ERR_REQUIRE_ESM importing SSEClientTransport from @modelcontextprotocol/sdk in a CJS project

Question

In a CommonJS project on Node.js v22.6.0, importing SSEClientTransport from @modelcontextprotocol/sdk/client/sse.js fails with ERR_REQUIRE_ESM because the SDK's CJS build requires the ESM package pkce-challenge. How can I use the SSE client transport from a CommonJS project, and is this fixed in a later SDK version?

Originally asked on GitHub: https://github.com/modelcontextprotocol/typescript-sdk/issues/203

AgentArmorHQ team · Accepted Answer

**Status: closed (completed).** No maintainer replied in the thread; one user reported that upgrading to `@modelcontextprotocol/sdk` v1.9.0 resolved the error, and another user confirmed a dynamic-import workaround.

**Root cause:** importing `SSEClientTransport` pulls in the SDK's CommonJS build (`dist/cjs/client/auth.js`), which does `require()` of `pkce-challenge`. That package's nearest `package.json` declares `"type": "module"`, so Node.js rejects the `require()` call with `ERR_REQUIRE_ESM` on Node.js v22.6.0. On Node.js 23 the same import path produced a related `ERR_REQUIRE_ASYNC_MODULE` error. Server-side imports from the SDK worked fine; the failure is specific to the client/auth code path.

**Workaround reported in the thread:** replace the top-level import with a dynamic import inside an async function:

```typescript
const SSEClientTransport = (await import('@modelcontextprotocol/sdk/client/sse.js')).SSEClientTransport;
```

**Also reported:** upgrading to `@modelcontextprotocol/sdk` v1.9.0 made the problem go away for one user.

AgentArmorHQ team · Answer

**What @wictorwilen tried on GitHub** (Worked)

Replaced the top-level import with a dynamic import inside an async function: `const SSEClientTransport = (await import('@modelcontextprotocol/sdk/client/sse.js')).SSEClientTransport;`.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/203#issuecomment-2739382675

AgentArmorHQ team · Answer

**What @henrytsuicsa tried on GitHub** (Worked)

Upgraded to `@modelcontextprotocol/sdk` v1.9.0; the user reported "seems problem solved".

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/203#issuecomment-2819889448

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

Used a top-level named import `import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";` in a CJS project on Node.js v22.6.0, which threw `ERR_REQUIRE_ESM` because `require()` hit the ESM file `pkce-challenge/dist/index.node.js`.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/203

AgentArmorHQ team · Answer

**What @anandrewfiserv tried on GitHub** (Didn't work)

On Node.js 23, the same import path produced a different error, `ERR_REQUIRE_ASYNC_MODULE`, instead of `ERR_REQUIRE_ESM`.

Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/203#issuecomment-2737504501

ERR_REQUIRE_ESM importing SSEClientTransport from @modelcontextprotocol/sdk in a CJS project

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ERR_REQUIRE_ESM importing SSEClientTransport from @modelcontextprotocol/sdk in a CJS project

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