Build error: SWC fails to read source map for dist/index.js with Bun

[haydenbleasel]

Description

When building a Next.js app that depends on chat@4.23.0 using Bun as the package manager, the SWC compiler fails with:

ERROR  failed to read input source map: failed to find input source map file "index.js.map"
  in "node_modules/.bun/chat@4.23.0/node_modules/chat/dist/index.js" file
  as either "node_modules/.bun/chat@4.23.0/node_modules/chat/dist/index.js.map"
  or with appended .map
    at crates/swc/src/lib.rs:398

Details

The source map file (dist/index.js.map) is present in the package and is valid JSON. The issue appears to be that SWC cannot resolve the source map through Bun's internal module cache path (node_modules/.bun/).

dist/index.js ends with:

//# sourceMappingURL=index.js.map

The file dist/index.js.map exists and is ~316KB of valid JSON, but SWC fails to locate it when the package is installed via Bun.

Reproduction

1. Create a Next.js app using Bun as the package manager
2. Install chat@4.23.0
3. Import from chat in any server-side code
4. Run next build

Environment

• chat: 4.23.0
• Next.js: 16
• Bun: 1.3
• macOS (Darwin 25.3.0)

Workaround

Adding chat to serverExternalPackages in next.config.ts avoids the error:

const nextConfig: NextConfig = {
  serverExternalPackages: ["chat"],
};

[haydenbleasel]

Suggested fix

The root cause is sourcemap: true in packages/chat/tsup.config.ts, which generates external .map files and appends //# sourceMappingURL=index.js.map to the output. When Bun installs the package, it stores files in its internal cache path (node_modules/.bun/chat@x.y.z/...) and SWC fails to resolve the relative source map reference through that indirection.

Option A — drop source maps from the published package (recommended):

export default defineConfig({
  entry: ["src/index.ts", "src/jsx-runtime.ts"],
  format: ["esm"],
  dts: true,
  clean: true,
  sourcemap: false,
});

Published npm packages rarely need source maps, and this eliminates ~300KB from the tarball.

Option B — use inline source maps:

export default defineConfig({
  entry: ["src/index.ts", "src/jsx-runtime.ts"],
  format: ["esm"],
  dts: true,
  clean: true,
  sourcemap: "inline",
});

This embeds the map as a base64 data URL directly in the JS file, so there's no external file path to resolve.