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-calls-fail-with-invalid-type-for-parameter-in-tool/70831 and https://forum.cursor.com/t/cursor-cannot-handle-mcp-server-params/52657/2, but neither of those showed when I initially searched the forums before posting. Leaving this topic open in case it has additional context. Steps to Reproduce Clone Test MCP Server: git clone https://github.com/lkingland/cursor-mcp-test Build: cd cursor-mcp-test && go build Configure Cursor MCP settings (~/.cursor/mcp.json): { "mcpServers": { "cursor-mcp-test": { "command": "/absolute/path/to/cursor-mcp-test" } } } Restart Cursor Ask Cursor to invoke “Example Tool” with: { "requiredParam": "test-value", "optionalStr": "optional-value" } A log of communication with the server is written to /tmp/cursor-mcp-test.log which shows the tool invocation never exits the client. This is therefore happening client-side in the MCP Client and is likely a problem of not supporting union set validation of the tool’s schema. Operating System MacOS Current Cursor Version (Menu → About Cursor → Copy) Version: 2.0.75 VSCode Version: 1.99.3 Commit: 9e7a27b76730ca7fe4aecaeafc58bac1e2c82120 Date: 2025-11-12T17:34:21.472Z Electron: 37.7.0 Chromium: 138.0.7204.251 Node.js: 22.20.0 V8: 13.8.258.32-electron.0 OS: Darwin arm64 25.0.0 Additional Information The test server uses the following to define an input: type MyToolInput struct { RequiredParam string `json:"requiredParam" jsonschema:"required,A required string parameter"` OptionalStr *string `json:"optionalStr,omitempty" jsonschema:"An optional string parameter"` } Which generates the tool schema: { "type": "object", "required": ["requiredParam"], "properties": { "requiredParam": { "type": "string", "description": "A required string parameter" }, "optionalStr": { "type": ["null", "string"], "description": "An optional string parameter" } }, "additionalProperties": false } The pattern "type": ["null", "string"] is the standard way to represent optional/nullable fields: Go MCP servers: pointer types (*string, *int, *bool) generate union types Python MCP servers: Optional[str] generates union types TypeScript MCP servers: string | null generates union types GitHub Issue: Error Invoking MCP Tools with Optional Parameters · Issue #3778 · cursor/cursor · GitHub Does this stop you from using Cursor No - Cursor works, but with this issue

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 field (e.g., "type": ["null", "string"]), which is the standard pattern for optional/nullable parameters.

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 ./pkg === RUN TestMyTool === RUN TestMyTool/with_optional_string === RUN TestMyTool/without_optional_string === RUN TestMyTool/with_explicit_null === RUN TestMyTool/missing_required_param === RUN TestMyTool/wrong_type_for_optional --- PASS: TestMyTool (0.00s) --- PASS: TestMyTool/with_optional_string (0.00s) --- PASS: TestMyTool/without_optional_string (0.00s) --- PASS: TestMyTool/with_explicit_null (0.00s) --- PASS: TestMyTool/missing_required_param (0.00s) --- PASS: TestMyTool/wrong_type_for_optional (0.00s) === RUN TestListTools server_test.go:128: optionalStr type: [null string] --- PASS: TestListTools (0.00s) PASS ok cursor-mcp-test/pkg (cached)

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 anyOf bugs. The fix should cover both cases, since both are valid representations of union types in JSON Schema. Thanks for the links to related threads and the GitHub issue - your test server repo will help verify the fix!

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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