fix(parser): accept Markdown-fenced Qwen tool calls

[xiaguan]

Summary

• accept one matching json or bare Markdown fence around Qwen XML tool-call JSON
• buffer fenced calls until the closing fence, validate the arguments as JSON, and commit one atomic tool-call event
• preserve the exact uncommitted model output through reset() when the fence is missing or the enclosed JSON is invalid
• keep existing unfenced Qwen streaming behavior unchanged

This is a review-only draft in a personal fork. It must not be submitted upstream until a human has reviewed every changed line and rerun the relevant tests.

Scope

This implements only the first deterministic repair case from openinfer-project/openinfer#733: an otherwise-valid Qwen tool-call object wrapped in one Markdown code fence.

It does not repair schema violations, change function names or argument values, enforce tool_choice, or add a retry controller.

Why this is not duplicate work

Searches for open PRs covering Qwen3XmlToolParser, Markdown fences, and fenced tool-call JSON found no matching implementation. The related open PRs found in broader searches (vllm-project#44993, vllm-project#48516, and vllm-project#36138) address structured-output grammar advancement around reasoning/speculative-decoding boundaries, not frontend parser recovery.

Validation

• cargo fmt --all -- --check
• cargo test -p vllm-parser --release --offline: 385 passed
• cargo clippy -p vllm-parser --all-targets --offline -- -D warnings
• relevant pre-commit hooks: typos, check-spdx-header, and rust-cargo-fmt
• independent toxic-reviewer pass: approved after fixes for invalid-JSON acceptance, per-chunk buffering, and invalid configuration combinations

New tests cover both accepted opening fences, chunked atomic commit, unmatched-fence recovery, and invalid-JSON recovery. Existing unfenced malformed-but-balanced argument behavior remains unchanged for compatibility.

Performance

Criterion was run from independent build targets pinned to the same CPU. Median main → patch results:

Existing path	Main	Patch	Change
mixed text/tool calls, parser reuse	3.6970 µs	3.7286 µs	+0.85%
mixed text/tool calls, parser create	4.1929 µs	4.2551 µs	+1.48%
long normal text, parser reuse	1.4283 ms	1.4099 ms	-1.29%
long normal text, parser create	1.4245 ms	1.4043 ms	-1.42%

No optimization win is claimed; these measurements are the ordinary-path regression check.

Model evaluation

No post-change model evaluation was run because this parser-only fixture requires deliberately inducing a malformed Markdown wrapper. The preceding Qwen3-8B evaluation on the existing unfenced path passed 44/44 ordinary tool-call checks; the new malformed-output behavior is covered deterministically at the parser boundary.

AI assistance disclosure

AI assistance was used to implement, test, benchmark, review, and draft this change. The commit is authored and signed off by OpenAI Codex. A human submitter must understand and defend the change end to end before any upstream PR is opened.

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[gemini-code-assist]

Code Review

This pull request introduces support for parsing Markdown-fenced JSON tool calls (e.g., wrapped in 'json' or '' blocks) within the JSON tool parser. It adds an 'allow_markdown_fence' configuration option, which is enabled for Qwen XML-wrapped tool calls and disabled for other models. The parser is updated to handle these fences by buffering and validating the fenced JSON arguments before emitting the tool call. Additionally, comprehensive unit tests have been added to verify the new functionality under various scenarios, including streaming and error recovery. I have no feedback to provide as there are no review comments.

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