Speed Up Tool::Schema Validation by 5x to 100x
## Motivation and Context `MCP::Tool::Schema` previously used the pure-Ruby `json-schema` gem for both construction-time metaschema validation and runtime argument / result validation. For deep schemas this cost ~32-100ms per construction (see issue #364). `json_schemer` (https://rubygems.org/gems/json_schemer, v2.5.0) is an actively maintained alternative that is much faster on this workload. Switching reduces cold-start warming pressure in horizontally scaled multi-tenant deployments (the scenario raised in #364) and brings runtime data validation down to sub-millisecond per call once the per-instance schemer is memoized. The implementation preserves the legacy behavior of the `json-schema` based code. Metaschema is pinned to draft-04 (`meta_schema:` option); the `$schema` dialect URI is still emitted in `to_h` as 2020-12 but runtime validation continues to use draft-04, exactly as before. Moving the runtime validator itself to 2020-12 is out of scope for this PR and warrants its own discussion. `format` keywords are not enforced (`format: false` option), matching what `json-schema`'s draft-04 path did. Malformed schemas (e.g. an invalid `pattern` regex) continue to surface as `ArgumentError, "Invalid JSON Schema: ..."`; the `RegexpError` `json_schemer` raises during construction is wrapped. `Symbol` values in argument data are coerced to strings inside the internal `stringify` helper so they validate against `type: "string"` the same way they did before. The public `Schema#schema` accessor is dropped. It was a refactor artifact from #198 (the consolidation of `InputSchema`'s `properties` / `required` readers into a single hash) with no callers in `lib/`; tests that needed the merged hash now read it through `to_h`. The accessor's only remaining role would have been to expose a mutation path that could desynchronize the memoized `@schemer`, which this change removes. Runtime dependency delta: drops `json-schema` (and `addressable`), adds `json_schemer`, `hana`, `regexp_parser`, and `simpleidn`. Each added gem is a single-purpose library tied to JSON Schema spec compliance. Closes #364. ## How Has This Been Tested? Side-by-side benchmark on representative schemas (simple, with `$ref` / `additionalProperties`, nested, depth 20, depth 40): construction-time metaschema validation is 4.7x to ~100x faster; runtime data validation with the memoized schemer is sub-millisecond across all sizes. Four existing tests had their stub targets or message assertions updated to track the new validator while preserving the original test intent. The "unexpected errors bubble up" tests in `input_schema_test.rb` and `output_schema_test.rb` now stub `JSONSchemer::Schema#validate` instead of `JSON::Validator.fully_validate`. The cache tests in `schema_test.rb` now stub `JSONSchemer::Schema#validate_schema`. The two "detailed error message" tests in `tool_test.rb` switched from asserting `json-schema`'s exact wording to format-agnostic substrings (`"properties/count/minimum"`, `"number"`). The "required arguments are converted to strings" test in `input_schema_test.rb` now reads the result through `to_h[:required]` instead of the removed `schema` accessor. Regression tests added for the legacy-behavior preservations above: `format` keyword is not enforced, invalid `pattern` raises `ArgumentError` (not `RegexpError`), and `Symbol` values validate against `type: "string"`. ## Breaking Changes `Schema#schema` is no longer a public method. The accessor had no callers outside the test suite, but anyone reading the internal representation directly (rather than through `to_h`) will need to switch. `Schema` / `InputSchema` / `OutputSchema` otherwise keep their constructor signatures, `to_h` output, `ValidationError` / `ArgumentError` raise points, and the `$schema` dialect emission. The validator error message wording inside `ArgumentError` and `ValidationError` now comes from `json_schemer` and still includes the JSON pointer path and a description of the mismatch.
This commit is contained in:
Koichi ITO
parent
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7 changed files with 81 additions and 42 deletions
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@ -12,9 +12,9 @@ module MCP
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end
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def missing_required_arguments(arguments)
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return [] unless schema[:required].is_a?(Array)
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return [] unless @schema[:required].is_a?(Array)
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(schema[:required] - arguments.keys.map(&:to_s))
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(@schema[:required] - arguments.keys.map(&:to_s))
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end
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def validate_arguments(arguments)
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@ -1,7 +1,7 @@
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# frozen_string_literal: true
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require "digest"
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require "json-schema"
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require "json_schemer"
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module MCP
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class Tool
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@ -38,11 +38,10 @@ module MCP
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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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# is still performed against the JSON Schema draft-04 metaschema.
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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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DRAFT4_META_SCHEMA_URI = "http://json-schema.org/draft-04/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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@ -51,7 +50,7 @@ module MCP
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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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other.is_a?(self.class) && @schema == other.instance_variable_get(:@schema)
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end
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def to_h
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@ -62,8 +61,38 @@ module MCP
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private
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def stringify(obj)
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case obj
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when Hash
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obj.each_with_object({}) { |(k, v), h| h[k.to_s] = stringify(v) }
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when Array
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obj.map { |v| stringify(v) }
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when Symbol
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obj.to_s
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else
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obj
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end
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end
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# Lazily built so a cache hit in `validate_schema!` avoids the schemer construction cost.
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# Memoized per Schema instance because schema content is fixed at construction,
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# so the compiled schemer is reusable across many `fully_validate` calls.
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#
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# `format: false` preserves the legacy behavior of the previous `json-schema` based implementation,
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# which did not enforce `format` keywords. `RegexpError` from a malformed `pattern` is re-raised as
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# `ArgumentError` so callers see the same exception class they used to.
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def schemer
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@schemer ||= JSONSchemer.schema(
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stringify(schema_for_validation),
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meta_schema: DRAFT4_META_SCHEMA_URI,
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format: false,
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)
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rescue RegexpError => e
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raise ArgumentError, "Invalid JSON Schema: #{e.message}"
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end
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def fully_validate(data)
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JSON::Validator.fully_validate(schema_for_validation, data)
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schemer.validate(stringify(data)).map { |validation_error| validation_error.fetch("error") }
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end
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def validate_schema!
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@ -75,16 +104,7 @@ module MCP
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key = Digest::SHA256.hexdigest(JSON.generate(target, max_nesting: false))
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return if VALIDATION_CACHE.validated?(key)
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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, target, schema_reader: schema_reader)
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errors = schemer.validate_schema.map { |validation_error| validation_error.fetch("error") }
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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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@ -92,9 +112,8 @@ module MCP
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VALIDATION_CACHE.store(key)
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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 `$schema` before validation so a dialect URI
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# (whether SDK-injected by `to_h` or user-supplied) does not break the validator.
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# `json_schemer` is pinned to the draft-04 metaschema, so strip top-level `$schema` before validation:
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# this preserves the legacy behavior of ignoring the advertised dialect URI when the SDK validates schemas.
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def schema_for_validation
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return @schema unless @schema.key?(:"$schema")
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@ -30,5 +30,5 @@ Gem::Specification.new do |spec|
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spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
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spec.require_paths = ["lib"]
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spec.add_dependency("json-schema", ">= 4.1")
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spec.add_dependency("json_schemer", ">= 2.4")
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end
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@ -7,7 +7,7 @@ module MCP
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class InputSchemaTest < ActiveSupport::TestCase
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test "required arguments are converted to strings" do
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input_schema = InputSchema.new(properties: { message: { type: "string" } }, required: [:message])
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assert_equal ["message"], input_schema.schema[:required]
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assert_equal ["message"], input_schema.to_h[:required]
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end
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test "to_h returns a hash representation of the input schema" do
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@ -139,10 +139,10 @@ module MCP
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test "unexpected errors bubble up from validate_arguments" do
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schema = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])
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JSON::Validator.stub(:fully_validate, ->(*) { raise "unexpected error" }) do
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assert_raises(RuntimeError) do
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schema.validate_arguments({ foo: "bar" })
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end
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JSONSchemer::Schema.any_instance.stubs(:validate).raises("unexpected error")
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assert_raises(RuntimeError) do
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schema.validate_arguments(foo: "bar")
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end
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end
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@ -200,6 +200,26 @@ module MCP
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schema6 = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"], additionalProperties: false)
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refute_equal schema1, schema6
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end
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test "format keyword is not enforced (legacy behavior)" do
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schema = InputSchema.new(
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properties: { email: { type: "string", format: "email" } },
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required: ["email"],
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)
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assert_nil(schema.validate_arguments(email: "not_an_email"))
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end
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test "invalid pattern raises ArgumentError, not RegexpError" do
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error = assert_raises(ArgumentError) do
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InputSchema.new(properties: { id: { type: "string", pattern: "[" } })
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end
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assert_includes error.message, "Invalid JSON Schema"
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end
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test "Symbol values in arguments are treated as strings" do
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schema = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])
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assert_nil(schema.validate_arguments(foo: :bar))
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end
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end
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end
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end
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@ -110,10 +110,10 @@ module MCP
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test "unexpected errors bubble up from validate_result" do
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schema = OutputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])
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JSON::Validator.stub(:fully_validate, ->(*) { raise "unexpected error" }) do
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assert_raises(RuntimeError) do
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schema.validate_result({ foo: "bar" })
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end
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JSONSchemer::Schema.any_instance.stubs(:validate).raises("unexpected error")
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assert_raises(RuntimeError) do
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schema.validate_result(foo: "bar")
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end
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end
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@ -10,7 +10,7 @@ module MCP
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end
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test "validates a schema once and reuses the result for identical schemas" do
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JSON::Validator.expects(:fully_validate).once.returns([])
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JSONSchemer::Schema.any_instance.expects(:validate_schema).once.returns([])
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schema = { properties: { validates_once: { type: "string" } } }
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InputSchema.new(schema)
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@ -18,7 +18,7 @@ module MCP
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end
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test "validates distinct schemas separately" do
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JSON::Validator.expects(:fully_validate).twice.returns([])
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JSONSchemer::Schema.any_instance.expects(:validate_schema).twice.returns([])
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InputSchema.new(properties: { distinct_a: { type: "string" } })
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InputSchema.new(properties: { distinct_b: { type: "string" } })
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@ -64,11 +64,11 @@ module MCP
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break
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end
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JSON::Validator.stub(:fully_validate, []) do
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assert_nothing_raised do
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InputSchema.new(schema)
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InputSchema.new(schema)
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end
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JSONSchemer::Schema.any_instance.stubs(:validate_schema).returns([])
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assert_nothing_raised do
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InputSchema.new(schema)
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InputSchema.new(schema)
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end
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end
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@ -167,8 +167,8 @@ module MCP
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end
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assert_includes error.message, "Invalid JSON Schema"
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assert_includes error.message, "#/properties/count/minimum"
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assert_includes error.message, "string did not match the following type: number"
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assert_includes error.message, "properties/count/minimum"
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assert_includes error.message, "number"
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end
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test ".define allows definition of simple tools with a block" do
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@ -431,8 +431,8 @@ module MCP
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end
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assert_includes error.message, "Invalid JSON Schema"
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assert_includes error.message, "#/properties/count/minimum"
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assert_includes error.message, "string did not match the following type: number"
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assert_includes error.message, "properties/count/minimum"
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assert_includes error.message, "number"
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end
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test "output_schema accepts $ref in schema" do
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