The correct option is Treat environment variables as case sensitive in every workflow. This gives the team consistent results across Ubuntu, macOS, and Windows and across Bash and PowerShell because every reference must match the defined name exactly.

This approach is right because Linux and macOS treat variable names as case sensitive while Windows does not. GitHub Actions does not normalize the casing for you, so the only portable practice is to pick one casing and use it consistently. If you define MY_VAR then you must always reference MY_VAR. Using my_var or My_Var may work on Windows but it will fail on Linux and macOS.

Use only uppercase names for all environment variables is incorrect because choosing uppercase is only a convention. It does not prevent bugs if you later reference the variable with a different case. Exact case matching is what ensures portability.

Ignore casing because GitHub Actions resolves environment variables in a uniform way is incorrect because Actions does not unify case behavior across runners. Ignoring case will cause subtle failures on Linux and macOS.

Defer to the operating system and shell behavior for how variables are matched is incorrect because that produces different results across platforms. The goal is consistent cross runner behavior, so you should enforce exact case matching in your workflows.

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