CJS Build Fails Due to Top-Level Await in SSEClientTransport Dependencies

[mayank-holofy]

When bundling my Electron/TypeScript project in CommonJS mode, the build fails with the error:

Top-level await is currently not supported with the "cjs" output format
It appears SSEClientTransport (and its transitive dependency on pkce-challenge) uses top-level await in its ESM code, which breaks CJS builds in esbuild (and other bundlers).

Steps to reproduce

Import SSEClientTransport from @modelcontextprotocol/sdk/client/sse.js in a project configured for CommonJS output.
Attempt to bundle using esbuild (or a similar CJS-only tool).
The build fails with the above error.
Expected behavior
A working CJS build without errors—or a documented ESM-only usage pattern for SSE.

Possible solutions

Provide a CJS-friendly build that avoids top-level await.
Use a lazy/async import that defers or removes top-level await from the main bundle.
Document that SSE requires ESM (so users can plan accordingly).
Environment

@modelcontextprotocol/sdk version: latest
Bundler: esbuild (CJS mode)
Node/Electron environment
Additional context
This prevents using @modelcontextprotocol/sdk’s SSE features in many Electron + Node.js CJS setups. A workaround is removing SSE usage altogether or switching the entire project to ESM.

[dimavedenyapin]

Having similar problem, can't use the sdk now.

[AmAzing129]

cuz pkce-challenge is not CJS-friendly, maybe use another library?

[Cloudkkk]

same problem

[RussellLuo]

Encountered the same problem. After consulting with my colleague (thanks to @jialudev), I got a temporary solution:

1. Install patch-package

npm install patch-package --save-dev

2. Fix pkce-challenge

Modify node_modules/pkce-challenge/dist/index.node.js as below:

--- a/node_modules/pkce-challenge/dist/index.node.js
+++ b/node_modules/pkce-challenge/dist/index.node.js
@@ -2,7 +2,9 @@ let crypto;
 crypto =
     globalThis.crypto?.webcrypto ?? // Node.js 16 REPL has globalThis.crypto as node:crypto
         globalThis.crypto ?? // Node.js 18+ 
-        (await import("node:crypto")).webcrypto; // Node.js 16 non-REPL
+        (async() => {
+            (await import("node:crypto")).webcrypto; // Node.js 16 non-REPL
+        })();
 /**
  * Creates an array of length `size` of random bytes
  * @param size

3. Generate a patch

npx patch-package pkce-challenge

4. Auto-apply the patch

Add the following line in package.json:

"scripts": {
  ...
  "postinstall": "patch-package"
}