Claude Code desktop app treats MCP optional params as required, making tools uncallable
richsad · ghdev · 2026-09-16
New issue #94608 in anthropics/claude-code reports that the desktop client misvalidates MCP tool schemas: params declared with "default": null but absent from required are treated as mandatory ("expected nonoptional"), and there's a catch-22 — supplying them to pass client validation then trips server-side mutually-exclusive checks, so such tools are completely uncallable since no value ("", {}, null) expresses absence.
Reproduced with ha-mcp (Home Assistant) via haconfigsetdashboard: the CLI on the same machine and same config works fine, confirming it's desktop-only. Author suspects a Zod v4 schema-conversion regression mapping default: null to required fields.
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