It Works on My Machine

Code that works in one developer’s environment fails in another, in CI, or in production. Environment differences make results unreproducible.
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  3 minute read  

What you are seeing

A developer runs the application locally and everything works. They push to CI and the build fails. Or a teammate pulls the same branch and gets a different result. Or a bug report comes in that nobody can reproduce locally.

The team spends hours debugging only to discover the issue is environmental: a different Node version, a missing system library, a different database encoding, or a service running on the developer’s machine that is not available in CI. The code is correct. The environments are different.

New team members experience this acutely. Setting up a development environment takes days of following an outdated wiki page, asking teammates for help, and discovering undocumented dependencies. Every developer’s machine accumulates unique configuration over time, making “works on my machine” a common refrain and a useless debugging signal.

Common causes

Snowflake Environments

When development environments are set up manually and maintained individually, each developer’s machine becomes unique. One developer installed Python 3.9, another has 3.11. One has PostgreSQL 14, another has 15. These differences are invisible until someone hits a version-specific behavior. Reproducible, containerized development environments eliminate the variance by ensuring every developer works in an identical setup.

Read more: Snowflake Environments

Manual Deployments

When environment setup is a manual process documented in a wiki or README, it is never followed identically. Each developer interprets the instructions slightly differently, installs a slightly different version, or skips a step that seems optional. The manual process guarantees divergence over time. Infrastructure as code and automated setup scripts ensure consistency.

Read more: Manual Deployments

Tightly Coupled Monolith

When the application has implicit dependencies on its environment (specific file paths, locally running services, system-level configuration), it is inherently sensitive to environmental differences. Well-designed code with explicit, declared dependencies works the same way everywhere. Code that reaches into its runtime environment for undeclared dependencies works only where those dependencies happen to exist.

Read more: Tightly Coupled Monolith

How to narrow it down

  1. Do all developers use the same OS, runtime versions, and dependency versions? If not, environment divergence is the most likely cause. Start with Snowflake Environments.
  2. Is the development environment setup automated or manual? If it is a wiki page that takes a day to follow, the manual process creates the divergence. Start with Manual Deployments.
  3. Does the application depend on local services, file paths, or system configuration that is not declared in the codebase? If the application has implicit environmental dependencies, it will behave differently wherever those dependencies differ. Start with Tightly Coupled Monolith.

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