🔌 fix: Preserve Ephemeral MCP Selections Across Model Switches

[danny-avila]

Summary

I fixed a regression where switching the provider/model mid-conversation (or on a new chat draft) silently dropped the ephemeral MCP selection — and tool toggles — from subsequent request payloads, while the MCP badge kept displaying the selection as active. Regenerating or sending after a switch ran without MCP tools.

Root cause: the no-spec branch of useApplyModelSpecEffects (introduced in #11796) reset ephemeralAgentByConvoId to null on every newConversation call when model specs are configured. That reset is paired with a BadgeRowContext localStorage refill — but the refill effect only re-runs when the storage suffix or active spec changes, so on in-place modular switches (same conversation id, non-spec → non-spec) nothing restored the state and submissions read a null ephemeral agent.

• Gated the no-spec reset to context transitions only — leaving a spec or moving to a different conversation key — which are exactly the cases where a BadgeRowContext refill is guaranteed to follow; in-place switches now preserve the ephemeral agent.
• Passed prevSpecName/prevConvoId into applyModelSpecEffects from newConversation via a snapshot read of the pre-switch conversation.
• Preserved the intended 🗂️ refactor: Artifacts via Model Specs & Scope Badge Persistence by Spec Context #11796 behaviors: starting a fresh chat from an existing conversation still refills from __defaults__, and leaving a spec still discards spec-applied values.
• Added jest coverage for all five branches of the no-spec path (in-place existing convo, in-place new draft, spec → non-spec, convo → new chat, no specs configured).
• Added an e2e spec asserting ephemeralAgent.mcp stays in the chat request payload after a model switch on a new chat and after regenerate on a switched conversation — verified both tests fail on the unfixed build and pass with the fix.
• Added a non-spec "Mock Provider D" endpoint to the e2e config so tests can switch between two real ephemeral endpoints, and widened the MockEndpoint helper type accordingly.

Change Type

• Bug fix (non-breaking change which fixes an issue)

Testing

Reproduced the original report first: new chat with an MCP server selected → first payload includes ephemeralAgent.mcp → switch provider/model → regenerate → payload omits ephemeralAgent entirely while the MCP select still shows the server. Also confirmed the new-chat variant (switch before the first send) drops it the same way.

• cd client && npx jest useApplyModelSpecAgents useMCPSelect useToolToggle agents.spec — 67 tests pass.
• npx playwright test --config=e2e/playwright.config.mock.ts mcp-ephemeral — 2 pass with the fix; both fail on the unfixed build with ephemeralAgent.mcp undefined in the captured request payload (regression-proof).
• Sanity-ran the selector-adjacent mock e2e specs (chat, mcp, model-switching, soft-default, model-spec-branding) — 15 tests pass with the e2e config additions.
• ESLint and sort-imports:check clean on all changed files.

Test Configuration:

• Node v24, mock e2e harness (playwright.config.mock.ts) with E2E_BASE_URL=http://localhost:3334 MOCK_LLM_PORT=8899; bug requires modelSpecs configured, which the e2e config already has.

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes

[danny-avila]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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