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V3 Streaming: Tool-Call Block-Delta Events Are Snapshots Causing Quadratic Size Growth

In langchain-core v3 streaming, tool-call argument deltas are emitted as full snapshots of accumulated state, leading to O(n^2) event sizes for large arguments. This causes severe performance degradation and memory overhead in LangGraph Server and any consumer of the event stream.

highConfidence 95%Langchain Affected V1.6.6

Origin Analysis

The compat bridge (chunks_to_events/achunks_to_events) builds tool-call block-delta events from per-index accumulated state instead of the incoming chunk, sending a full snapshot of name, id, and all argument characters on every chunk. Text and data deltas correctly send only the new slice. This mismatch causes quadratic growth: n characters of arguments streamed in chunks of c produce about n²/(2c) characters in events.

Run the provided reproduction script with langchain-core 1.6.6. It streams a fake model with tool_call_chunks of increasing argument sizes and measures the total serialized size of content-block-delta events. Text deltas grow linearly; tool-call deltas grow quadratically (e.g., 64k chars → 0.42MB text vs 128.82MB tool-call).

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 changes the public event stream schema: tool-call content-block-delta events will no longer contain a full snapshot of args. Any external consumer that relies on receiving the complete accumulated args in each event will break. Rollback: revert to snapshot emission and shallow-merge consumer. Version compatibility: the patch must be applied consistently to both the bridge and all consumers; mixing old and new versions within the same deployment will cause incorrect tool-call argument reconstruction. Concurrency: ensure thread-safety of any shared accumulated state when patching async versions.