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Default JSON Schema validators reject declared draft-07, which the spec permits #2532
Description
Both default validators (AjvJsonSchemaValidator and CfWorkerJsonSchemaValidator) throw on any tool schema that declares a non-2020-12 dialect:
Tool 'example' has an invalid outputSchema: JSON Schema declares an unsupported dialect
("$schema": "http://json-schema.org/draft-07/schema#"). The default validator supports
JSON Schema 2020-12 only.
Per the spec, 2020-12 is only the default when $schema is absent — an explicitly declared draft-07 dialect is valid on all protocol versions (the spec's own examples include it). So schemas the spec permits currently hard-fail tools/call in the client.
The practical impact is wide: zod-to-json-schema, which v1 SDK servers use for outputSchema, stamps "$schema": ".../draft-07/schema#" on every schema. Any v2-SDK client talking to any v1-SDK server with an output schema fails on every tool call, with no server-side fix available short of rewriting the server.
Repro:
import { AjvJsonSchemaValidator } from "@modelcontextprotocol/client/validators/ajv";
new AjvJsonSchemaValidator().getValidator({
$schema: "http://json-schema.org/draft-07/schema#",
type: "object",
}); // throws "unsupported dialect"Expected: validate the schema using the dialect it declares (or at minimum draft-07, given its ubiquity via zod), rather than rejecting it.
Seen on @modelcontextprotocol/client@2.0.0-beta.4. Happy to contribute a fix — we currently work around this with a dispatching validator that routes on the declared $schema.
Activity
Sakshamm-Goyal commentedon Jul 22, 2026
I’d like to take this on with a narrow bug fix and regression coverage. I’ll first reproduce the declared draft-07 path in both default validators, then keep the change limited to dialect selection/registration required for standards-compliant validation. Please assign it to me if that approach is welcome; I’ll wait for assignment before implementation.
Kaif10 commentedon Jul 24, 2026
I ran into this too and can reproduce it on
@modelcontextprotocol/client@2.0.0-beta.4. Root cause is that both default validators hard-require the 2020-12 meta-schema and throw for any other declared$schema, whereas per spec 2020-12 is only the default when$schemais absent — an explicitly declared dialect (notably draft-07, whichzod-to-json-schemastamps on every v1-server output schema) should be honored.The fix I have in mind is to route by the declared dialect rather than reject it: keep 2020-12 as the default when
$schemais absent, but when a schema declares a dialect the underlying validator supports (draft-07 in particular — Ajv's default build is draft-07, with 2020-12 viaajv/dist/2020), compile it against that dialect instead of throwing. That restores v2-client ↔ v1-server interop without loosening validation. I'd add tests covering an explicit draft-07 schema, an absent-$schemaschema (still 2020-12), and a genuinely unsupported dialect (still rejected).@chelojimenez — are you still planning to submit your fix? I don't want to step on it. If not (or if a maintainer would rather I take it), I'm happy to open a small PR with tests. Either way I've confirmed the repro.
claude commentedon Aug 19, 2026
Closing: fixed in #2534, released in
2.0.0Thanks for the detailed report and repro — this was exactly the v2-client ↔ v1-server interop gap described. It was fixed in #2534 (merged 2026-07-27, ahead of the stable
2.0.0release later that day): both default validators (AjvJsonSchemaValidatorandCfWorkerJsonSchemaValidator) now dispatch on the declared dialect instead of rejecting it — declared draft-07/draft-06 compiles with classic Ajv, declared 2019-09 with Ajv 2019, and 2020-12 stays the default when$schemais absent; only genuinely unknown dialects still produce the typed error. This is also noted in the migration guide (docs/migration/upgrade-to-v2.md): "a declared draft-07/06$schemadispatches automatically". If you still hit this on@modelcontextprotocol/client@2.0.0or later, please open a new issue.This comment and close were reviewed and approved by maintainer @felixweinberger — it is a human-approved action, not an unattended automated one.
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Remove outputSchema: Claude Desktop rejects the draft-07 dialect the …