[v6.0] fix(langchain): reset streamEvents text between model invocations#16789
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…5254) ## Background When `toUIMessageStream` consumes LangGraph **`streamEvents`** (v2), `processStreamEventsEvent` tracks streaming text with `textStarted` / `textMessageId` (and reasoning with `reasoningStarted` / `reasoningMessageId`). Those flags were only cleared at the end of the whole stream, not when a **new** chat model run started. If a graph performs multiple LLM calls in one run (e.g. tool use then model again), **`on_chat_model_start`** can fire again while `textStarted` is still `true`. Subsequent **`text-delta`** chunks then reuse the first text part’s id, so later prose appends to that first block. Tool events still append **new** tool parts after it, so the assistant `UIMessage`’s `parts` no longer reflect chronological order in the UI. ## Summary - On **`on_chat_model_start`**, if the previous model turn left streams open, emit **`reasoning-end`** and **`text-end`** with the correct ids, then reset `reasoningStarted` / `reasoningMessageId` and `textStarted` / `textMessageId`. - After that, set **`messageId`** from `event.run_id` or `event.data.run_id` as before. This matches the intent of ending open streams when a new model invocation begins, so the next **`text-start`** / **`reasoning-start`** pair applies to the new turn. ## Manual Verification - Use a LangGraph agent with **`streamEvents`** (v2) that **invokes the chat model more than once** in the same run (e.g. tools then model again), with text on both segments. - Pipe the iterator through **`toUIMessageStream`** and consume the UI message stream. - Confirm the resulting assistant message has **multiple text parts in order** (one per model segment) and that **tool parts** appear in the order events occurred, not all text merged into the first text part. ## Checklist - [ ] All commits are signed (PRs with unsigned commits cannot be merged) - [ ] Tests have been added / updated (for bug fixes / features) - [ ] Documentation has been added / updated (for bug fixes / features) - [ ] A _patch_ changeset for relevant packages has been added (for bug fixes / features - run `pnpm changeset` in the project root) - [x] I have reviewed this pull request (self-review) ## Future Work Optional: add an automated test that simulates two `on_chat_model_start` / `on_chat_model_stream` sequences and asserts the emitted chunk sequence includes **`text-end`** before the second segment’s **`text-start`**. --------- Co-authored-by: Lars Grammel <lars.grammel@gmail.com> (cherry picked from commit 3711955)
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Backport of #15254 to
release-v6.0(clean cherry-pick).on_chat_model_startdid not resettextStarted/reasoningStarted, so multi-invocation streamEvents runs appended later text to the first text part.Part of the v6.0 backport tracking issue #16767.