Affected version
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In langchain_core, concatenating message chunks with '+' and the merge_message_runs function silently discard the 'name' attribute and fail to fallback to the trailing chunk's 'id' when the leading chunk's id is None, causing loss of metadata during streaming and in multi-agent scenarios.
In langchain-core's _compat_bridge._iter_protocol_blocks, tool_call content blocks lacking an explicit index are keyed by ("__lc_no_index__", block_type, position_in_chunk). When Gemini streams parallel function calls as separate chunks each containing one complete tool_call block at position 0, the keys collide. _accumulate then overwrites the earlier block with the later one, silently dropping the first tool call. This leads to incorrect model behavior and, with Gemini 3, the lost call may carry the thought_signature, causing upstream validation issues or silent execution of only one of the requested tools.
The v2 implementation of astream_events fails to emit terminal error events (on_chain_error, on_llm_error, on_chat_model_error, on_retriever_error) when a tracked run raises an exception. Only the corresponding start event is emitted, leaving consumers unable to reliably determine that a run terminated unsuccessfully.