License Expression¶

PEP 639 defined a new pyproject.toml’s license value and added a corresponding core metadata License-Expression field. This specification defines which license expressions are acceptable.

Specification¶

License can be defined as a text string that is a valid SPDX license expression, as documented in the SPDX specification, either Version 2.2 or a later compatible version.

A license expression can use the following license identifiers:

Any SPDX-listed license short-form identifiers that are published in the SPDX License List, version 3.17 or any later compatible version.
The custom LicenseRef-[idstring] string(s), where [idstring] is a unique string containing letters, numbers, . and/or -, to identify licenses that are not included in the SPDX license list. The custom identifiers must follow the SPDX specification, clause 10.1 of the given specification version.

Examples of valid license expressions:

MIT
BSD-3-Clause
MIT AND (Apache-2.0 OR BSD-2-Clause)
MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)
GPL-3.0-only WITH Classpath-Exception-2.0 OR BSD-3-Clause
LicenseRef-Special-License OR CC0-1.0 OR Unlicense
LicenseRef-Proprietary

Examples of invalid license expressions:

Use-it-after-midnight  # No `LicenseRef` prefix
Apache-2.0 OR 2-BSD-Clause  # 2-BSD-Clause is not a valid SPDX identifier
LicenseRef-License with spaces  # spaces are not allowed
LicenseRef-License_with_underscores  # underscore are not allowed

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Last updated on May 21, 2026

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License Expression
- Specification