[Bugfix] Fix streaming boundary delta losing content and role fields in DelegatingParser

[QingZhou-YangHY]

Purpose

Fixes #43221

When a streaming delta spans the reasoning/tool-call boundary (e.g., contains both </think> and <tool_call> in the same chunk), Qwen3ReasoningParser.extract_reasoning_streaming may return a DeltaMessage with both reasoning and content fields set.

Previously in DelegatingParser.parse_delta, only reasoning was saved before _extract_tool_calls_streaming overwrote delta_message. The content field (e.g., the <tool_call> prefix text) and role field were silently discarded, causing streaming reasoning tokens to be truncated.

Changes

• In vllm/parser/abstract_parser.py, DelegatingParser.parse_delta: save all three fields (reasoning, content, role) from the boundary delta_message before the tool parser overwrites it.
• After the tool parser returns, merge the saved fields back:
  • If the tool parser also returns content, the two strings are concatenated in order (saved_content + tool_content).
  • reasoning and role are restored unconditionally if they were set.

Test Plan

Manually verified with a mock test that reproduces the bug:

• Before fix: result.content is None (the <tool_call> prefix is lost)
• After fix: result.content == "<tool_call>" and result.reasoning == "think about this" are both preserved

The existing tests/parser/test_streaming.py suite continues to pass.

[gemini-code-assist]

Code Review

This pull request updates parse_delta in vllm/parser/abstract_parser.py to save and restore content and role attributes in addition to reasoning before they are potentially overwritten by the tool parser. Feedback focuses on removing unnecessary getattr calls in favor of direct attribute access, as delta_message is a statically typed Pydantic model where these attributes are guaranteed to exist.

[gemini-code-assist]

Since delta_message is statically typed as DeltaMessage | None (which is a Pydantic model), its attributes reasoning, content, and role are guaranteed to exist when it is not None. Using getattr is unnecessary, bypasses static type checking (e.g., mypy/pyright), and is inconsistent with how attributes are accessed elsewhere in this file (e.g., delta_message.tool_calls). Direct attribute access is cleaner and more idiomatic.

Suggested change

	saved_reasoning = (
	getattr(delta_message, "reasoning", None) if delta_message else None
	)
	saved_content = (
	getattr(delta_message, "content", None) if delta_message else None
	)
	saved_role = getattr(delta_message, "role", None) if delta_message else None
	saved_reasoning = delta_message.reasoning if delta_message else None
	saved_content = delta_message.content if delta_message else None
	saved_role = delta_message.role if delta_message else None

[gemini-code-assist]

Since delta_message is guaranteed to be an instance of DeltaMessage at this point, we can access delta_message.content directly instead of using getattr.

Suggested change

	current_content = getattr(delta_message, "content", None)
	if current_content:
	delta_message.content = saved_content + current_content
	else:
	delta_message.content = saved_content
	if delta_message.content:
	delta_message.content = saved_content + delta_message.content
	else:
	delta_message.content = saved_content

[mergify]

Hi @QingZhou-YangHY, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[QingZhou-YangHY]

Closing this PR as it is a duplicate of #42691, which was merged on May 23, 2026 and already fixes the same root cause (reasoning/content/role fields being discarded when the tool parser overwrites delta_message on a boundary delta). Apologies for the noise!