fix(core): properly handle empty or missing arguments for ToolCallChunk

[amznero]

Description: Streaming tool call chunks: if chunk["args"] == None, the tool call is incorrectly marked as an InvalidToolCall.

Fixes #32016

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
langchain	🛑 Canceled (Inspect)			Jul 14, 2025 4:40pm

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32017 will not alter performance

_{Comparing amznero:bugfix_tool_call_args (62099a1) with master (d96b75f)}

⚠️ Unknown Walltime execution environment detected

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Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32017 will not alter performance

_{Comparing amznero:bugfix_tool_call_args (62099a1) with master (d96b75f)}

Summary

✅ 14 untouched benchmarks

[mdrxy]

Passing this off to @eyurtsev for final review/approval

[eyurtsev]

Why is this change necessary?

An empty dict represents no args currently. I'd rather not add a second way to represent the same information

[mdrxy]

Yes, an empty dict should represent a completed tool call that has no arguments.

None is used exclusively to represent the in-progress state where arguments have not yet been received during streaming.

Using an empty dictionary for both the "no arguments" state and the "arguments are coming" state makes them indistinguishable. We wouldn't know how to interpret the message, e.g. two scenarios to illustrate this:

1. Final State (No Arguments): The model finishes a tool call and intends for it to have zero args. It correctly sends args={}. We'd see this and execute the tool immediately.
2. In-Transit State (Streaming): The model starts a tool call and is about to stream the arguments. If it used {} as a temporary placeholder, we'd see args={}

If we went back to args_ = parse_partial_json(chunk["args"]) if chunk["args"] != "" else {}, it doesn't handle the case where chunk["args"] is literally None. When a streaming chunk arrives with args=None, chunk["args"] != "" evaluates to True (since None != ""). The code then tries to execute parse_partial_json(None), which results in a TypeError

Even if it mapped None to {} we still lose the ability to distinguish between the two different scenarios.