1fc698bb8e
## Motivation and Context MCP 2025-11-25 (SEP-1613) establishes JSON Schema 2020-12 as the default dialect for MCP schema definitions. https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1613 The Ruby SDK was emitting tool input and output schemas with no `$schema` field, leaving the dialect implicit and inconsistent with the TypeScript SDK (`packages/core/src/util/standardSchema.ts`, which emits with `target: 'draft-2020-12'`) and the Python SDK (which preserves `$schema` in its types and conformance fixtures). `Tool::Schema#to_h` now includes `"$schema": "https://json-schema.org/draft/2020-12/schema"` unless the user supplied a different `$schema` value, so clients see the canonical dialect URI on the wire. Runtime metaschema and argument validation remain on draft-04 because the `json-schema` gem does not yet support 2020-12. Switching the runtime validator is left as a separate change because it requires either replacing the gem or waiting for upstream 2020-12 support. ## How Has This Been Tested? Updated `Tool::Schema#to_h` expectations across `input_schema_test`, `output_schema_test`, `tool_test`, and the `tools/list` server tests to reflect the new `$schema` field. Added focused regression tests: - `to_h` preserves a user-supplied `$schema` URI rather than overwriting it (covered for both symbol-keyed input and the string-keyed Hash form that goes through the initializer's `symbolize_names` normalization) - `validate_arguments` / `validate_result` continue to work when the user supplies a 2020-12 `$schema` URI on the schema (the validator strips the unknown top-level dialect URI before invoking the draft-04 validator, instead of raising `JSON::Schema::SchemaError`) - `tools/list` JSON-RPC response includes the 2020-12 `$schema` on both `inputSchema` and `outputSchema` (transport-boundary assertion) `bundle exec rake test` and `bundle exec rake rubocop` both pass. ## Breaking Changes Additive change. Tolerant consumers of the `tools/list` payload are unaffected. Consumers that strictly assert hash equality on the emitted `inputSchema` / `outputSchema` need to account for the new `$schema` key, but no schema semantics change. Schemas using 2020-12-only keywords (e.g. `prefixItems`, nested `$schema` on subschemas) are still rejected by the draft-04 metaschema validator, as before. Lifting that restriction depends on adopting a 2020-12-capable validator and is out of scope for this change.
66 lines
2.3 KiB
Ruby
66 lines
2.3 KiB
Ruby
# frozen_string_literal: true
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require "json-schema"
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module MCP
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class Tool
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class Schema
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# JSON Schema 2020-12 is the default dialect for MCP schema definitions
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# per MCP 2025-11-25 (SEP-1613). Note: emission only — runtime validation
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# is still performed against the JSON Schema draft-04 metaschema because
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# the `json-schema` gem does not yet support 2020-12.
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JSON_SCHEMA_2020_12_URI = "https://json-schema.org/draft/2020-12/schema"
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attr_reader :schema
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def initialize(schema = {})
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@schema = JSON.parse(JSON.dump(schema), symbolize_names: true)
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@schema[:type] ||= "object"
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validate_schema!
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end
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def ==(other)
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other.is_a?(self.class) && schema == other.schema
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end
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def to_h
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return @schema if @schema.key?(:"$schema")
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{ "$schema": JSON_SCHEMA_2020_12_URI }.merge(@schema)
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end
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private
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def fully_validate(data)
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JSON::Validator.fully_validate(schema_for_validation, data)
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end
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def validate_schema!
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gem_path = File.realpath(Gem.loaded_specs["json-schema"].full_gem_path)
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schema_reader = JSON::Schema::Reader.new(
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accept_uri: false,
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accept_file: ->(path) { File.realpath(path.to_s).start_with?(gem_path) },
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)
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metaschema_path = Pathname.new(JSON::Validator.validator_for_name("draft4").metaschema)
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# Converts metaschema to a file URI for cross-platform compatibility
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metaschema_uri = JSON::Util::URI.file_uri(metaschema_path.expand_path.cleanpath.to_s.tr("\\", "/"))
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metaschema = metaschema_uri.to_s
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errors = JSON::Validator.fully_validate(metaschema, schema_for_validation, schema_reader: schema_reader)
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if errors.any?
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raise ArgumentError, "Invalid JSON Schema: #{errors.join(", ")}"
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end
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end
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# The `json-schema` gem's draft-04 validator cannot resolve newer or unknown `$schema`
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# dialect URIs. Strip the top-level `$schema` before validation so a dialect URI
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# (whether SDK-injected by `to_h` or user-supplied) does not break the validator.
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def schema_for_validation
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return @schema unless @schema.key?(:"$schema")
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copy = @schema.dup
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copy.delete(:"$schema")
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copy
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end
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end
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end
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end
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