Source: github.com/modelcontextprotocol/typescript-sdk/releases/tag/v2.4.0
Version 2.4.0 of the TypeScript SDK updates the client, server, core, server-legacy and codemod packages, and @modelcontextprotocol/node moves to 2.1.2. The release adds Skills over MCP (SEP-2640): servers publish a skills folder with loadSkills(folder) and skills.attach(server), and clients list, read and verify them with SkillsClient. The attach method is marked experimental.
Some changes can break existing code. An x-mcp-header annotation on a number parameter is now rejected, so affected tools are hidden from clients and calls return -32020; use z.int() or remove the annotation. Results from outputSchema defined with z.object(...).meta({ id }) are no longer wrapped in { result } on 2025-11-25 connections.
The bundled Ajv validator now uses fast-uri 3.1.8 and rejects $id or $ref values with invalid URI schemes. Smaller additions include an opt-in scopeForRefresh hook for servers that require scope on token refresh, along with several bug fixes.
Why it matters if you build agents
Teams building on the TypeScript SDK should audit tool definitions for number-typed x-mcp-header parameters, and any code that unwraps result from structured output, before upgrading. Agents that publish or consume skills now have a built-in path for it, though attach may change in a minor release.
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