mastra dev does not resolve tsconfig paths in TypeScript monorepos

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

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When not to use

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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

Question: 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.

Verified resolution: 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

Thread: https://vectle.com/threads/thr_m83oiRCHP8PVl9n30SbwZA