Add tests for MCPServer.read_resource exception handling (#1957)

Author: Kludex

src/mcp/server/experimental/request_context.py

@@ -91,7 +91,7 @@ def validate_task_mode(
             error = ErrorData(code=METHOD_NOT_FOUND, message="This tool does not support task-augmented invocation")
 
         if error is not None and raise_error:
-            raise MCPError(code=error.code, message=error.message)
+            raise MCPError.from_error_data(error)
 
         return error
 

src/mcp/server/lowlevel/server.py

@@ -795,12 +795,7 @@ async def _handle_request(
 
             await message.respond(response)
         else:  # pragma: no cover
-            await message.respond(
-                types.ErrorData(
-                    code=types.METHOD_NOT_FOUND,
-                    message="Method not found",
-                )
-            )
+            await message.respond(types.ErrorData(code=types.METHOD_NOT_FOUND, message="Method not found"))
 
         logger.debug("Response sent")
 

src/mcp/server/mcpserver/resources/resource_manager.py

@@ -82,10 +82,8 @@ def add_template(
         return template
 
     async def get_resource(
-        self,
-        uri: AnyUrl | str,
-        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
-    ) -> Resource | None:
+        self, uri: AnyUrl | str, context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None
+    ) -> Resource:
         """Get resource by URI, checking concrete resources first, then templates."""
         uri_str = str(uri)
         logger.debug("Getting resource", extra={"uri": uri_str})

src/mcp/server/mcpserver/server.py

@@ -347,16 +347,18 @@ async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContent
         """Read a resource by URI."""
 
         context = self.get_context()
-        resource = await self._resource_manager.get_resource(uri, context=context)
-        if not resource:  # pragma: no cover
+        try:
+            resource = await self._resource_manager.get_resource(uri, context=context)
+        except ValueError:
             raise ResourceError(f"Unknown resource: {uri}")
 
         try:
             content = await resource.read()
             return [ReadResourceContents(content=content, mime_type=resource.mime_type, meta=resource.meta)]
-        except Exception as e:  # pragma: no cover
-            logger.exception(f"Error reading resource {uri}")
-            raise ResourceError(str(e))
+        except Exception as exc:
+            logger.exception(f"Error getting resource {uri}")
+            # If an exception happens when reading the resource, we should not leak the exception to the client.
+            raise ResourceError(f"Error reading resource {uri}") from exc
 
     def add_tool(
         self,

tests/issues/test_141_resource_templates.py

@@ -2,6 +2,7 @@
 
 from mcp import Client
 from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.exceptions import ResourceError
 from mcp.types import (
     ListResourceTemplatesResult,
     TextResourceContents,
@@ -54,10 +55,10 @@ def get_user_profile_missing(user_id: str) -> str:  # pragma: no cover
     assert result_list[0].mime_type == "text/plain"
 
     # Verify invalid parameters raise error
-    with pytest.raises(ValueError, match="Unknown resource"):
+    with pytest.raises(ResourceError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts")  # Missing post_id
 
-    with pytest.raises(ValueError, match="Unknown resource"):
+    with pytest.raises(ResourceError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts/456/extra")  # Extra path component