Update MCP resource methods to raise upstream server exceptions rather than try to cast to None | []

Author: fennb

pydantic_ai_slim/pydantic_ai/mcp.py

@@ -347,7 +347,7 @@ def capabilities(self) -> ServerCapabilities:
                 f'The `{self.__class__.__name__}.capabilities` is only instantiated after initialization.'
             )
         return self._server_capabilities
-      
+
     @property
     def instructions(self) -> str | None:
         """Access the instructions sent by the MCP server during initialization."""
@@ -504,18 +504,16 @@ async def list_resource_templates(self) -> list[ResourceTemplate]:
         return [_mcp.map_from_mcp_resource_template(t) for t in result.resourceTemplates]
 
     @overload
-    async def read_resource(
-        self, uri: str
-    ) -> str | messages.BinaryContent | list[str | messages.BinaryContent] | None: ...
+    async def read_resource(self, uri: str) -> str | messages.BinaryContent | list[str | messages.BinaryContent]: ...
 
     @overload
     async def read_resource(
         self, uri: Resource
-    ) -> str | messages.BinaryContent | list[str | messages.BinaryContent] | None: ...
+    ) -> str | messages.BinaryContent | list[str | messages.BinaryContent]: ...
 
     async def read_resource(
         self, uri: str | Resource
-    ) -> str | messages.BinaryContent | list[str | messages.BinaryContent] | None:
+    ) -> str | messages.BinaryContent | list[str | messages.BinaryContent]:
         """Read the contents of a specific resource by URI.
 
         Args:
@@ -524,26 +522,17 @@ async def read_resource(
         Returns:
             The resource contents. If the resource has a single content item, returns that item directly.
             If the resource has multiple content items, returns a list of items.
-            Returns `None` if the server does not support resources or the resource is not found.
 
         Raises:
-            MCPError: If the server returns an error other than resource not found.
+            MCPError: If the server returns an error.
         """
         resource_uri = uri if isinstance(uri, str) else uri.uri
         async with self:  # Ensure server is running
-            if not self.capabilities.resources:
-                return None
             try:
                 result = await self._client.read_resource(AnyUrl(resource_uri))
             except mcp_exceptions.McpError as e:
-                # As per https://modelcontextprotocol.io/specification/2025-06-18/server/resources#error-handling
-                if e.error.code == -32002:
-                    return None
                 raise MCPError.from_mcp_sdk_error(e) from e
 
-        if not result.contents:
-            return None
-
         return (
             self._get_content(result.contents[0])
             if len(result.contents) == 1
@@ -646,12 +635,8 @@ async def _map_tool_result_part(
             resource = part.resource
             return self._get_content(resource)
         elif isinstance(part, mcp_types.ResourceLink):
-            result = await self.read_resource(str(part.uri))
-            # Rather than hide an invalid resource link, we raise an error so it's consistent with any
-            # other error that could happen during resource reading.
-            if result is None:
-                raise MCPError(message=f'Invalid ResourceLink {part.uri} returned by tool', code=-32002)
-            return result
+            # read_resource will raise MCPError if the resource is not found or has any other error
+            return await self.read_resource(str(part.uri))
         else:
             assert_never(part)
 

tests/test_mcp.py

@@ -1607,25 +1607,6 @@ async def test_read_resource_template(run_context: RunContext[int]):
         assert content == snapshot('Hello, Alice!')
 
 
-async def test_read_resource_not_found(mcp_server: MCPServerStdio) -> None:
-    """Test that read_resource returns None for MCP spec error code -32002 (resource not found).
-
-    As per https://modelcontextprotocol.io/specification/2025-06-18/server/resources#error-handling
-
-    Note: We mock this because FastMCP uses error code 0 instead of -32002, which is non-standard.
-    """
-    mcp_error = McpError(error=ErrorData(code=-32002, message='Resource not found'))
-
-    async with mcp_server:
-        with patch.object(
-            mcp_server._client,  # pyright: ignore[reportPrivateUsage]
-            'read_resource',
-            new=AsyncMock(side_effect=mcp_error),
-        ):
-            result = await mcp_server.read_resource('resource://missing')
-            assert result is None
-
-
 async def test_read_resource_error(mcp_server: MCPServerStdio) -> None:
     """Test that read_resource converts McpError to MCPError for generic errors."""
     mcp_error = McpError(
@@ -1648,7 +1629,7 @@ async def test_read_resource_error(mcp_server: MCPServerStdio) -> None:
 
 
 async def test_read_resource_empty_contents(mcp_server: MCPServerStdio) -> None:
-    """Test that read_resource returns None when server returns empty contents."""
+    """Test that read_resource returns empty list when server returns empty contents."""
     from mcp.types import ReadResourceResult
 
     # Mock a result with empty contents
@@ -1661,7 +1642,7 @@ async def test_read_resource_empty_contents(mcp_server: MCPServerStdio) -> None:
             new=AsyncMock(return_value=empty_result),
         ):
             result = await mcp_server.read_resource('resource://empty')
-            assert result is None
+            assert result == []
 
 
 async def test_list_resources_error(mcp_server: MCPServerStdio) -> None:
@@ -1961,7 +1942,7 @@ async def test_capabilities(mcp_server: MCPServerStdio) -> None:
 
 
 async def test_resource_methods_without_capability(mcp_server: MCPServerStdio) -> None:
-    """Test that resource methods return empty values when resources capability is not available."""
+    """Test that resource list methods return empty values when resources capability is not available."""
     async with mcp_server:
         # Mock the capabilities to not support resources
         mock_capabilities = ServerCapabilities(resources=False)
@@ -1974,9 +1955,7 @@ async def test_resource_methods_without_capability(mcp_server: MCPServerStdio) -
             result = await mcp_server.list_resource_templates()
             assert result == []
 
-            # read_resource should return None
-            result = await mcp_server.read_resource('resource://test')
-            assert result is None
+
 async def test_instructions(mcp_server: MCPServerStdio) -> None:
     with pytest.raises(
         AttributeError, match='The `MCPServerStdio.instructions` is only available after initialization.'