How long can a Windows environment variable be?

Windows is a rather amusing operating system, full of internal contradictions. Environment-variable length limits have long been a source of trouble. RapidEE used to flag values longer than 2,047 characters, and later 4,096. So I wanted to check how things behave on newer versions of Windows…

I tested Windows 11 (build 26200) and Windows Server 2022 (build 20348). On both machines, the native SetEnvironmentVariableW API created a temporary variable in the process environment with a 1,000,000-character value, and PowerShell read it in full. That was the largest size I tested. So far, so good.

However, that’s where things got weird.

1. In my tests, cmd.exe stopped reliably expanding values around 8,191 characters.
2. PowerShell read the value in full. However, Windows PowerShell 5.1 refused to create a 32,767-character value through Environment.SetEnvironmentVariable, while PowerShell 7 on my machine created a value of one million characters.

In these tests, I was able to set a value one million characters long, but individual applications and APIs have their own limits.

So RapidEE now warns when a value exceeds 8,191 characters as a compatibility threshold for cmd.exe.