Fix MCP TypeScript SDK EvalError: AJV Code Generation Blocked on Cloudflare Workers, Deno Deploy, and Edge Environments

For environments where upgrading the MCP SDK is not possible (e.g., locked dependencies), swap the bundled AJV v6 validator with @cfworker/json-schema v4.1.1, a JSON Schema validator specifically designed for Cloudflare Workers. This validator uses pure interpretation without dynamic code generation, making it fully compatible with Workers' security model. Note: this requires understanding MCP SDK internals.

1. Install @cfworker/json-schema: npm install @cfworker/json-schema
2. Create a custom validator adapter implementing the MCP SDK's validation interface
3. Configure the MCP server to use the custom validator instead of the default AJV-based one
4. Test elicitation with the new validator on Cloudflare Workers

npm install @cfworker/json-schema

npm view @cfworker/json-schema version

wrangler dev

// Custom validator using @cfworker/json-schema for Cloudflare Workers
import { Validator } from '@cfworker/json-schema';

const cfValidator = {
  validate(schema: object, data: unknown): { valid: boolean; errors?: string[] } {
    const validator = new Validator(schema as any);
    const result = validator.validate(data);
    return {
      valid: result.valid,
      errors: result.errors?.map(e => e.error)
    };
  }
};

// Pass to MCP server configuration
const server = new McpServer({
  name: 'my-server',
  version: '1.0.0'
}, { validator: cfValidator });

Cloudflare Workers now supports MCP elicitation natively through the McpAgent class. Call `this.elicitInput()` on the McpAgent instance instead of `this.server.server.elicitInput()` on the raw MCP server object. This bypasses the AJV validation issue entirely by routing through Cloudflare's built-in elicitation handling. Use this approach if you cannot upgrade the MCP SDK or need Cloudflare-specific features.

1. Ensure your Cloudflare Worker uses McpAgent from 'agents-mcp' package
2. Replace calls to this.server.server.elicitInput({...}) with this.elicitInput({...})
3. Verify the McpAgent class is imported and extended correctly
4. Deploy to Cloudflare Workers and test elicitation flow

npm list agents-mcp

wrangler deploy

// ✅ CORRECT: Use McpAgent.elicitInput() on Cloudflare Workers
import { McpAgent } from 'agents-mcp';

export class MyAgent extends McpAgent {
  async someMethod() {
    const response = await this.elicitInput({
      message: 'Please provide your name',
      schema: { type: 'object', properties: { name: { type: 'string' } } }
    });
    // ✅ Works on Cloudflare Workers — no AJV involved
  }
}

// ❌ WRONG: Calling on raw MCP server triggers AJV v6 code generation
// const response = await this.server.server.elicitInput({...}); // EvalError!

The MCP TypeScript SDK permanently re-applied the AJV v6→v8 upgrade starting in v1.21.0 (npm dependency: ajv ^8.17.1). Upgrading to v1.21.0 or later eliminates the EvalError on Cloudflare Workers because AJV v8's default mode does not use `new Function`. This is the simplest fix — no code changes required for most MCP servers.

1. Check current MCP SDK version: npm list @modelcontextprotocol/sdk
2. If version is below 1.21.0, upgrade: npm install @modelcontextprotocol/sdk@latest
3. Verify AJV dependency is v8: npm list ajv (should show ajv@8.x.x, not ajv@6.x.x)
4. Test elicitation on Cloudflare Workers: wrangler dev
5. Deploy: wrangler deploy

npm list @modelcontextprotocol/sdk

npm install @modelcontextprotocol/sdk@latest

npm list ajv

npm view @modelcontextprotocol/sdk version

wrangler dev

wrangler deploy