# What Is a Build Matrix? Testing Across Versions and Platforms

> A build matrix runs the same job across many combinations - OS, language version, dependency set. Learn how matrices work and how they multiply cost.

Source: https://latchkey.dev/learn/ci-cd-concepts/what-is-a-build-matrix  
Updated: 2026-06-25

A matrix turns one job definition into many parallel jobs - one per combination of the variables you list - so you test across versions and platforms without copy-pasting jobs.

A build matrix is a compact way to fan a single job out across a grid of configurations. It is how you prove your code works on multiple OSes, language versions, or dependency sets at once.

## How a matrix works

You declare variables with lists of values; the CI system generates the Cartesian product - one job per combination - and runs them in parallel. A 3-OS × 3-version matrix expands to nine jobs, each an isolated run of the same steps.

```2 × 3 = 6 jobs
strategy:
  matrix:
    os: [ubuntu-latest, macos-latest]
    node: [18, 20, 22]
```

## Why it is useful

- Catch version- and platform-specific bugs before users do.
- Parallelize coverage instead of running combinations serially.
- Express broad compatibility testing in a few lines of config.

## The cost multiplier

A matrix multiplies billable minutes by the number of combinations. A wide matrix of short jobs is also punished by per-job minute rounding. Trim combinations that add little signal (use `include`/`exclude` to test only meaningful pairs) rather than blindly testing the full grid.

## Matrix and fail-fast

By default many systems cancel the rest of the matrix when one job fails (`fail-fast`). That saves minutes but hides whether the failure is specific to one combination. Disable fail-fast when you need to see the full pass/fail grid across all configurations.

## FAQ

### What is What is a build Matrix? testing across versions and platforms?

A build matrix is a compact way to fan a single job out across a grid of configurations. It is how you prove your code works on multiple OSes, language versions, or dependency sets at once.

### How a matrix works?

You declare variables with lists of values; the CI system generates the Cartesian product - one job per combination - and runs them in parallel. A 3-OS × 3-version matrix expands to nine jobs, each an isolated run of the same steps.

### The cost multiplier?

A matrix multiplies billable minutes by the number of combinations. A wide matrix of short jobs is also punished by per-job minute rounding. Trim combinations that add little signal (use include/exclude to test only meaningful pairs) rather than blindly testing the full grid.

### Matrix and fail-fast?

By default many systems cancel the rest of the matrix when one job fails (fail-fast). That saves minutes but hides whether the failure is specific to one combination. Disable fail-fast when you need to see the full pass/fail grid across all configurations.

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Latchkey runs CI/CD that repairs its own failures. Agent entry points: https://latchkey.dev/agent.txt, https://latchkey.dev/openapi.json, https://latchkey.dev/llms.txt