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Bad Request: Unsupported protocol version: 2026-07-28

Node.js · updated 2026-11-12

Returned with HTTP 400 (code -32000) by HTTP MCP servers built on @modelcontextprotocol/sdk 1.x when a request's MCP-Protocol-Version header names a version the SDK does not know. SDK 1.32.2, the last 1.x release, supports 2025-11-25, 2025-06-18, 2025-03-26, 2024-11-05 and 2024-10-07. MCP 2026-07-28 is the first stateless revision: it drops the initialize handshake, so a client speaking it sends the version on every request, and the Python SDK 2.x lists it as its latest version.

HTTP/1.1 400 Bad Request
Content-Type: application/json

{"jsonrpc":"2.0","error":{"code":-32000,"message":"Bad Request: Unsupported protocol version: 2026-07-28 (supported versions: 2025-11-25, 2025-06-18, 2025-03-26, 2024-11-05, 2024-10-07)"},"id":null}

Quick fix

If clients still connect, there is nothing to fix today: a client that supports both eras treats a 400 without a modern error body as a legacy server and falls back to initialize, which your 1.x server answers. To serve clients that speak only 2026-07-28, move the server to the v2 package (@modelcontextprotocol/server), which supports both. Do not fake the new protocol's UnsupportedProtocolVersionError (-32022) from a 1.x server: that error tells a client the server is modern, so it stops falling back.

  1. Check whether the client connected anyway: a dual-era client logs this 400, then retries with initialize and works.
  2. If a client only speaks 2026-07-28, it cannot use a 1.x server at all. Plan the move to @modelcontextprotocol/server 2.x.
  3. Until then, leave the 400 as it is; don't rewrite the header or the error code.
  4. Log the MCP-Protocol-Version header on rejected requests (transport.onerror receives the message) so you can see how many clients need the new protocol.
typescript
import { WebStandardStreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js';

const transport = new WebStandardStreamableHTTPServerTransport({ sessionIdGenerator: undefined });
// The SDK calls onerror before it answers a 400, with the reason in the message.
transport.onerror = (err) => {
  console.log('mcp rejected', {
    protocolVersion: request.headers.get('mcp-protocol-version'),
    userAgent: request.headers.get('user-agent'),
    error: err.message,
  });
};
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Why it happens

A client on the newest protocol

MCP 2026-07-28 removed initialize and sends the version with every request. Clients built on the newest SDKs try it first.

A server on SDK 1.x

@modelcontextprotocol/sdk 1.x stops at 2025-11-25; only the v2 packages know 2026-07-28.

A hand-written client or test

A curl test or custom client that hard-codes a version header the server does not support.

Other fixes

  • The Python MCP client 2.x, connecting to a 1.x TypeScript server, still works: it sends no version header on initialize and settles on 2025-11-25.
  • Under 2026-07-28 a modern server answers an unsupported version with error -32022 and the list of versions it supports, so the client can retry.

Same error, other wording

  • Bad Request: Unsupported protocol version: 2026-07-28 (supported versions: 2025-11-25, 2025-06-18, 2025-03-26, 2024-11-05, 2024-10-07)
  • Unsupported protocol version (code -32022, from a 2026-07-28 server)

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Related errors

More in MCP servers and clients

References: MCP specification 2026-07-28: versioning and compatibility, MCP specification 2026-07-28: key changes, MCP TypeScript SDK repository

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