fix(chat): preserve messages after summarization

[ggnnggez]

Summary

Fixed #2965

• Preserve visible chat history when summarization resets the live message stream.
• Match DeerFlow summarization update keys by the stable SummarizationMiddleware.before_model suffix instead of one exact class name.
• Add frontend coverage for hidden control-message dedupe and DeerFlow summarization update keys.
• Add a backend regression guard for the frontend/backend summarization update-key contract.

Root Cause

The backend emits summarization updates under DeerFlowSummarizationMiddleware.before_model, while the frontend only listened for SummarizationMiddleware.before_model. As a result, the frontend missed the summarization reset update and did not move visible messages into local history before RemoveMessage(remove_all) cleared the live stream.

Validation

pnpm test tests/unit/core/threads/message-merge.test.ts
pnpm lint
uv run pytest tests/test_lead_agent_model_resolution.py -k summarization_middleware_preserves_frontend_update_key_contract
uv run pytest tests/test_lead_agent_model_resolution.py -k "create_summarization_middleware"
commit hook: frontend check and backend lint passed

Note: pnpm typecheck was previously blocked by stale Next generated route types under .next/types versus .next/dev/types, unrelated to this change.

[Copilot]

Pull request overview

Fixes a frontend/backend contract mismatch that caused chat history to disappear from the active “new conversation” view after summarization-triggered stream resets, by broadening the frontend’s summarization update-key matching and improving message deduplication around hidden control messages.

Changes:

• Frontend: preserve visible history across summarization resets by matching any *.SummarizationMiddleware.before_model update key and moving pre-reset messages into history before the live stream clears.
• Frontend: adjust message deduplication/overlap logic to ensure hidden control messages don’t evict visible messages with the same identity.
• Tests: add frontend unit coverage for hidden-message dedupe + update-key matching; add backend regression test to guard the update-key suffix contract.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
frontend/src/core/threads/hooks.ts	Matches summarization updates by stable suffix, preserves visible messages on reset, and avoids hidden-message dedupe issues.
frontend/tests/unit/core/threads/message-merge.test.ts	Adds unit tests for hidden-message ID reuse and DeerFlow summarization update-key matching.
backend/tests/test_lead_agent_model_resolution.py	Adds regression guard ensuring summarization middleware class name keeps the SummarizationMiddleware.before_model suffix.

[WillemJiang]

@ggnnggez Here are some review comments.

1. Loose key matching in getSummarizationMiddlewareMessages — endsWith("SummarizationMiddleware.before_model") could theoretically match a future unrelated key that happens to share the suffix. Consider adding a more specific check (e.g., key === "SummarizationMiddleware.before_model" || key === "DeerFlowSummarizationMiddleware.before_model") or at least a comment documenting the known keys. The current approach is fine for now given it's unlikely, but it's fragile against accidental matches.
2. mergeMessages filtering hidden messages from overlap — The change at line ~113 filters hidden messages out of threadMessages when building the overlap set. This is correct for the summarization case, but it is worth verifying that there are no other call sites where hidden messages in thread messages are meaningful for overlap detection.
3. Backend test is a naming contract, not a behavior test — The backend test only checks type(middleware).name ends with SummarizationMiddleware. It doesn't verify that the middleware actually emits the right update keys at runtime. If someone renames the class, this test passes but the contract breaks silently. The PR description acknowledges this ("Remove this test once the frontend and backend have a stronger explicit contract"), which is fair.

[ggnnggez]

Thanks for the review. Addressed in e584efd

1. Replaced suffix matching with an explicit allowlist for SummarizationMiddleware.before_model and DeerFlowSummarizationMiddleware.before_model, plus a negative test for suffix-sharing keys.
2. Verified mergeMessages has a single production call site in the UI stream merge path. Added coverage around hidden/visible overlap and comments clarifying that hidden messages in this path are UI control messages, not task/tracing records.
3. Added a runtime create_agent(...).stream(stream_mode="updates") regression test that asserts the actual DeerFlowSummarizationMiddleware.before_model update key and summary reset payload are emitted.