Nitro-in-Vite: intermediate chunks re-transformed, duplicate class registration crashes SSR with "Cannot redefine property: classId"

[ryantbrown]

Description

In the Nitro-in-Vite pipeline (e.g. TanStack Start with nitro/vite), the workflow SWC transform runs a second time over Nitro's own intermediate chunks. Any serialization class that was correctly registered in the first pass gets a second inlined registration IIFE appended in the final server bundle. Since the IIFE defines classId as non-configurable (#1480), the second Object.defineProperty on the same class throws at module load, and every SSR request 500s:

TypeError: Cannot redefine property: classId
    at Object.defineProperty (<anonymous>)
    at file:///var/task/_ssr/dist-D8JevQNu.mjs:15277:9
    at ModuleJob.run (node:internal/modules/esm/module_job:439:25)
    ...
    at async loadEntries (file:///var/task/_ssr/ssr.mjs:13810:52)

The final chunk contains both registrations for the same class object — first the correct package-derived id, then a chunk-path-derived one (on workflow@5.0.0-beta.31 we see the path-derived registration up to 3×):

// pass 1 — correct
})(GatewayEmbeddingModel, "class//@ai-sdk/gateway@4.0.16//GatewayEmbeddingModel");

// pass 2 — re-transform of Nitro's intermediate chunk, crashes at load
})(GatewayEmbeddingModel, "class//./node_modules/.nitro/vite/services/ssr/assets/dist-Bhk9b09n//GatewayEmbeddingModel");

Root cause

packages/nitro/src/index.ts unshifts workflowTransformPlugin in rollup:before with only the pre-built workflow bundles excluded:

nitro.hooks.hook('rollup:before', (_nitro, config) => {
  config.plugins.unshift(
    workflowTransformPlugin({
      // Exclude pre-built workflow bundles from re-transformation
      exclude: [workflowBuildDir],
    })
  );
});

Nitro's second build pass consumes the Vite environment build's output (node_modules/.nitro/vite/services/ssr/assets/*). Those chunks still contain serde markers (static [WORKFLOW_SERIALIZE], Symbol.for("workflow-serialize")) from bundled dependencies, so shouldTransformFile matches and the chunk is transformed again. The same gap exists for the Vite-level transform plugin, which also runs in the Nitro environment.

Reproduction conditions

1. TanStack Start (or any Nitro-in-Vite app) + workflow/vite
2. ssr.noExternal: true (bundle dependencies into SSR chunks — needed e.g. under bun's isolated installs)
3. Any dependency with WDK serde-marked classes in the SSR graph — @ai-sdk/gateway (GatewayLanguageModel et al.) or @vercel/sandbox

The workbench apps don't hit this because they externalize SSR deps, so serde-marked code never sits inside an intermediate chunk.

Reproduced on: workflow@5.0.0-beta.30 and 5.0.0-beta.31, nitro@3.0.260610-beta, vite@8.1.4, @tanstack/react-start@1.168.27, bun 1.3.12, nitro({ preset: "vercel" }).

Suggested fix

Exclude Nitro's intermediate build assets from re-transformation (mirroring the existing workflowBuildDir exclusion), or make the transform idempotent for already-registered classes (e.g. skip emitting a registration when the module already contains the registry IIFE for that class, or guard the inlined defineProperty).

Workaround

Wrapping every workflow:transform plugin instance (the Vite-level one and the one unshifted in rollup:before) to return null for ids containing /.nitro/ restores a correct bundle: each class registered exactly once under its package-derived id, server boots cleanly.

How did you test your changes?

n/a — bug report. Verified the mechanism by importing the built chunk directly (node -e "import('./_ssr/dist-….mjs')"), which reproduces the crash, and confirming the double registration in the emitted code.

[NathanColosimo]

Hi @ryantbrown , first thing I would try is not bun, that has some known issues with workflow.

• Try bun 1.4 beta / canary latest
• try node 24 if that's possible

edit: its still an issue with node, looking into this

[NathanColosimo]

@ryantbrown The fix should be on the next 5.x beta so wait for that to land and let us know if still having any issues!