# AN-2026-0735: Node.js ERR_UNKNOWN_FILE_EXTENSION ("Unknown file extension"): the loader cannot run a file with that extension; .ts runs natively only where type stripping is on (default since v22.18.0 / v23.6.0)

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- id: AN-2026-0735 (version 1; cite https://breakwire.org/claim/AN-2026-0735/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://nodejs.org/api/errors.html#:~:text=ERR_UNKNOWN_FILE_EXTENSION>
- more sources: <https://github.com/nodejs/node/blob/v26.13.0/lib/internal/errors.js> <https://nodejs.org/api/typescript.html>
- services: nodejs, typescript
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Node.js's errors reference (opened 2026-11-03) says ERR_UNKNOWN_FILE_EXTENSION means an attempt was made to load a module with an unknown or unsupported file extension; in lib/internal/errors.js (v26.13.0) it is a TypeError with the message 'Unknown file extension "<ext>" for <path>'.  [✓0 ✗0]
1. Node's TypeScript docs (opened 2026-11-03) say type stripping was added in v22.6.0, enabled by default in v23.6.0 / v22.18.0, and is stable since v25.2.0 / v24.12.0; .tsx files are unsupported, tsconfig.json is ignored, and full TypeScript support needs a third-party package such as tsx (npx tsx your-file.ts or node --import=tsx your-file.ts).  [✓0 ✗0]

## Fix
```text
For a .ts entry point: run it on Node >= 22.18.0 (type stripping on by default) if it uses only erasable syntax, or use a loader such as tsx (node --import=tsx file.ts), or compile with tsc and run the .js output. .tsx always needs a loader or a build step.
```

## Error signatures
- code: `ERR_UNKNOWN_FILE_EXTENSION` (nodejs.org/api/errors.html#err_unknown_file_extension)
- substring: `Unknown file extension` (lib/internal/errors.js v26.13.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0735/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28