fix: streaming tool calls drop for Qwen3.6 bracket format

[mikepixelmagic-dev]

Summary

Fixes two bugs that cause Qwen3.6 [Calling tool: name({...})] streaming tool calls to leak into text content instead of emitting structured tool_calls.

Bug 1 — server.py _stream_responses_request fast-path gate

The Responses API streaming path still uses the old "<" not in delta_text gate to decide whether to engage the tool parser. That gate only matches <tool_call> / <function= shapes — bracket-format deltas start with [, so they skip the parser entirely and get emitted as plain text.

The other 4 streaming paths (Anthropic messages and OpenAI chat completions, reasoning and non-reasoning branches) were already refactored to use _streaming_tool_markup_possible() in prior PRs (see refs below) but this path was missed.

Fix: use _streaming_tool_markup_possible(tool_accumulated_text + delta_text), matching the other 4 paths.

Bug 2 — qwen_tool_parser.extract_tool_calls_streaming closing-marker check

Detection of a completed bracket-format tool call looks for )] or </tool_call> in delta_text only. In practice those closing markers routinely span token boundaries — e.g. ) arrives in one delta and ] in the next — so the check never fires, the parser returns None for every chunk, and the entire call gets suppressed without ever being emitted as structured tool_calls.

Fix: check current_text (accumulated) instead of delta_text, so the close is detected reliably regardless of token splits.

Reproduction

Serve Qwen3.6-35B-A3B-8bit with:

vllm-mlx serve mlx-community/Qwen3.6-35B-A3B-8bit \
  --enable-auto-tool-choice \
  --tool-call-parser qwen \
  --reasoning-parser qwen3

Run a streaming chat completion with multiple tools (e.g. a create_file tool with path and content parameters). The model emits a bracket-format tool call. Without these fixes, the client receives [Calling tool: create_file({"path": "...", "content": "..."})] as content with finish_reason: null and zero tool_calls deltas. With these fixes, a proper structured tool_calls delta is emitted and finish_reason: tool_calls is set.

A 40-turn drift test cycling through 5 tools (get_weather, read_file, web_search, calculator, create_file) held 4 turns before drifting pre-fix, and held all 40 turns post-fix with no drift.

Related work

• auto tool parser: bare bracket format [func({...})] not recognized #146 / fix(auto-parser): support bare bracket tool calls #305 added bare-bracket recognition to the auto parser
• fix(server): stream structured tool calls without parser flags #304 refactored generic streaming tool calls
• fix: suppress tool call XML from streaming text content (#129) #232 added XML suppression from streaming text

This PR completes the bracket-format coverage by fixing the one remaining Responses API path and the qwen tool parser closing-marker detection.

Test plan

• Streaming chat completion with Qwen3.6 and a create_file-style tool emits structured tool_calls
• Multi-turn drift test (40 turns, 5 tools) completes without format drift
• Other streaming paths (Anthropic messages, OpenAI chat, Responses API) still pass existing tool-call tests