Error: "Server does not support completions (required for ${method})"

[dyoshikawa]

Currently, following error is occured.

file:///workspace/node_modules/.pnpm/@modelcontextprotocol+sdk@1.22.0/node_modules/@modelcontextprotocol/sdk/dist/esm/server/index.js:134
                    throw new Error(`Server does not support completions (required for ${method})`);
                          ^

Error: Server does not support completions (required for completion/complete)
    at Server.assertRequestHandlerCapability (/workspace/node_modules/.pnpm/@modelcontextprotocol+sdk@1.22.0/node_modules/@modelcontextprotocol/sdk/src/server/index.ts:242:27)

Reproduction Code

import { FastMCP } from "fastmcp";

const server = new FastMCP({
  name: "My MCP Server",
  version: "1.0.0",
  instructions:
    "...,
});

server.addTool(myAnyTool);

void server.start({
  transportType: "stdio",
});

Temporaly Solution

@modelcontextprotocol/sdk@1.22.0 seems to have some issues.

Downgrading @modelcontextprotocol/sdk to 1.21.2 , it works.

In my case, I use pnpm, so I specified @modelcontextprotocol/sdk version explicitly.

// package.json
"dependencies": {
+ "@modelcontextprotocol/sdk": "1.21.2",
}

And I ran pnpm install, pnpm-lock.yaml was changed, so it was resolved.

[yujeongJeon]

I went through the same issue and analyzed it like this.

Issue: completions capability not set, causing error with MCP SDK 1.22.0

Description

When using FastMCP with @modelcontextprotocol/sdk version 1.22.0, the server fails to start with the following error:

Error: Server does not support completions (required for completion/complete)
at Server.assertRequestHandlerCapability
at Server.setRequestHandler
at FastMCPSession.setupCompleteHandlers

Root Cause

This issue was introduced in MCP SDK 1.22.0 by this commit:
modelcontextprotocol/typescript-sdk@f59996a
modelcontextprotocol/typescript-sdk#1086

The assertRequestHandlerCapability method now requires the completions capability to be set when registering the completion/complete request handler.

MCP SDK 1.22.0 (@modelcontextprotocol/sdk/dist/esm/server/index.js:134):

assertRequestHandlerCapability(method) {
    switch (method) {
        case 'completion/complete':
            if (!this._capabilities.completions) {
                throw new Error(`Server does not support completions (required for ${method})`);
            }
            break;
        // ...
    }
}

However, FastMCP's FastMCPSession constructor does not set the completions capability, even though it always calls setupCompleteHandlers().

Add the completions capability in the FastMCPSession constructor:

// ...
this.#capabilities.logging = {};
this.#capabilities.completions = {}; // Add this line

Workaround

Currently, downgrading to @modelcontextprotocol/sdk@1.21.2 works as a temporary workaround, as this version does not enforce the capability check.

[Crunchyman-ralph]

Related to #209

[punkpeye]

Fixed in v3.23.1 via #209