diff --git a/tests/tool_use/test_responses_request_validations.py b/tests/tool_use/test_responses_request_validations.py
new file mode 100644
index 00000000000..63a1828c500
--- /dev/null
+++ b/tests/tool_use/test_responses_request_validations.py
@@ -0,0 +1,184 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+from pydantic import ValidationError
+
+from vllm.entrypoints.openai.responses.protocol import ResponsesRequest
+
+SAMPLE_TOOL = {
+    "type": "function",
+    "name": "get_weather",
+    "description": "Get current weather",
+    "parameters": {
+        "type": "object",
+        "properties": {"location": {"type": "string", "description": "City name"}},
+        "required": ["location"],
+    },
+}
+
+NAMED_TOOL_CHOICE = {
+    "type": "function",
+    "name": "get_weather",
+}
+
+
+def test_responses_request_with_no_tools():
+    # tools key is not present — defaults tool_choice to "none"
+    request = ResponsesRequest.model_validate({"input": "Hello", "model": "test-model"})
+    assert request.tool_choice == "none"
+
+    # tools key present but empty
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tools": []}
+    )
+    assert request.tool_choice == "none"
+
+
+def test_responses_request_no_tools_tool_choice_none():
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tool_choice": "none"}
+    )
+    assert request.tool_choice == "none"
+
+
+def test_responses_request_no_tools_tool_choice_auto():
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tool_choice": "auto"}
+    )
+    assert request.tool_choice == "none"
+
+
+@pytest.mark.parametrize("tools", [None, []])
+def test_responses_request_required_without_tools(tools):
+    kwargs = {"input": "Hello", "model": "test-model", "tool_choice": "required"}
+    if tools is not None:
+        kwargs["tools"] = tools
+    with pytest.raises(
+        ValidationError, match="Tool choice 'required' must be specified"
+    ):
+        ResponsesRequest.model_validate(kwargs)
+
+
+def test_responses_request_named_tool_choice_without_tools():
+    with pytest.raises(ValidationError, match="not found in 'tools' parameter"):
+        ResponsesRequest.model_validate(
+            {
+                "input": "Hello",
+                "model": "test-model",
+                "tool_choice": NAMED_TOOL_CHOICE,
+            }
+        )
+
+
+def test_responses_request_with_tools_default_tool_choice():
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tools": [SAMPLE_TOOL]}
+    )
+    assert request.tool_choice == "auto"
+
+
+def test_responses_request_with_tools_tool_choice_none():
+    request = ResponsesRequest.model_validate(
+        {
+            "input": "Hello",
+            "model": "test-model",
+            "tools": [SAMPLE_TOOL],
+            "tool_choice": "none",
+        }
+    )
+    assert request.tool_choice == "none"
+
+
+def test_responses_request_named_tool_choice_matching():
+    request = ResponsesRequest.model_validate(
+        {
+            "input": "Hello",
+            "model": "test-model",
+            "tools": [SAMPLE_TOOL],
+            "tool_choice": NAMED_TOOL_CHOICE,
+        }
+    )
+    assert request.tool_choice.type == "function"
+    assert request.tool_choice.name == "get_weather"
+
+
+def test_responses_request_named_tool_choice_not_matching():
+    with pytest.raises(ValidationError, match="not found in 'tools' parameter"):
+        ResponsesRequest.model_validate(
+            {
+                "input": "Hello",
+                "model": "test-model",
+                "tools": [SAMPLE_TOOL],
+                "tool_choice": {"type": "function", "name": "nonexistent"},
+            }
+        )
+
+
+def test_responses_request_with_tools_tool_choice_auto():
+    request = ResponsesRequest.model_validate(
+        {
+            "input": "Hello",
+            "model": "test-model",
+            "tools": [SAMPLE_TOOL],
+            "tool_choice": "auto",
+        }
+    )
+    assert request.tool_choice == "auto"
+
+
+def test_responses_request_with_tools_tool_choice_required():
+    request = ResponsesRequest.model_validate(
+        {
+            "input": "Hello",
+            "model": "test-model",
+            "tools": [SAMPLE_TOOL],
+            "tool_choice": "required",
+        }
+    )
+    assert request.tool_choice == "required"
+
+
+def test_responses_request_empty_tools_tool_choice_none():
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tools": [], "tool_choice": "none"}
+    )
+    assert request.tool_choice == "none"
+
+
+def test_responses_request_empty_tools_tool_choice_auto():
+    request = ResponsesRequest.model_validate(
+        {"input": "Hello", "model": "test-model", "tools": [], "tool_choice": "auto"}
+    )
+    assert request.tool_choice == "none"
+
+
+@pytest.mark.parametrize(
+    "tool_choice",
+    [
+        {"type": "function"},
+        {"type": "function", "name": ""},
+    ],
+)
+def test_responses_request_named_tool_choice_missing_name(tool_choice):
+    with pytest.raises(ValidationError, match="not found in 'tools' parameter"):
+        ResponsesRequest.model_validate(
+            {
+                "input": "Hello",
+                "model": "test-model",
+                "tools": [SAMPLE_TOOL],
+                "tool_choice": tool_choice,
+            }
+        )
+
+
+def test_responses_request_empty_tools_named_tool_choice():
+    with pytest.raises(ValidationError, match="not found in 'tools' parameter"):
+        ResponsesRequest.model_validate(
+            {
+                "input": "Hello",
+                "model": "test-model",
+                "tools": [],
+                "tool_choice": NAMED_TOOL_CHOICE,
+            }
+        )
diff --git a/vllm/entrypoints/openai/responses/protocol.py b/vllm/entrypoints/openai/responses/protocol.py
index 79f5894fb91..96876e3f00f 100644
--- a/vllm/entrypoints/openai/responses/protocol.py
+++ b/vllm/entrypoints/openai/responses/protocol.py
@@ -492,6 +492,46 @@ class ResponsesRequest(OpenAIBaseModel):
         data["input"] = processed_input
         return data
 
+    @model_validator(mode="before")
+    @classmethod
+    def check_tool_usage(cls, data):
+        if not isinstance(data, dict):
+            return data
+
+        tools = data.get("tools")
+        tool_choice = data.get("tool_choice", "auto")
+        has_tools = tools is not None and len(tools) > 0
+        is_named_tool_choice = (
+            isinstance(tool_choice, dict) and tool_choice.get("type") == "function"
+        )
+
+        if not has_tools:
+            if tool_choice in ("auto", "none"):
+                data["tool_choice"] = "none"
+            elif tool_choice == "required":
+                raise VLLMValidationError(
+                    "Tool choice 'required' must be specified with 'tools' parameter.",
+                    parameter="tool_choice",
+                )
+            elif is_named_tool_choice:
+                raise VLLMValidationError(
+                    "Tool choice 'function' not found in 'tools' parameter.",
+                    parameter="tool_choice",
+                )
+        elif is_named_tool_choice and tools is not None:
+            tool_name = tool_choice.get("name")
+            tool_names = {
+                t.get("name") if isinstance(t, dict) else getattr(t, "name", None)
+                for t in tools
+            }
+            if not tool_name or tool_name not in tool_names:
+                raise VLLMValidationError(
+                    "Tool choice 'function' not found in 'tools' parameter.",
+                    parameter="tool_choice",
+                )
+
+        return data
+
 
 class ResponsesResponse(OpenAIBaseModel):
     id: str = Field(default_factory=lambda: f"resp_{random_uuid()}")
diff --git a/vllm/entrypoints/openai/responses/serving.py b/vllm/entrypoints/openai/responses/serving.py
index 6af25c9bcce..6d018f9c5b5 100644
--- a/vllm/entrypoints/openai/responses/serving.py
+++ b/vllm/entrypoints/openai/responses/serving.py
@@ -718,9 +718,10 @@ class OpenAIServingResponses(OpenAIServing):
         request: ResponsesRequest,
         prev_response: ResponsesResponse | None,
     ):
-        if request.tool_choice != "auto":
+        if request.tool_choice not in ("auto", "none"):
             raise NotImplementedError(
-                "Only 'auto' tool_choice is supported in response API with Harmony"
+                "Only 'auto' or 'none' tool_choice is supported "
+                "in response API with Harmony"
             )
 
         arrival_time = time.time()