[Bugfix] Fix Qwen3Coder prev_tool_call_arr double-emission on parse failure

[ToastyTheBot]

Summary

When _parse_xml_function_call fails during streaming (returns None or throws), prev_tool_call_arr still holds the "{}" placeholder from the header-sent step. The serving layer's remaining-args check then sees "{}" as the expected argument and computes a wrong remainder, causing {"arguments": "{}"} to be emitted twice.

Context

We run Qwen3.6-27B-FP8 with MTP=3 in production and observed a suite of tool-calling issues with speculative decoding. After applying several open PRs together, tool call reliability improved dramatically:

• [Bugfix] Fix Qwen3 reasoning parser: raw text tags, transition loss, end detection, token counting, withhold recovery #40783 — fragmented <think/> tag handling in reasoning parser
• [Bugfix][ToolParser] Fix Qwen3 XML and Coder streaming tool call parser regressions #40861 — structural parsing fixes in Qwen3Coder tool parser
• Fix Qwen3 reasoning tool calls embedded inside think #39055 — embedded tool call recovery from reasoning
• [Bugfix] Grammar was ignored when reasoning ended within speculated tokens #36138 — grammar ignored when reasoning ends within speculated tokens
• [BugFix] Fix streaming tool call empty fields with MTP: Pydantic null serialization + qwen3coder early return #39598 — Pydantic null serialization in streaming tool call deltas

We also opened two other sibling draft PRs in an attempt to perfectly fix the issue:

• [Bugfix] Detect MTP truncation at reasoning-to-tool-call boundary #41467 — Detect MTP truncation at reasoning-to-tool-call boundary
• fix(spec decode): suppress EOS at draft positions in rejection sampler #41493 — Suppress EOS at draft positions in rejection sampler

This PR addresses a remaining edge case where XML parsing fails mid-stream and the placeholder "{}" leaks through to the client.

Root Cause Analysis

During streaming tool call parsing, the Qwen3CoderToolParser sends a tool call header with "{}" as the placeholder arguments in prev_tool_call_arr. After the header is sent, incremental text is parsed via _parse_xml_function_call to extract the actual arguments.

When _parse_xml_function_call fails (returns None or throws an exception), prev_tool_call_arr still contains the "{}" placeholder. The serving layer's _should_check_for_unstreamed_tool_arg_tokens check then sees "{}" as the expected arguments and computes a remainder of zero — but the incremental streamed_args_for_tool contains the actual streamed text. This mismatch causes {"arguments": "{}"} to be emitted as a delta, double-emitting the empty arguments.

Changes

• Adds a parse_succeeded flag to track whether _parse_xml_function_call returned a valid result
• After the try/except block, if parsing failed, copies the incrementally streamed arguments from streamed_args_for_tool into prev_tool_call_arr plus the closing } (appended because line 1335 adds } to streamed_args_for_tool after the fallback runs — without it the serving layer's remainder check loses the brace and produces broken JSON)
• Uses an explicit flag to avoid false positives where "{}" is a legitimate empty-parameter result

Why This Is Safe

• Only activates on the failure path (_parse_xml_function_call returns None or throws)
• The success path (normal parsing) is completely unchanged
• The fallback produces the same result as a successful parse would have — the streamed arguments are copied directly
• Bounds checks on current_tool_index prevent index out of range

Reproduction

Stream a tool-calling request with the qwen3_coder parser where the model's XML output is malformed enough to cause _parse_xml_function_call to fail. The double-emission of "{}" appears in the streamed deltas.

Test Plan

• Verify streaming tool calls with qwen3_coder parser still work normally when parsing succeeds
• Verify that when _parse_xml_function_call fails, the streamed arguments are used as fallback instead of "{}"
• Run pytest tests/tool_parsers/test_qwen3coder_tool_parser.py

[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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[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism in the qwen3coder_tool_parser to handle failures in XML function call parsing by utilizing incrementally streamed arguments. Feedback indicates that the fallback value should include a closing brace to ensure the serving layer correctly identifies the remainder and maintains JSON integrity.