Ajv defaults: non-standard "format": "json" in third-party schemas (e.g. Notion) warns twice per property — register it like #2601

[Shallow-dusty]

Context

This is the same class of noise as #2601 (google-duration), with an instance found in the wild.

The Notion MCP server's post-page tool annotates two string properties with "format": "json" — a value that is defined neither by JSON Schema nor by ajv-formats. Compiling that inputSchema through the default validator instances of @modelcontextprotocol/client@2.0.0 (createDefaultAjvInstance: { strict: false, allErrors: true } + ajv-formats) logs, per compile:

unknown format "json" ignored in schema at path "#/properties/icon"
unknown format "json" ignored in schema at path "#/properties/icon"
unknown format "json" ignored in schema at path "#/properties/cover"
unknown format "json" ignored in schema at path "#/properties/cover"

Two lines per property, because allErrors: true makes Ajv compile the format check twice — i.e. connecting to that one server costs 4 lines of client-side warning output per session.

It is not purely cosmetic: with Ajv's own defaults (strict: true) the same schema throws during compile (Error: unknown format "json" ignored in schema at path "#/properties/icon"), so any integrator who wants a strict validator cannot load that tool.

Ask

1. Register json as a no-op format (ajv.addFormat("json", true)) in createDefaultAjvInstance, for all three dialect engines, alongside the google-duration registration proposed in fix: register google-duration format to silence unknown format warnings #2601; or
2. Downgrade the unknown-format message to debug-level output, so quirks in third-party schemas don't surface as user-visible warnings.

Note on custom validator providers

Consumers that pass their own Ajv instance to AjvJsonSchemaValidator bypass the default instances entirely, so registering formats there does not help them — e.g. a downstream extension that constructs its own validator provider still gets the warnings until it registers json itself. Worth a line in the validator docs.

Environment / reproduction

@modelcontextprotocol/client 2.0.0 · ajv 8.20.0 · node 24.12.0

import Ajv2020 from "ajv/dist/2020.js";
import addFormats from "ajv-formats";

const ajv = new Ajv2020({ strict: false, allErrors: true });  // mirrors createDefaultAjvInstance
addFormats(ajv);

ajv.compile({
  type: "object",
  properties: {
    icon:  { type: "string", format: "json" },
    cover: { type: "string", format: "json" },
  },
});
// -> the four "unknown format" lines above

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Filed after reproducing the behaviour locally with the reporter's coding agent; outputs above are from that run and can be provided on request.

[iamhuman-cheolheelee]

The registration point is createDefaultAjvInstance in packages/core-internal/src/validators/ajvProvider.ts, which is the same spot #2601 changes for google-duration. Since #2601 is still open, the least churn is probably to add ajv.addFormat('json', true) in that PR, or right after it merges, instead of opening a second PR that would conflict with it. I can do the follow-up if maintainers would rather keep #2601 as it is.