diff --git a/astrbot/core/provider/sources/openai_source.py b/astrbot/core/provider/sources/openai_source.py
index 64e3a6645a..9b73819a38 100644
--- a/astrbot/core/provider/sources/openai_source.py
+++ b/astrbot/core/provider/sources/openai_source.py
@@ -1,5 +1,6 @@
 import asyncio
 import base64
+import binascii
 import copy
 import inspect
 import json
@@ -55,6 +56,17 @@
 )
 class ProviderOpenAIOfficial(Provider):
     _ERROR_TEXT_CANDIDATE_MAX_CHARS = 4096
+    # 部分 OpenAI 兼容中转站会校验 data URL 的 MIME 类型是否和图片字节一致。
+    # 这里统一维护格式映射，确保本地文件和 `base64://` 图片引用使用相同声明。
+    _IMAGE_FORMAT_MIME_TYPES = {
+        "JPEG": "image/jpeg",
+        "PNG": "image/png",
+        "GIF": "image/gif",
+        "WEBP": "image/webp",
+        "BMP": "image/bmp",
+        "TIFF": "image/tiff",
+        "AVIF": "image/avif",
+    }
 
     @classmethod
     def _truncate_error_text_candidate(cls, text: str) -> str:
@@ -195,25 +207,55 @@ def _encode_image_file_to_data_url(
                 raise
             return None
 
-        try:
-            with PILImage.open(BytesIO(image_bytes)) as image:
-                image.verify()
-                image_format = str(image.format or "").upper()
-        except (OSError, UnidentifiedImageError):
+        image_format = cls._detect_image_format(image_bytes)
+        if image_format is None:
             if mode == "strict":
                 raise ValueError(f"Invalid image file: {image_path}")
             return None
 
-        mime_type = {
-            "JPEG": "image/jpeg",
-            "PNG": "image/png",
-            "GIF": "image/gif",
-            "WEBP": "image/webp",
-            "BMP": "image/bmp",
-        }.get(image_format, "image/jpeg")
+        mime_type = cls._image_format_to_mime_type(image_format)
         image_bs64 = base64.b64encode(image_bytes).decode("utf-8")
         return f"data:{mime_type};base64,{image_bs64}"
 
+    @classmethod
+    def _detect_image_format(cls, image_bytes: bytes) -> str | None:
+        """返回 Pillow 校验后的图片格式，非法图片返回 None。"""
+        try:
+            # verify() 只校验图片容器，不完整解码像素。
+            # 这里仅需要可信的格式标签，因此这种方式足够且开销较小。
+            with PILImage.open(BytesIO(image_bytes)) as image:
+                image.verify()
+                return str(image.format or "").upper()
+        except (OSError, UnidentifiedImageError):
+            return None
+
+    @classmethod
+    def _image_format_to_mime_type(cls, image_format: str | None) -> str:
+        """将 Pillow 图片格式映射为 data URL 使用的 MIME 类型。"""
+        # 未识别格式保持历史 JPEG 兜底，兼容传入任意 `base64://` 内容的旧调用方。
+        return cls._IMAGE_FORMAT_MIME_TYPES.get(
+            str(image_format or "").upper(), "image/jpeg"
+        )
+
+    @classmethod
+    def _base64_image_ref_to_data_url(cls, image_ref: str) -> str:
+        """将 `base64://` 图片引用转换为带真实 MIME 的 data URL。"""
+        raw_base64 = image_ref.removeprefix("base64://")
+        mime_type = "image/jpeg"
+        try:
+            # 平台适配器可能通过 `base64://` 传入 PNG/GIF/WebP 等图片字节，
+            # 但不会额外携带 MIME 元数据。发送 OpenAI 请求前先识别真实格式，
+            # 避免把 PNG 等图片错误声明为 JPEG。
+            image_bytes = base64.b64decode(raw_base64)
+        except (binascii.Error, ValueError):
+            # 对错误或非图片 base64 保持旧行为：继续返回 JPEG data URL，
+            # 避免让历史调用方因为格式识别失败而直接抛异常。
+            pass
+        else:
+            image_format = cls._detect_image_format(image_bytes)
+            mime_type = cls._image_format_to_mime_type(image_format)
+        return f"data:{mime_type};base64,{raw_base64}"
+
     @staticmethod
     def _file_uri_to_path(file_uri: str) -> str:
         """Normalize file URIs to paths.
@@ -242,7 +284,7 @@ async def _image_ref_to_data_url(
         mode: Literal["safe", "strict"] = "safe",
     ) -> str | None:
         if image_ref.startswith("base64://"):
-            return image_ref.replace("base64://", "data:image/jpeg;base64,")
+            return self._base64_image_ref_to_data_url(image_ref)
 
         if image_ref.startswith("http"):
             image_path = await download_image_by_url(image_ref)
diff --git a/tests/test_openai_source.py b/tests/test_openai_source.py
index 950e2ea162..b5587ffb14 100644
--- a/tests/test_openai_source.py
+++ b/tests/test_openai_source.py
@@ -1,4 +1,6 @@
+import base64
 import builtins
+from io import BytesIO
 from types import SimpleNamespace
 
 import pytest
@@ -1031,6 +1033,27 @@ async def test_resolve_image_part_supports_base64_scheme():
         await provider.terminate()
 
 
+@pytest.mark.asyncio
+async def test_resolve_image_part_preserves_base64_png_mime_type():
+    provider = _make_provider()
+    try:
+        image_buffer = BytesIO()
+        PILImage.new("RGBA", (1, 1), (255, 0, 0, 255)).save(
+            image_buffer,
+            format="PNG",
+        )
+        image_base64 = base64.b64encode(image_buffer.getvalue()).decode("ascii")
+
+        image_part = await provider._resolve_image_part(f"base64://{image_base64}")
+
+        assert image_part == {
+            "type": "image_url",
+            "image_url": {"url": f"data:image/png;base64,{image_base64}"},
+        }
+    finally:
+        await provider.terminate()
+
+
 @pytest.mark.asyncio
 async def test_prepare_chat_payload_materializes_context_localhost_file_uri_image_urls(
     tmp_path,