fix(server): ACP mode picks reach agents that name their mode option differently

[juliusmarminge]

A user's pick from an ACP Registry agent's own mode picker could silently fail to apply. runtime.setMode always sent session/set_config_option with the id mode. An agent whose mode option has a different id (for example permission-mode with category: "mode") got a request for an option it doesn't have. An ACP v1 agent that advertises only modes (gemini-cli) got a config-option request it doesn't handle.

This is the agent-agnostic part of #13629 that survives its spec-only reshape (see "Replaces" below).

What changed

• AcpSessionRuntime.setMode follows the ACP spec:
  • If the session has a category: "mode" config option, it sends session/set_config_option under that option's own id. It then takes the current mode from the agent's answer instead of assuming the switch happened.
  • If the session advertises only modes (ACP v1), it sends session/set_mode.
• effect-acp gains agent.setSessionMode. It is v1-only like setSessionModel, and on a v2 session it fails with method-not-found. ACP v2 removed session/set_mode.
• Nothing per-agent. The agent's mode picker stays visible, registry agents keep T3 answering session/request_permission by the thread's mode, and nothing new reads the runtime mode.

Replaces #13629

With the per-agent parts removed (the mode table, the per-agent sessionModeForPolicy, the goose set_config_option → set_mode fallback, omitModes, and enforcement: "native" for registry agents), this transport fix is all that's left of #13629. I propose closing #13629 in favour of this PR. Its open High finding, about claiming native enforcement when the live session lacks the mapped mode, goes away with the native claim.

Verification

In apps/server unless noted, with TMPDIR under /home:

• AcpRegistryAdapterV2.test.ts > the agent's own mode picker drives the real registry adapter against scripted ACP v1 agents (acp-replay-agent.ts), and each script must be consumed exactly:
  • switches an agent that only advertises modes with session/set_mode: a stored autoEdit pick sends session/set_mode.
  • switches a mode config option under its own id: a stored auto pick sends session/set_config_option with configId: "permission-mode".
  • With the previous setMode, both fail.
• vp test run on AcpRegistryAdapterV2, AcpAdapterV2, AntigravityAdapterV2, GrokAdapterV2, AntigravityAcpSupport, AcpSessionConfig, AcpRegistryProbe, AntigravityTextGeneration: 229 passed. Antigravity's mode switching goes through the same setMode.
• OrchestratorReplayFixtures -t "grok|antigravity|acpRegistry|registry": 21 passed. No ACP fixture switches modes.
• packages/effect-acp: vp test run src/client.test.ts 29/29. The v1 flow sends session/set_mode, and a v2 session gets -32601.
• tsc --noEmit for apps/server and packages/effect-acp: clean. vp run knip:check: clean. vp lint on touched files: only the pre-existing inline-schema warning in effect-acp/src/client.ts.
• Not run: live registry agents.

Model: Claude Opus 5.5 (Claude Code)

🤖 Generated with Claude Code

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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	—	20.7 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	1	—	8	✅

Baseline: unavailable · PR result: 95d2f85 · 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.

[macroscopeapp]

Approvability

Verdict: Approved at 95d2f85

Macroscope's review found this PR approvable — This is a contained ACP compatibility fix that corrects mode selection for agents using alternate config IDs or ACP v1 mode transport. The new transport path is protocol-gated, existing defaults and unrelated paths remain unchanged, and both cases are covered by focused tests.

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