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Bug: Streaming Tool Call Fires Prematurely When First SSE Chunk Has Empty Args

When streaming tool calls from providers like OpenRouter or deepseek-v3.2, the first SSE chunk may contain an empty args string. AIMessageChunk incorrectly treats this as a complete tool call with empty arguments, causing agents to invoke the tool prematurely and often crash with ValidationError.

highConfidence 85%LangChain Affected V0.3.x

Origin Analysis

In langchain_core.messages.ai.AIMessageChunk._init_tool_calls, the logic for parsing args treats an empty string (args_str == '') the same as None, both resulting in args = {}. This causes the tool call to be created immediately after the first chunk, before actual arguments have been streamed.

1. Import AIMessageChunk from langchain_core.messages.\n2. Create an AIMessageChunk with tool_call_chunks=[{'name': 'get_weather', 'args': '', 'id': 'call_abc', 'index': 0}].\n3. Access the tool_calls property.\n4. Observe that it returns [{'name': 'get_weather', 'args': {}, 'id': 'call_abc', 'type': 'tool_call'}] instead of [].

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 fix may drop tool calls that legitimately have no arguments if the provider sends an empty string instead of '{}'. Providers should send '{}' for no-argument tools. Additionally, partial JSON strings that are non-empty but invalid are still coerced to {}, which could cause issues for fragmented streams with incomplete JSON in intermediate chunks. Rollback: remove the early continue to revert to previous behavior. Test thoroughly with providers that may send empty args for no-arg tools.