[V1] guidance backend for structured output + auto fallback mode (#14779)

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Co-authored-by: Loc Huynh <jc1da.3011@gmail.com>
Co-authored-by: Michal Moskal <michal@moskal.me>

requirements/common.txt

@@ -18,7 +18,7 @@ pillow  # Required for image processing
 prometheus-fastapi-instrumentator >= 7.0.0
 tiktoken >= 0.6.0  # Required for DBRX tokenizer
 lm-format-enforcer >= 0.10.11, < 0.11
-llguidance >= 0.7.2, < 0.8.0; platform_machine == "x86_64" or platform_machine == "arm64" or platform_machine == "aarch64"
+llguidance >= 0.7.9, < 0.8.0; platform_machine == "x86_64" or platform_machine == "arm64" or platform_machine == "aarch64"
 outlines == 0.1.11
 lark == 1.2.2
 xgrammar == 0.1.16; platform_machine == "x86_64" or platform_machine == "aarch64"

tests/v1/entrypoints/llm/test_struct_output_generate.py

@@ -13,7 +13,7 @@ from vllm.entrypoints.llm import LLM
 from vllm.outputs import RequestOutput
 from vllm.sampling_params import GuidedDecodingParams, SamplingParams
 
-GUIDED_DECODING_BACKENDS_V1 = ["xgrammar"]
+GUIDED_DECODING_BACKENDS_V1 = ["xgrammar", "guidance"]
 MODELS_TO_TEST = [
     "Qwen/Qwen2.5-1.5B-Instruct", "mistralai/Ministral-8B-Instruct-2410"
 ]
@@ -30,12 +30,13 @@ def test_guided_json_completion(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=1.0,
-                                     max_tokens=1000,
-                                     guided_decoding=GuidedDecodingParams(
-                                         json=sample_json_schema,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=1.0,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(json=sample_json_schema))
     outputs = llm.generate(prompts=[
         f"Give an example JSON for an employee profile "
         f"that fits this schema: {sample_json_schema}"
@@ -111,13 +112,14 @@ def test_guided_json_object(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=1.0,
-                                     max_tokens=100,
-                                     n=2,
-                                     guided_decoding=GuidedDecodingParams(
-                                         json_object=True,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=1.0,
+        max_tokens=100,
+        n=2,
+        guided_decoding=GuidedDecodingParams(json_object=True))
 
     outputs = llm.generate(
         prompts=("Generate a JSON object with curly braces for a person with "
@@ -137,12 +139,20 @@ def test_guided_json_object(
 
             # Parse to verify it is valid JSON
             parsed_json = json.loads(generated_text)
-            assert isinstance(parsed_json, dict)
+            allowed_types: tuple[type, ...] = (dict, )
+            if guided_decoding_backend == "xgrammar":
+                # TODO - we are currently too permissive with xgrammar and
+                # allow # any valid json (typically comes back as a list or
+                # object).  We can fix this by specifying a jsonschema of
+                # {"type": "object"}, # but we need this fix in a release
+                # first: https://github.com/mlc-ai/xgrammar/pull/264
+                allowed_types = (dict, list)
+            assert isinstance(parsed_json, allowed_types)
 
 
 @pytest.mark.skip_global_cleanup
 @pytest.mark.parametrize("guided_decoding_backend",
-                         GUIDED_DECODING_BACKENDS_V1)
+                         GUIDED_DECODING_BACKENDS_V1 + ["auto"])
 @pytest.mark.parametrize("model_name", MODELS_TO_TEST)
 def test_guided_json_unsupported_schema(
     monkeypatch: pytest.MonkeyPatch,
@@ -151,21 +161,43 @@ def test_guided_json_unsupported_schema(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=1.0,
-                                     max_tokens=1000,
-                                     guided_decoding=GuidedDecodingParams(
-                                         json=unsupported_json_schema,
-                                         backend=guided_decoding_backend))
-    with pytest.raises(ValueError,
-                       match="The provided JSON schema contains features "
-                       "not supported by xgrammar."):
-        llm.generate(prompts=[
-            f"Give an example JSON for an employee profile "
-            f"that fits this schema: {unsupported_json_schema}"
-        ] * 2,
-                     sampling_params=sampling_params,
-                     use_tqdm=True)
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=1.0,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(json=unsupported_json_schema))
+    if guided_decoding_backend == "xgrammar":
+        with pytest.raises(ValueError,
+                           match="The provided JSON schema contains features "
+                           "not supported by xgrammar."):
+            llm.generate(prompts=[
+                f"Give an example JSON for an employee profile "
+                f"that fits this schema: {unsupported_json_schema}"
+            ] * 2,
+                         sampling_params=sampling_params,
+                         use_tqdm=True)
+    else:
+        # This should work for both "guidance" and "auto".
+
+        outputs = llm.generate(
+            prompts=("Give an example JSON object for a grade "
+                     "that fits this schema: "
+                     f"{unsupported_json_schema}"),
+            sampling_params=sampling_params,
+            use_tqdm=True)
+        assert outputs is not None
+        for output in outputs:
+            assert output is not None
+            assert isinstance(output, RequestOutput)
+            generated_text = output.outputs[0].text
+            assert generated_text is not None
+            print(generated_text)
+
+            # Parse to verify it is valid JSON
+            parsed_json = json.loads(generated_text)
+            assert isinstance(parsed_json, dict)
 
 
 @pytest.mark.skip_global_cleanup
@@ -179,13 +211,14 @@ def test_guided_grammar_ebnf(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=0.8,
-                                     top_p=0.95,
-                                     max_tokens=1000,
-                                     guided_decoding=GuidedDecodingParams(
-                                         grammar=sample_sql_ebnf,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=0.8,
+        top_p=0.95,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(grammar=sample_sql_ebnf))
     outputs = llm.generate(
         prompts=("Generate a sql statement that selects col_1 from "
                  "table_1 where it is equal to 1"),
@@ -222,13 +255,14 @@ def test_guided_grammar_lark(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=0.8,
-                                     top_p=0.95,
-                                     max_tokens=1000,
-                                     guided_decoding=GuidedDecodingParams(
-                                         grammar=sample_sql_lark,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=0.8,
+        top_p=0.95,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(grammar=sample_sql_lark))
     outputs = llm.generate(
         prompts=("Generate a sql statement that selects col_1 from "
                  "table_1 where it is equal to 1"),
@@ -269,16 +303,15 @@ def test_guided_grammar_ebnf_invalid(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=0.8,
-                                     top_p=0.95,
-                                     max_tokens=1000,
-                                     guided_decoding=GuidedDecodingParams(
-                                         grammar="not a grammar",
-                                         backend=guided_decoding_backend))
-    with pytest.raises(ValueError,
-                       match="Failed to convert the grammar "
-                       "from Lark to EBNF."):
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=0.8,
+        top_p=0.95,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(grammar="not a grammar"))
+    with pytest.raises(ValueError, match="Failed to convert the grammar "):
         llm.generate(
             prompts=("Generate a sql statement that selects col_1 from "
                      "table_1 where it is equal to 1"),
@@ -298,12 +331,13 @@ def test_guided_regex(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=0.8,
-                                     top_p=0.95,
-                                     guided_decoding=GuidedDecodingParams(
-                                         regex=sample_regex,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=0.8,
+        top_p=0.95,
+        guided_decoding=GuidedDecodingParams(regex=sample_regex))
     outputs = llm.generate(
         prompts=[
             f"Give an example IPv4 address with this regex: {sample_regex}"
@@ -335,12 +369,13 @@ def test_guided_choice_completion(
     model_name: str,
 ):
     monkeypatch.setenv("VLLM_USE_V1", "1")
-    llm = LLM(model=model_name, max_model_len=1024)
-    sampling_params = SamplingParams(temperature=0.8,
-                                     top_p=0.95,
-                                     guided_decoding=GuidedDecodingParams(
-                                         choice=sample_guided_choice,
-                                         backend=guided_decoding_backend))
+    llm = LLM(model=model_name,
+              max_model_len=1024,
+              guided_decoding_backend=guided_decoding_backend)
+    sampling_params = SamplingParams(
+        temperature=0.8,
+        top_p=0.95,
+        guided_decoding=GuidedDecodingParams(choice=sample_guided_choice))
     outputs = llm.generate(
         prompts="The best language for type-safe systems programming is ",
         sampling_params=sampling_params,

vllm/config.py

@@ -2800,12 +2800,17 @@ class DecodingConfig:
         return hash_str
 
     def __post_init__(self):
-        valid_guided_backends = [
-            'outlines', 'lm-format-enforcer', 'xgrammar', 'guidance'
+        v0_valid_guided_backends = [
+            'outlines', 'lm-format-enforcer', 'xgrammar'
         ]
+        v1_valid_guided_backends = ['xgrammar', 'guidance', 'auto']
 
         backend = GuidedDecodingParams(
             backend=self.guided_decoding_backend).backend_name
+        if envs.VLLM_USE_V1:
+            valid_guided_backends = v1_valid_guided_backends
+        else:
+            valid_guided_backends = v0_valid_guided_backends
         if backend not in valid_guided_backends:
             raise ValueError(f"Invalid guided_decoding_backend '{backend}',"
                              f" must be one of {valid_guided_backends}")

vllm/engine/arg_utils.py

@@ -391,16 +391,13 @@ class EngineArgs:
             default='xgrammar',
             help='Which engine will be used for guided decoding'
             ' (JSON schema / regex etc) by default. Currently support '
-            'https://github.com/outlines-dev/outlines, '
-            'https://github.com/mlc-ai/xgrammar, and '
-            'https://github.com/noamgat/lm-format-enforcer.'
-            ' Can be overridden per request via guided_decoding_backend'
-            ' parameter.\n'
-            'Backend-specific options can be supplied in a comma-separated '
-            'list following a colon after the backend name. Valid backends and '
-            'all available options are: [xgrammar:no-fallback, '
-            'xgrammar:disable-any-whitespace, '
-            'outlines:no-fallback, lm-format-enforcer:no-fallback]')
+            'https://github.com/mlc-ai/xgrammar and '
+            'https://github.com/guidance-ai/llguidance.'
+            'Valid backend values are "xgrammar", "guidance", and "auto". '
+            'With "auto", we will make opinionated choices based on request'
+            'contents and what the backend libraries currently support, so '
+            'the behavior is subject to change in each release. '
+            'The default is xgrammar.')
         parser.add_argument(
             '--logits-processor-pattern',
             type=nullable_str,
@@ -1539,9 +1536,9 @@ class EngineArgs:
                                recommend_to_remove=False)
             return False
 
-        # Only support Xgrammar for guided decoding so far.
+        # Xgrammar and Guidance are supported.
         SUPPORTED_GUIDED_DECODING = [
-            "xgrammar", "xgrammar:disable-any-whitespace"
+            "xgrammar", "xgrammar:disable-any-whitespace", "guidance", "auto"
         ]
         if self.guided_decoding_backend not in SUPPORTED_GUIDED_DECODING:
             _raise_or_fallback(feature_name="--guided-decoding-backend",

vllm/v1/engine/processor.py

@@ -4,7 +4,6 @@ import time
 from collections.abc import Mapping
 from typing import Optional, Union
 
-import vllm.platforms
 from vllm.config import VllmConfig
 from vllm.inputs import (INPUT_REGISTRY, InputRegistry, ProcessorInputs,
                          PromptType, SingletonInputsAdapter)
@@ -20,7 +19,10 @@ from vllm.prompt_adapter.request import PromptAdapterRequest
 from vllm.sampling_params import SamplingParams
 from vllm.transformers_utils.tokenizer_group import BaseTokenizerGroup
 from vllm.v1.engine import EngineCoreRequest
-from vllm.v1.structured_output.utils import validate_structured_output_request
+from vllm.v1.structured_output.backend_guidance import (
+    validate_guidance_grammar)
+from vllm.v1.structured_output.utils import (
+    validate_structured_output_request_xgrammar)
 
 
 class Processor:
@@ -120,7 +122,9 @@ class Processor:
         if not params.guided_decoding or not self.decoding_config:
             return
 
-        supported_backends = ["xgrammar", "xgrammar:disable-any-whitespace"]
+        supported_backends = [
+            "xgrammar", "xgrammar:disable-any-whitespace", "guidance", "auto"
+        ]
         engine_level_backend = self.decoding_config.guided_decoding_backend
         if engine_level_backend not in supported_backends:
             raise ValueError(f"Only {supported_backends} structured output is "
@@ -134,10 +138,31 @@ class Processor:
         else:
             params.guided_decoding.backend = engine_level_backend
 
-        if vllm.platforms.current_platform.is_tpu():
-            raise ValueError("Structured output is not supported on TPU.")
+        # Request content validation
 
-        validate_structured_output_request(params)
+        if engine_level_backend == "xgrammar":
+            # xgrammar with no fallback
+            validate_structured_output_request_xgrammar(params)
+            params.guided_decoding.backend = "xgrammar"
+        elif engine_level_backend == "auto":
+            # "auto" is an opt-in to opinionated behavior where we try to
+            # choose a backend based on request contents. This is not the
+            # default as it is less predictable and subject to change
+            # between releases as feature support changes.
+            try:
+                validate_structured_output_request_xgrammar(params)
+                params.guided_decoding.backend = "xgrammar"
+            except ValueError:
+                # The request includes some jsonschema feature(s) that
+                # are not supported in xgrammar. Fall back to guidance.
+                params.guided_decoding.backend = "guidance"
+
+        if params.guided_decoding.backend == "guidance":
+            # TODO ideally we would have the LLTokenizer here as Lark syntax
+            # allows <|special_token|> and similar, see
+            # https://github.com/guidance-ai/llguidance/blob/main/docs/syntax.md#special-tokens
+            # Without tokenizer these are disallowed in grammars.
+            validate_guidance_grammar(params, tokenizer=None)
 
     def process_inputs(
         self,

vllm/v1/structured_output/__init__.py

@@ -7,6 +7,7 @@ from typing import TYPE_CHECKING, Optional
 
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
+from vllm.v1.structured_output.backend_guidance import GuidanceBackend
 from vllm.v1.structured_output.backend_types import (StructuredOutputBackend,
                                                      StructuredOutputGrammar)
 
@@ -50,6 +51,8 @@ class StructuredOutputManager:
                     XgrammarBackend)
 
                 self.backend = XgrammarBackend(self.vllm_config)
+            elif backend_name == "guidance":
+                self.backend = GuidanceBackend(self.vllm_config)
             else:
                 raise ValueError(
                     f"Unsupported structured output backend: {backend_name}")

vllm/v1/structured_output/backend_guidance.py

@@ -0,0 +1,164 @@
+# SPDX-License-Identifier: Apache-2.0
+
+import os
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Optional
+
+import torch
+
+from vllm.config import VllmConfig
+from vllm.logger import init_logger
+from vllm.sampling_params import SamplingParams
+from vllm.transformers_utils.tokenizer_group import init_tokenizer_from_configs
+from vllm.utils import LazyLoader
+from vllm.v1.structured_output.backend_types import (StructuredOutputBackend,
+                                                     StructuredOutputGrammar,
+                                                     StructuredOutputOptions)
+from vllm.v1.structured_output.request import get_structured_output_key
+
+if TYPE_CHECKING:
+    import llguidance
+    import llguidance.hf as llguidance_hf
+    import llguidance.torch as llguidance_torch
+else:
+    llguidance = LazyLoader("llguidance", globals(), "llguidance")
+    llguidance_hf = LazyLoader("llguidance.hf", globals(), "llguidance.hf")
+    llguidance_torch = LazyLoader("llguidance.torch", globals(),
+                                  "llguidance.torch")
+
+logger = init_logger(__name__)
+
+
+class GuidanceBackend(StructuredOutputBackend):
+
+    def __init__(self, vllm_config: VllmConfig):
+        self.vllm_config = vllm_config
+        tokenizer_group = init_tokenizer_from_configs(
+            model_config=vllm_config.model_config,
+            scheduler_config=vllm_config.scheduler_config,
+            parallel_config=vllm_config.parallel_config,
+            lora_config=vllm_config.lora_config)  # type: ignore[arg-type]
+        tokenizer_group.ping()
+        self.vllm_config = vllm_config
+        self.vocab_size = vllm_config.model_config.get_vocab_size()
+
+        tokenizer = tokenizer_group.get_lora_tokenizer(None)
+        self.ll_tokenizer = llguidance_hf.from_tokenizer(tokenizer, None)
+
+    def compile_grammar(self, request_type: StructuredOutputOptions,
+                        grammar_spec: str) -> StructuredOutputGrammar:
+        self.serialized_grammar = serialize_guidance_grammar(
+            request_type, grammar_spec)
+
+        ll_matcher = llguidance.LLMatcher(
+            self.ll_tokenizer,
+            self.serialized_grammar,
+            log_level=int(os.environ.get("LLGUIDANCE_LOG_LEVEL", "1")),
+        )
+
+        r = GuidanceGrammar(
+            ll_matcher=ll_matcher,
+            ll_tokenizer=self.ll_tokenizer,
+            vocab_size=self.vocab_size,
+        )
+
+        r.check_error()
+        return r
+
+    def allocate_token_bitmask(self, max_num_seqs: int):
+        return llguidance_torch.allocate_token_bitmask(
+            max_num_seqs, self.ll_tokenizer.vocab_size)
+
+
+@dataclass
+class GuidanceGrammar(StructuredOutputGrammar):
+    ll_matcher: llguidance.LLMatcher
+    ll_tokenizer: llguidance.LLTokenizer
+    vocab_size: int
+    printed_error: bool = False
+    terminated: bool = False
+
+    def check_error(self):
+        if not self.printed_error:
+            err = self.ll_matcher.get_error()
+            if err:
+                self.printed_error = True
+                logger.warning("LLMatcher error: %s", err)
+
+    def accept_tokens(self, request_id: str, tokens: list[int]) -> bool:
+        """Accepts a list of tokens and advances the parser.
+
+        Returns True if the parser was advanced successfully.
+        Returns False if the parser failed to advance.
+        """
+
+        if self.ll_tokenizer.eos_token in tokens:
+            self.terminated = True
+
+        if self.ll_matcher.is_stopped():
+            return True
+
+        # TODO - Add jump decoding support in the future:
+        # self.ll_matcher.compute_ff_bytes() - this should always work
+        # self.ll_matcher.compute_ff_tokens() - this only works for
+        #   "canonical" tokenizers
+        # For conversion between the two, see
+        # https://github.com/guidance-ai/llguidance/blob/main/docs/fast_forward.md
+
+        r = self.ll_matcher.consume_tokens(tokens)
+
+        self.check_error()
+
+        return r
+
+    def fill_bitmask(self, bitmask: torch.Tensor, idx: int) -> None:
+        # this will automatically return [EOS] mask if the matcher is stopped
+        # or otherwise in an error state
+        llguidance_torch.fill_next_token_bitmask(self.ll_matcher, bitmask, idx)
+        self.check_error()
+
+    def is_terminated(self) -> bool:
+        return self.terminated
+
+    def reset(self):
+        # This method may be not needed anymore? TODO
+        self.ll_matcher.reset()
+
+
+def serialize_guidance_grammar(request_type: StructuredOutputOptions,
+                               grammar_spec: str) -> str:
+    if request_type == StructuredOutputOptions.JSON:
+        # TODO: make whitespace_flexible configurable
+        return llguidance.LLMatcher.grammar_from_json_schema(
+            grammar_spec, defaults={
+                "whitespace_flexible": True,
+            })
+    elif request_type == StructuredOutputOptions.JSON_OBJECT:
+        return llguidance.LLMatcher.grammar_from_json_schema(
+            '{"type": "object"}', defaults={
+                "whitespace_flexible": True,
+            })
+    else:
+        if request_type == StructuredOutputOptions.REGEX:
+            tp = "regex"
+        elif request_type == StructuredOutputOptions.GRAMMAR:
+            tp = "grammar"
+        elif request_type == StructuredOutputOptions.CHOICE:
+            tp = "choice"
+        else:
+            logger.error("Validation should have already occurred. "
+                         "Please file an issue.")
+            raise ValueError("grammar is not of valid supported types. "
+                             f"({request_type!s})")
+        return llguidance.grammar_from(tp, grammar_spec)
+
+
+def validate_guidance_grammar(
+        sampling_params: SamplingParams,
+        tokenizer: Optional[llguidance.LLTokenizer] = None) -> None:
+    tp, grm = get_structured_output_key(sampling_params)
+    guidance_grm = serialize_guidance_grammar(tp, grm)
+    err = llguidance.LLMatcher.validate_grammar(guidance_grm,
+                                                tokenizer=tokenizer)
+    if err:
+        raise ValueError(f"Grammar error: {err}")

vllm/v1/structured_output/request.py

@@ -53,25 +53,30 @@ class StructuredOutputRequest:
 
     @functools.cached_property
     def structured_output_key(self) -> StructuredOutputKey:
-        params = self.sampling_params.guided_decoding
-        assert params is not None, "params can't be None."
-        if params.json is not None:
-            if not isinstance(params.json, str):
-                json_str = json.dumps(params.json)
-            else:
-                json_str = params.json
-            return (StructuredOutputOptions.JSON, json_str)
-        elif params.json_object:
-            return (StructuredOutputOptions.JSON_OBJECT, "")
-        elif params.regex is not None:
-            return (StructuredOutputOptions.REGEX, params.regex)
-        elif params.choice is not None:
-            if not isinstance(params.choice, str):
-                json_str = json.dumps(params.choice)
-            else:
-                json_str = params.choice
-            return (StructuredOutputOptions.CHOICE, json_str)
-        elif params.grammar is not None:
-            return (StructuredOutputOptions.GRAMMAR, params.grammar)
+        return get_structured_output_key(self.sampling_params)
+
+
+def get_structured_output_key(
+        sampling_params: SamplingParams) -> StructuredOutputKey:
+    params = sampling_params.guided_decoding
+    assert params is not None, "params can't be None."
+    if params.json is not None:
+        if not isinstance(params.json, str):
+            json_str = json.dumps(params.json)
         else:
-            raise ValueError("No valid structured output parameter found")
+            json_str = params.json
+        return (StructuredOutputOptions.JSON, json_str)
+    elif params.json_object:
+        return (StructuredOutputOptions.JSON_OBJECT, "")
+    elif params.regex is not None:
+        return (StructuredOutputOptions.REGEX, params.regex)
+    elif params.choice is not None:
+        if not isinstance(params.choice, str):
+            json_str = json.dumps(params.choice)
+        else:
+            json_str = params.choice
+        return (StructuredOutputOptions.CHOICE, json_str)
+    elif params.grammar is not None:
+        return (StructuredOutputOptions.GRAMMAR, params.grammar)
+    else:
+        raise ValueError("No valid structured output parameter found")

vllm/v1/structured_output/utils.py

@@ -239,7 +239,7 @@ def choice_as_grammar(choice: list[str]) -> str:
     return grammar
 
 
-def validate_structured_output_request(
+def validate_structured_output_request_xgrammar(
         sampling_params: SamplingParams) -> None:
     """Validate that the request is supported by structured output.