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Parallel Tool Calls Without Index Collide On Positional Fallback Key And One Is Silently Dropped

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.

highConfidence 95%LangChain Affected V1.4.8 Affected V1.6.2

Origin Analysis

The positional fallback key for content blocks without an index uses the block's position within its chunk. Since Gemini emits each parallel tool call in its own chunk with a single tool_call block at position 0, all such blocks receive the identical key ("__lc_no_index__", 'tool_call', 0). When these blocks are accumulated, the later block replaces the earlier one, causing data loss.

1. Import _accumulate and _iter_protocol_blocks from langchain_core.language_models._compat_bridge. 2. Create two AIMessageChunk objects, each containing one tool_call block with different ids (e.g., 'a' and 'b') and same type 'tool_call', no index. 3. Pass each through _iter_protocol_blocks to obtain (key, block_dict) tuples. 4. Observe that both keys are identical: ("__lc_no_index__", 'tool_call', 0). 5. Call _accumulate with the two block dictionaries; inspect the result: it will contain only the second call's id and name, proving the first was dropped.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This change assumes that tool_call blocks always carry a unique id when present. If a stream erroneously reuses the same id for two different calls, they will merge incorrectly. Additionally, if a provider fragments a single tool_call across multiple chunks and only some chunks include the id, the chunks without id will fall back to positional keying and may not merge with the id-keyed chunks, potentially breaking argument accumulation for such providers. Rollback recommendation: if this fix causes regressions with providers that stream tool_call arguments in fragments, revert to the original positional fallback and address the collision via a more comprehensive chunk-consolidation mechanism.