Add support for Sampling

[dclark27]

Sampling is a feature supported by both the MCP Protocol writ large and the modelcontextprotocol/typescript-sdk.

Currently, using methods like server.server.createMessage result in a timeout. Please either add support for sampling, or return early in the mcp-api-handler that the tool is not supported. The Inspector never gets notified of a request for sampling.

Sampling and Elicitation are at the heart of the MCP server I am building, so support for these 2 features would be so greatly appreciated! Also happy to create a PR for this.

Steps to reproduce

import { createMcpHandler } from 'mcp-handler';
import z from 'zod';

const handler = createMcpHandler(
	(server) => {
		server.registerTool(
			'summarize',
			{
				description: 'Summarize any text using an LLM',
				inputSchema: {
					text: z.string().describe('Text to summarize'),
				},
			},
			async ({ text }) => {
				// Call the LLM through MCP sampling
				const response = await server.server.createMessage({
					messages: [
						{
							role: 'user',
							content: {
								type: 'text',
								text: `Please summarize the following text concisely:\n\n${text}`,
							},
						},
					],
					maxTokens: 500,
				});

				return {
					content: [
						{
							type: 'text',
							text:
								response.content.type === 'text'
									? response.content.text
									: 'Unable to generate summary',
						},
					],
				};
			},
		);
	},
	{
		// Optional server options
	},
	{
		// Optional redis config
		redisUrl: process.env.REDIS_URL,
		basePath: '/api', // this needs to match where the [transport] is located.
		maxDuration: 60,
		verboseLogs: true,
	},
);
export { handler as GET, handler as POST };

Example video

Screen.Recording.2025-07-14.at.3.49.13.PM.mov

[dclark27]

I believe this might be an issue with the Redis instance. I have one running but if I change the port of the REDIS_URL to something made up I get no output that it's not working.

[dclark27]

Ok, one more update here -- looks like sampling works fine, but you need to enable the legacy SSE transport method and URL. Looking at mcp-api-handler, the redis server is never started and stateful transactions like this and Elicitation do not work (I can't show a video of elicitation as we are still waiting on modelcontextprotocol/inspector#535).

Screen.Recording.2025-07-15.at.10.56.33.AM.mov

Screen.Recording.2025-07-15.at.10.55.44.AM.mov

[quuu]

hey @dclark27 , would love to support stateful transactions on this! not sure the best way right now as i built this mainly to deploy in serverless environments, hence the redis for managing state and message passing

open to any suggestions!

[dclark27]

hey @quuu !

I don't think you guys are too far off. The SDK docs only list auth as the requirement for a stateful server that uses the HTTPStreamable mcp transport method.

I think we are just missing something in the api handler to handle the requests, though it's not clear to me what exactly is missing (I've never built a server before, so I'm trying to get as caught up as I can). Happy to fork this and see if I can figure out what's missing though.

[quuu]

ah @dclark27, just confirming that you mean sampling uses the Redis instance to work?

[dclark27]

@QUUUY yes sampling works fine with a redis instance over SSE transport