[Bugfix] Preserve reasoning, content, and role fields on streaming boundary delta transitions

[Alex-ai-future]

Purpose

This PR fixes a bug in DelegatingParser.parse_delta where reasoning content (and other metadata) is silently overwritten and lost when a single streaming token chunk spans across the transition boundary from the reasoning phase to the content/tool-call phase (which is extremely common during Speculative Decoding / MTP).

Root Cause

When a boundary-spanning chunk (e.g. think about this</think><tool_call>{\"name\") arrives:

1. The reasoning block in parse_delta extracts the reasoning content (" think about this") and assigns it to delta_message.
2. It detects the end of the reasoning phase and transitions the state (state.reasoning_ended = True).
3. In the same call, since the state has transitioned, the tool-call parser is immediately invoked on the remaining portion of the chunk.
4. The tool-call parser returns a new DeltaMessage (for the tool calls) and overwrites the delta_message variable, completely discarding the reasoning/content extracted in step 1.

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Comparative Analysis & Why This Is Not a Duplicate

There are two open PRs addressing this issue: #42691 and #43055. This PR does not duplicate them; instead, it proposes a superior, best-of-both-worlds architectural approach:

Dimension	PR #42691	PR #43055	This PR (Our Solution)
Boundary Detection	Uses is_reasoning_end_streaming (fixes token-split bugs)	Uses is_reasoning_end (vulnerable to token-split bugs)	Uses is_reasoning_end_streaming (robust)
Preserved Fields	Only reasoning (drops content or role if set)	reasoning, content, role	reasoning, content, role (complete)
Code Architecture	Reuses delta_message via ad-hoc save/restore variables	Reuses delta_message via ad-hoc save/restore variables	Decoupled variables (reasoning_delta & tool_delta) with clean merge at the end
No-Tool Transition	Supported	Not handled	Supported

Key Improvements in This PR:

1. Decoupled Variable Design: Instead of mutably overwriting and restoring fields on the same delta_message object back-and-forth, we isolate the outputs of the reasoning and tool-calling phases into separate reasoning_delta and tool_delta variables. We then perform a clean, pure merge at the end of the function.
2. Robust Boundary Check + All-Field Merging: We combine the streaming-aware boundary check is_reasoning_end_streaming from [Bugfix] Fix reasoning dropped on streaming boundary deltas #42691 (to handle </think> being sliced across token boundaries) with the complete field restoration (reasoning, content, and role) from [Bugfix] Preserve reasoning in streaming deltas spanning phase boundary #43055.

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Proposed Changes

vllm/parser/abstract_parser.py

• Overrode is_reasoning_end_streaming in DelegatingParser to properly delegate to the underlying reasoning parser's streaming-aware method.
• Updated parse_delta to separate phase outputs and cleanly merge all set fields (reasoning, content, role) at the final step.
• Handled the no-tool-parser transition case by updating delta_text and delta_token_ids to the remaining content when reasoning ends, while preserving the content of the reasoning chunk if no tool parser is active.

tests/parser/test_streaming.py

• Added test_parse_delta_transition_chunk regression test to simulate a single boundary-spanning chunk containing both the end of reasoning (</think>) and a tool call.

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Test Plan

Automated Tests

Run the streaming parser test suite bypassing the local PyTorch MPS allocator cleanup issue:

.venv/bin/python -m pytest tests/parser/test_streaming.py -v --noconftest

Results:

======================== 6 passed, 2 warnings in 17.25s ========================

Linters and Typecheckers

Ran all pre-commit hooks on modified files:

.venv/bin/pre-commit run --files vllm/parser/abstract_parser.py tests/parser/test_streaming.py

Purpose

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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[gemini-code-assist]

Code Review

This pull request enhances the streaming parser to correctly handle transitions between reasoning and tool call phases within a single data chunk, supported by a new test case. A significant issue was identified in the parse_delta logic where failing to properly reset the current text when reasoning ends could lead to reasoning content leaking into subsequent messages; a unified approach for updating the current text was suggested to resolve this.

[gemini-code-assist]

The logic for updating current_text when reasoning ends is incomplete. If _tool_parser is None and reasoning_delta.content is also None (or reasoning_delta is None), current_text is not updated and remains the full accumulated text (including the reasoning part). This causes delta_text to be incorrect at line 702 and state.previous_text to be corrupted at line 757. This can lead to the entire reasoning block being leaked into the content field in subsequent calls when the parser hits the fallback at line 755.

A unified approach ensures current_text is always correctly transitioned to the content part (or an empty string if no content exists) regardless of whether a tool parser is present.

                if reasoning_delta and reasoning_delta.content:
                    current_text = reasoning_delta.content
                    if self._tool_parser:
                        reasoning_delta.content = None
                else:
                    current_text = ""