diff --git a/python/sglang/srt/entrypoints/openai/protocol.py b/python/sglang/srt/entrypoints/openai/protocol.py
index e473976a8..b39ad2427 100644
--- a/python/sglang/srt/entrypoints/openai/protocol.py
+++ b/python/sglang/srt/entrypoints/openai/protocol.py
@@ -53,6 +53,7 @@ from pydantic import (
     BaseModel,
     ConfigDict,
     Field,
+    StrictBool,
     field_serializer,
     field_validator,
     model_serializer,
@@ -219,7 +220,10 @@ class JsonSchemaResponseFormat(BaseModel):
     description: Optional[str] = None
     # use alias to workaround pydantic conflict
     schema_: Optional[Dict[str, object]] = Field(alias="schema", default=None)
-    strict: Optional[bool] = None
+    # The OpenAI wire contract accepts JSON booleans only; StrictBool rejects
+    # the values lax pydantic would coerce ("yes", "on", 0, 1, ...), matching
+    # OpenAI's 422 behavior. Omitted (None) keeps its meaning.
+    strict: Optional[StrictBool] = None
 
 
 class ResponseFormat(BaseModel):
diff --git a/test/registered/unit/entrypoints/openai/test_protocol.py b/test/registered/unit/entrypoints/openai/test_protocol.py
index 245cdc3b6..eb3d60dbf 100644
--- a/test/registered/unit/entrypoints/openai/test_protocol.py
+++ b/test/registered/unit/entrypoints/openai/test_protocol.py
@@ -114,6 +114,42 @@ class TestCompletionRequest(unittest.TestCase):
 class TestChatCompletionRequest(unittest.TestCase):
     """Test ChatCompletionRequest protocol model"""
 
+    def test_json_schema_strict_requires_json_boolean(self):
+        base_request = {
+            "model": "test-model",
+            "messages": [{"role": "user", "content": "Hello"}],
+            "response_format": {
+                "type": "json_schema",
+                "json_schema": {
+                    "name": "answer",
+                    "schema": {"type": "object"},
+                },
+            },
+        }
+
+        for strict in (True, False, None):
+            with self.subTest(strict=strict):
+                response_format = dict(base_request["response_format"])
+                response_format["json_schema"] = {
+                    **response_format["json_schema"],
+                    "strict": strict,
+                }
+                request = ChatCompletionRequest.model_validate(
+                    {**base_request, "response_format": response_format}
+                )
+                self.assertIs(request.response_format.json_schema.strict, strict)
+
+        for strict in ("yes", "false", 0, 1):
+            with self.subTest(strict=strict), self.assertRaises(ValidationError):
+                response_format = dict(base_request["response_format"])
+                response_format["json_schema"] = {
+                    **response_format["json_schema"],
+                    "strict": strict,
+                }
+                ChatCompletionRequest.model_validate(
+                    {**base_request, "response_format": response_format}
+                )
+
     def test_basic_chat_completion_request(self):
         """Test basic chat completion request"""
         messages = [{"role": "user", "content": "Hello"}]