vllm - 💡(How to fix) Fix [Bug]: explicit `parallel_tool_calls: null` is filtered like false instead of the documented "true" default [1 pull requests]

🐛 Describe the bug

An explicit parallel_tool_calls: null is treated like false, trimming the response to a single tool call. The docs define only true and false; explicit null is unspecified:

Setting it to true (the default) allows returning more than one tool call per request.

Since true is the documented default and null conveys "unspecified," the natural reading is that an explicit null should resolve to that default, just like an omitted field. If so, this is a bug: an omitted field correctly defaults to true, but explicit null falls through as falsy and is filtered like false.

Context

Introduced in #26233 (which also set the True default). Surfaced while porting the field to the Rust frontend in #44760, where unwrap_or(true) treats null as the default. This issue tracks the alignment on the Python side.

Current Behavior

Root Cause

The field defaults to True: https://github.com/vllm-project/vllm/blob/4128605ad4197551a5ae4de08ede38f2348d1eb5/vllm/entrypoints/openai/chat_completion/protocol.py#L233

…but the filter gates on truthiness, so an explicit null (None) is falsy and filtered the same as false: https://github.com/vllm-project/vllm/blob/4128605ad4197551a5ae4de08ede38f2348d1eb5/vllm/entrypoints/serve/utils/tool_calls_utils.py#L19-L37

Two mismatches:

1. The docstring says filter "when parallel_tool_calls is False", but if request.parallel_tool_calls: also fires on None.
2. Since true is the default, an unspecified value (null) should resolve to the default, not to false.

Expected

parallel_tool_calls: null should behave like an omitted field (default true, keep all tool calls). Minimal fix: gate on request.parallel_tool_calls is False rather than truthiness, matching the docstring.

cc @njhill @chaunceyjiang

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