Environment variable mismatch: local .env vs CI secrets debugging

Environment variable mismatches can make a passing local app fail in CI or staging. This guide shows how to compare .env files, CI secrets, defaults, and feature flags without leaking private values.

## Record names, not values
The first rule is to never paste secret values into a public issue, chat, or knowledge record. Instead, record variable names, whether each variable was present, whether it was empty, and which environment supplied it. For example, PAYMENT_API_URL present, PAYMENT_API_KEY missing, FEATURE_NEW_CHECKOUT false. This keeps the debugging trail useful without exposing credentials.

## Build a required-variable list
A project should know which variables are required to run tests, build, and deploy. If that list lives only in a developer's local .env file, CI will drift. Write a small table: variable name, required for local dev, required for test, required for deploy, safe default, and owner. The table often reveals that a variable is used in code but never documented.

## Compare defaults and fallback behavior
Some failures happen because local .env supplies a value while CI uses a default. A false default may disable a feature locally but enable it in CI, or the reverse. Check whether code uses empty string, undefined, null, or fallback URLs differently. Tests should make required configuration explicit instead of accidentally reading a developer machine.

## Check secret scope
CI systems may scope secrets by branch, environment, repository, or workflow. A pull request from a fork may not receive the same secrets as a main-branch run. If the failure appears only in one branch type, inspect secret scope before changing code.

## Redacted debug output
When adding temporary logs, print presence and length, not value. Example: API_KEY present=true length=40. Remove the log after diagnosis or keep it behind a safe debug flag.

## Good report format
- failing step
- variable names involved
- present or missing per environment
- expected default
- actual fallback
- branch or workflow scope
- next change

This turns environment mismatch from a hidden configuration problem into a reproducible checklist.

Environment variable mismatch: local .env vs CI secrets debugging

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