[Bugfix] Constrain tools + response_format with a union grammar in Qwen3XMLToolParser

[areslp]

Purpose

Fixes silently dropped tool calls when tools and response_format are combined. Same defect as the still-open upstream issue vllm-project/vllm#39929.

The bug

A chat request carrying both tools and a response_format (json_schema or json_object) with tool_choice absent or "auto" silently loses its tool calls: tool_calls comes back empty, HTTP 200, no warning, and the model returns a fabricated schema-conforming answer instead of calling the tool. Output tokens collapse from thousands to a few dozen.

Root cause

1. to_sampling_params (entrypoints/openai/chat_completion/protocol.py) converts response_format into a grammar constraining the entire output, never consulting tools / tool_choice.
2. get_json_schema_from_tools() (tool_parsers/utils.py) returns None for "auto", so no tool grammar is built — "auto" tool calling normally works by free generation plus post-hoc text parsing.
3. Only the answer-schema grammar is installed, and the literal <tool_call> prefix falls outside its allowed token set. The model is not "choosing not to call" — it physically cannot emit the opening token.
4. The asymmetry: ToolParser.adjust_request sets the tool schema and clears response_format on the required / named paths, so the tool grammar wins there. Only "auto" leaves response_format governing decoding.

The json_object case shows this most clearly. The model tries to call the tool, and the grammar deforms the call into a JSON object returned as plain content:

{"name": "get_secret_token", "arguments": {"city": "Paris"}}

The intent survives; the wire format does not, so nothing parses it.

Qwen3XMLToolParser overrides neither adjust_request nor get_structural_tag, so it inherits this wholesale. That is the entire gap — Qwen3CoderToolParser already overrides get_structural_tag for the same wire format.

The fix

Override adjust_request in Qwen3XMLToolParser. It intervenes only when the request is a ChatCompletionRequest with non-empty tools, tool_choice == "auto", and a json_schema/json_object response_format. Everything else returns super().adjust_request(request) unchanged.

In that one case it installs an xgrammar OrFormat union as structured_outputs.structural_tag:

output        ::= optional-whitespace ( tool-branch | answer-branch )
tool-branch   ::= ( "<tool_call>\n<function=NAME>\n" params-xml "\n</function>\n</tool_call>" )+   sep "\n"
answer-branch ::= JSON matching response_format

and clears response_format — mandatory, otherwise to_sampling_params derives a second constraint and the single-constraint validation raises through replace().

Both branches stay available on every turn, so a multi-turn agent loop works: tool-call turns early, schema answer on the final turn once tool results are back.

Three non-obvious design points

Tool branch shape. It reuses get_model_structural_tag(model="qwen_3_5", ...) with tool_choice="required" — a TagsWithSeparatorFormat — rather than the TriggeredTagsFormat that "auto" would produce. Measured on a Qwen3 tokenizer (vocab 248077), allowed tokens at position 0:

grammar	allowed @ pos 0	% vocab
answer schema alone	2	0.00%
tools alone, TagsWithSeparator	3	0.00%
tools alone, TriggeredTags	248075	100.00%
union via TagsWithSeparator	5	0.00%
union via TriggeredTags	248075	100.00%

TriggeredTagsFormat permits free text before the trigger, so the union's mask degenerates to nearly everything and the answer branch becomes unreachable. This also explains why "auto" tool calling works at all today: it is fully unconstrained plus post-hoc parsing.

Leading whitespace must be allowed. The chat template emits </think>\n\n before content, and with enable_in_reasoning=False the grammar binds at the token immediately after </think>. Without the whitespace prefix, the tool-vs-answer decision is taken at a token whose natural probability mass is masked out, decided by residual mass instead — that passes smoke tests and picks the wrong branch intermittently under load. With it, position 0 goes from 5 to 164 allowed tokens, still tight, and whitespace alone cannot terminate the match.

Parallel-call separator. structural_tag_registry.py hardcodes separator="" for the qwen_3_5 required branch, but the chat template emits \n<tool_call> between parallel calls, so a real </tool_call>\n<tool_call> sequence would be rejected and parallel calling would silently collapse to a single call. This PR corrects it at the use site via model_copy. The registry itself deviates from both the chat template and xgrammar's own builder here (the deepseek_v4 builder correctly uses "\n"), which also affects the strict-mode required path — left for a separate change to keep this PR focused.

Self-validation and graceful degradation

The union compiles every tool's parameters through the qwen_xml converter, which the "auto" path never did before — a new exposure surface. A schema feature xgrammar cannot express (regex lookahead, for example) would raise and, under backend="auto", fall back to the guidance backend, which raises KeyError: 'triggers' on v2 structural tags.

So adjust_request self-validates with Grammar.from_structural_tag and, on failure, retries with tool parameters left unconstrained before finally falling back to the previous behaviour with a warning. The relaxed retry is faithful rather than lossy: today's "auto" path does not constrain tool arguments at all, and the XML call structure stays constrained either way.

Known limitations

• Requests that set the native structured_outputs field without response_format hit the same mechanism and are not fixed here. When structured_outputs already carries a constraint, this code deliberately skips the union and defers to the base path rather than stacking a second constraint.
• The Responses API (text.format) has the identical bug and is untouched.
• get_structural_tag is deliberately not added. Under VLLM_ENFORCE_STRICT_TOOL_CALLING=1 it would activate base Step 1, which installs a structural tag but never clears response_format, breaking required / named + response_format where they work today. Qwen3CoderToolParser has that same latent issue; base Step 1 should be fixed before parity is worth restoring.

Test Plan

Serve a Qwen3 XML tool-calling model with --enable-auto-tool-choice --tool-call-parser qwen3_xml --reasoning-parser qwen3, then issue the request below. The tool returns a value the model cannot possibly know, so any content-only answer is necessarily fabricated — that makes the failure unambiguous:

curl -s localhost:8000/v1/chat/completions -H 'Content-Type: application/json' -d '{
  "model": "<qwen3-xml-model>",
  "messages": [{"role": "user", "content": "What is the secret token for Paris? Use the tool."}],
  "tools": [{"type": "function", "function": {
    "name": "get_secret_token",
    "description": "Returns the random secret token for a city.",
    "parameters": {"type": "object", "properties": {"city": {"type": "string"}}, "required": ["city"]}}}],
  "response_format": {"type": "json_schema", "json_schema": {"name": "a", "strict": true, "schema": {
    "type": "object", "properties": {"token": {"type": "string"}},
    "required": ["token"], "additionalProperties": false}}},
  "temperature": 0
}'

The full matrix sweeps the same request across response_format in {none, json_schema strict, json_schema lax, json_object, {"type":"text"}} × tool_choice in {absent, "auto", "required", named, "none", explicit null}, plus streaming, two-tool, no-tools, and a no-tool-needed prompt that must still produce a schema answer.

Test Result

15-case matrix against a live server (temperature: 0, seed: 42):

#	response_format	tool_choice	before	after
01	none	absent	1 call	1 call
02	json_schema strict	absent	0, fabricated	1 call
03	json_schema lax	absent	0, fabricated	1 call
04	json_object	absent	0, call deformed into content	1 call
05	{"type":"text"}	absent	1 call	1 call
06	json_schema strict	"auto"	0, fabricated	1 call
07	json_schema strict	"required"	1 call	1 call
08	json_schema strict	named	1 call	1 call
09	json_schema strict	absent, streaming	0 deltas	1 logical call
10	json_schema strict	"none"	schema answer	unchanged
11	none	"none"	raw XML in content	unchanged
12	answer-branch schema	absent	schema answer	schema answer
13	json_schema strict, 2 tools	absent	0, fabricated token	1 call
14	no tools + schema	—	schema answer	unchanged
15	json_schema strict	explicit null	schema answer	unchanged

7 broken cases fixed, 8 working cases unchanged. Case 12 is the important one: the answer branch stays reachable when no tool is needed, so the union genuinely selects a branch rather than always taking the tool path.

Additional live checks:

• Parallel calls with response_format return 2 calls (Paris + London) — this is what the separator correction buys.
• A tool whose parameter carries a regex lookahead still returns a correct call via the relaxed retry, with the warning logged; no hard fallback, no traceback.
• No tracebacks, KeyError, or Invalid structural tag in the server log across the whole sweep.

Offline, 31 assertions against the real tokenizer cover branch acceptance and rejection ({"nope": 1} rejected), stop-token reachability after each branch, the mask counts in the table above, the enable_in_reasoning=True wrapper, and byte-identical equivalence with the base adjust_request for every non-intervened request shape (auto without response_format, required, named, "none").

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