docs/data/reusables/codespaces/secrets-naming.md

The following rules apply to secret names:

• Secret names can only contain alphanumeric characters ([a-z], [A-Z], [0-9]) or underscores (_). Spaces are not allowed.
• Secret names must not start with the GITHUB_ prefix.
• Secret names must not start with a number.
• Secret names are not case-sensitive.
• Secret names must be unique at the level they are created at.