Support ImageContent and AudioContent in Message class

Message.content now accepts ImageContent and AudioContent in addition to
TextContent and EmbeddedResource, matching MCP's ContentBlock type. This
fixes ProxyPrompt.render() silently JSON-serializing image/audio content
instead of preserving it.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fastmcp/prompts/prompt.py

@@ -17,8 +17,10 @@ if TYPE_CHECKING:
 import mcp.types
 from mcp import GetPromptResult
 from mcp.types import (
+    AudioContent,
     EmbeddedResource,
     Icon,
+    ImageContent,
     PromptMessage,
     TextContent,
 )
@@ -61,7 +63,7 @@ class Message(pydantic.BaseModel):
     """
 
     role: Literal["user", "assistant"]
-    content: TextContent | EmbeddedResource
+    content: TextContent | ImageContent | AudioContent | EmbeddedResource
 
     def __init__(
         self,
@@ -72,13 +74,18 @@ class Message(pydantic.BaseModel):
 
         Args:
             content: The message content. str passes through directly.
-                     TextContent and EmbeddedResource pass through.
+                     TextContent, ImageContent, AudioContent, and
+                     EmbeddedResource pass through.
                      Other types (dict, list, BaseModel) are JSON-serialized.
             role: The message role, either "user" or "assistant".
         """
         # Handle already-wrapped content types
-        if isinstance(content, (TextContent, EmbeddedResource)):
-            normalized_content: TextContent | EmbeddedResource = content
+        if isinstance(
+            content, (TextContent, ImageContent, AudioContent, EmbeddedResource)
+        ):
+            normalized_content: (
+                TextContent | ImageContent | AudioContent | EmbeddedResource
+            ) = content
         elif isinstance(content, str):
             normalized_content = TextContent(type="text", text=content)
         else:

tests/prompts/test_prompt.py

@@ -493,6 +493,36 @@ class TestMessage:
         assert isinstance(mcp_msg.content, TextContent)
         assert mcp_msg.content.text == "Hello"
 
+    def test_message_passthrough_image_content(self):
+        """Test Message passes through ImageContent without JSON serialization."""
+        from mcp.types import ImageContent
+
+        img = ImageContent(type="image", data="base64data", mimeType="image/png")
+        msg = Message(img, role="user")
+        assert isinstance(msg.content, ImageContent)
+        assert msg.content.data == "base64data"
+        assert msg.content.mimeType == "image/png"
+
+    def test_message_passthrough_audio_content(self):
+        """Test Message passes through AudioContent without JSON serialization."""
+        from mcp.types import AudioContent
+
+        audio = AudioContent(type="audio", data="base64audio", mimeType="audio/wav")
+        msg = Message(audio, role="user")
+        assert isinstance(msg.content, AudioContent)
+        assert msg.content.data == "base64audio"
+        assert msg.content.mimeType == "audio/wav"
+
+    def test_message_image_content_to_mcp_prompt_message(self):
+        """Test that ImageContent round-trips through to_mcp_prompt_message."""
+        from mcp.types import ImageContent
+
+        img = ImageContent(type="image", data="base64data", mimeType="image/png")
+        msg = Message(img, role="user")
+        mcp_msg = msg.to_mcp_prompt_message()
+        assert isinstance(mcp_msg.content, ImageContent)
+        assert mcp_msg.content.data == "base64data"
+
 
 class TestPromptResult:
     def test_promptresult_from_string(self):

tests/server/providers/proxy/test_proxy_server.py

@@ -130,6 +130,25 @@ def fastmcp_server():
     def welcome(name: str) -> str:
         return f"Welcome to FastMCP, {name}!"
 
+    @server.prompt
+    def image_prompt():
+        """A prompt that returns an image."""
+        from fastmcp.prompts.prompt import Message, PromptResult
+
+        return PromptResult(
+            messages=[
+                Message("Here is an image:"),
+                Message(
+                    content=mcp_types.ImageContent(
+                        type="image",
+                        data="iVBORw0KGgoAAAANSUhEUg==",
+                        mimeType="image/png",
+                    ),
+                    role="user",
+                ),
+            ]
+        )
+
     return server
 
 
@@ -656,6 +675,22 @@ class TestPrompts:
             param_names = [arg.name for arg in welcome_prompt.arguments or []]
             assert "extra" in param_names
 
+    async def test_proxy_prompt_preserves_image_content(
+        self, fastmcp_server: FastMCP, proxy_server: FastMCPProxy
+    ):
+        """Test that ProxyPrompt preserves ImageContent without lossy conversion."""
+        async with Client(fastmcp_server) as client:
+            result = await client.get_prompt("image_prompt")
+        async with Client(proxy_server) as client:
+            proxy_result = await client.get_prompt("image_prompt")
+
+        # The proxy result should match the original exactly
+        assert proxy_result == result
+        # Verify the image content is preserved as ImageContent, not JSON text
+        assert isinstance(proxy_result.messages[1].content, mcp_types.ImageContent)
+        assert proxy_result.messages[1].content.data == "iVBORw0KGgoAAAANSUhEUg=="
+        assert proxy_result.messages[1].content.mimeType == "image/png"
+
 
 async def test_proxy_handles_multiple_concurrent_tasks_correctly(
     proxy_server: FastMCPProxy,