fix: respect resource mime type in responses

The server was ignoring mime types set on resources, defaulting to text/plain
for strings and application/octet-stream for bytes. Now properly preserves
the specified mime type in both FastMCP and low-level server implementations.

Note that this is breaks backwards compatibility as it changes the return
values of read_resource() on FastMCP. It is BC compatible on lowlevel since
it only extends the callback.

Github-Issue: #152
Reported-by: eiseleMichael

Author: dsp-ant

CLAUDE.md

@@ -23,6 +23,30 @@ This file is intended to be used by an LLM such as Claude.
   git commit -am "\"fix: my commit message\""
   ```
 
+- For commits fixing bugs or adding features based on user reports add:
+  ```bash
+  git commit --trailer "Reported-by:<name>"
+  ```
+  Where `<name>` is the name of the user.
+
+- For commits related to a Github issue, add
+  ```bash
+  git commit --trailer "Github-Issue:<number>"
+  ```
+- NEVER ever mention a `co-authored-by` or similar aspects. In particular, never
+  mention the tool used to create the commit message or PR.
+
+## Pull Requests
+
+- Create a detailed message of what changed. Focus on the high level description of
+  the problem it tries to solve, and how it is solved. Don't go into the specifics of the
+  code unless it adds clarity.
+
+- Always add `jerome3o-anthropic` and `jspahrsummers` as reviewer.
+
+- NEVER ever mention a `co-authored-by` or similar aspects. In particular, never
+  mention the tool used to create the commit message or PR.
+
 ## Python Tools
 
 ### Ruff

src/mcp/server/fastmcp/server.py

@@ -197,14 +197,16 @@ async def list_resource_templates(self) -> list[MCPResourceTemplate]:
             for template in templates
         ]
 
-    async def read_resource(self, uri: AnyUrl | str) -> str | bytes:
+    async def read_resource(self, uri: AnyUrl | str) -> tuple[str | bytes, str]:
         """Read a resource by URI."""
+
         resource = await self._resource_manager.get_resource(uri)
         if not resource:
             raise ResourceError(f"Unknown resource: {uri}")
 
         try:
-            return await resource.read()
+            content = await resource.read()
+            return (content, resource.mime_type)
         except Exception as e:
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
@@ -606,7 +608,7 @@ async def report_progress(
             progress_token=progress_token, progress=progress, total=total
         )
 
-    async def read_resource(self, uri: str | AnyUrl) -> str | bytes:
+    async def read_resource(self, uri: str | AnyUrl) -> tuple[str | bytes, str]:
         """Read a resource by URI.
 
         Args:

src/mcp/server/lowlevel/server.py

@@ -252,32 +252,55 @@ async def handler(_: Any):
         return decorator
 
     def read_resource(self):
-        def decorator(func: Callable[[AnyUrl], Awaitable[str | bytes]]):
+        def decorator(
+            func: Callable[[AnyUrl], Awaitable[str | bytes | tuple[str | bytes, str]]],
+        ):
             logger.debug("Registering handler for ReadResourceRequest")
 
             async def handler(req: types.ReadResourceRequest):
                 result = await func(req.params.uri)
+
+                def create_content(data: str | bytes, mime_type: str):
+                    match data:
+                        case str() as data:
+                            return types.TextResourceContents(
+                                uri=req.params.uri,
+                                text=data,
+                                mimeType=mime_type,
+                            )
+                        case bytes() as data:
+                            import base64
+
+                            return types.BlobResourceContents(
+                                uri=req.params.uri,
+                                blob=base64.urlsafe_b64encode(data).decode(),
+                                mimeType=mime_type,
+                            )
+
                 match result:
-                    case str(s):
-                        content = types.TextResourceContents(
-                            uri=req.params.uri,
-                            text=s,
-                            mimeType="text/plain",
+                    case str() | bytes() as data:
+                        default_mime = (
+                            "text/plain"
+                            if isinstance(data, str)
+                            else "application/octet-stream"
                         )
-                    case bytes(b):
-                        import base64
-
-                        content = types.BlobResourceContents(
-                            uri=req.params.uri,
-                            blob=base64.urlsafe_b64encode(b).decode(),
-                            mimeType="application/octet-stream",
+                        content = create_content(data, default_mime)
+                        return types.ServerResult(
+                            types.ReadResourceResult(
+                                contents=[content],
+                            )
+                        )
+                    case (data, mime_type):
+                        content = create_content(data, mime_type)
+                        return types.ServerResult(
+                            types.ReadResourceResult(
+                                contents=[content],
+                            )
+                        )
+                    case _:
+                        raise ValueError(
+                            f"Unexpected return type from read_resource: {type(result)}"
                         )
-
-                return types.ServerResult(
-                    types.ReadResourceResult(
-                        contents=[content],
-                    )
-                )
 
             self.request_handlers[types.ReadResourceRequest] = handler
             return func

tests/issues/test_152_resource_mime_type.py

@@ -98,9 +98,9 @@ async def handle_list_resources():
     @server.read_resource()
     async def handle_read_resource(uri: AnyUrl):
         if str(uri) == "test://image":
-            return base64_string
+            return (base64_string, "image/png")
         elif str(uri) == "test://image_bytes":
-            return image_bytes
+            return (bytes(image_bytes), "image/png")
         raise Exception(f"Resource not found: {uri}")
 
     # Test that resources are listed with correct mime type

tests/server/fastmcp/servers/test_file_server.py

@@ -88,7 +88,9 @@ async def test_list_resources(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_dir(mcp: FastMCP):
-    files = await mcp.read_resource("dir://test_dir")
+    files, mime_type = await mcp.read_resource("dir://test_dir")
+    assert mime_type == "text/plain"
+
     files = json.loads(files)
 
     assert sorted([Path(f).name for f in files]) == [
@@ -100,7 +102,7 @@ async def test_read_resource_dir(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_file(mcp: FastMCP):
-    result = await mcp.read_resource("file://test_dir/example.py")
+    result, _ = await mcp.read_resource("file://test_dir/example.py")
     assert result == "print('hello world')"
 
 
@@ -117,5 +119,5 @@ async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
     await mcp.call_tool(
         "delete_file", arguments=dict(path=str(test_dir / "example.py"))
     )
-    result = await mcp.read_resource("file://test_dir/example.py")
+    result, _ = await mcp.read_resource("file://test_dir/example.py")
     assert result == "File not found"

tests/server/fastmcp/test_server.py

@@ -563,8 +563,8 @@ def test_resource() -> str:
 
         @mcp.tool()
         async def tool_with_resource(ctx: Context) -> str:
-            data = await ctx.read_resource("test://data")
-            return f"Read resource: {data}"
+            data, mime_type = await ctx.read_resource("test://data")
+            return f"Read resource: {data} with mime type {mime_type}"
 
         async with client_session(mcp._mcp_server) as client:
             result = await client.call_tool("tool_with_resource", {})