## Motivation and Context
SEP-2106 (modelcontextprotocol/modelcontextprotocol#2106, merged for the 2026-07-28 spec release)
makes tool `inputSchema` and `outputSchema` conform to the full JSON Schema 2020-12 vocabulary:
an input schema keeps `type: "object"` at the root but may use any 2020-12 keyword below it;
an output schema may be ANY valid schema (object, array, primitive, or a root-level composition);
and `CallToolResult.structuredContent` widens from an object to any JSON value. The SEP also adds
resource bounds: `$ref` resolution is restricted (same-document only in the reference implementation) and
composition-heavy documents must be bounded to avoid excessive validation cost.
This follows the TypeScript SDK's reference implementation (typescript-sdk#2249; the Python SDK tracks
the same work in python-sdk#2792):
- `Tool::Schema` now validates against the JSON Schema 2020-12 metaschema rather than the draft-04 metaschema.
The draft-04 pin was a stopgap from when the SDK used the `json-schema` gem, which did not support 2020-12;
`json_schemer` does, so `$defs`/`$ref` and the rest of the 2020-12 vocabulary now resolve natively.
This matches the dialect the SDK already advertises in emitted schemas and the Python SDK's behavior,
whose `jsonschema.validate` selects the validator from the schema's `$schema`.
- `Tool::Schema` moves root-type defaulting into an overridable `apply_default_root_type!` hook.
`InputSchema` keeps the historical `type: "object"` default; `OutputSchema` now applies it only
when no root schema keyword (`type`, `$ref`, `oneOf`, `anyOf`, `allOf`, `not`, `if`, `const`, `enum`) is present.
The previous unconditional default merged `type: "object"` into root combinators such as `{ oneOf: [...] }`,
producing a wrong schema, so that case is a bug fix.
- `Tool::Schema` enforces the TypeScript SDK's schema bounds at construction time: only same-document
`$ref`/`$dynamicRef`s (starting with `#`, so schema handling can never trigger network or file access),
`MAX_SCHEMA_DEPTH = 64` nesting levels, and `MAX_SUBSCHEMA_COUNT = 10_000` subschema objects,
all raising `ArgumentError` on violation.
- `Server#call_tool` mirrors non-object `structuredContent` into `content` as serialized JSON text
when the tool provided no content blocks, so pre-SEP clients that only read `content` still receive the data.
Object results and explicit content are untouched.
Resolves#377.
## How Has This Been Tested?
- `test/mcp/tool/output_schema_test.rb`: root-level `oneOf`, `$ref`+`$defs`, primitive, and `enum` schemas serialize
without an injected `type` and validate results correctly; the `properties`-only shorthand still serializes
with `type: "object"` (wire-format regression); explicit `type: "array"` keeps working.
- `test/mcp/tool/input_schema_test.rb`: an input schema using `$defs`, `$ref`, `oneOf`, `if`/`then`, and `allOf`
keeps its object root and round-trips all keywords; a draft-04-only boolean `exclusiveMinimum` is rejected
under the 2020-12 dialect while the numeric form is accepted.
- `test/mcp/tool/schema_test.rb`: depth and subschema-count bound violations raise `ArgumentError`;
non-same-document `$ref`s (remote URI, sibling file) are rejected while `#/$defs/...` is accepted.
The previous unbounded-depth caching test is replaced, since the depth bound now rejects such documents by design.
- `test/mcp/server_test.rb`: `tools/call` with array `structuredContent` and no content gains
the serialized TextContent fallback; explicit content is not overwritten; object `structuredContent` gets no fallback.
`bundle exec rake` (tests, RuboCop, and conformance baseline, including the `json-schema-2020-12` server scenario) passes.
## Breaking Changes
Three narrow behavior changes, all intentional per the SEP:
- Runtime validation now uses the JSON Schema 2020-12 metaschema instead of draft-04.
Schemas that rely on draft-04-only syntax are rejected at construction time. The practical case is the boolean
`exclusiveMinimum`/`exclusiveMaximum` form (deprecated since draft-06), which must now be the numeric form;
the Python SDK rejects it the same way. Other draft-04 spellings (`definitions`, `id`, `dependencies`) still validate,
since 2020-12 tolerates unknown keywords.
- Schemas that exceed the new resource bounds (nesting deeper than 64, more than 10,000 subschema objects) or
use a non-same-document `$ref`/`$dynamicRef` now raise `ArgumentError` at construction time.
Previously such documents were accepted (external references were already never fetched, only ignored).
- An `OutputSchema` whose root declares a schema keyword other than `type` (e.g. `oneOf`) no longer has `type: "object"`
merged into it. The old output was an invalid hybrid schema, so no conforming consumer could have relied on it.
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