fix(antigravity): accept supported legacy handshake versions

[juliusmarminge]

Antigravity 1.1.1 echoes protocol version 2 while returning legacy ACP fields. V2 requests 2, but the installer required 1, so the correct pinned runtime failed with a generic identity error.

Accept the known versions 1 and 2 while retaining the identity, release, and capability checks. The existing response-shape negotiation continues to select legacy session messages. Other protocol versions remain rejected; there is no global downgrade.

Verification:

• 52 focused installer and Effect ACP client tests pass. The supplied handshake fixture now covers installation and legacy authentication, MCP definitions, new/load/resume sessions, model selection, elicitation, streamed updates, prompt completion, and logout.
• Protocol 3, wrong identity, and wrong release remain rejected.
• Server and Effect ACP typechecks, targeted lint, and formatting pass.

Before: the version-2 fixture failed installation. After: it passes while prompt completion still uses the legacy RPC result. No credentials, real provider sessions, or runtime archive downloads were used. Authenticated Antigravity chat remains unverified. This is backend behavior, so screenshots do not apply.

Base: t3code/codex-turn-mapping. Bottom of the requested two-PR provider diagnostics stack.

Model: GPT-6. Harness: Codex.

[macroscopeapp]

Approvability

Verdict: Approved at 552dd42

Macroscope's review found this PR approvable — The PR narrowly fixes Antigravity installation validation for a supported legacy handshake and adds focused coverage for accepted and rejected protocol versions. Existing wire-format selection remains unchanged, keeping the runtime risk limited.

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[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: 552dd42 · 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: 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.