PydanticSchemaGenerationError: Unable to generate pydantic-core schema for Image type

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

The Image type in question is imported like this: from mcp.server.fastmcp.utilities.types import Image

We are upgrading our dependency on mcp from 1.9.4 to 1.10.1.

Once on 1.10.1 our application and tests began to fail with error like this:

pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema
for <class 'mcp.server.fastmcp.utilities.types.Image'>. Set `arbitrary_types_allowed=True` in
the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your
type to fully support it.

I found a test tool function among this repo's unit tests, that is pretty close to the mixed content
use case in our server. So I tweaked it to create a repro case for you:

diff --git a/tests/server/fastmcp/test_server.py b/tests/server/fastmcp/test_server.py
index c30930f..20c25af 100644
--- a/tests/server/fastmcp/test_server.py
+++ b/tests/server/fastmcp/test_server.py
@@ -194,12 +194,12 @@ def image_tool_fn(path: str) -> Image:
     return Image(path)
 
 
-def mixed_content_tool_fn() -> list[ContentBlock]:
-    return [
-        TextContent(type="text", text="Hello"),
-        ImageContent(type="image", data="abc", mimeType="image/png"),
+def mixed_content_tool_fn() -> tuple[str, Image, AudioContent]:
+    return (
+        "Hello",
+        Image(data=b"abc", format="png"),
         AudioContent(type="audio", data="def", mimeType="audio/wav"),
-    ]
+    )
 
 
 class TestServerTools:
@@ -311,7 +311,7 @@ class TestServerTools:
             assert content1.text == "Hello"
             assert isinstance(content2, ImageContent)
             assert content2.mimeType == "image/png"
-            assert content2.data == "abc"
+            assert content2.data == "YWJj"
             assert isinstance(content3, AudioContent)
             assert content3.mimeType == "audio/wav"
             assert content3.data == "def"

test_tool_mixed_content fails with PydanticSchemaGenerationError on 1.10.1 but succeeds on 1.9.4.

Python & MCP Python SDK

Python 3.12, mcp 1.10.1

Empieza por tests/server/fastmcp/test_server.py, especialmente por test_tool_mixed_content y la reproducción de mixed_content_tool_fn. Lee el tipo Image importado de mcp.server.fastmcp.utilities.types y sigue cómo se procesa su anotación de retorno de tupla en mcp 1.10.1. Se considera terminado cuando la prueba de contenido mixto pasa sin PydanticSchemaGenerationError y se conservan las aserciones esperadas sobre el contenido de imagen y audio.