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Inconsistent `Generations` Shape In Chat Model Streaming `On_llm_error` Callbacks

BaseChatModel.stream() and astream() emit malformed LLMResult.generations to on_llm_error: when no chunks are produced, generations is [[]]; when partial chunks exist, it is [[chunk], []] instead of the correct [] or [[chunk]] respectively.

highConfidence 95%Langchain-Core Affected V1.4.8

Origin Analysis

PR #30626 introduced _generate_response_from_error(e), which returns a flat list of ChatGeneration. The stream/astream exception handlers wrap this list unconditionally in an outer list, then prepend accumulated chunks if present. This yields an extra empty inner list when the error has no HTTP response (common case), producing [[]] with no chunks or [[chunk], []] with partial chunks.

Provided in issue: run the FakeListChatModel loop with error_on_chunk_number values 0-3; observe generations shape from callback. See inconsistency.

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 shape of the callback payload in edge cases. Downstream consumers that have adapted to the malformed shape (e.g., expecting an extra empty row) may break. Ensure all async paths are updated identically; otherwise sync and async behavior diverges. Thoroughly test with integrations that raise HTTP errors (which may provide error metadata). Rollback: revert to previous unconditional wrapping if regressions appear, but this perpetuates the bug.