[Bugfix] Fix Gemma4 streaming tool calls with accumulated parser state

[bevenky]

Purpose

Fix Gemma4 streaming tool-call corruption when streamed deltas contain split tool-call boundaries, multiple tool-call transitions, or MTP/speculative token-id chunks with missing text. These cases can produce missing or malformed streamed tool-call arguments.

The streaming parser now uses accumulated Gemma4 text as structured state and diffs that state per tool-call index. In particular, it:

• keeps a Gemma4-local accumulated text buffer,
• parses visible tool-call regions into structured state,
• emits tool-call headers as soon as the function name is known,
• streams only stable, append-only partial argument prefixes,
• withholds unstable split suffixes such as partial delimiters, trailing numeric/exponent fragments, booleans, nulls, arrays, and nested objects until they can be emitted safely,
• repairs missing Gemma4 tool boundary text from token IDs for MTP/speculative paths,
• keeps non-tool content behind a cursor with partial_tag_overlap,
• keeps Gemma4 streaming state private so the generic terminal backfill path does not rewrite complete Gemma4 deltas,
• exposes only the first call when parallel_tool_calls=false.

This follows an accumulated-buffer / structured-diff approach, similar in spirit to robust tool parsers such as llama.cpp, while keeping the implementation inside vLLM's Gemma4 parser and OpenAI-compatible response structures.

Related work:

• Fixes [Bug]: Gemma4 + MTP speculative decoding drops first tool-call arguments in streaming multi-tool auto-tool-choice #41967.
• Alternative to [Bugfix] Fix Gemma4 MTP streaming multi-tool calls #42006 and [Bugfix] Rewrite Gemma4 streaming tool parser #42237.
• Covers the streaming numeric corruption class from [Bugfix] Fix Gemma4ToolParser streaming float corruption #42128 / [Bug] Gemma4ToolParser streams incorrect float values (e.g., 108.2 → 108.02) #42047, extended to unstable split strings, booleans, nulls, arrays, nested objects, and exponents.

Why this is not duplicating existing PRs:

• [Bugfix] Fix Gemma4 MTP streaming multi-tool calls #42006 keeps the old parser model and replays split segments. This PR replaces the event-order-dependent path with accumulated structured parsing.
• [Bugfix] Rewrite Gemma4 streaming tool parser #42237 has the same broad direction, but does not cover token-id repair, private Gemma streaming state, or the parallel_tool_calls=false behavior validated here.
• [Bugfix] Fix Gemma4ToolParser streaming float corruption #42128 fixes a narrow float case; this PR covers that class as part of the larger streaming parser fix.

AI assistance was used to help draft and validate this change. The submitter should review every changed line before marking this PR ready for review.

Test Plan

Added parser regression tests for:

• complete tool call in one delta,
• multiple complete calls in one delta,
• close-then-open in one delta,
• split start delimiter after a completed call,
• split string suffixes such as src/main. + rs and e + xplore,
• stable partial argument streaming before the Gemma4 end marker,
• partial string delimiter overlap withholding,
• nested object/array partial argument prefixes,
• split floats, booleans, nulls, and other unstable bare values,
• syntax-aware partial JSON suffix trimming,
• parallel_tool_calls=false.

Test Result

PYTHONPATH=/workspace/vllm-main-check \
  /workspace/gemma4_stream_patch_20260510/pytest_sys_venv/bin/python \
  -m pytest tests/tool_parsers/test_gemma4_tool_parser.py -q
# 72 passed

PYTHONPATH=/workspace/vllm-main-check \
  /workspace/gemma4_stream_patch_20260510/pytest_sys_venv/bin/python \
  -m pytest tests/reasoning/test_gemma4_reasoning_parser.py \
            tests/tool_parsers/test_gemma4_tool_parser.py -q
# 103 passed

git diff --check
# passed

Concurrent production-shaped replay used the existing repro_vllm_stream_tool_payload.py harness with 160 requests / 80 concurrency per row:

engine	thinking	max tokens	parallel_tool_calls	stream=true	stream=false
non-MTP	off	500	false	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
non-MTP	off	500	true	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
non-MTP	on	1200	false	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
non-MTP	on	1200	true	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
MTP	off	500	false	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
MTP	off	500	true	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
MTP	on	1200	false	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing
MTP	on	1200	true	160/160 ok, 0 malformed, 0 missing	160/160 ok, 0 malformed, 0 missing

Also replayed the existing edge-case harness, run_tool_stream_edge_cases.py, at 160 requests / 80 concurrency for MTP and non-MTP. It reported no malformed-argument or argument-before-header failures. Remaining red rows were model-compliance misses where the model chose fewer tool calls than the case expected, not parser corruption.

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[gemini-code-assist]

Code Review

This pull request refactors the Gemma4ToolParser to enhance streaming reliability, particularly for speculative and multi-token prediction (MTP) scenarios where text deltas may be omitted. It introduces a mechanism to repair missing text from token IDs and improves the handling of type-unstable partial values, such as numeric literals ending in decimals or exponents. Feedback suggests that the current implementation of _diff_streaming_tool_calls unnecessarily delays the emission of tool call arguments until the call is complete, which negatively impacts the incremental streaming experience. It is recommended to allow partial argument delivery if the parsing logic for unstable values is sufficiently robust.

[gemini-code-assist]

The current implementation of _diff_streaming_tool_calls skips any tool call that is not marked as complete. This means that tool call arguments are only emitted once the entire tool call is finished (i.e., when the <tool_call|> tag is encountered). While this approach is robust against the type-instability issues mentioned in the PR description, it significantly degrades the streaming experience by preventing incremental argument delivery.

If the partial=True logic in _parse_gemma4_args is correctly implemented to withhold unstable trailing values, it should be safe to stream partial arguments. Consider removing this continue to allow incremental argument streaming, which is the expected behavior for streaming tool calls in vLLM.

[bevenky]

Thanks, agreed. I updated the parser in 1833701 to remove the complete-only gate and stream partial arguments incrementally when they are stable and append-only. The partial Gemma4 argument path now withholds unstable suffixes, trims only syntactic trailing JSON closers for incomplete calls, and refuses to emit a delta if the next parsed argument string would rewrite already-emitted bytes. I also added regression coverage for stable partial strings, delimiter overlap, nested object/array prefixes, and syntax-aware suffix trimming, then reran the 160/80 MTP and non-MTP replay matrix for stream=true/false, parallel_tool_calls=true/false, and thinking on/off.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[abinggo]

Thanks @bevenky for the systematic refactor — the structured-diff approach with _is_unstable_partial_bare_value is a clean generalization of the narrow float fix in #42128. Happy to close mine in favor of this once it lands.

A few observations from reading through the diff:

Confirmed coverage

The float corruption case from #42128 is handled end-to-end:

• _is_unstable_partial_bare_value is a strict superset of my endswith(".") check
• test_trailing_dot_float_partial_withheld covers both dict and array paths
• test_streaming_trailing_dot_float_split_across_chunks gives end-to-end validation
• The separate _NUMBER_LITERAL_RE fix also addresses a latent bug where exponent-form numbers like "1e3" were silently coerced to strings in non-streaming mode. Nice catch.

Test coverage gap

_is_unstable_partial_bare_value withholds four classes of unstable values: trailing dot, trailing exponent/sign (1e, 1e+), partial booleans (tru, fals), and partial nulls (nu, nul). The current tests pin only the trailing-dot branch. Since this function is the single source of truth for partial-value stability, it'd be worth pinning the other branches too. Suggested additions (for TestParseArgs):

def test_partial_bool_withheld(self):
    assert _parse_gemma4_args("flag:tru", partial=True) == {}
    assert _parse_gemma4_args("flag:fals", partial=True) == {}
    assert _parse_gemma4_args("flag:true") == {"flag": True}

def test_partial_null_withheld(self):
    assert _parse_gemma4_args("x:nu", partial=True) == {}
    assert _parse_gemma4_args("x:nul", partial=True) == {}
    assert _parse_gemma4_args("x:null") == {"x": None}

def test_partial_exponent_withheld(self):
    assert _parse_gemma4_args("score:1e", partial=True) == {}
    assert _parse_gemma4_args("score:1e+", partial=True) == {}
    assert _parse_gemma4_args("score:1e3") == {"score": 1000.0}

And for TestParseArray:

def test_partial_bare_literal_withheld(self):
    assert _parse_gemma4_array("tru", partial=True) == []
    assert _parse_gemma4_array("nu", partial=True) == []
    assert _parse_gemma4_array("1e", partial=True) == []
    # Stable elements before unstable tail are kept
    assert _parse_gemma4_array("42,tru", partial=True) == [42]

Minor observation

_compute_arguments_diff has a structure-regression branch:

if prev_streamed and not arguments_json.startswith(prev_streamed):
    return None

This silently waits for self-healing. Correct for the tested cases, but a logger.debug here would help diagnose unexpected structural regressions in production — otherwise the client sees a stalled diff with no trace. Non-blocking.

Happy to send these test additions as a follow-up PR against your branch if that's easier than inlining, or you can lift them directly into #42300.

[bevenky]

@abinggo Thanks, added direct coverage for the remaining unstable partial literal classes in dict and array parsing. I’m leaving the debug log out for now since the branch is intentionally silent self-healing.

[abinggo]

@bevenky Thanks for the quick turnaround on the tests, and agreed on keeping the branch silent — self-healing by design, extra logging would add more noise than signal.

One thing I wanted to ask — with the four tests going in verbatim and #42128 being covered by this, it genuinely feels like shared work on the same bug. Would you be open to adding Co-authored-by: abinggo <107740309+abinggo@users.noreply.github.com> to the commit? The refactor is absolutely yours — I just think it'd be nice to have the collaboration reflected in git history. I'd really appreciate it if you'd consider this.

[chaunceyjiang]

/cc @bbrowning @sfeng33 PTAL.

[bbrowning]

Wow, quite a rewrite of the parser here. That immediately makes me hesitant to even consider merging or approving this, as opposed to small incremental fixes that are easily understandable. I do appreciate the unit tests added, but what real-world testing has been done? Does this raise or lower the score on any common eval suites for this model with vLLM? Have you run this against one or more agent harnesses that was exhibiting problems before and is now fixed?

[abinggo]

@bbrowning Noting your preference for small incremental fixes — I had #42128 open for exactly this narrow float corruption case (~20 lines, zero refactor, isolated to the partial-value withholding check in _parse_gemma4_args / _parse_gemma4_array). I closed it earlier in favor of this broader refactor, but given your concerns I'll reopen it as the small-scope alternative.

Not meant to compete with #42300 — the broader refactor still has real value for the other bug classes (partial bool/null, trailing exponent/sign, MTP token-id repair). Just making the narrow fix available if reviewers prefer the incremental path.

[whytem]

Thanks for putting this together. This PR looks like it consolidates several issues that already have active PRs/discussions, especially #42006, #42128, and #42237. I think it may be better to let those narrower discussions reach their natural conclusion before replacing them with one larger combined PR.

There are definitely valuable hardening fixes here, though. From reviewing the tests, many of them do not seem inherently dependent on adopting the more invasive accumulated parser state model. My concern is that bundling them together risks conflating separable parser correctness fixes with the larger architectural choice, and those smaller improvements could be harder to land if maintainers decide not to pursue the full rewrite.

My suggestion would be to split out the independent parser hardening pieces:

• exponent numeric literal parsing and partial exponent withholding could extend [Bugfix] Fix Gemma4ToolParser streaming float corruption #42128, or become a small follow-up PR in the same area
• partial string delimiter trimming inside Gemma arg parsing could be its own focused parser fix
• syntax-aware partial JSON suffix trimming could also be reviewed independently

The parallel_tool_calls=false fix doesn't appear to be needed. vLLM already correctly handles this above the parser level at vllm/entrypoints/openai/utils.py (line 22) and this IMO is a better solution than silently dropping content at the parser level.

The token-ID repair path is the one piece I do not fully understand yet. It sounds like it may target an MTP/speculative decoding edge case where delta_token_ids contain Gemma tool boundary tokens that are missing from delta_text. If so, it would be useful to contribute that minimal failing test case to the #42006 discussion as well, since it may affect the choice between segment replay and parser-owned accumulated text.

So overall I think this PR contains useful work, but I’d personally prefer to split the independent fixes out from the larger parser rewrite decision.

[bevenky]

@bbrowning @whytem Fair points on having a smaller surface area and splitting the PR. The reason I took a larger surface area here is, there are quite a few cases/issues open and were breaking for us too and fixing them one by one was a lot of band-aid work vs redoing the parser logic (inspired by llama.cpp). Regarding the question on the test cases, I did not use a public suite, but used an internal suite that failed on higher concurrency. I am happy to add that to this PR after cleaning up any internal data. Is there any public test suite you would recommend I test before-after on? @bbrowning

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @bevenky.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork