diff --git a/src/llm_models/model_client/gemini_client.py b/src/llm_models/model_client/gemini_client.py
index b52474e4..33afacaa 100644
--- a/src/llm_models/model_client/gemini_client.py
+++ b/src/llm_models/model_client/gemini_client.py
@@ -98,7 +98,10 @@ def _convert_messages(
             content: List[Part] = []
             for item in message.content:
                 if isinstance(item, tuple):
-                    image_format = "jpeg" if item[0].lower() == "jpg" else item[0].lower()
+                    image_format = item[0].lower()
+                    # 规范 JPEG MIME 类型后缀，统一使用 image/jpeg
+                    if image_format in ("jpg", "jpeg"):
+                        image_format = "jpeg"
                     content.append(Part.from_bytes(data=base64.b64decode(item[1]), mime_type=f"image/{image_format}"))
                 elif isinstance(item, str):
                     content.append(Part.from_text(text=item))
diff --git a/src/llm_models/model_client/openai_client.py b/src/llm_models/model_client/openai_client.py
index 5793d4f9..5372d95b 100644
--- a/src/llm_models/model_client/openai_client.py
+++ b/src/llm_models/model_client/openai_client.py
@@ -61,10 +61,16 @@ def _convert_messages(messages: list[Message]) -> list[ChatCompletionMessagePara
             content = []
             for item in message.content:
                 if isinstance(item, tuple):
+                    image_format = item[0].lower()
+                    # 规范 JPEG MIME 类型后缀，统一使用 image/jpeg
+                    if image_format in ("jpg", "jpeg"):
+                        mime_suffix = "jpeg"
+                    else:
+                        mime_suffix = image_format
                     content.append(
                         {
                             "type": "image_url",
-                            "image_url": {"url": f"data:image/{item[0].lower()};base64,{item[1]}"},
+                            "image_url": {"url": f"data:image/{mime_suffix};base64,{item[1]}"},
                         }
                     )
                 elif isinstance(item, str):