[Bugfix] Qwen3Coder streaming: emit args when whole tool body lands in one delta

[alexbi29]

Summary

Fix a streaming-tool-call bug in Qwen3CoderToolParser where the entire
<function=NAME>…</function></tool_call> body collapses into a single delta
after is_tool_call_started flipped on the previous delta. In that situation
the parser emits the header and returns, never reaches the param loop or
</function> block, and the final streamed arguments ends up as "{}".

Reproduces with:

• --tool-call-parser qwen3_coder
• --stream-interval 20
• --speculative-config '{"method":"mtp","num_speculative_tokens":3}'

…on Qwen3.6-27B (and per the linked issue, Qwen3.5 w8a8 on Ascend with similar
settings). Client-side this surfaces as Validation failed for tool "bash": command: must have required properties command / AI_TypeValidationError
because tool args arrive as {}.

Root cause

In extract_tool_calls_streaming, each "phase" (header, opening "{", param
fragments, closing "}") returns its own DeltaMessage and exits early. With
small completions plus large --stream-interval and/or MTP, a single delta
can carry the whole body once the previous delta consumed the
<tool_call> start token. The parser:

1. emits the header and returns; the same call never enters the body section,
2. never updates prev_tool_call_arr[i]["arguments"] from the initial
   "{}" placeholder,
3. never appends anything to streamed_args_for_tool[i].

The serving-layer remaining_call backfill at finish then computes
expected_call.replace(actual_call, "", 1) == "{}" and sends that as the
final args.

Fix

Accumulate header / "{" / param fragments / "}" into a single pending
DeltaMessage instead of returning after each phase. The end of the function
emits one combined delta. When the phases arrive across separate deltas
(normal non-burst streaming) the behavior is unchanged — each call sets
exactly one of the pending fields and returns it.

prev_tool_call_arr[i]["arguments"] is still updated inside the </function>
block, so the serving layer's remaining_call computation now sees the real
parsed args and there is nothing left to backfill.

Why this is not a duplicate of existing PRs

• [BugFix] Fix streaming tool call empty fields with MTP: Pydantic null serialization + qwen3coder early return #39598 — same family of bugs, but its parser fix targets only the
  params + </function> collapse: it removes the early return after the
  param loop but leaves the header phase and json_started phase early
  returns in place. With the entire body in one delta (header not yet sent),
  the header-phase early return still fires and the rest of the body is
  dropped. This PR additionally collapses the header and "{" phases into
  the same accumulator, fixing the strictly larger case. (PR [BugFix] Fix streaming tool call empty fields with MTP: Pydantic null serialization + qwen3coder early return #39598's tests
  all keep the header in its own delta, so they don't cover this scenario.)
• [Bugfix] Fix streaming tool call args blanked when entire call arrives in one delta #39615 — serving-layer fix for the replace("", "", 1) no-op when all
  args land in one delta. Doesn't help here because prev_tool_call_arr
  itself is left stale at "{}", so there is no real expected_call to
  recover; the parser must update it.
• [Bugfix] Fix Qwen3Coder prev_tool_call_arr double-emission on parse failure #41466 / [Bugfix][ToolParser] Fix Qwen3 XML and Coder streaming tool call parser regressions #40861 / [Bugfix] Fix IndexError in Qwen3CoderToolParser streaming #34495 / [Tool Parser] Qwen3Coder: stop string streaming at next <parameter=> boundary #37829 / [Bugfix][Tool Parser] Fix Qwen3 Coder parser to stream tool call arguments #32536 — different code paths
  (parse-failure double emission, anyOf/oneOf, IndexError, parameter
  streaming boundaries). Not the whole-body-in-one-delta path.

Test plan

.venv/bin/python -m pytest tests/tool_parsers/test_qwen3coder_tool_parser.py -v

55 passed (53 pre-existing + 2 new):

• test_streaming_whole_body_after_start_token — the bug case: a single
  delta delivers \n<function=…>\n<parameter=…>…</parameter>\n</function>\n</tool_call>
  after a prior delta consumed <tool_call>. Asserts the reconstructed
  args equal the expected object and that prev_tool_call_arr[0]["arguments"]
  is updated (no stale "{}").
• test_streaming_multi_param_single_chunk (existing) — multi-param-in-one
  -delta case, unchanged.

End-to-end verified against a running Qwen3.6-27B vLLM instance with the
flags above:

• Before: final stream chunk has arguments: "{}".
• After: chunks deliver {"command": "ls /tmp" followed by }.

AI assistance disclosure

This fix was developed with the assistance of Claude (Anthropic). The
submitter reviewed every line, reproduced the bug end-to-end against a
running model, verified upstream PRs don't already cover this case, and ran
the test suite locally.

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[gemini-code-assist]

Code Review

This pull request updates the qwen3coder_tool_parser.py to correctly handle streaming tool calls where multiple components, such as the function header and arguments, arrive in a single delta. This is achieved by accumulating the parsed components into local variables and emitting a single DeltaMessage at the end of the processing loop, rather than returning early. A regression test, test_streaming_whole_body_after_start_token, has also been added to ensure the parser correctly handles these collapsed deltas. I have no feedback to provide.