fix(chat): link agent attribution to its source thread

[juliusmarminge]

Agent-created threads show "Sent by another agent" as plain text, so users cannot return to the thread that sent the prompt.

Store an optional sending thread ID on agent messages and expose the attribution as a source-thread button on web, desktop, and mobile. Track the source for MCP thread creation and sends, delegated tasks, and native subagent prompts, and preserve it through queueing and steering. Older messages without a sender ID retain their existing attribution.

Validation: 262 focused tests passed after rebasing, including native subagent coverage. Server, web, contracts, and mobile typechecks passed. The built-in Browser panel verified navigation to the source in the current environment using disposable synthetic data. Lint and React Doctor reported existing issues outside the additions. Native mobile UI was not exercised.

Final head: 22442fb0e27, based on t3code/codex-turn-mapping at 72a0226be4a. Both UI review threads are resolved. Macroscope declined automatic approval because the source metadata spans runtime and multiple clients; human review is needed.

CI blocker: Test Server 2 fails in SelectionRestart.integration.test.ts, “restarts selection as a new attempt and retries after old-session cleanup.” The identical failure reproduces on untouched base revision 72a0226be4a, with baseline server code and contracts. The final CI run has the same sole test failure; the other executed validation jobs passed. This PR does not change that retry behavior. After the review fix, all 66 timeline tests, web typecheck, and targeted lint passed; source navigation was verified again.

Before, actual base revision. The attribution is plain text.

After, PR head with the same fixture, viewport, and scroll. The attribution is a button; its appearance changes on hover or keyboard focus.

Recording: open the sending thread

Model: GPT-6. Harness: Codex.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.4 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	20.8 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	2	—	8	✅

Baseline: unavailable · PR result: 22442fb · Source CI: failure

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 106.1 KiB
• Claude decoded thread snapshot: 106.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This change adds source-thread attribution as a new interactive capability and propagates new metadata across MCP, delegation, native subagents, queueing, provider adapters, web, and mobile. The additive contract shape is backward compatible, but the cross-cutting runtime and user-interface impact exceeds a narrowly self-contained fix.

^{You can add or adjust custom eligibility rules. Learn more.}

Dismissing prior approval to re-evaluate 783c352

Dismissing prior approval to re-evaluate 22442fb