Allow $ref in Tool::Schema for protocol version 2025-11-25#242
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koic merged 1 commit intomodelcontextprotocol:mainfrom Mar 1, 2026
Merged
Allow $ref in Tool::Schema for protocol version 2025-11-25#242koic merged 1 commit intomodelcontextprotocol:mainfrom
$ref in Tool::Schema for protocol version 2025-11-25#242koic merged 1 commit intomodelcontextprotocol:mainfrom
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`Tool::Schema` rejected `$ref` with an `ArgumentError`. Earlier MCP spec versions (2024-11-05 through 2025-06-18) did not define a JSON Schema dialect or reference the `$ref` keyword. `inputSchema` was described only as "JSON Schema defining expected parameters" with no further constraints: - https://modelcontextprotocol.io/specification/2024-11-05/server/tools - https://modelcontextprotocol.io/specification/2025-06-18/server/tools The MCP spec 2025-11-25 introduced a "JSON Schema Usage" section that adopts JSON Schema 2020-12 as the default dialect. Since `$ref` is a core keyword in 2020-12, it is now allowed for protocol version 2025-11-25 and later. - https://modelcontextprotocol.io/specification/2025-11-25/basic#json-schema-usage - https://modelcontextprotocol.io/specification/2025-11-25/server/tools - https://json-schema.org/draft/2020-12/release-notes For backward compatibility with older protocol versions (2025-06-18 and earlier), `Server#validate!` continues to raise `ArgumentError` when a tool input schema contains `$ref`. This preserves the existing behavior for servers targeting those spec versions.
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atesgoral
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Motivation and Context
Tool::Schemarejected$refwith anArgumentError. Earlier MCP spec versions (2024-11-05 through 2025-06-18) did not define a JSON Schema dialect or reference the$refkeyword.inputSchemawas described only as "JSON Schema defining expected parameters" with no further constraints:The MCP spec 2025-11-25 introduced a "JSON Schema Usage" section that adopts JSON Schema 2020-12 as the default dialect. Since
$refis a core keyword in 2020-12, it is now allowed for protocol version 2025-11-25 and later.How Has This Been Tested?
Added a repro test to verify the fix and prevent regression.
Breaking Changes
None. For backward compatibility with older protocol versions (2025-06-18 and earlier),
Server#validate!continues to raiseArgumentErrorwhen a tool input schema contains$ref. This preserves the existing behavior for servers targeting those spec versions.Types of changes
Checklist