feat: add WebSocket server transport

[KhizarA77]

Add a WebSocket server transport (WebSocketServerTransport) to complement the existing WebSocketClientTransport, allowing MCP servers to communicate over WebSockets in addition to stdio and Streamable HTTP.

Motivation and Context

The SDK currently supports:

• stdio-based transports, and
• Streamable HTTP (SSE/JSON) transports,
  as well as a WebSocket client transport.

However, there is no corresponding server-side WebSocket transport. This makes it harder to:

• host MCP servers behind a WebSocket endpoint, or
• integrate with environments where WebSockets are the preferred or only option.

This change introduces WebSocketServerTransport, which implements the shared Transport interface and allows MCP servers to accept WebSocket connections using the mcp subprotocol.

How Has This Been Tested?

• Ran existing test cases
  Example you can write once you’ve tested:

• Ran npm run build and npm test.

• Manually tested a simple MCP server using WebSocketServerTransport:

  • Started a server on ws://localhost:8787/mcp.
  • Connected using WebSocketClientTransport.
  • Verified JSON-RPC requests and responses flowed correctly in both directions.

Breaking Changes

No breaking changes are expected:

• This adds a new transport implementation and an export.
• Existing transports (stdio and Streamable HTTP) and APIs are unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• New file: src/server/websocket.ts
  • Implements WebSocketServerTransport using the ws library.
  • Accepts a single WebSocket client per transport instance.
  • Enforces the mcp subprotocol and validates incoming messages with JSONRPCMessageSchema.
• Exported WebSocketServerTransport from src/server/index.ts so it can be imported via @modelcontextprotocol/sdk/server.
• Designed to mirror the existing StdioServerTransport/WebSocketClientTransport patterns for consistency.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1280

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1280

commit: 65d6551

[changeset-bot]

⚠️ No Changeset found

Latest commit: 65d6551

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[felixweinberger]

Thanks for this. Per the discussion in #142, we're moving away from WebSocket transport in the SDK rather than adding to it, to avoid transport proliferation. The Transport interface is exported for custom implementations. If SEP-1288 lands at the spec level we'd revisit. This also targets the pre-monorepo src/ path. Closing.