NFT does not walk ESM conditional exports branches for @swc/helpers #580

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1. styfle commented on May 18, 2026

   

   styfle

   on May 18, 2026

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   Please create a minimal repo without Next.js so we can verify if the bug is in Next.js or not
2. chanson commented on May 18, 2026

   

   chanson

   on May 18, 2026

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   We're also hitting this in a Next.js 15 app on Vercel using Node 22.x. In our case it's @reduxjs/toolkit (transitive dependency via recharts) rather than @swc/helpers, but seems to be the same root cause where nft traces the CJS path but Node resolves to the ESM entry at runtime.

   @reduxjs/toolkit exports map:

     {
       "module-sync": { "default": "./dist/redux-toolkit.modern.mjs" },
       "import": { "default": "./dist/redux-toolkit.modern.mjs" },
       "default": { "default": "./dist/cjs/index.js" }
     }
   

   Runtime resolves to redux-toolkit.modern.mjs, which then imports from redux/dist/redux.mjs, redux-thunk/dist/redux-thunk.mjs, etc. — all of which have the same CJS/ESM split in their exports maps. None of the ESM files are included in the SSR bundle.

   This started after a deploy today, which I'm guessing was our first deploy with Node 22.22.3 rolled out on Vercel. We confirmed by rolling back to a deploy from last week (which continued to work) vs reverting the change we deployed today and redeploying, resulting in the same error. The relevant Node changes appear to be nodejs/node#59679 (separate cache for require(esm) in imported CJS) and nodejs/node#62080 (path normalization in finalizeResolution).

   We worked around it two ways:

   1. outputFileTracingIncludes covering the full dist/** for each affected package
   2. Dynamically importing the affected components to keep them out of the SSR bundle entirely
3. Diego4523BJJ commented on May 18, 2026

   

   Diego4523BJJ

   on May 18, 2026

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   Please create a minimal repo without Next.js so we can verify if the bug is in Next.js or not

   Minimal repro without Next.js, as requested.

   package.json

   {
     "name": "nft-swc-helpers-esm-bug-repro",
     "version": "1.0.0",
     "private": true,
     "scripts": {
       "trace:cjs": "node trace.mjs entry.cjs",
       "trace:esm": "node trace.mjs entry.mjs"
     },
     "dependencies": {
       "@swc/helpers": "0.5.21",
       "@vercel/nft": "^0.27.0"
     }
   }

   entry.cjs (mimics Next.js / SWC compiled output for Node Lambda)

   const helper = require("@swc/helpers/_/_interop_require_default");
   console.log("helper:", typeof helper);

   entry.mjs (control — Node ESM context)

   import { _ } from "@swc/helpers/_/_interop_require_default";
   console.log("interop helper loaded:", typeof _);

   trace.mjs

   import { nodeFileTrace } from "@vercel/nft";
   import { resolve } from "node:path";
   
   const entry = resolve(process.argv[2] ?? "entry.cjs");
   const result = await nodeFileTrace([entry]);
   const swc = [...result.fileList].filter((f) => f.includes("@swc/helpers")).sort();
   const hasCjs = swc.some((f) => f.endsWith("cjs/_interop_require_default.cjs"));
   const hasEsm = swc.some((f) => f.endsWith("esm/_interop_require_default.js"));
   
   console.log("@swc/helpers files traced:");
   for (const f of swc) console.log("  " + f);
   console.log("  cjs included:", hasCjs);
   console.log("  esm included:", hasEsm);
   process.exit(hasEsm ? 0 : 1);

   Run:

   npm install
   npm run trace:cjs   # exits 1 — bug reproduced
   npm run trace:esm   # exits 0 — bug absent
   

   npm run trace:cjs output (current @vercel/nft behaviour):

   @swc/helpers files traced:
     node_modules/@swc/helpers/_/_interop_require_default/package.json
     node_modules/@swc/helpers/cjs/_interop_require_default.cjs
     node_modules/@swc/helpers/package.json
     cjs included: true
     esm included: false   <-- bug
   

   npm run trace:esm output (no bug):

   @swc/helpers files traced:
     node_modules/@swc/helpers/_/_interop_require_default/package.json
     node_modules/@swc/helpers/cjs/_interop_require_default.cjs
     node_modules/@swc/helpers/esm/_interop_require_default.js
     node_modules/@swc/helpers/package.json
     cjs included: true
     esm included: true
   

   The differentiator is the importer's module system. CJS require() makes NFT walk only the default (CJS) condition of the subpath's exports map. Under Node 22.22.3+ (nodejs/node#59679, nodejs/node#62080), the same require() at runtime resolves via the import condition because @swc/helpers/package.json declares "type": "module" — so the ESM file is what actually gets loaded. NFT and Node's resolver disagree, and the ESM sibling is missing from the lambda bundle.

   This matches @chanson's @reduxjs/toolkit observation above — same pattern, different package.
4. styfle commented on May 21, 2026

   

   styfle

   on May 21, 2026

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   You're using nft@0.27.0 which is quite old. Try upgrading to nft@1.5.0 and it works as expected.

   • PR fix(deps): bump @vercel/nft@1.10.2 next.js#93979
5. styfle

   closed this as not plannedon May 21, 2026