fix(desktop): prevent a second server from breaking active threads

[t3dotgg]

Opening desktop while the background server is running can start a second server on port 3774 against the same database. The second server marks running threads as lost and cannot resume their Codex sessions because the original server still owns them. Follow-up messages then fail with already has an active writer.

Desktop now refuses startup when its default port is occupied instead of scanning for another port. The error explains how to use the running server or a separate T3 home. Explicit port settings retain their current behavior.

This brings #9003 onto current main after confirming the exact failure in a live thread. Credit to @maxibotstef for the original fix. This covers the automatic desktop fallback path. General server directory locking is separate.

Validation: all 5 focused tests passed, desktop typecheck passed, and targeted lint and format checks passed. No browser or desktop UI was launched.

Created with GPT-6 Astra in Codex.

Closes #6097

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This changes packaged desktop startup’s default port-conflict behavior: the app now refuses to start rather than selecting a fallback port, to avoid competing with an existing server. The change is narrowly scoped and tested, but the altered default startup behavior warrants human review.

Not approved because:

• Monthly spending limit reached (workspace setting). Approvability relies on correctness review in order to determine eligibility

^{Review your spending limits in Billing settings. You can add or adjust custom eligibility rules. Learn more.}

[t3dotgg]

Note

🤖 GPT-6 Astra responding on behalf of Theo

@cursor review

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.6 KiB	+53 B (+0.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	0 B (0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+53 B (+0.8%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.0 KiB	57.1 KiB	+88 B (+0.2%)	66.4 KiB	✅
Codex	Live turn messages	8	10	+2 (+25.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+16 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	+3 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+13 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.9 KiB	+44 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	9	10	+1 (+11.1%)	21	✅

Baseline: 223ff44 · PR result: 4a3bda1 · Source CI: success

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: 113.9 KiB
• Claude decoded thread snapshot: 114.5 KiB

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

[shivamhwp]

Note: GPT-6 on behalf of shivam (@shivamhwp).

Please correct the startup error's recovery guidance. It suggests using a different T3CODE_HOME, but port selection still refuses startup whenever 3773 is occupied and no explicit port is configured, regardless of the data directory. Changing only T3 home therefore repeats the same error. For a separate environment, the guidance needs both a separate home and an explicit free T3CODE_PORT; changing only the port must not be presented as safe for an already-owned home.

[tomasmach]

Me on behalf of me:

Please merge this, been hitting this error pretty often, thanks!