feat: add jsonSchemaAdapter for plain JSON Schema support — closes #54

[lucamorettibuilds]

Summary

Closes #54

Adds a jsonSchemaAdapter() utility that wraps plain JSON Schema objects with a Standard Schema v1 interface. This lets users define tool parameters using raw JSON Schema — no Zod, Yup, or other validation library required.

How it works

1. jsonSchemaAdapter(schema) returns a StandardSchemaV1-compatible object
2. Runtime validation uses AJV (lazy-loaded, optional peer dep)
3. Fast-path: The adapter exposes __jsonSchema so FastMCP can skip the toJsonSchema() conversion and use the raw schema directly — zero overhead
4. ajv-formats is optionally detected for format validation (e.g. email, uri)

Usage

import { FastMCP, jsonSchemaAdapter } from "fastmcp";

const server = new FastMCP({ name: "Example" });

server.addTool({
  name: "greet",
  description: "Greet a user",
  parameters: jsonSchemaAdapter({
    type: "object",
    properties: {
      name: { type: "string" },
    },
    required: ["name"],
  }),
  execute: async ({ name }) => `Hello, ${name}!`,
});

Changes

File	What
src/jsonSchemaAdapter.ts	The adapter function + JsonSchemaObject type
src/jsonSchemaAdapter.test.ts	5 tests: valid input, invalid input, __jsonSchema fast-path, vendor metadata, nested objects
src/FastMCP.ts	Re-export from barrel + fast-path detection in inputSchema resolution

Design decisions

• AJV is a peer dep, not a direct dep — keeps the fastmcp bundle lightweight. Users who want JSON Schema support install ajv themselves. The adapter throws a clear error message if AJV is missing.
• __jsonSchema fast-path — avoids the toJsonSchema() roundtrip since we already have valid JSON Schema. FastMCP checks for this property before falling back to the standard conversion.
• No changes to existing behavior — all existing Zod/Valibot/ArkType schemas continue to work exactly as before.

Related

• Issue: Add support for ajv as a validator implementation #54
• Requested by multiple users who want to use JSON Schema directly from OpenAPI specs or LLM-generated schemas

[lucamorettibuilds]

Hey @punkpeye — CI is blocked on first-time contributor approval (action_required). Could you approve the workflow run when you get a chance? The implementation wraps ajv in a Standard Schema–compatible adapter so existing Zod tools continue working while plain JSON Schema objects can be passed as schema directly. Happy to adjust anything.