fix(sessions): stop co-located sessions from sharing one preview/topic

[muqsitnawaz]

Bug

agents sessions --active showed the identical preview + topic for every session in one cwd — 6 editor tabs, or two worktree siblings, all rendered the same activity and looked like duplicate cards.

Root cause (debugged + verified by 2 independent reviewers)

findClaudeSessionFile fell through to the newest .jsonl in the cwd whenever a session's <id>.jsonl wasn't found, so N distinct sessions collapsed onto ONE file — and preview/topic derive solely from that file. The id goes stale because an editor caches the launch uuid in live-terminals.json, but Claude rotates its transcript uuid on resume/compact, so the cached id no longer matches any file.

Fix

• pickSessionFile(dir, id?): a concrete id returns its own file or undefined, never a sibling's; the newest-file fallback is gated to the no-id case.
• listTerminalsActive resolves each tab's exact id from the pid registry first (mirroring the headless path).
• classifyActivity: no resolvable file ⇒ idle, not running.
• Regression test active.pickfile.test.ts (real temp files, no mocks).

Verification

Live fleet: 6 codium tabs went from 6× identical preview → 6× distinct (blank, idle). tsc clean; active + pickfile tests green (7 pass).

Known limitation / follow-up: previews are now blank (not each tab's real content) because the pid registry doesn't cover editor-launched pids and the cached ids are stale. Showing the real per-tab preview needs the upstream fix (editor registers the rotated uuid, or the pid registry covers those pids).

Note: opened during a merge-freeze for the #724 restructure; hold merge until all-clear + rebase onto the new apps/cli/ layout.

[prix-cloud]

Code Reviewer

Verdict: Ready to merge — but hold per author's own instruction (rebase onto apps/cli/ from #724 first)

Build: tsc — exit 0, no errors. dist/lib/session/active.js freshly emitted.
Tests: 407 run — 399 pass, 8 fail. All 8 failures are pre-existing: 3 in relative-time.test.ts (vi.setSystemTime is a vitest API, bun doesn't have it) and 5 in __tests__/db.test.ts (short_id NOT NULL + home-dir path assertion). Neither file was touched by this PR (last commits: 7d3a8e6, 3c5dce9). New pickfile test: 5/5 pass. Existing active.test.ts: 2/2 pass.

Instructions read: /workspace/task-05788a68/AGENTS.md (root). No per-directory AGENTS.md or CLAUDE.md under src/lib/session/.

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Changes that work well

The extraction of pickSessionFile is clean and the exported symbol makes it directly testable. The fix is minimal and surgical — three distinct logical changes, each targeting one of the three places the bug manifested, and the comment on each explains exactly what the pre-fix behavior was. The test file covers the four cases that matter:

1. concrete id → own file
2. concrete id absent → undefined, not a sibling (the bug)
3. two absent ids → both undefined (no collapse)
4. absent id → newest-file fallback preserved

Real temp files, no mocks — consistent with the repo's "real services only" testing rule.

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One thing to verify

sessionId vs sessionFile can diverge when the pid registry resolves a rotated uuid.

In listTerminalsActive (active.ts:450–464):

const resolvedId = readPidSessionEntry(t.pid)?.sessionId ?? t.sessionId;
const sessionFile = findSessionFileForKind(t.kind, t.cwd ?? undefined, resolvedId);
// ...
sessionId: t.sessionId ?? sessionIdFromFile(sessionFile),   // ← still t.sessionId

When the pid registry has a fresh (post-rotate) uuid and findSessionFileForKind returns its .jsonl, the ActiveSession record carries sessionId = t.sessionId (the old stale uuid) while sessionFile points to the new uuid's file. The mismatch means labelMap.get(t.sessionId) may miss a /rename label that was set under the new uuid.

This is a pre-existing pattern (the headless path has the same shape), and the PR description already acknowledges it as a known limitation — upstream fix is the editor registering the rotated uuid. Not a blocker; noting it so it lands in the follow-up ticket.

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Merge hold

Per the PR description: hold merge until #724 restructure all-clear + rebase onto the new apps/cli/ layout. This is the author's own note, not a review finding — surfacing it here so it isn't missed in the merge queue.

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Reviewed by Code Reviewer — actually ran the build and tests on this branch.

[muqsitnawaz]

Superseded by #728 — the identical fix rebuilt on the post-#724 apps/cli/ layout (this branch predates the restructure and now conflicts). Landing #728 instead.