Test Plan: Session-scoped model configurations

[pwang347]

Refs: #320393, #321405

• anyOS @digitarald
• anyOS @hediet
• anyOS @joshspicer

Complexity: 3

Author: @pwang347

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Context: Previously, we stored model configurations as globals in chatLanguageModels.json. This did NOT align with the behaviour for model selections themselves, which follow a hierarchy of 1. saved per session to avoid breaking cache 2. stored as default selected per location + session type and 3. using a global value.

What Changed: We should now expect that wherever a model is shown, the associated config should be either the appropriate default value or match the previously used value there.

Test Cases

For the test cases here, we are focused on the Local agent. There are known issues with Copilot CLI context selection.

1. Default context behaviour is correct

• Open VS Code with --transient flag
• Select the Local Agent
• Select GPT 5.5
• Verify that the picker defaults to the smaller context value
• Send a message and verify that the context widget shows the correct context value (not 1M)
• Open chat debug logs, and verify that the maxPromptTokens is 272000

2. Default value after initial change is persisted

• In chat panel, choose a model and modify the thinking effort / context size
• Select a different harness and do the same
• Reload the window
• Verify that the values for both harnesses are the same as the last set values

3. Configurations are persisted per session

• Send two chat messages, each with the same model but different model configurations (thinking effort / context size)
• Reload the window
• Clicking into each of the chats should show the previously selected configurations for each
• Reload the window and start a new chat with the model; the default config should be whatever was last set

[hediet]

Clicking into each of the chats should show the previously selected configurations for each

I assume this means the first message should show the most recent model configuration, and not the one the message was sent with?