fix(acp): custom models now reach the agent instead of failing to open

[t3dotgg]

A model added to an ACP agent's custom models showed up in the composer, but every run with it failed as a generic provider-session-open-failure. The session runtime only accepted model ids the agent advertised, so it rejected the custom id with -32602 before sending anything. Cline's free tier (cline-free/*) is the real case: the agent does not advertise it, but cline --acp accepts it.

Fix

• The ACP registry adapter passes the instance's configured custom model slugs to the session runtime, using the same trimmed slugs the composer catalog shows.
• The runtime accepts those values for the model config option only, and lets the agent validate them. Everything else keeps the existing guard.
• No agent-specific branches. Other runtime users (Grok, Antigravity, Cursor, probe, auth) do not pass custom models, so their behavior is unchanged.

Proof

• New test in AcpJsonRpcConnection.test.ts against the mock ACP agent: a configured custom model is sent with session/set_config_option, and an unconfigured unknown id is still rejected. It fails on main with the same -32602 error as the issue and passes with the fix.
• Registry path: I temporarily set customModels: [" cline-free/custom-model "] in the real-process registry adapter test and opened a session with that model. Without the adapter change it failed with ProviderAdapterOpenSessionError / -32602. With it, the session opened. I did not commit this edit.
• Typecheck for provider-acp, provider-acp-registry, and apps/server. Lint and format on the changed files. provider-acp-registry tests (65/65), ACP adapter tests (123/123), and AcpJsonRpcConnection.test.ts (44/44) pass.
• Reviewed with sol-loop: 1 round with GPT-6.1 Sol, approved with no findings.

Fixes #18164

Credit to #18169 for the diagnosis and the Cline probe that showed the agent accepts these ids.

Model and harness: Claude Opus 5.5 in Claude Code via T3 Code.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Approved at b15b74f

Macroscope's review found this PR approvable — This is a small, self-contained fix that passes already-configured custom model slugs to ACP sessions and preserves rejection of unconfigured values and existing callers. The focused test covers both the newly enabled path and the existing validation guard.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Path: .coderabbit.config.ts
• Review profile: CHILL
• Plan: Team
• Run ID: 086ea7a2-9f91-41c0-8dd1-f62a76ab7cad

📥 Commits

Reviewing files that changed from the base of the PR and between 173a970 and b15b74f.

📒 Files selected for processing: 3

• apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts
• packages/provider-acp-registry/src/server/adapter.ts
• packages/provider-acp/src/server/AcpSessionRuntime.ts

Included review availability: This review used your included allowance. 2 included reviews remain after this review. Your included PR review attempts over the past 7 days set your current allowance at 10 reviews per hour.

📝 Walkthrough

The ACP registry adapter now passes configured custom model IDs to the session runtime. The runtime accepts those IDs for model configuration even when the agent does not advertise them. A test checks that configured models are sent and unconfigured models are rejected.

Changes: ACP custom model configuration

Layer / File(s)	Summary
Runtime model validation packages/provider-acp/src/server/AcpSessionRuntime.ts	AcpSessionRuntimeOptions adds customModels. Model validation accepts a configured ID for a model option when that ID is absent from the agent-advertised values. Other unadvertised values remain rejected with -32602.
Registry wiring and validation test packages/provider-acp-registry/src/server/adapter.ts, apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts	The adapter passes configured custom model slugs to the runtime. The test verifies that a configured custom model is sent and an unconfigured model is not sent.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to b15b7

Configured custom model IDs now reach the agent, as the linked issue requests, while unconfigured IDs are still rejected. No merge-blocking risk was identified.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary fix: allowing custom ACP models to reach the agent instead of failing during session opening.
Description check		The description explains the problem, expected behavior, implementation across components, focused scope, linked issue, and verification results. The Fix and Proof headings provide the required Change…
Linked Issues check		Issue [#18164] requires configured Cline ACP custom model IDs to reach the agent while unconfigured IDs remain rejected. The registry adapter reads customModels with readCustomModelEntries, which …
Out of Scope Changes check		The changes stay within the linked issue scope. They modify the ACP registry adapter, add the narrowly scoped runtime exception, and add coverage for configured and unconfigured model IDs. Other ACP r…

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

🧪 Generate unit tests (beta)

• Commit to this branch
• Create a new PR

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	4.9 KiB	−23 B (−0.5%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	−23 B (−1.9%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.8 KiB	−41 B (−0.2%)	29.3 KiB	✅
Codex	Live turn messages	2	1	−1 (−50.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 173a970 · PR result: b15b74f · 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: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

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