[Bugfix] Gemma4 streaming parser for multi-boundary tool deltas

[yasu-oh]

Purpose

Fixes #41967.

This PR fixes a Gemma4 streaming tool parser bug where a single streamed delta can contain multiple tool-call boundary events, such as closing one tool call and starting the next one in the same delta.

This can happen when vLLM emits larger streaming chunks, for example with MTP/speculative decoding or high-throughput scheduling. In that case, the existing Gemma4 streaming parser processes the whole incoming delta with one pass through the current state machine. Since only one parser branch is selected per delta, a delta that crosses multiple tool-call boundaries can cause tool-call argument fragments to be missed or attributed incorrectly.

This PR takes a deliberately narrower approach than the related PRs:

• Related: [Bugfix] Fix Gemma4 MTP streaming multi-tool calls #42006
• Related: [Bugfix] Rewrite Gemma4 streaming tool parser #42237
• Related: [Bugfix] Fix Gemma4 streaming tool calls with accumulated parser state #42300
• Related: [Fix] Gemma4 tool parser: support multiple tool calls in single delta and refactor streaming extraction #43037

The goal is to fix the broken multi-boundary case without refactoring the Gemma4 streaming parser.

The main design goals are:

• Do not rewrite the Gemma4 streaming parser state machine.
• Do not change the normal streaming path for ordinary deltas.
• Only add special handling when a single delta contains multiple Gemma4 tool-call boundary tokens.
• Reuse the existing parser logic by replaying the multi-boundary delta as smaller delimiter-aligned segments.
• Merge the resulting DeltaMessages back into one response for the original upstream delta.
• Keep the change easy to review and easy to revert.

For ordinary deltas with zero or one tool-call boundary token, the existing _extract_streaming() path is still used directly. The new segmented path is only used for multi-boundary deltas, which minimizes the risk of affecting unrelated Gemma4 streaming behavior.

AI assistance was used to help prepare this change. I reviewed the changed lines and ran the tests listed below.

Test Plan

Run the focused Gemma4 multi-boundary streaming regression test:

/app/venv/bin/python -m pytest \
  tests/tool_parsers/test_gemma4_tool_parser.py::TestStreamingExtraction::test_streaming_mtp_chunk_with_multiple_tool_boundaries -q

Run the full Gemma4 tool parser test file:

/app/venv/bin/python -m pytest tests/tool_parsers/test_gemma4_tool_parser.py -q

Run changed-file lint and formatting checks:

/app/venv/bin/python -m ruff check \
  vllm/tool_parsers/gemma4_tool_parser.py \
  tests/tool_parsers/test_gemma4_tool_parser.py

/app/venv/bin/python -m ruff format --check \
  vllm/tool_parsers/gemma4_tool_parser.py \
  tests/tool_parsers/test_gemma4_tool_parser.py

git diff --check -- \
  vllm/tool_parsers/gemma4_tool_parser.py \
  tests/tool_parsers/test_gemma4_tool_parser.py

Run pre-commit hooks on the changed files:

/app/venv/bin/pre-commit run --files \
  vllm/tool_parsers/gemma4_tool_parser.py \
  tests/tool_parsers/test_gemma4_tool_parser.py

The added regression coverage checks multi-boundary streamed deltas including:

• a delta that closes one tool call and starts the next one
• a delta that closes one tool call and completes the next one
• a first streamed delta that contains two complete tool calls

These cases cover the boundary-crossing behavior without broadening the production change.

Test Results

Focused regression test:

3 passed, 2 warnings in 0.75s

Full Gemma4 parser test file:

54 passed, 2 warnings in 13.19s

Ruff check:

All checks passed!

Ruff format check:

2 files already formatted

git diff --check:

No issues.

Pre-commit on changed files:

ruff check..........................................................................................Passed
ruff format.........................................................................................Passed
typos...............................................................................................Passed
clang-format....................................................................(no files to check)Skipped
markdownlint-cli2...............................................................(no files to check)Skipped
Lint GitHub Actions workflow files..............................................(no files to check)Skipped
pip-compile.....................................................................(no files to check)Skipped
pip-compile-rocm................................................................(no files to check)Skipped
pip-compile-xpu.................................................................(no files to check)Skipped
pip-compile-docs................................................................(no files to check)Skipped
reformat test/nightly-torch.txt to be in sync with test/cuda.in.................(no files to check)Skipped
Run mypy for Python 3.10............................................................................Passed
Lint shell scripts..............................................................(no files to check)Skipped
Lint PNG exports from excalidraw................................................(no files to check)Skipped
Check SPDX headers..................................................................................Passed
Check root lazy imports.............................................................................Passed
Check for spaces in all filenames...................................................................Passed
Update Dockerfile dependency graph..................................................................Passed
Test non-root entrypoint wrapper................................................(no files to check)Skipped
Check for forbidden imports.........................................................................Passed
Prevent new 'torch.cuda' APIs call..................................................................Passed
Validate configuration has default values and that each field has a docstring.......................Passed
Validate docker/versions.json matches Dockerfile................................(no files to check)Skipped
Check attention backend documentation is up to date.................................................Passed
Check for boolean ops in with-statements............................................................Passed
Rust - Normalize Cargo manifests with autoinherit...............................(no files to check)Skipped
Rust - Sort Cargo manifest sections.............................................Skipped
Rust - Format code..............................................................Skipped
Suggestion..........................................................................................Passed

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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. Not applicable; this is a narrowly scoped parser bug fix with regression tests.

[claude]

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