Table of contents

• Why testing matters more in monorepos
• Core types of tests in monorepos
• Test organization and isolation
• Optimizing CI pipelines for monorepos
• Test ownership and visibility
• How Graphite helps with monorepo workflows
• Conclusion

Why testing matters more in monorepos

In a monorepo, many services, libraries, and apps live in a shared codebase. This setup makes collaboration easier but introduces a higher risk of regressions. A single commit can break unrelated projects if tests aren’t scoped or structured well. If you're considering this transition, see this guide: Migrating to Monorepo: A Step-by-Step Guide for practical advice.

Testing protects against tight coupling, ensures changes don’t create unintended side effects, and keeps CI fast. With many contributors working on different areas, a good testing strategy is critical to keep mainline builds stable and feedback loops short.

Core types of tests in monorepos

Unit tests

Validate isolated functions, classes, or modules. They run fast and are highly targeted. In a monorepo, they form the first defense against breaking changes. Use mocks and dependency injection to keep tests deterministic and focused.

Integration tests

Check how modules work together. This might involve verifying API endpoints, middleware layers, or microservice communication. Since all components live in one repo, integration testing becomes easier but more essential. Contract testing is useful here—ensure service interfaces don’t break.

End-to-end (e2e) tests

Simulate real user or system behavior from frontend to backend. While slower and harder to maintain, these catch errors across boundaries. In monorepos, E2E tests often span multiple services—run them sparingly, focusing on critical workflows.

Snapshot tests

Capture and compare output like UI renders or structured JSON. They flag unexpected changes and are particularly useful in fast-moving repos. Use them to protect shared components, exports, or API schemas.

Dependency-based (selective) testing

Instead of running all tests on every commit, run only those affected by code changes. This speeds up CI dramatically. Tools like Nx, Turborepo, and Bazel help automate dependency-aware testing. You can also script your CI pipeline to detect changed paths and only trigger matching tests. For more on optimizing CI/CD in monorepos, check out these guides: How to implement CI/CD strategies for monorepos and Monorepo with GitHub Actions.

Test organization and isolation

Keep tests co-located with code (e.g. Button.tsx and Button.test.tsx) or in a mirrored test/ directory. Either approach works—consistency matters most.

Isolate tests by service or module. Use in-memory databases or containers to prevent test contamination. Avoid shared state and global mocks.

For cross-project tests like E2E or system-level integration, keep them in a separate tests/ or e2e/ folder. This enforces boundaries and clarifies ownership.

Optimizing CI pipelines for monorepos

CI must scale with your repo. Avoid blanket test runs—use selective strategies:

• run tests based on changed files or dependency graphs
• parallelize test jobs to reduce runtime
• use caching for dependencies and unchanged test results
• set up merge queues to serialize changes and test them against the latest main branch

A merge queue like Graphite's stack-aware system helps maintain green builds while letting teams push faster. Each queued PR is tested in context, preventing conflicts and flaky failures post-merge. For more on performance and build optimization, see How monorepos might impact build times and what you can do about it. For dependency management, see Strategies for effectively managing dependencies in a monorepo.

Test ownership and visibility

Assign test ownership per service or module using tools like GitHub’s CODEOWNERS. When a test fails, it’s clear who should fix it. This also helps scale responsibility across teams.

Track test coverage using tools like Codecov or SonarQube. Use monorepo-specific flags or scopes so you can view per-project coverage. Surface this data in PRs to keep developers accountable and aware.

Maintain high trust in your test suite by rooting out flakiness and enforcing cleanup. Unreliable tests erode team confidence and slow releases.

How Graphite helps with monorepo workflows

Graphite improves testing workflows by promoting small, focused PRs. Large PRs slow down reviews and break tests unpredictably. With Graphite’s stacked PR model, teams break work into sequential, testable chunks.

Each PR in a stack is reviewed and tested independently, catching issues early. This improves test scoping and prevents regressions from landing in a bulk merge.

Graphite also offers stack-aware merge queue. When multiple PRs are stacked, it rebases, tests, and merges them in the correct order. This keeps mainline clean, reduces conflicts, and eliminates manual rebasing.

Conclusion

Testing in monorepos presents unique challenges, but with the right strategies, it can become a powerful advantage for teams. By scoping tests, optimizing CI pipelines, and leveraging tools like Graphite, Nx, and Bazel, you can keep your codebase stable and your feedback loops fast—even as your organization grows. Assigning clear test ownership, tracking coverage, and rooting out flakiness will help maintain trust in your test suite. As monorepos continue to gain popularity, adopting these best practices and modern workflows will set your team up for long-term success.