chore: create union for working with message content (#939)

examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py

@@ -41,14 +41,7 @@ def main(
     app = Server("mcp-streamable-http-stateless-demo")
 
     @app.call_tool()
-    async def call_tool(
+    async def call_tool(name: str, arguments: dict) -> list[types.Content]:
-        name: str, arguments: dict
-    ) -> list[
-        types.TextContent
-        | types.ImageContent
-        | types.AudioContent
-        | types.EmbeddedResource
-    ]:
         ctx = app.request_context
         interval = arguments.get("interval", 1.0)
         count = arguments.get("count", 5)

examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.py

@@ -45,14 +45,7 @@ def main(
     app = Server("mcp-streamable-http-demo")
 
     @app.call_tool()
-    async def call_tool(
+    async def call_tool(name: str, arguments: dict) -> list[types.Content]:
-        name: str, arguments: dict
-    ) -> list[
-        types.TextContent
-        | types.ImageContent
-        | types.AudioContent
-        | types.EmbeddedResource
-    ]:
         ctx = app.request_context
         interval = arguments.get("interval", 1.0)
         count = arguments.get("count", 5)

examples/servers/simple-tool/mcp_simple_tool/server.py

@@ -7,9 +7,7 @@ from mcp.shared._httpx_utils import create_mcp_http_client
 
 async def fetch_website(
     url: str,
-) -> list[
+) -> list[types.Content]:
-    types.TextContent | types.ImageContent | types.AudioContent | types.EmbeddedResource
-]:
     headers = {
         "User-Agent": "MCP Test Server (github.com/modelcontextprotocol/python-sdk)"
     }
@@ -31,14 +29,7 @@ def main(port: int, transport: str) -> int:
     app = Server("mcp-website-fetcher")
 
     @app.call_tool()
-    async def fetch_tool(
+    async def fetch_tool(name: str, arguments: dict) -> list[types.Content]:
-        name: str, arguments: dict
-    ) -> list[
-        types.TextContent
-        | types.ImageContent
-        | types.AudioContent
-        | types.EmbeddedResource
-    ]:
         if name != "fetch":
             raise ValueError(f"Unknown tool: {name}")
         if "url" not in arguments:

src/mcp/server/fastmcp/prompts/base.py

@@ -7,18 +7,16 @@ from typing import Any, Literal
 import pydantic_core
 from pydantic import BaseModel, Field, TypeAdapter, validate_call
 
-from mcp.types import AudioContent, EmbeddedResource, ImageContent, TextContent
+from mcp.types import Content, TextContent
-
-CONTENT_TYPES = TextContent | ImageContent | AudioContent | EmbeddedResource
 
 
 class Message(BaseModel):
     """Base class for all prompt messages."""
 
     role: Literal["user", "assistant"]
-    content: CONTENT_TYPES
+    content: Content
 
-    def __init__(self, content: str | CONTENT_TYPES, **kwargs: Any):
+    def __init__(self, content: str | Content, **kwargs: Any):
         if isinstance(content, str):
             content = TextContent(type="text", text=content)
         super().__init__(content=content, **kwargs)
@@ -29,7 +27,7 @@ class UserMessage(Message):
 
     role: Literal["user", "assistant"] = "user"
 
-    def __init__(self, content: str | CONTENT_TYPES, **kwargs: Any):
+    def __init__(self, content: str | Content, **kwargs: Any):
         super().__init__(content=content, **kwargs)
 
 
@@ -38,7 +36,7 @@ class AssistantMessage(Message):
 
     role: Literal["user", "assistant"] = "assistant"
 
-    def __init__(self, content: str | CONTENT_TYPES, **kwargs: Any):
+    def __init__(self, content: str | Content, **kwargs: Any):
         super().__init__(content=content, **kwargs)
 
 

src/mcp/server/fastmcp/server.py

@@ -52,10 +52,8 @@ from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from mcp.shared.context import LifespanContextT, RequestContext, RequestT
 from mcp.types import (
     AnyFunction,
-    AudioContent,
+    Content,
-    EmbeddedResource,
     GetPromptResult,
-    ImageContent,
     TextContent,
     ToolAnnotations,
 )
@@ -256,9 +254,7 @@ class FastMCP:
             request_context = None
         return Context(request_context=request_context, fastmcp=self)
 
-    async def call_tool(
+    async def call_tool(self, name: str, arguments: dict[str, Any]) -> Sequence[Content]:
-        self, name: str, arguments: dict[str, Any]
-    ) -> Sequence[TextContent | ImageContent | AudioContent | EmbeddedResource]:
         """Call a tool by name with arguments."""
         context = self.get_context()
         result = await self._tool_manager.call_tool(name, arguments, context=context)
@@ -842,12 +838,12 @@ class FastMCP:
 
 def _convert_to_content(
     result: Any,
-) -> Sequence[TextContent | ImageContent | AudioContent | EmbeddedResource]:
+) -> Sequence[Content]:
     """Convert a result to a sequence of content objects."""
     if result is None:
         return []
 
-    if isinstance(result, TextContent | ImageContent | AudioContent | EmbeddedResource):
+    if isinstance(result, Content):
         return [result]
 
     if isinstance(result, Image):

src/mcp/server/lowlevel/server.py

@@ -384,9 +384,7 @@ class Server(Generic[LifespanResultT, RequestT]):
         def decorator(
             func: Callable[
                 ...,
-                Awaitable[
+                Awaitable[Iterable[types.Content]],
-                    Iterable[types.TextContent | types.ImageContent | types.AudioContent | types.EmbeddedResource]
-                ],
             ],
         ):
             logger.debug("Registering handler for CallToolRequest")

src/mcp/types.py

@@ -667,11 +667,14 @@ class EmbeddedResource(BaseModel):
     model_config = ConfigDict(extra="allow")
 
 
+Content = TextContent | ImageContent | AudioContent | EmbeddedResource
+
+
 class PromptMessage(BaseModel):
     """Describes a message returned as part of a prompt."""
 
     role: Role
-    content: TextContent | ImageContent | AudioContent | EmbeddedResource
+    content: Content
     model_config = ConfigDict(extra="allow")
 
 
@@ -787,7 +790,7 @@ class CallToolRequest(Request[CallToolRequestParams, Literal["tools/call"]]):
 class CallToolResult(Result):
     """The server's response to a tool call."""
 
-    content: list[TextContent | ImageContent | AudioContent | EmbeddedResource]
+    content: list[Content]
     isError: bool = False
 
 

tests/issues/test_88_random_error.py

@@ -11,7 +11,7 @@ from anyio.abc import TaskStatus
 from mcp.client.session import ClientSession
 from mcp.server.lowlevel import Server
 from mcp.shared.exceptions import McpError
-from mcp.types import AudioContent, EmbeddedResource, ImageContent, TextContent
+from mcp.types import Content, TextContent
 
 
 @pytest.mark.anyio
@@ -31,7 +31,7 @@ async def test_notification_validation_error(tmp_path: Path):
     slow_request_complete = anyio.Event()
 
     @server.call_tool()
-    async def slow_tool(name: str, arg) -> Sequence[TextContent | ImageContent | AudioContent | EmbeddedResource]:
+    async def slow_tool(name: str, arg) -> Sequence[Content]:
         nonlocal request_count
         request_count += 1
 

tests/server/fastmcp/test_server.py

@@ -8,7 +8,7 @@ from pydantic import AnyUrl
 from starlette.routing import Mount, Route
 
 from mcp.server.fastmcp import Context, FastMCP
-from mcp.server.fastmcp.prompts.base import EmbeddedResource, Message, UserMessage
+from mcp.server.fastmcp.prompts.base import Message, UserMessage
 from mcp.server.fastmcp.resources import FileResource, FunctionResource
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.shared.exceptions import McpError
@@ -18,6 +18,8 @@ from mcp.shared.memory import (
 from mcp.types import (
     AudioContent,
     BlobResourceContents,
+    Content,
+    EmbeddedResource,
     ImageContent,
     TextContent,
     TextResourceContents,
@@ -192,7 +194,7 @@ def image_tool_fn(path: str) -> Image:
     return Image(path)
 
 
-def mixed_content_tool_fn() -> list[TextContent | ImageContent | AudioContent]:
+def mixed_content_tool_fn() -> list[Content]:
     return [
         TextContent(type="text", text="Hello"),
         ImageContent(type="image", data="abc", mimeType="image/png"),