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https://github.com/langchain-ai/langchain/pull/31286 included an update to the return type for `BaseChatModel.(a)stream`, from `Iterator[BaseMessageChunk]` to `Iterator[BaseMessage]`. This change is correct, because when streaming is disabled, the stream methods return an iterator of `BaseMessage`, and the inheritance is such that an `BaseMessage` is not a `BaseMessageChunk` (but the reverse is true). However, LangChain includes a pattern throughout its docs of [summing BaseMessageChunks](https://python.langchain.com/docs/how_to/streaming/#llms-and-chat-models) to accumulate a chat model stream. This pattern is implemented in tests for most integration packages and appears in application code. So https://github.com/langchain-ai/langchain/pull/31286 introduces mypy errors throughout the ecosystem (or maybe more accurately, it reveals that this pattern does not account for use of the `.stream` method when streaming is disabled). Here we revert just the change to the stream return type to unblock things. A fix for this should address docs + integration packages (or if we elect to just force people to update code, be explicit about that).