[Bug]: Antigravity adapter claims manual compaction support although ACP 1.3.0 treats /compact as model input #22205

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server — Antigravity adapter's manual-compaction capability.

Summary

Antigravity's CLI has /compact, but the Antigravity ACP 1.3.0 runtime used by T3 does not handle it as a native compaction command. T3's Antigravity adapter nevertheless sets supportsCompaction: true.

The missing Compact context button is consistent with the runtime's advertised capabilities. Adding compact to the command catalog would expose an action that does not perform native compaction. The bug is the adapter's unsupported compaction capability and the resulting possibility of reporting successful compaction for an ordinary model response.

Steps to reproduce

The runtime behavior was verified directly over ACP in a disposable workspace and profile, using a copied credential file and no existing conversations:

1. Start the managed Antigravity ACP 1.3.0 executable.
2. Send initialize, authenticate with oauth-personal, and session/new with an empty MCP server list.
3. Send a session/prompt containing synthetic records and a marker to remember; wait for completion and record usage_update.
4. Send a standalone /compact text block through session/prompt.
5. Inspect the response and subsequent usage updates.

Expected behavior

T3 should expose manual compaction only when its provider runtime can perform it. Unsupported compaction requests should fail clearly, without being sent to the model or recorded as successful compaction. The context-meter action should remain unavailable until Antigravity exposes a working manual-compaction path through ACP.

Actual behavior and runtime evidence

In the isolated ACP test:

• Agent version: 1.3.0.
• Advertised commands: plan and logout.
• Initial prompt: assistant replied READY; context usage was 15,393 / 1,048,576 tokens.
• /compact: assistant generated an ordinary summary of the synthetic records; context usage increased to 15,697 tokens. No compaction update was emitted.
• Follow-up asking for the marker succeeded; context usage increased again to 15,881 tokens.

Inspection of the Python source bundled in the installed ACP runtime confirms that its command parser recognizes the advertised built-in plan command, with logout handled separately. /compact is passed through as ordinary model input. No manual-compaction handler was found in that source.

T3 source diagnosis

• AntigravityAdapterV2 sets supportsCompaction: true.
• The shared ACP adapter consequently exposes compactThread, forwarding /compact as a prompt.
• Its turn-finalization path emits a completed Context compacted item when that prompt completes successfully and the flavor enables compaction. Ordinary prompt completion does not prove native compaction occurred.
• The UI visibility check requires compact in the advertised command catalog, which correctly keeps the button hidden for this runtime.

The false-success path above is traced from T3 source; the live reproduction exercised the native ACP runtime directly, not the complete T3 UI path.

Suggested fix

Disable Antigravity's unsupported manual-compaction capability and add regression coverage at the provider-session boundary. Document the ACP limitation. Do not inject a synthetic compact catalog entry unless a working native compaction path is implemented and verified.

Impact

Minor bug or occasional failure — T3 can treat an ordinary assistant summary as successful context compaction even though context usage has not decreased.

Version or commit

Source investigated: main at 069e63b11758. Installed T3 Code version: 0.1.1-nightly.20261020.3386.

Environment

• Antigravity ACP: 1.3.0
• OS: macOS 27.0.1

Related issues

• [Bug]: Gemini models through Antigravity do not report context window usage #15291 concerns missing Antigravity context usage and is closed.
• [Bug]: /compact on ACP registry agents (e.g. Devin) fails at turn start — adapter never sets supportsCompaction #16663 concerns ACP registry agents advertising native compaction while their adapter cannot dispatch it; this is a different capability mismatch.

Workaround

Start a new thread when a fresh context is needed. Do not rely on /compact sent through Antigravity ACP 1.3.0 to reduce context usage.