[Frontend] Support strict mode for tool calling with ResponsesAPI (#45396)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

Author: chaunceyjiang

tests/entrypoints/openai/responses/conftest.py

@@ -390,7 +390,6 @@ def server_with_store(default_server_args):
         env_dict={
             "VLLM_ENABLE_RESPONSES_API_STORE": "1",
             "VLLM_SERVER_DEV_MODE": "1",
-            "VLLM_ENFORCE_STRICT_TOOL_CALLING": "0",
         },
     ) as remote_server:
         yield remote_server

vllm/parser/abstract_parser.py

@@ -438,8 +438,7 @@ def _apply_structural_tag(
         self, request: ChatCompletionRequest | ResponsesRequest
     ) -> ChatCompletionRequest | ResponsesRequest:
         if (
-            not isinstance(request, ChatCompletionRequest)
-            or self._tool_parser is None
+            self._tool_parser is None
             or self._tool_parser.structural_tag_model is None
             or not request.tools
         ):
@@ -448,7 +447,10 @@ def _apply_structural_tag(
         need_tool_calling = (
             request.tool_choice == "auto"
             or request.tool_choice == "required"
-            or isinstance(request.tool_choice, ChatCompletionNamedToolChoiceParam)
+            or isinstance(
+                request.tool_choice,
+                (ChatCompletionNamedToolChoiceParam, ToolChoiceFunction),
+            )
         )
         if not need_tool_calling:
             return request
@@ -464,7 +466,10 @@ def _apply_structural_tag(
         request.structured_outputs = StructuredOutputsParams(
             structural_tag=structural_tag,
         )
-        request.response_format = None
+        if isinstance(request, ResponsesRequest):
+            request.text = None
+        else:
+            request.response_format = None
         return request
 
     def extract_reasoning_streaming(

vllm/reasoning/abs_reasoning_parsers.py

@@ -181,9 +181,8 @@ def prepare_structured_tag(
     ) -> str | None:
         """
         Instance method that is implemented for preparing the structured tag
-        Otherwise, None is returned
         """
-        return None
+        return original_tag
 
 
 class ReasoningParserManager:

vllm/tool_parsers/abstract_tool_parser.py

@@ -165,7 +165,10 @@ def adjust_request(
         return request
 
     def get_structural_tag(
-        self, request: ChatCompletionRequest, *, reasoning: bool = False
+        self,
+        request: ChatCompletionRequest | ResponsesRequest,
+        *,
+        reasoning: bool = False,
     ):
         if self.structural_tag_model is None:
             return None

vllm/tool_parsers/structural_tag_registry.py

@@ -1,9 +1,14 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from collections.abc import Callable
-from typing import Any, Literal
-
+from collections.abc import Callable, Sequence
+from typing import Any, Literal, TypeAlias
+
+from openai.types.responses import FunctionTool
+from openai.types.responses.response import ToolChoice as ResponsesToolChoice
+from openai.types.responses.tool import Tool as ResponsesTool
+from openai.types.responses.tool_choice_allowed import ToolChoiceAllowed
+from openai.types.responses.tool_choice_function import ToolChoiceFunction
 from xgrammar import StructuralTag, normalize_tool_choice
 from xgrammar import get_model_structural_tag as get_xgrammar_model_structural_tag
 from xgrammar.openai_tool_call_schema import (
@@ -25,11 +30,15 @@
     ChatCompletionToolsParam,
 )
 
-ToolChoice = (
-    Literal["none", "auto", "required"] | ChatCompletionNamedToolChoiceParam | None
+ToolChoice: TypeAlias = (
+    Literal["none", "auto", "required"]
+    | ChatCompletionNamedToolChoiceParam
+    | ResponsesToolChoice
+    | None
 )
-SimplifiedToolChoice = Literal["auto", "required", "forced"]
-StructuralTagBuilder = Callable[
+AllowedToolRef: TypeAlias = dict[str, object]
+SimplifiedToolChoice: TypeAlias = Literal["auto", "required", "forced"]
+StructuralTagBuilder: TypeAlias = Callable[
     [
         list[FunctionToolParam],
         list[BuiltinToolParam],
@@ -77,7 +86,7 @@ def decorator(func: StructuralTagBuilder) -> StructuralTagBuilder:
 
 def get_model_structural_tag(
     model: str,
-    tools: list[ChatCompletionToolsParam] | None,
+    tools: Sequence[ChatCompletionToolsParam | ResponsesTool] | None,
     tool_choice: ToolChoice,
     reasoning: bool,
 ) -> StructuralTag | None:
@@ -86,8 +95,8 @@ def get_model_structural_tag(
     if not tools or tool_choice == "none":
         return None
 
-    dumped_tools = [_model_dump(tool) for tool in tools]
-    dumped_tool_choice = _model_dump(tool_choice)
+    dumped_tools = [_dump_tool_for_xgrammar(tool) for tool in tools]
+    dumped_tool_choice = _dump_tool_choice_for_xgrammar(tool_choice)
 
     if model in _VLLM_STRUCTURAL_TAG_REGISTRY:
         function_tools, builtin_tools, simplified_tool_choice = normalize_tool_choice(
@@ -113,12 +122,72 @@ def get_model_structural_tag(
     )
 
 
-def _model_dump(value: Any) -> Any:
-    """Convert vLLM/Pydantic request objects to xgrammar's dict protocol."""
+def _dump_tool_for_xgrammar(
+    tool: ChatCompletionToolsParam | ResponsesTool,
+) -> dict[str, Any]:
+    """Convert tool objects to xgrammar's Chat Completions tool protocol."""
+
+    if isinstance(tool, FunctionTool):
+        function: dict[str, Any] = {"name": tool.name}
+        if tool.description is not None:
+            function["description"] = tool.description
+        if tool.parameters is not None:
+            function["parameters"] = tool.parameters
+        if tool.strict is not None:
+            function["strict"] = tool.strict
+        return {"type": "function", "function": function}
+    dumped_tool = tool.model_dump(mode="json", exclude_none=True)
+    if isinstance(tool, ChatCompletionToolsParam):
+        return dumped_tool
+    return dict(dumped_tool)
+
+
+def _dump_tool_choice_for_xgrammar(
+    tool_choice: ToolChoice,
+) -> dict[str, Any] | str | None:
+    """Convert tool_choice objects to xgrammar's expected protocol."""
+
+    if tool_choice is None:
+        return None
 
-    if hasattr(value, "model_dump"):
-        return value.model_dump(exclude_none=True)
-    return value
+    if isinstance(tool_choice, str):
+        return tool_choice
+
+    if isinstance(tool_choice, ChatCompletionNamedToolChoiceParam):
+        return tool_choice.model_dump(mode="json", exclude_none=True)
+
+    if isinstance(tool_choice, ToolChoiceFunction):
+        return {
+            "type": "function",
+            "function": {"name": tool_choice.name},
+        }
+
+    if isinstance(tool_choice, ToolChoiceAllowed):
+        return {
+            "type": "allowed_tools",
+            "allowed_tools": {
+                "mode": tool_choice.mode,
+                "tools": [
+                    _dump_allowed_tool_ref_for_xgrammar(tool)
+                    for tool in tool_choice.tools
+                ],
+            },
+        }
+
+    return tool_choice.model_dump(mode="json", exclude_none=True)
+
+
+def _dump_allowed_tool_ref_for_xgrammar(tool_ref: AllowedToolRef) -> AllowedToolRef:
+    if (
+        tool_ref.get("type") == "function"
+        and "function" not in tool_ref
+        and "name" in tool_ref
+    ):
+        return {
+            "type": "function",
+            "function": {"name": tool_ref["name"]},
+        }
+    return tool_ref
 
 
 def _get_function_parameters(function) -> dict[str, Any] | bool: