## TL;DR

Verified resolution: upgrade to mastra 0.4.3-alpha.1 (or later): contributor wardpeet confirmed "Published in latest alpha, with support for ts paths" and the reporter replied "i installed the latest alpha (0.4.3-alpha.1) and this worked for me. thank you!".

## Steps

1. In a TypeScript monorepo using nx, running `mastra dev` failed to resolve imports that used custom `paths` aliases from the root tsconfig.json (for example `@my-project/constants` pointing at `libs/constants/src/index.ts`), even though the paths worked fine everywhere else in the project. Importing workspace packages also failed with `TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts"`. A direct `tsx ./src/main.ts` run worked, but that blocked use of the Mastra Playground.

2. This was fixed by the Mastra team adding tsconfig paths support to the dev server. Verified resolution: upgrade to mastra 0.4.3-alpha.1 (or later): contributor wardpeet confirmed "Published in latest alpha, with support for ts paths" and the reporter replied "i installed the latest alpha (0.4.3-alpha.1) and this worked for me. thank you!". A stopgap if you cannot upgrade: transpile your internal packages first (e.g. turborepo with watch mode) so `mastra dev` imports built output instead of raw .ts. Source: https://github.com/mastra-ai/mastra/issues/1996

## When to use

You are seeing this: Importing workspace packages also failed with TypeError [ERR\_UNKNOWN\_FILE\_EXTENSION]: Unknown file extension ".ts". Use this skill when you run into "mastra dev does not resolve tsconfig paths in TypeScript monorepos".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

Versions mentioned in the source: mastra 0.4.3-alpha.1. If you are on something much newer or older, the details may have shifted.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

## Sources

- https://github.com/mastra-ai/mastra/issues/1996