Add SPDX or full copyright headers to the top of any source file. Pick a license, year, and holder; the tool wraps the text in the right comment syntax for your language and respects shebang lines, BOMs, and existing headers.
SPDX-License-Identifier: is a machine-readable tag (SPDX 2.0) that compliance scanners like FOSSology, ScanCode, ORT, and REUSE-tool recognize. One short line in every source file is enough for automatic license inventory across a repo. The SPDX License List is the canonical source of identifiers — common ones: MIT, Apache-2.0, GPL-3.0-or-later, BSD-3-Clause, MPL-2.0, ISC.
The "replace existing top header" option scans for a leading run of comment lines (single style, terminated by a blank line) and swaps it. It only touches the top of the file — comments deeper in the source are never modified.