diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 58490e4430..a9a123aec5 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -536,8 +536,14 @@ def _build_request(
         if is_body_allowed:
             if isinstance(json_data, bytes):
                 kwargs["content"] = json_data
-            else:
-                kwargs["json"] = json_data if is_given(json_data) else None
+            elif kwargs.get("data") is None and not files:
+                payload = json_data if is_given(json_data) else None
+                kwargs["content"] = json.dumps(
+                    payload,
+                    ensure_ascii=False,
+                    separators=(",", ":"),
+                ).encode("utf-8")
+                headers.setdefault("Content-Type", "application/json")
             kwargs["files"] = files
         else:
             headers.pop("Content-Type", None)
diff --git a/tests/lib/chat/test_completions.py b/tests/lib/chat/test_completions.py
index afad5a1391..3a18e8ef8a 100644
--- a/tests/lib/chat/test_completions.py
+++ b/tests/lib/chat/test_completions.py
@@ -4,6 +4,7 @@
 from typing import List, Optional
 from typing_extensions import Literal, TypeVar
 
+import httpx
 import pytest
 from respx import MockRouter
 from pydantic import Field, BaseModel
@@ -161,6 +162,65 @@ class Location(BaseModel):
     )
 
 
+@pytest.mark.respx(base_url=base_url)
+def test_parse_pydantic_schema_preserves_unicode(client: OpenAI, respx_mock: MockRouter) -> None:
+    class EmojiModel(BaseModel):
+        """Emoji docstring 😍"""
+
+        message: str = Field(description="Say hi 😍")
+
+    body_capture: dict[str, str] = {}
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        body_capture["content"] = request.content.decode()
+        return httpx.Response(
+            200,
+            json={
+                "id": "chatcmpl-emoji",
+                "object": "chat.completion",
+                "created": 0,
+                "model": "gpt-4o-mini",
+                "choices": [
+                    {
+                        "index": 0,
+                        "message": {
+                            "role": "assistant",
+                            "content": '{"message":"hi 😍"}',
+                            "refusal": None,
+                        },
+                        "logprobs": None,
+                        "finish_reason": "stop",
+                    }
+                ],
+                "usage": {
+                    "prompt_tokens": 0,
+                    "completion_tokens": 0,
+                    "total_tokens": 0,
+                    "completion_tokens_details": {"reasoning_tokens": 0},
+                },
+                "system_fingerprint": "fp_test",
+            },
+        )
+
+    respx_mock.post("/chat/completions").mock(side_effect=handler)
+
+    client.chat.completions.parse(
+        model="gpt-4o-mini",
+        messages=[
+            {
+                "role": "user",
+                "content": "Say hi",
+            }
+        ],
+        response_format=EmojiModel,
+    )
+
+    body = body_capture.get("content", "")
+    assert body, "expected request body to be captured"
+    assert "Say hi 😍" in body
+    assert "\\ud83d" not in body
+
+
 @pytest.mark.respx(base_url=base_url)
 def test_parse_pydantic_model_optional_default(
     client: OpenAI, respx_mock: MockRouter, monkeypatch: pytest.MonkeyPatch