Describe the Bug Cursor Desktop’s MCP Client yields an error when attempting to invoke an MCP server tool with optional parameters. Example error message: Parameter 'optionalStr' must be of type null,string, got string UPDATE: May be a duplicate of https://forum.cursor.com/t/mcp-server-tool-cal…

According to the MCP specification (2025-06-18) : A Tool’s inputSchema is “A JSON Schema object defining the expected parameters for the tool” The MCP specification requires inputSchema to be a valid JSON Schema object. JSON Schema explicitly supports union types via array notation for the type f…

Per JSON Schema specification , a value validates successfully if it matches any of the types in a union array. Verification The same parameters work correctly with other MCP clients. Unit tests using the official Go MCP SDK all pass (see tests the cursor-mcp-test repo outlined in OP) $ go test -v…

Hey, thanks for the report. This is a known issue in the Cursor MCP client when handling union types in parameter schemas. Good news: we’ll release a fix soon for similar union type validation problems. Your issue with the array “type”: [“null”, “string”] looks like the same root cause as the anyO…

That’s great news; thanks @deanrie

I too would love to know when this fix may arrive. Thanks all.

Error Invoking MCP Tools with Optional Parameters

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Pierre_Allyndree (Pierre Allyndree) November 26, 2025, 10:11am 6

Thanks @deanrie Can you share when that will be fixed?

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Error Invoking MCP Tools with Optional Parameters

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