normalizeHeaders references the bare global `HeadersInit`, breaking Node-only consumers with skipLibCheck: false

Summary

shared/transport.d.ts references the bare global HeadersInit, which is a DOM lib type. Node-only consumers compiling with "skipLibCheck": false and no "DOM" in lib fail to build.

File/line: dist/esm/shared/transport.d.ts:7

export declare function normalizeHeaders(headers: HeadersInit | undefined): Record<string, string>;

Reproduction

@modelcontextprotocol/sdk@1.30.0 + @types/node@22.20.1, with "lib": ["ES2023"], "types": ["node"], "skipLibCheck": false:

node_modules/@modelcontextprotocol/sdk/dist/esm/shared/transport.d.ts(7,51):
  error TS2304: Cannot find name 'HeadersInit'.

Affects every published 1.x from 1.23.0-beta.0 through 1.30.0 (current latest).

Why @types/node cannot fix this

@types/node@22's fetch-globals shim (node_modules/@types/node/web-globals/fetch.d.ts) binds Headers, Request, RequestInit, Response and ResponseInit globally from undici-types — but deliberately not HeadersInit.

Per the review thread on DefinitelyTyped PR #74414 (@Renegade334): HeadersInit is a web-IDL typedef, which maps to a TypeScript type alias, and type aliases cannot be declaration-merged — unlike the dictionary/interface-shaped Headers/Request/RequestInit/Response, which can. So @types/node structurally cannot bind this global the way it binds the others.

That leaves the fix on the SDK side: don't reference the bare global.

Suggested fix

Change the signature to avoid the DOM-only global:

export declare function normalizeHeaders(headers: RequestInit['headers'] | undefined): Record<string, string>;

This is same-behaviour and depends only on a global @types/node does bind. It's also exactly what the v2 rewrite already does (#1279).

Blast radius is small: src/shared/transport.ts and its compiled output — 2 files.

Why a 1.x backport matters

The v2 rewrite already fixes this, but v2 is outside the peer range that @anthropic-ai/claude-agent-sdk pins (^1.29.0), so consumers coming through the Agent SDK can't upgrade to it today. Without a 1.x backport, the only options are a local ambient shim, adding "DOM" to lib (which drags every browser global into a Node service), or skipLibCheck: true (which stops type-checking all declaration files).

Happy to open a PR for the one-line change if that's useful.

Inspect normalizeHeaders in src/shared/transport.ts and its compiled declaration at dist/esm/shared/transport.d.ts. Verify the Node-only reproduction with skipLibCheck false and no DOM lib, then confirm the published declaration no longer requires the unresolved global HeadersInit.