fix(server): ACP threads accept a model change after their session detaches #35951

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

When negotiated session capabilities are absent, same-instance model-selection planning now uses apply_on_next_turn instead of calling the transition planner with current-instance capabilities. A test covers a settled thread with no recorded sessions.

Changes: Model selection planning

Layer / File(s)	Summary
Plan model changes without negotiated capabilities apps/server/src/orchestration-v2/ProviderSwitchService.ts, apps/server/src/orchestration-v2/ProviderSwitchService.test.ts	When negotiated session capabilities are absent, the plan uses apply_on_next_turn. The test checks that a settled thread with no recorded sessions uses switch_model_in_session for an acpSelectionTransition plan and releases no provider sessions.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: maria-rcks, t3dotgg

Merge Risk: Minimal · up to 21d76

This change lets an ACP thread accept a model change after its session detaches, applying the saved selection when the next session starts instead of rejecting it. No concrete regression was found in the reviewed change, so it looks ready to merge.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title names the main change: ACP threads accept a model change after their session detaches. It is a conventional commit with the fix(server) scope. The changeset adds a planner path that treats a…
Linked Issues check		[#35938] The issue asks that a model change on an ACP thread with no live session be stored and applied when the next session starts, not rejected. The diff in apps/server/src/orchestration-v2/Provide…
Out of Scope Changes check		The only code change is the planner branch in ProviderSwitchService.plan and one regression test in ProviderSwitchService.test.ts. Both serve #35938. No unrelated change was found in the whole-PR diff…
Description check		The description explains the problem (settling emits provider-session.detached, removing the session record, so ACP planning rejects a model change), the change (skip the adapter when no session recor…

Full details: Title check

Explanation

The title names the main change: ACP threads accept a model change after their session detaches. It is a conventional commit with the fix(server) scope. The changeset adds a planner path that treats a model change with no session record as apply_on_next_turn, which matches the title.

Full details: Linked Issues check

Explanation

[#35938] The issue asks that a model change on an ACP thread with no live session be stored and applied when the next session starts, not rejected. The diff in apps/server/src/orchestration-v2/ProviderSwitchService.ts returns { type: "apply_on_next_turn" } when negotiatedCapabilities is undefined. Before the change, the adapter received negotiatedCapabilities ?? currentInstance.value.capabilities, and the base ACP capabilities set supportsModelSwitchInSession to false, so acpSelectionTransition rejected the change. The new test 'applies a model change on next turn after a settled thread's session detached' asserts switch_model_in_session with no released sessions for a settled thread whose session records are empty. The existing dead-record rejection test is kept. Residual uncertainty: the claim that the ACP adapter applies the saved model selection on session load or resume was not verified in this review. The planner-level behavior is verified by the test; the end-to-end apply-on-resume behavior is taken from the PR description.

Full details: Out of Scope Changes check

Explanation

The only code change is the planner branch in ProviderSwitchService.plan and one regression test in ProviderSwitchService.test.ts. Both serve #35938. No unrelated change was found in the whole-PR diff against the merge base 0a333a69e7. Note for the author: the PR description says the planner skips the adapter when no session record exists. The code instead skips the adapter when negotiatedCapabilities is undefined. That value is also undefined when a session record exists without a capabilities field, including a live session. In that case the change now returns apply_on_next_turn without asking the adapter. This wider condition is related to the objective but is not described in the PR. Whether a live session can lack capabilities was not established from the contracts schema.

Full details: Description check

Explanation

The description explains the problem (settling emits provider-session.detached, removing the session record, so ACP planning rejects a model change), the change (skip the adapter when no session record exists and treat the change as apply_on_next_turn), and the scope (links issue #35938 and gives a reproduction on main). It also gives a detailed verification section with focused test results and manual repro steps, plus evidence links, and it names the tools and models used. Required template sections are all filled in. Minor gaps: the Problem section is folded into the body rather than under a '## Problem' heading, the title is not shown in the provided description, and the evidence includes external links whose availability cannot be verified here. The author also lists the work agents, which the template asks for. Overall the description is mostly complete and specific.

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