## 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.
Due to overly aggressive detection by RuboCop Minitest, the distinction between `[]` and `{}` was uniformly replaced with `assert_empty`,
making it impossible to differentiate between them.
As a result of https://github.com/rubocop/rubocop-minitest/issues/344, where the `Minitest/AssertEmptyLiteral` cop was disabled,
the return value has been expressed using explicit literal forms.
This refactoring extracts common schema validation logic into a base Schema
class, eliminating ~100 lines of duplicate code between InputSchema and
OutputSchema. It also adds support for the JSON Schema `additionalProperties`
keyword to allow fine-grained control over extra properties in tool arguments.
Previously `additionalParameters` could be set only on OutputSchema.
Key changes:
- Created MCP::Tool::Schema base class consolidating:
- Schema validation against JSON Schema Draft 4 metaschema
- Deep key transformation (string to symbol conversion)
- $ref disallowance checking
- Common validation methods
- Refactored InputSchema to inherit from Schema:
- Simplified constructor to accept full schema hash
- Changed required fields storage from symbols to strings
- Removed ~40 lines of duplicated validation logic
- Refactored OutputSchema to inherit from Schema:
- Removed ~54 lines of duplicated validation logic
- Now shares all common functionality with InputSchema
- Updated Tool.input_schema setter:
- Simplified to pass full hash directly to InputSchema
- Added additionalProperties support:
- By default, tools now allow additional properties (JSON Schema default)
- Can explicitly set `additionalProperties: false` to disallow extras
- Added comprehensive test coverage for both behaviors
When defining a tool without any arguments taken, it should be allowed
to not have to define the `input_schema` at all, and the default value
for such a scenario should be `{ type: "object" }` as per the schema.
16d91abb68/schema/2025-06-18/schema.ts (L896-L900)
