fix(acp): restore empty-valued default agent choices

[amv146]

Problem

GitHub Copilot advertises its default agent with an empty ACP option value, which T3 drops from the picker. With custom agents installed, the OpenCode-only plan filter also runs on Copilot and can select the first custom agent, leaving no way to switch back.

Fixes #16898.

Change

• Represent exact empty choices and their current selections as __acp_default__, escaping native values that begin with the same prefix to avoid collisions. Whitespace-only, padded, and overlong values remain excluded.
• Resolve encoded selections against the agent's advertised native values before ACP dispatch. Shared contracts and the wire protocol stay unchanged.
• Restrict the plan-agent filter to the OpenCode driver so other providers retain their advertised choices and current selection.

Existing saved custom-agent selections are not reset automatically; users can select the provider's default once after updating.

Scope and approval

Implements the sentinel mapping and OpenCode-only filtering direction confirmed in the maintainer triage comment. Both changes address the missing/defaulted ACP agent selection described there. No pairing/authentication changes are included.

Verification

Focused automated checks

Run from the worktree root on Windows with Node 22.22.0 using pnpm exec vp, with the inherited MCP environment left intact:

pnpm exec vp test run apps\server\src\provider\acp\AcpSessionConfig.test.ts apps\web\src\providerModels.test.ts apps\web\src\components\chat\composerProviderState.test.tsx apps\server\src\orchestration-v2\Adapters\AcpAdapterV2.test.ts apps\server\src\orchestration-v2\Adapters\AcpRegistryAdapterV2.test.ts -t "acpProviderOptionDescriptors|getProviderModelCapabilities|getComposerProviderState|decodes ACP sentinels|the agent's own mode picker"

Passed: 44 tests across 5 files; 122 unrelated cases excluded by the name filter. Coverage includes empty defaults surviving the existing contracts, invalid choices remaining excluded, native sentinel-like selections retaining their meaning, the 256-character boundary, synthetic session modes, ACP choices remaining unchanged when plan mode is off, and existing OpenCode filtering. The mock-agent integration starts with a saved literal selection, switches through the empty default, custom and sentinel-like values, and returns to the default while asserting native ACP request values and updated labels. Existing registry mode-selection cases also pass.

pnpm exec vp run --filter t3 --filter '@t3tools/web' typecheck

Passed for server and web; existing Effect suggestions remain.

Formatting, lint, and whitespace checks for the five locally modified files:

$files = @(git diff --name-only --diff-filter=ACM | ForEach-Object { $_.Replace('/', '\') })
pnpm exec vp fmt --check @files
pnpm exec vp lint @files
git diff --check

Passed. Lint reported three existing warnings in AcpAdapterV2.test.ts: two unused bindings and an array-find preference. No repository-wide checks were run.

Default agent showing up after my change

Native desktop-shell behavior, mobile, and remote/tunnel modes were not tested. The legacy-value collision cases are covered by the focused automated tests, not by a real provider advertising those artificial values.

Model: GPT-6 Astra (gpt-6-astra). Harness: T3 Code acpRegistry (GitHub Copilot).

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This focused ACP fix changes the agent choices and current values presented and dispatched as provider defaults, including behavior for non-OpenCode providers. The implementation is narrowly scoped and tested, but the default-selection change warrants human review.

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[coderabbitai]

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📒 Files selected for processing (2)

• apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
• apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts

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📝 Walkthrough

Walkthrough

ACP option descriptors now preserve empty and sentinel-prefixed values. Session configuration maps encoded selections to advertised native values. Plan-agent filtering applies only to opencode.

Changes

ACP Option Handling

Layer / File(s)	Summary
Encode ACP option values apps/server/src/provider/acp/AcpSessionConfig.ts, apps/server/src/provider/acp/AcpSessionConfig.test.ts	Empty choices and current values use an encoded sentinel. Sentinel-prefixed native values remain distinct. Tests cover empty values, sentinel collisions, session modes, and choice filtering.
Apply encoded ACP selections apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts, apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts, apps/server/scripts/acp-mock-agent.ts	The adapter matches persisted selections against encoded advertised values and applies matching native option or mode values. The mock agent and adapter test cover empty and sentinel-prefixed profile values.
Limit plan-agent filtering to opencode apps/web/src/providerModels.ts, apps/web/src/providerModels.test.ts, apps/web/src/components/chat/composerProviderState.test.tsx	Plan-agent filtering applies only to opencode when plan mode is disabled. Tests cover provider capabilities and opencode composer state.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: bil0000

Merge Risk: ⚪ Minimal · up to b0d76

The ACP default choice and literal provider values are preserved, and no actionable merge-blocking risk remains after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to b0d76

The fix restores advertised default choices without adding an arbitrary configuration dispatch path. No new security bypass was established. Provider rejection, concurrent option updates, and downgrade behavior retain some uncertainty.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The direct changed outcome is selection of advertised configuration values or modes within the active ACP session. The native requests retain the runtime's started session identity; the changed matching logic does not introduce a separate destination or arbitrary RPC method.

Trust Boundaries and Controls

• observed — Caller-supplied selections do not directly determine native string dispatch: the adapter requires an advertised match. The runtime additionally checks known configuration options for type and allowed values. These checks use separate snapshots, not an established atomic advertisement-to-write transaction.

Resilience and Maintainability Implications

• observed — Turn startup and thread resumption retain serialized runtime transitions. Interruption-driven restart clears cached selection and session setup, and configuration or mode state updates remain tied to successful native responses. These existing controls provide recovery boundaries but do not resolve the rejected-write caching behavior.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly identifies the main change: restoring ACP agent choices that use empty values.
Description check		The description includes the required Problem, Change, Scope and approval, and Verification sections. It explains the issue, implementation, maintainer approval, focused test results, limitations, and…
Linked Issues check		The PR satisfies the coding requirements in #16898. AcpSessionConfig.ts preserves an empty ACP choice with an encoded non-empty ID, escapes native values with the reserved prefix, keeps valid curren…
Out of Scope Changes check		The changes remain within #16898. The mock-agent profile and added tests provide coverage for the sentinel mapping and native dispatch behavior. The provider-model change directly prevents the reporte…

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[coderabbitai]

Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts:
- Around line 6323-6334: Update the empty-choice marker used by
encodeAcpOptionValue so it cannot collide with persisted legacy ACP option
values, which were limited to 256 characters. Choose a marker longer than that
limit and preserve the existing encoding and configureSession matching behavior.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Path: .coderabbit.config.ts
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📥 Commits

Reviewing files that changed from the base of the PR and between d81afa0 and 1899185.

📒 Files selected for processing (8)

• apps/server/scripts/acp-mock-agent.ts
• apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
• apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts
• apps/server/src/provider/acp/AcpSessionConfig.test.ts
• apps/server/src/provider/acp/AcpSessionConfig.ts
• apps/web/src/components/chat/composerProviderState.test.tsx
• apps/web/src/providerModels.test.ts
• apps/web/src/providerModels.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.

[juliusmarminge]

Note

This comment is posted by Julius' dot

The direction matches the maintainer's triage of #16898, and the focused tests cover sentinel decoding and legacy-value collisions. The remaining gap is UI verification: the attached image shows the default Copilot option after the fix, but neither this PR nor #16898 includes the required before image.

Closing for missing before/after evidence. Please attach a matching before screenshot showing the missing default alongside the existing after image, and record switching from a custom agent back to the default so the changed selection behavior is visible. Then request reconsideration.