Backport: fix(langchain): recognize Python AIMessageChunk type in RemoteGraph streams#15604
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…treams (#15196) ## Background `@ai-sdk/langchain`'s `toUIMessageStream` adapter handles plain JSON message objects from LangGraph `RemoteGraph` streams. The type guard `isAIMessageChunk` and the `values`-event `tool_calls` extraction in `processLangGraphEvent` both match `obj.type === 'ai'`, which is the discriminator used by TypeScript `langchain-core`. Python `langchain-core` serializes streaming chunks with `type === 'AIMessageChunk'` ([Python source](https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/messages/ai.py)) instead of the TypeScript `type === 'ai'` ([TypeScript source](https://github.com/langchain-ai/langchainjs/blob/main/libs/langchain-core/src/messages/ai.ts)). Plain Python message objects fall through both gates. User-visible symptom: when streaming from a Python LangGraph server (`langgraph new --python`, LangSmith Cloud Python deployments, RemoteGraph against a Python deployment), `start`, `start-step`, `finish-step`, and `finish` arrive in the UI stream, but `text-start` / `text-delta` / `text-end` and tool-call events never do. The model output is silently lost. ## Summary `packages/langchain/src/utils.ts`: - `isAIMessageChunk`: also match `obj.type === 'AIMessageChunk'` for plain message objects; JSDoc updated to document both discriminators. - `processLangGraphEvent` `values` branch: also recognize `obj.type === 'AIMessageChunk'` when extracting `tool_calls` from non-streamed messages. The change is intentionally narrow — no behavior change for TypeScript callers, only widens the accepted discriminator for plain RemoteGraph objects. ## Manual Verification Reproduced against `@ai-sdk/langchain@2.0.162` (stable) and `@ai-sdk/langchain@3.0.0-canary.131` (canary) using a standalone async-iterable mock that emits `['messages', [chunk, metadata]]` events with both shapes (`type: 'ai'` vs `type: 'AIMessageChunk'`): - Before fix: TypeScript shape produced 2 `text-delta` UIMessageChunks, Python shape produced 0. - After fix (patched `dist/index.js` locally): both shapes produced identical output (`text-start`, 2 `text-delta`, `text-end`). Then verified at the source level: the three new regression tests fail on unfixed `main`, all 190 langchain tests pass with the fix applied. ## Checklist - [x] All commits are signed (PRs with unsigned commits cannot be merged) - [x] Tests have been added / updated (for bug fixes / features) - [x] Documentation has been added / updated (for bug fixes / features) — JSDoc on `isAIMessageChunk` - [x] 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 `isToolMessageType` (same file) uses the equivalent `obj.type === 'tool'` pattern. Python `langchain-core` also uses the `'tool'` literal for `ToolMessage`, so it is not affected by this bug — but if future Python versions emit class-name discriminators (`'ToolMessage'`), a parallel guard would be needed. Out of scope here. I deliberately did not add a runnable example under `examples/` (per AGENTS.md "When to Deviate"): exercising the bug end-to-end requires a Python LangGraph server, which is impractical for a TypeScript monorepo example, and the new unit tests serve as a deterministic, in-process reproduction. Happy to add one if maintainers prefer. ## Related Issues Fixes #14341. Co-authored-by: cristiandrei1234 <cristiandrei1234@users.noreply.github.com> Co-authored-by: Lars Grammel <lars.grammel@gmail.com>
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May 26, 2026
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This is an automated backport of #15196 to the release-v6.0 branch. FYI @cristiandrei1234