Claude Desktop rejects every MCP tool: "outputSchema ... unsupported dialect (draft-07)". Cause and working fix

Error (Claude Desktop and Cowork; the Claude Code CLI was not affected when last tested):

Tool 'count' has an invalid outputSchema: JSON Schema declares an unsupported dialect
("$schema": "http://json-schema.org/draft-07/schema#"). The default validator supports
JSON Schema 2020-12 only; pass a pre-configured Ajv instance to AjvJs

Every tool on the server fails before the call reaches it.

Versions seen: Claude Desktop 1.37937.1, 1.52386.3, 2.7032.0, updater 2.9939.4. Servers built with @modelcontextprotocol/sdk 1.29.0 to 1.32.1 (server-filesystem 2026.7.4 to 2026.8.31, Anthropic's Filesystem extension, mongodb-mcp-server, MongoDB Atlas, Roam, Veeam and more).

Cause: the app validates outputSchema with the MCP SDK v2 default validator, which accepts only JSON Schema 2020-12 and refuses any other declared $schema. McpServer in SDK 1.x always declares draft-07 (typescript-sdk issue 2721).

Reproduction (no app needed, 27 Oct 2026, Node 26.8.1): a server built with @modelcontextprotocol/sdk 1.29.0 and zod 4.4.3 registers count with an outputSchema; its tools/list answer carries "$schema": "http://json-schema.org/draft-07/schema#". new AjvJsonSchemaValidator().getValidator(schema) from @modelcontextprotocol/server 2.0.0-beta.2 throws the error above. With $schema deleted the same schema is accepted and still validates ({"total":3} passes, {"total":"x"} fails). Converted with target: "draft-2020-12" it is accepted too.

Fix for users (confirmed by several reporters): run the server behind a small stdio proxy that removes $schema from each tool's inputSchema and outputSchema in the tools/list answer, configured in claude_desktop_config.json with the built-in extension switched off. Watch out with server-filesystem: recent versions replace your configured folders with the roots the app announces, so check list_allowed_directories after the tools load.

Fix for server authors: publish outputSchema without $schema, or with https://json-schema.org/draft/2020-12/schema. McpServer 1.x has no option for this yet.

Still open: the app accepting draft-07, or the SDK change in typescript-sdk 2721.

Evidence: anthropics/claude-code issues 86142, 94351, 88882; modelcontextprotocol/typescript-sdk issue 2721; anthropics/claude-ai-mcp issue 1016.