fix: Raise resource not found error

src/mcp/server/fastmcp/exceptions.py

@@ -1,5 +1,9 @@
 """Custom exceptions for FastMCP."""
 
+from typing import Any
+
+from mcp.types import INTERNAL_ERROR, ErrorData
+
 
 class FastMCPError(Exception):
     """Base error for FastMCP."""
@@ -10,7 +14,23 @@ class ValidationError(FastMCPError):
 
 
 class ResourceError(FastMCPError):
-    """Error in resource operations."""
+    """Error in resource operations.
+
+    Defaults to INTERNAL_ERROR (-32603), but can be set to RESOURCE_NOT_FOUND (-32002)
+    for resource not found errors per MCP spec.
+    """
+
+    error: ErrorData
+
+    def __init__(self, message: str, code: int = INTERNAL_ERROR, data: Any | None = None):
+        """Initialize ResourceError with error code and message.
+
+        Args:
+            message: Error message
+            code: Error code (defaults to INTERNAL_ERROR -32603, use RESOURCE_NOT_FOUND -32002 for not found)
+        """
+        super().__init__(message)
+        self.error = ErrorData(code=code, message=message, data=data)
 
 
 class ToolError(FastMCPError):

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

@@ -7,10 +7,11 @@
 
 from pydantic import AnyUrl
 
+from mcp.server.fastmcp.exceptions import ResourceError
 from mcp.server.fastmcp.resources.base import Resource
 from mcp.server.fastmcp.resources.templates import ResourceTemplate
 from mcp.server.fastmcp.utilities.logging import get_logger
-from mcp.types import Annotations, Icon
+from mcp.types import RESOURCE_NOT_FOUND, Annotations, Icon
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
@@ -98,9 +99,9 @@ async def get_resource(
                 try:
                     return await template.create_resource(uri_str, params, context=context)
                 except Exception as e:
-                    raise ValueError(f"Error creating resource from template: {e}")
+                    raise ResourceError(f"Error creating resource from template: {e}")
 
-        raise ValueError(f"Unknown resource: {uri}")
+        raise ResourceError(f"Unknown resource: {uri}", code=RESOURCE_NOT_FOUND)
 
     def list_resources(self) -> list[Resource]:
         """List all registered resources."""

src/mcp/server/fastmcp/server.py

@@ -61,7 +61,17 @@
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared.context import LifespanContextT, RequestContext, RequestT
-from mcp.types import Annotations, AnyFunction, ContentBlock, GetPromptResult, Icon, ToolAnnotations
+from mcp.shared.exceptions import McpError
+from mcp.types import (
+    RESOURCE_NOT_FOUND,
+    Annotations,
+    AnyFunction,
+    ContentBlock,
+    ErrorData,
+    GetPromptResult,
+    Icon,
+    ToolAnnotations,
+)
 from mcp.types import Prompt as MCPPrompt
 from mcp.types import PromptArgument as MCPPromptArgument
 from mcp.types import Resource as MCPResource
@@ -367,9 +377,12 @@ 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)
+        try:
+            resource = await self._resource_manager.get_resource(uri, context=context)
+        except ResourceError as e:
+            raise McpError(error=e.error)
         if not resource:
-            raise ResourceError(f"Unknown resource: {uri}")
+            raise McpError(error=ErrorData(code=RESOURCE_NOT_FOUND, message=f"Unknown resource: {uri}"))
 
         try:
             content = await resource.read()

src/mcp/types.py

@@ -156,6 +156,7 @@ class JSONRPCResponse(BaseModel):
 METHOD_NOT_FOUND = -32601
 INVALID_PARAMS = -32602
 INTERNAL_ERROR = -32603
+RESOURCE_NOT_FOUND = -32002
 
 
 class ErrorData(BaseModel):

tests/issues/test_141_resource_templates.py

@@ -2,6 +2,7 @@
 from pydantic import AnyUrl
 
 from mcp.server.fastmcp import FastMCP
+from mcp.shared.exceptions import McpError
 from mcp.shared.memory import (
     create_connected_server_and_client_session as client_session,
 )
@@ -57,10 +58,10 @@ def get_user_profile_missing(user_id: str) -> str:
     assert result_list[0].mime_type == "text/plain"
 
     # Verify invalid parameters raise error
-    with pytest.raises(ValueError, match="Unknown resource"):
+    with pytest.raises(McpError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts")  # Missing post_id
 
-    with pytest.raises(ValueError, match="Unknown resource"):
+    with pytest.raises(McpError, match="Unknown resource"):
         await mcp.read_resource("resource://users/123/posts/456/extra")  # Extra path component
 
 

tests/server/fastmcp/resources/test_resource_manager.py

@@ -4,7 +4,9 @@
 import pytest
 from pydantic import AnyUrl, FileUrl
 
+from mcp.server.fastmcp.exceptions import ResourceError
 from mcp.server.fastmcp.resources import FileResource, FunctionResource, ResourceManager, ResourceTemplate
+from mcp.types import RESOURCE_NOT_FOUND
 
 
 @pytest.fixture
@@ -113,8 +115,10 @@ def greet(name: str) -> str:
     async def test_get_unknown_resource(self):
         """Test getting a non-existent resource."""
         manager = ResourceManager()
-        with pytest.raises(ValueError, match="Unknown resource"):
+        with pytest.raises(ResourceError) as exc_info:
             await manager.get_resource(AnyUrl("unknown://test"))
+        assert exc_info.value.error.code == RESOURCE_NOT_FOUND
+        assert "Unknown resource" in str(exc_info.value)
 
     def test_list_resources(self, temp_file: Path):
         """Test listing all resources."""