jun 7, 2026 3:57am

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7f11ace3-ee5d-4f8b-8280-741091ea59dc

📥 Commits

Reviewing files that changed from the base of the PR and between 45babda and c4408f5.

📒 Files selected for processing (2)

• src/config.test.ts
• src/multiplexer/session-launch.test.ts

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📝 Walkthrough

Walkthrough

This PR adds test coverage for backend session ID handling across two test suites. The first test verifies that the default Claude configuration properly initializes a session ID flag and resume arguments. The second test confirms that session creation generates a unique ID, binds it into the command, and persists the state.

Changes

Backend Session ID Test Coverage

Layer / File(s)	Summary
Config session ID initialization test src/config.test.ts	Verifies that the default Claude config includes a --session-id flag with {sessionId} placeholder, resume arguments reference the session ID, and resumeByBackendSessionId is enabled.
Session launch and persistence test src/multiplexer/session-launch.test.ts	Confirms that createSession generates a UUID-pattern backend session ID, injects it into the spawned command as a --session-id argument, and persists state through host.saveState.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 A session blooms with IDs bright,
Each test a checkpoint, burning light,
Config and launch dance hand-in-hand,
State saved safe across the land!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main objective: adding tests to lock the proactive backend session ID guarantee to prevent regressions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/lock-proactive-backend-id-capture

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