[BugFix] Fix FunctionGemma streaming tool-call arguments corrupted once a 2nd key arrives

[chuenchen309]

Why is this not duplicating an existing PR?

gh pr list --repo vllm-project/vllm --state open --search "functiongemma" and
gh api search/issues -f q='repo:vllm-project/vllm is:open functiongemma' show
no open PR/issue referencing FunctionGemma or touching this file. The only
match, #47254, is an unrelated Rust-frontend FunctionGemma implementation
that only touches rust/src/** — confirmed via gh pr view 47254 --json files, no overlap with vllm/tool_parsers/functiongemma_tool_parser.py.

Bug

FunctionGemmaToolParser.extract_tool_calls_streaming() diffs
json.dumps(current_args) against the last-sent string purely by length:

if len(current_args_json) > len(prev_streamed):
    diff = current_args_json[len(prev_streamed):]

This assumes prev_streamed is always a strict prefix of the new
json.dumps() output. But json.dumps() of a complete dict always ends
with a closing } — so as soon as a second argument key arrives, that
stale } from the previous (shorter) dict sits in the middle of
prev_streamed, breaking the prefix assumption. The client-side
concatenation of streamed argument deltas then contains a premature closing
brace followed by more content: invalid JSON.

Repro (also the added regression test)

Streaming these 5 deltas (a normal token-by-token pattern once more than one
argument key is involved):

'<start_function_call>call:get_weather{'
'ci'
'ty:<escape>NYC<escape>'
'unit:<escape>celsius<escape>'
'}<end_function_call>'

previously reconstructed on the client side to:

'{"city": "NYC"} "unit": "celsius"}'   # invalid JSON

This reproduces with any FunctionGemma tool call that has 2+ arguments
split across streaming deltas — not an edge case, this is the normal
streaming pattern.

Fix

Mid-stream, the argument dict isn't closed yet (more keys may still arrive),
so strip the trailing } that json.dumps() always adds before treating it
as the diffable/stored state — the real closing brace is only ever sent
once, from the "function call just ended" branch, which already computes
the true final arguments. Compute the diff via find_common_prefix()
(already used elsewhere in vllm/tool_parsers/utils.py for the same
partial-JSON-diffing purpose) instead of a raw length slice, so a
same-length or shorter update also diffs correctly. The same length-based
diff — and now the same fix — was duplicated in the "function call just
ended" branch; both call sites are updated.

Testing

• Independently reproduced the corruption against the actual installed
  vllm package (VLLM_USE_PRECOMPILED=1 uv pip install -e . --torch-backend=auto) via object.__new__(FunctionGemmaToolParser) +
  manual streaming state — not just static analysis.
• Confirmed the fix produces valid, correctly-ordered JSON for both a 2-key
  and 3-key streaming case.
• Added TestExtractToolCallsStreaming to
  tests/tool_parsers/test_functiongemma_tool_parser.py (previously zero
  streaming-path test coverage existed for this parser) covering the exact
  repro plus a 3-key variant. Confirmed red→green: reverting only the source
  change reproduces json.decoder.JSONDecodeError: Extra data for both new
  tests; reapplying passes all 17 tests in the file (15 pre-existing + 2
  new).
• pre-commit run ruff-check, ruff-format, mypy-3.12 (all --files
  scoped to the 2 changed files): pass.

Model-eval N/A — this only changes streaming-argument diff bookkeeping
logic, not model output/accuracy.

AI-Generated disclosure

Found via an AI-assisted code review pass (Claude Code) over
vllm/tool_parsers/. I independently reproduced the corruption against the
real installed package, traced the root cause (the length-diff-vs-json.dumps
closing-brace mismatch), verified the fix, and ran the tests above myself
before submitting.

[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.

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