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Advertise JSON Schema 2020-12 dialect on emitted tool schemas

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## 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.
This commit is contained in:
Koichi ITO 2026-05-01 02:17:31 +09:00
commit 1fc698bb8e
1 changed files with 196 additions and 22 deletions
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60
test/mcp/tool/input_schema_test.rb
View file

@ -13,7 +13,12 @@ module MCP
test "to_h returns a hash representation of the input schema" do
input_schema = InputSchema.new(properties: { message: { type: "string" } }, required: ["message"])
assert_equal(
{ type: "object", properties: { message: { type: "string" } }, required: ["message"] },
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
type: "object",
properties: { message: { type: "string" } },
required: ["message"],
},
input_schema.to_h,
)
end
@ -21,11 +26,52 @@ module MCP
test "to_h returns a hash representation of the input schema with additionalProperties set to false" do
input_schema = InputSchema.new(properties: { message: { type: "string" } }, required: ["message"], additionalProperties: false)
assert_equal(
{ type: "object", properties: { message: { type: "string" } }, required: ["message"], additionalProperties: false },
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
type: "object",
properties: { message: { type: "string" } },
required: ["message"],
additionalProperties: false,
},
input_schema.to_h,
)
end
test "to_h preserves user-supplied $schema dialect" do
input_schema = InputSchema.new(
"$schema": "https://json-schema.org/draft/2019-09/schema",
properties: { message: { type: "string" } },
)
assert_equal "https://json-schema.org/draft/2019-09/schema", input_schema.to_h[:"$schema"]
end
test "validate_arguments works when user supplies a 2020-12 $schema" do
input_schema = InputSchema.new(
"$schema": "https://json-schema.org/draft/2020-12/schema",
properties: { foo: { type: "string" } },
required: ["foo"],
)
assert_nil(input_schema.validate_arguments(foo: "bar"))
assert_raises(InputSchema::ValidationError) do
input_schema.validate_arguments({ foo: 123 })
end
end
test "to_h preserves user-supplied $schema given via string key" do
# The initializer normalizes input through `JSON.parse(...,
# symbolize_names: true)`, so a string-keyed `"$schema"` should
# arrive at `schema_for_validation` the same as a symbol-keyed one.
input_schema = InputSchema.new(
{
"$schema" => "https://json-schema.org/draft/2020-12/schema",
"properties" => { "foo" => { "type" => "string" } },
"required" => ["foo"],
},
)
assert_equal "https://json-schema.org/draft/2020-12/schema", input_schema.to_h[:"$schema"]
assert_nil(input_schema.validate_arguments(foo: "bar"))
end
test "missing_required_arguments returns an array of missing required arguments" do
input_schema = InputSchema.new(properties: { message: { type: "string" } }, required: ["message"])
assert_equal ["message"], input_schema.missing_required_arguments({})
@ -38,7 +84,15 @@ module MCP
test "valid schema initialization" do
schema = InputSchema.new(properties: { foo: { type: "string" } }, required: ["foo"])
assert_equal({ type: "object", properties: { foo: { type: "string" } }, required: ["foo"] }, schema.to_h)
assert_equal(
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
type: "object",
properties: { foo: { type: "string" } },
required: ["foo"],
},
schema.to_h,
)
end
test "invalid schema raises argument error" do