[V1] Add `structural_tag` support using xgrammar (#17085)

examples/online_serving/openai_chat_completion_structured_outputs_structural_tag.py

+# SPDX-License-Identifier: Apache-2.0
+from openai import OpenAI
+
+# This example demonstrates the `structural_tag` response format.
+# It can be used to specify a structured output format that occurs between
+# specific tags in the response. This example shows how it could be used
+# to enforce the format of a tool call response, but it could be used for
+# any structured output within a subset of the response.
+
+
+def main():
+    client = OpenAI(
+        base_url="http://localhost:8000/v1",
+        api_key="-",
+    )
+
+    messages = [{
+        "role":
+        "user",
+        "content":
+        """
+You have access to the following function to retrieve the weather in a city:
+
+    {
+        "name": "get_weather",
+        "parameters": {
+            "city": {
+                "param_type": "string",
+                "description": "The city to get the weather for",
+                "required": True
+            }
+        }
+    }
+
+If a you choose to call a function ONLY reply in the following format:
+<{start_tag}={function_name}>{parameters}{end_tag}
+where
+
+start_tag => `<function`
+parameters => a JSON dict with the function argument name as key and function
+              argument value as value.
+end_tag => `</function>`
+
+Here is an example,
+<function=example_function_name>{"example_name": "example_value"}</function>
+
+Reminder:
+- Function calls MUST follow the specified format
+- Required parameters MUST be specified
+- Only call one function at a time
+- Put the entire function call reply on one line
+- Always add your sources when using search results to answer the user query
+
+You are a helpful assistant.
+
+Given the previous instructions, what is the weather in New York City, Boston,
+and San Francisco?
+"""
+    }]
+
+    response = client.chat.completions.create(
+        model="meta-llama/Llama-3.1-8B-Instruct",
+        messages=messages,
+        response_format={
+            "type":
+            "structural_tag",
+            "structures": [{
+                "begin": "<function=get_weather>",
+                "schema": {
+                    "type": "object",
+                    "properties": {
+                        "city": {
+                            "type": "string"
+                        }
+                    }
+                },
+                "end": "</function>"
+            }],
+            "triggers": ["<function="]
+        })
+    print(response)
+
+
+if __name__ == "__main__":
+    main()

tests/v1/entrypoints/llm/test_struct_output_generate.py

@@ -350,6 +350,7 @@ def test_structured_output(
         temperature=1.0,
         max_tokens=1000,
         guided_decoding=GuidedDecodingParams(json=json_schema))
+
     outputs = llm.generate(
         prompts="Generate a description of a frog using 50 characters.",
         sampling_params=sampling_params,
@@ -368,6 +369,106 @@ def test_structured_output(
         output_json = json.loads(generated_text)
         jsonschema.validate(instance=output_json, schema=json_schema)
 
+    #
+    # Test 11: Generate structured output using structural_tag format
+    #
+    structural_tag_config = {
+        "type":
+        "structural_tag",
+        "structures": [{
+            "begin": "<function=get_weather>",
+            "schema": {
+                "type": "object",
+                "properties": {
+                    "city": {
+                        "type": "string"
+                    }
+                }
+            },
+            "end": "</function>"
+        }],
+        "triggers": ["<function="]
+    }
+
+    sampling_params = SamplingParams(
+        temperature=0.0,
+        max_tokens=100,
+        guided_decoding=GuidedDecodingParams(
+            structural_tag=json.dumps(structural_tag_config)))
+
+    prompt = """
+You have access to the following function to retrieve the weather in a city:
+         
+    {
+        "name": "get_weather",
+        "parameters": {
+            "city": {
+                "param_type": "string",
+                "description": "The city to get the weather for",
+                "required": True
+            }
+        }
+    }
+         
+If a you choose to call a function ONLY reply in the following format:
+<{start_tag}={function_name}>{parameters}{end_tag}
+where
+
+start_tag => `<function`
+parameters => a JSON dict with the function argument name
+              as key and function argument value as value.
+end_tag => `</function>`
+
+Here is an example,
+<function=example_function_name>{"example_name": "example_value"}</function>
+
+Reminder:
+- Function calls MUST follow the specified format
+- Required parameters MUST be specified
+- Only call one function at a time
+- Put the entire function call reply on one line
+- Always add your sources when using search results to answer the user query
+
+You are a helpful assistant.
+         
+Given the previous instructions, what is the weather in New York City?
+"""
+
+    # Change this once other backends support structural_tag
+    if guided_decoding_backend.startswith("xgrammar"):
+        outputs = llm.generate(prompts=prompt,
+                               sampling_params=sampling_params,
+                               use_tqdm=True)
+        assert outputs is not None
+    else:
+        outputs = []
+
+    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
+
+        # Search for function call pattern in the response
+        function_call_pattern = r'<function=get_weather>(.*?)</function>'
+        matches = re.findall(function_call_pattern, generated_text)
+
+        if not matches:
+            print(f"Warning: No function calls found in response: "
+                  f"{generated_text!r}")
+            continue
+
+        # Take the first function call if multiple are found
+        json_str = matches[0]
+        try:
+            json_content = json.loads(json_str)
+            assert "city" in json_content
+            assert isinstance(json_content["city"], str)
+            print(f"Found valid function call: {generated_text!r}")
+        except (json.JSONDecodeError, AssertionError) as e:
+            pytest.fail("Invalid function call format: "
+                        f"{generated_text!r}\nError: {str(e)}")
+
 
 @pytest.mark.skip_global_cleanup
 @pytest.mark.parametrize("model_name, tokenizer_mode",

vllm/entrypoints/llm.py

@@ -1396,7 +1396,9 @@ class LLM:
             grammar=guided_options.guided_grammar,
             json_object=guided_options.guided_json_object,
             backend=guided_options.guided_decoding_backend,
-            whitespace_pattern=guided_options.guided_whitespace_pattern)
+            whitespace_pattern=guided_options.guided_whitespace_pattern,
+            structural_tag=guided_options.structural_tag,
+        )
         return params
 
     def _run_engine(

vllm/entrypoints/openai/protocol.py

@@ -2,6 +2,7 @@
 
 # Adapted from
 # https://github.com/lm-sys/FastChat/blob/168ccc29d3f7edc50823016105c024fe2282732a/fastchat/protocol/openai_api_protocol.py
+import json
 import re
 import time
 from argparse import Namespace
@@ -139,12 +140,30 @@ class JsonSchemaResponseFormat(OpenAIBaseModel):
     strict: Optional[bool] = None
 
 
+class StructuralTag(OpenAIBaseModel):
+    begin: str
+    # schema is the field, but that causes conflicts with pydantic so
+    # instead use structural_tag_schema with an alias
+    structural_tag_schema: Optional[dict[str, Any]] = Field(default=None,
+                                                            alias="schema")
+    end: str
+
+
+class StructuralTagResponseFormat(OpenAIBaseModel):
+    type: Literal["structural_tag"]
+    structures: list[StructuralTag]
+    triggers: list[str]
+
+
 class ResponseFormat(OpenAIBaseModel):
-    # type must be "json_schema", "json_object" or "text"
+    # type must be "json_schema", "json_object", or "text"
     type: Literal["text", "json_object", "json_schema"]
     json_schema: Optional[JsonSchemaResponseFormat] = None
 
 
+AnyResponseFormat = Union[ResponseFormat, StructuralTagResponseFormat]
+
+
 class StreamOptions(OpenAIBaseModel):
     include_usage: Optional[bool] = True
     continuous_usage_stats: Optional[bool] = False
@@ -227,7 +246,7 @@ class ChatCompletionRequest(OpenAIBaseModel):
     max_completion_tokens: Optional[int] = None
     n: Optional[int] = 1
     presence_penalty: Optional[float] = 0.0
-    response_format: Optional[ResponseFormat] = None
+    response_format: Optional[AnyResponseFormat] = None
     seed: Optional[int] = Field(None, ge=_LONG_INFO.min, le=_LONG_INFO.max)
     stop: Optional[Union[str, list[str]]] = Field(default_factory=list)
     stream: Optional[bool] = False
@@ -340,6 +359,11 @@ class ChatCompletionRequest(OpenAIBaseModel):
         description=(
             "If specified, the output will follow the context free grammar."),
     )
+    structural_tag: Optional[str] = Field(
+        default=None,
+        description=(
+            "If specified, the output will follow the structural tag schema."),
+    )
     guided_decoding_backend: Optional[str] = Field(
         default=None,
         description=(
@@ -476,6 +500,12 @@ class ChatCompletionRequest(OpenAIBaseModel):
                 json_schema = self.response_format.json_schema
                 assert json_schema is not None
                 self.guided_json = json_schema.json_schema
+            elif self.response_format.type == "structural_tag":
+                structural_tag = self.response_format
+                assert structural_tag is not None and isinstance(
+                    structural_tag, StructuralTagResponseFormat)
+                s_tag_obj = structural_tag.model_dump(by_alias=True)
+                self.structural_tag = json.dumps(s_tag_obj)
 
         guided_decoding = GuidedDecodingParams.from_optional(
             json=self._get_guided_json_from_tool() or self.guided_json,
@@ -485,6 +515,7 @@ class ChatCompletionRequest(OpenAIBaseModel):
             json_object=guided_json_object,
             backend=self.guided_decoding_backend,
             whitespace_pattern=self.guided_whitespace_pattern,
+            structural_tag=self.structural_tag,
         )
 
         return SamplingParams.from_optional(
@@ -742,12 +773,13 @@ class CompletionRequest(OpenAIBaseModel):
             "If true (the default), special tokens (e.g. BOS) will be added to "
             "the prompt."),
     )
-    response_format: Optional[ResponseFormat] = Field(
+    response_format: Optional[AnyResponseFormat] = Field(
         default=None,
-        description=
-        ("Similar to chat completion, this parameter specifies the format of "
-         "output. Only {'type': 'json_object'}, {'type': 'json_schema'} or "
-         "{'type': 'text' } is supported."),
+        description=(
+            "Similar to chat completion, this parameter specifies the format "
+            "of output. Only {'type': 'json_object'}, {'type': 'json_schema'}"
+            ", {'type': 'structural_tag'}, or {'type': 'text' } is supported."
+        ),
     )
     guided_json: Optional[Union[str, dict, BaseModel]] = Field(
         default=None,

vllm/model_executor/guided_decoding/guided_fields.py

@@ -27,14 +27,15 @@ class GuidedDecodingRequest:
     guided_decoding_backend: Optional[str] = None
     guided_whitespace_pattern: Optional[str] = None
     guided_json_object: Optional[bool] = None
+    structural_tag: Optional[str] = None
 
     def __post_init__(self):
         """Validate that some fields are mutually exclusive."""
-        guide_count = sum([
-            self.guided_json is not None, self.guided_regex is not None,
-            self.guided_choice is not None, self.guided_grammar is not None,
-            self.guided_json_object is not None
-        ])
+        guide_count = sum(x is not None
+                          for x in (self.guided_json, self.guided_regex,
+                                    self.guided_choice, self.guided_grammar,
+                                    self.guided_json_object,
+                                    self.structural_tag))
         if guide_count > 1:
             raise ValueError(
                 "You can only use one kind of guided decoding but multiple are "

vllm/sampling_params.py

@@ -38,6 +38,7 @@ class GuidedDecodingParams:
     """These are other options that can be set"""
     backend: Optional[str] = None
     whitespace_pattern: Optional[str] = None
+    structural_tag: Optional[str] = None
 
     @staticmethod
     def from_optional(
@@ -48,9 +49,10 @@ class GuidedDecodingParams:
         json_object: Optional[bool] = None,
         backend: Optional[str] = None,
         whitespace_pattern: Optional[str] = None,
+        structural_tag: Optional[str] = None,
     ) -> Optional["GuidedDecodingParams"]:
-        if all(arg is None
-               for arg in (json, regex, choice, grammar, json_object)):
+        if all(arg is None for arg in (json, regex, choice, grammar,
+                                       json_object, structural_tag)):
             return None
         # Extract json schemas from pydantic models
         if isinstance(json, (BaseModel, type(BaseModel))):
@@ -63,6 +65,7 @@ class GuidedDecodingParams:
             json_object=json_object,
             backend=backend,
             whitespace_pattern=whitespace_pattern,
+            structural_tag=structural_tag,
         )
 
     @property

vllm/v1/structured_output/backend_guidance.py

@@ -194,6 +194,9 @@ def serialize_guidance_grammar(
             tp = "grammar"
         elif request_type == StructuredOutputOptions.CHOICE:
             tp = "choice"
+        elif request_type == StructuredOutputOptions.STRUCTURAL_TAG:
+            raise ValueError("Structural tag is not supported "
+                             "for guidance backend yet")
         else:
             logger.error("Validation should have already occurred. "
                          "Please file an issue.")

vllm/v1/structured_output/backend_types.py

@@ -12,6 +12,7 @@ class StructuredOutputOptions(enum.Enum):
     REGEX = enum.auto()
     GRAMMAR = enum.auto()
     CHOICE = enum.auto()
+    STRUCTURAL_TAG = enum.auto()
 
 
 StructuredOutputKey = tuple[StructuredOutputOptions, str]

vllm/v1/structured_output/backend_xgrammar.py

@@ -108,6 +108,16 @@ class XgrammarBackend(StructuredOutputBackend):
             ctx = self.compiler.compile_grammar(grammar_spec)
         elif request_type == StructuredOutputOptions.REGEX:
             ctx = self.compiler.compile_regex(grammar_spec)
+        elif request_type == StructuredOutputOptions.STRUCTURAL_TAG:
+            s_tag = json.loads(grammar_spec)
+            tags = [
+                xgr.StructuralTagItem(
+                    begin=s["begin"],
+                    schema=json.dumps(s["schema"]),
+                    end=s["end"],
+                ) for s in s_tag["structures"]
+            ]
+            ctx = self.compiler.compile_structural_tag(tags, s_tag["triggers"])
         else:
             logger.error(
                 "Validation should have already occurred. Please file an issue."
@@ -272,3 +282,18 @@ def validate_xgrammar_grammar(sampling_params: SamplingParams) -> None:
             xgr.Grammar.from_ebnf(gd_params.grammar)
         except Exception as e:
             raise ValueError("Invalid grammar specification.") from e
+        return
+
+    if gd_params.structural_tag:
+        try:
+            s_tag = json.loads(gd_params.structural_tag)
+            tags = [
+                xgr.StructuralTagItem(
+                    begin=s["begin"],
+                    schema=json.dumps(s["schema"]),
+                    end=s["end"],
+                ) for s in s_tag["structures"]
+            ]
+            xgr.Grammar.from_structural_tag(tags, s_tag["triggers"])
+        except Exception as e:
+            raise ValueError("Invalid structural tag specification.") from e

vllm/v1/structured_output/request.py

@@ -78,5 +78,7 @@ def get_structured_output_key(
         return (StructuredOutputOptions.CHOICE, json_str)
     elif params.grammar is not None:
         return (StructuredOutputOptions.GRAMMAR, params.grammar)
+    elif params.structural_tag is not None:
+        return (StructuredOutputOptions.STRUCTURAL_TAG, params.structural_tag)
     else:
         raise ValueError("No valid structured output parameter found")