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

SDKのバリデーターモジュール内のcreateDefaultAjvInstance関数を確認してください。この問題は、警告を抑制するためにAjvインスタンスに'json'のno-opフォーマットを追加することに関するものです。参考として、issue #2601で'google-duration'がどのように登録されたかを確認してください。変更には、デフォルトのバリデーターの設定を修正して、3つの方言エンジン（Ajv2020、Ajv2019、Ajv）すべてにajv.addFormat('json', true)を含めることが含まれます。変更後、提供された再現コードでテストを行い、警告が消えていることを確認してください。

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 #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

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