Backstage "yarn build" failed in CI

Backstage builds the app with a bundler and the backend with the backstage-cli. A failed yarn build almost always has a specific cause in the log above the summary line: a missing module, a config error, or an out-of-memory kill.

What this error means

The build step ends with "error Command failed with exit code 1" after a bundler or backstage-cli trace pointing at a module, config, or memory problem.

yarn

$ backstage-cli repo build --all
Building app...
Failed to compile.
Module not found: Error: Can't resolve './components/Missing' in '/app/packages/app/src'
error Command failed with exit code 1.

Common causes

A broken import or missing file

A module the bundler cannot resolve (a renamed file, a missing plugin) fails the compile with "Module not found".

A downstream cause: memory or config

The build can also fail from an out-of-memory kill or a missing app-config value; the real reason is the trace above the "Command failed" line.

How to fix it

Read the failure above the summary line

Scroll up to the first "Module not found", "error TS", or "heap out of memory".
Fix that specific cause (import, dependency, memory, or config).
Re-run the build to confirm it compiles.

Build backend and app explicitly

Use the repo build so workspaces compile in the correct order before bundling.

Terminal

yarn tsc
yarn build:backend
yarn build:all

How to prevent it

Run the clean type check before build so import errors surface first.
Provide required app-config values in the build environment.
Set the heap size for large monorepo builds so memory does not kill the job.