# Mutation testing at Mercado Libre: From pilot to everyday guardrail

DevFeed: [Mutation testing at Mercado Libre: From pilot to everyday guardrail](<https://devfeed.tech/articles/mutation-testing-at-mercado-libre-from-pilot-to-everyday-guardrail-22555.md>)

Original publisher: [Read original article](<https://medium.com/mercadolibre-tech/mutation-testing-at-mercado-libre-from-pilot-to-everyday-guardrail-2ffe0a273f0e?source=rss----5011f85401f0---4>)

Author: Luram Archanjo

Published: 2026-01-14T12:19:03Z

Content type: article

Language: en

Sources: [Mercado Libre Tech](<https://devfeed.tech/sources/mercado-libre-tech.md>)

Topics: [mutation-testing](<https://devfeed.tech/topics/mutation-testing.md>), [Testing](<https://devfeed.tech/topics/testing.md>), [GitHub](<https://devfeed.tech/topics/github.md>), [Pull Request](<https://devfeed.tech/topics/pull-request.md>), [test-coverage](<https://devfeed.tech/topics/test-coverage.md>)

Tags: [ci-cd-pipeline](<https://devfeed.tech/tags/ci-cd-pipeline.md>), [code-quality](<https://devfeed.tech/tags/code-quality.md>), [github](<https://devfeed.tech/tags/github.md>), [github-action](<https://devfeed.tech/tags/github-action.md>), [mutation-testing](<https://devfeed.tech/tags/mutation-testing.md>), [pull-requests](<https://devfeed.tech/tags/pull-requests.md>), [software-engineering](<https://devfeed.tech/tags/software-engineering.md>), [software-quality-testing](<https://devfeed.tech/tags/software-quality-testing.md>), [test-coverage](<https://devfeed.tech/tags/test-coverage.md>), [testing](<https://devfeed.tech/tags/testing.md>)

## AI overview

Mercado Envíos integrated mutation testing into Mercado Libre's Fury pipelines using a multi-stack GitHub Action, delta-first execution focused on modified code, and gradual team-level gates. The article explains how this approach supplements coverage by identifying weak test assertions and aims to improve pull request quality and reduce regressions.

## Source excerpt

Beyond coverage: How we integrated mutation testing into Fury pipelines and turned it into an actionable quality indicator for teams At Mercado Envíos (ME), we saw that high test coverage didn't always prevent regressions -- pull requests (PRs) were green, but we still broke production. In this article, we share how we brought mutation testing into our pipelines on Fury, our internal developer platform (IDP). We used a single multi-stack GitHub Action, a delta-first approach, and gradual team-level gates. We walk through the design decisions, challenges, and lessons learned that helped us turn this technique into an everyday guardrail that improves test effectiveness and PR quality. Introduction: Green PR, still an incident Friday, end of the day. You've got a green PR, pipelines are passing, and coverage looks great. You merge with confidence. On Monday, production goes down because of an edge case no one asserted. Sounds familiar? Same here. Coverage tells us what runs, not whether tests fail where they should. You can have 95-100% line coverage and still let a subtle regression slip -- a slightly wrong if condition, an unexpected default value, a branch with no assertions. That's where mutation testing comes in. Instead of assuming tests are good just because they pass, you intentionally introduce small code changes (mutants) and check whether the test suite kills them. In mutation testing, "killing a mutant" means your test suite fails when run on the mutated code, proving it can detect the injected defect. If mutants survive, you get a concrete indicator of weak assertions exactly in the area you're changing. At Mercado Envíos (ME), we moved from a one-off experiment to making mutation testing part of teams' daily flow. We integrated it into our standard pipelines on Fury, Mercado Libre's internal developer platform. We did it without breaking delivery: a single, multi-stack GitHub Action; delta-first execution focused on modified code; and team-level gates with