[BUG] Claude Code SDK: SDK MCP server faills to connect due to closed stream

[codylund]

Environment

• Platform (select one):
  • Anthropic API
  • AWS Bedrock
  • Google Vertex AI
  • Other: Claude Code SDK
• Claude CLI version: 1.0.96
• Operating System: macOS 15.1
• Terminal: Terminal App

Bug Description

The new SDK MCP server type does not work with basic string prompts.
Running with DEBUG=true, I see the following error:

[ERROR] MCP server "test" Failed to connect SDK MCP server: Error: Stream closed

Steps to Reproduce

Below is a basic sample:

import { createSdkMcpServer, query, tool } from "@anthropic-ai/claude-code";
import { z } from "zod";

const mcp = createSdkMcpServer({
  name: "test",
  tools: [
    tool(
      "add",
      "Add two numbers",
      {
        a: z.number(),
        b: z.number(),
      },
      async ({ a, b }) => {
        return {
          content: [
            {
              type: "text",
              text: `${a + b}`,
            },
          ],
        };
      }
    ),
  ],
});

for await (const message of query({
  //prompt: generateMessages(),
  prompt: "Use the tool to add 1 and 2 and return the result.",
  options: {
    mcpServers: {
      test: mcp,
    },
    allowedTools: ["mcp__test__add"],
  },
})) {
  console.log(JSON.stringify(message));
}

Expected Behavior

Prompt should run with successful connection and access to the SDK MCP server.

Actual Behavior

Claude CLI process exits with an error.

[selenehyun]

I encountered the same issue today and was able to work around it with a minimal adjustment:

1. Refactored into an MCP Server using @modelcontextprotocol/sdk (this was straightforward since the code format is nearly identical).
2. Updated the mcpServers definition as shown below.

Before:

for await (const message of query({
    prompt: ...,
    options: {
        allowedTools: [
            ...
            'mcp__test',
        ],
        mcpServers: {
            'test': testMcpServer,
        },
    },
})) {

After:

for await (const message of query({
    prompt: ...,
    options: {
        allowedTools: [
            ...
            'mcp__test',
        ],
        mcpServers: {
            'test': {
                type: 'stdio',
                command: 'npx',
                args: ['-y', 'tsx', 'src/mcps/test.ts'],
            },
        },
    },
})) {

With this approach, I was able to get things running in under five minutes with only minimal changes.
Depending on your setup, you may need to use a built JS artifact instead of tsx, but for my use case this was sufficient.

[codylund]

@selenehyun I'm glad to hear this works for you, but this does not run the local MCP server in the same process. That's my main motivation for using the SDK type.

[codylund]

This still reproduces with v1.0.96...

[bolahanna44]

same issue for me

[ShuangLiu1992]

Broken with v1.0.98 as well

[xpluscal]

same issue

[tom-n-terra]

same issue , even without the allowedTools prop

[tariqkb]

Trying the example here https://docs.anthropic.com/en/docs/claude-code/sdk/sdk-typescript#creating-custom-tools but it doesn't seem to work either:

[ERROR] MCP server "my-custom-tools" Failed to connect SDK MCP server: Error: Stream closed

[codylund]

FWIW, I have the SDK MCP server type working in a single Bun-based project using v1.0.98, but I can't get it working in any other project. I have no idea why it works yet...

[ShuangLiu1992]

@codylund by any other project, do you mean projects using node or deno?