fix(acp): accept SDK elicitation requests

[shivamhwp]

Depends on #11298, which fixes a typecheck regression on main from #11029. The diff against that base contains only ACP changes.

ACP agents using elicitation/create receive "Method not found" even when T3 has a registered question handler. The shared client now accepts that SDK method and returns its flat action and content response. It also routes elicitation/complete to the existing completion handler.

The legacy session/elicitation methods retain their nested response format. Compatibility lives beside the RPC definitions because the generated schema is pinned to v0.11.3. Providers still supply their own question handler; missing handlers and malformed requests return protocol errors.

Refs #9048. #10893 carries an Oh My Pi-specific workaround; this handles the shared transport separately from adding that provider. The adapter-facing generated types remain compatible, and this change covers session-scoped form and URL requests.

Validation: 44 focused client, agent, protocol and server-runtime tests pass, including flat accept/decline/cancel results, metadata, both notification names, malformed requests and legacy exchanges. A separate exchange with the official @agentclientprotocol/sdk 1.4.0 passes: its AgentSideConnection.createElicitation receives the accepted answer, and completeElicitation reaches T3's completion handler. Package and server typechecks and scoped lint pass. No live Oh My Pi process or native client UI was exercised.

Model: GPT-6. Harness: Codex in T3 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	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	57.1 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	10	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.8 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: 0838f78 · 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: 114.6 KiB

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

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ℹ️ Recent review info

⚙️ Run configuration

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Plan: Advanced

Run ID: a26dddfd-9f91-474f-b7e0-17c5be56e48c

📥 Commits

Reviewing files that changed from the base of the PR and between e1c94f7 and 44f78f2.

📒 Files selected for processing (4)

• packages/effect-acp/src/client.test.ts
• packages/effect-acp/src/client.ts
• packages/effect-acp/src/protocol.ts
• packages/effect-acp/src/rpc.ts

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📝 Walkthrough

Walkthrough

Adds "elicitation/create" RPC support and accepts "elicitation/complete" notifications. The client shares elicitation handling across method aliases and validates payloads, response shapes, URL mode, malformed requests, and missing handlers.

Changes

Elicitation support

Layer / File(s)	Summary
Elicitation RPC contract packages/effect-acp/src/rpc.ts	Defines and registers CreateElicitationRpc with compatible request, success, and error schemas.
Client request routing packages/effect-acp/src/client.ts	Decodes "elicitation/create" requests, maps invalid payloads to protocol errors, and invokes the shared elicitation handler.
Notification aliases and validation packages/effect-acp/src/protocol.ts, packages/effect-acp/src/client.test.ts	Accepts both completion notification methods and tests response shapes, URL mode, validation failures, and missing handlers.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant CreateElicitationRpc
  participant decodeElicitationRequest
  participant coreHandlers.elicitation
  CreateElicitationRpc->>decodeElicitationRequest: Decode request payload
  decodeElicitationRequest-->>CreateElicitationRpc: Return decoded request
  CreateElicitationRpc->>coreHandlers.elicitation: Handle elicitation
  coreHandlers.elicitation-->>CreateElicitationRpc: Return action and optional _meta

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Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to 44f78

No actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 4…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: accepting SDK elicitation requests in ACP.
Description check	✅ Passed	The description explains what changed, why it changed, compatibility constraints, scope, and validation results. It does not use the exact template headings or checklist, but it includes the required …

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/acp-elicitation-compat

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[macroscopeapp]

Approvability

Verdict: Approved at 0838f78

Macroscope's review found this PR approvable — This is a localized ACP compatibility fix that routes SDK elicitation aliases through the existing handler while preserving legacy behavior. Its runtime impact is narrow and directly covered by focused tests.

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