Optional MCP params required

“Invalid input: expected nonoptional, received undefined”

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Fading. No reports and thumbs-up in the last 7 days and 1 the week before, down from 13 a week at the mid-September peak. A workaround is known.

MCP tool calls rejected when optional parameters are omitted. Client-side validator rejects omitted parameters that the tool's JSON Schema marks optional with a default, before reaching the server.

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Where: MCP servers
Error text: Invalid input: expected nonoptional, received undefined
Versions: 2.1.273 to 2.1.302
Platforms: macOS
How bad: Breaks a normal workflow. Some reports say it used to work. Some reports say it blocks them completely.

Original reports6, newest first

Oct 26, 2026MCP tools with required anyOf: [X, null] params are unusable: JSON null is never sent (regression of #81911, auto-closed)#105223
Sep 18, 2026MCP Calls#95383
Sep 17, 2026MCP tool-call validation treats optional/nullable parameters as required, and literal "null" is never transmitted as JSON null#95239
Sep 17, 2026MCP tool-call validation rejects omission of any optional/defaulted parameter, regardless of type#95212
Sep 16, 2026MCP client rejects tool calls omitting optional parameters; deadlocks on tools with mutually-exclusive parameter groups#94718
Sep 15, 2026Desktop app rejects MCP calls that omit optional parameters (expected, nonoptional, rerecived undefined)#94608

Optional MCP params required

Is this your bug? What gets counted gets fixed. Pile on.

Is this your bug?

Original reports6, newest first

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