Preserve selected Agent Host model for new Copilot CLI sessions in Agents window

[Chulong-Li]

Fixes #316044

Summary

This fixes an Agent Host model-selection race where the model picker could show the intended model, such as GPT-5.5 XHigh or Claude Opus 4.7, while a new Copilot CLI session still briefly used or displayed the backend default model, Claude Sonnet 4.6 Medium.

The change ensures that new untitled Agent Host sessions receive the selected model before the first message is sent, and that eagerly-created backend sessions are updated with the selected model as soon as they are available. This keeps the picker, session cache, and backend session summary aligned during new session startup.

Root cause

The first-send path only includes userSelectedModelId and the corresponding model configuration when the NewSession already has a selected model.

For new untitled Agent Host sessions, the picker could resolve and display a model from current picker state or stored profile state without pushing that model into the backing session before send. In that case, the first request was sent without userSelectedModelId, allowing the backend to fall back to its default model.

Cached session model state was also only updated from dedicated model-change actions. When a session summary update included changes.model, the cached modelId and modelSelection could remain stale.

There was one related startup flicker: NewSession.eagerCreate() creates the backend session before the selected model is propagated to it. The backend can initially summarize the session with its default model, Claude Sonnet 4.6 Medium, before the later first-turn SessionModelChanged action corrects it.

Figure 1.1: The session automatically switches to Claude Sonnet 4.6 Medium even though GPT-5.5 XHigh was initially selected

Fix

• Applies the resolved picker model to new untitled Agent Host sessions before first send.
• Uses the current picker selection before falling back to the stored profile model for new untitled sessions.
• Avoids changing sessions that already have an explicit model.
• Keeps picker state scoped to the Agent Host resource scheme.
• Updates cached session model state when SessionSummaryChanged includes changes.model, including explicit model clearing.
• Queues the selected model while eager backend session creation is in flight.
• Dispatches SessionModelChanged once eager createSession() completes, or immediately if the eager backend session already exists, preventing the transient Claude Sonnet 4.6 Medium flicker.

Validation

Figure 2.1: Start a new Copilot CLI session with GPT-5.5 XHigh selected in the model picker



Figure 2.2: During processing, the UI continues to display the user-selected model instead of briefly falling back to Claude Sonnet 4.6 Medium



Figure 2.3: After completion, the session still displays the user-selected model rather than Claude Sonnet 4.6 Medium

[Copilot]

Pull request overview

This PR fixes a model-selection race in the Agents window for Agent Host (Copilot CLI) sessions by ensuring the user-selected model is applied to new/untitled sessions early enough to avoid the backend briefly using its default model, and by keeping cached session model state in sync when summaries change.

Changes:

• Queue and flush model selection to the eagerly-created backend session once createSession() completes (or immediately if already created).
• Prefer the current picker selection over stored profile state when resolving the model for new/untitled sessions, while scoping picker state per Agent Host resource scheme.
• Update cached session model state when SessionSummaryChanged includes changes.model (including explicit clears), with added test coverage.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/vs/sessions/contrib/providers/agentHost/browser/baseAgentHostSessionsProvider.ts	Queues/flushes model changes for eager session creation and updates cached model state on summary changes.
src/vs/sessions/contrib/providers/agentHost/browser/agentHostModelPicker.ts	Resolves model using current picker state before stored state for untitled sessions; scopes state by resource scheme and avoids redundant pushes.
src/vs/sessions/contrib/providers/agentHost/test/browser/localAgentHostSessionsProvider.test.ts	Adds coverage for queued vs immediate model dispatch around eager create and for summary-driven cached model updates/clears.
src/vs/sessions/contrib/providers/agentHost/test/browser/agentHostModelPicker.test.ts	Adds coverage for preferring current picker model over stored model for new/untitled sessions.