Elicitation

Elicitation lets a tool request input from the client mid-execution. This is useful for confirmations, additional information, or interactive flows.

Requirements

Elicitation requires both adapters to be configured:

const transport = new StreamableHttpTransport({
  sessionAdapter: new InMemorySessionAdapter({ maxEventBufferSize: 1024 }),
  clientRequestAdapter: new InMemoryClientRequestAdapter({ defaultTimeoutMs: 30000 })
});

Basic Example

import { z } from "zod";

server.tool("delete_record", {
  inputSchema: z.object({
    recordId: z.string(),
    tableName: z.string(),
  }),
  handler: async (args, ctx) => {
    // Check if client supports elicitation
    if (!ctx.client.supports("elicitation")) {
      throw new Error("Elicitation not supported");
    }

    // Request confirmation from client
    const response = await ctx.elicit({
      message: `Delete record "${args.recordId}" from "${args.tableName}"?`,
      schema: z.object({ confirmed: z.boolean() })
    });

    if (response.action === "accept" && response.content.confirmed) {
      await deleteFromDatabase(args.tableName, args.recordId);
      return { content: [{ type: "text", text: "Record deleted" }] };
    }

    return { content: [{ type: "text", text: "Deletion cancelled" }] };
  }
});

How It Works

1. Tool execution starts: Handler begins processing
2. Elicitation request: Handler calls ctx.elicit()
3. Server pauses: Execution waits for client response
4. Client responds: Client sends confirmation/input
5. Execution resumes: Handler continues with the response
6. Result returned: Tool completes normally

Distributed Deployments

For serverless or multi-instance deployments, implement a custom ClientRequestAdapter using persistent storage.

The Cloudflare KV adapter works by:

1. Storing request metadata in KV - Only serializable data
2. Keeping local promise handlers - In the instance that created the request
3. Polling for responses - Each instance polls KV to see if responses arrived
4. Cross-instance coordination - Any instance can resolve/reject requests by updating KV
5. Automatic cleanup - Handles timeouts and cleans up both local state and KV entries

See examples/cloudflare-worker-kv/src/mcp/client-request-adapter.ts for the full implementation.

Next Steps

• Adapters - Understand adapter interfaces
• Sampling - Request LLM completions