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Add a one-line SPDX-License-Identifier: comment near the top of each file your project policy covers, using the exact identifier or expression from the current SPDX License List. Keep the full license text and required notices in the repository; the tag makes a file’s declared licensing terms easier to identify, but it does not prove that the declaration is legally correct.
What an SPDX license ID does
An SPDX license identifier is file-level metadata that states the license under which a source file is declared to be distributed. SPDX guidance says to place the tag in a comment at or near the top of the file, on a single line. The standard tag is deliberately recognizable to software tools, and the identifier can travel with a file when it is reused outside its original repository.
SPDX describes file-level identifiers as a way to make licensing more precise, concise, language-neutral, portable, and amenable to machine processing. They also point to standardized license entries for lookup and cross-reference. For these reasons, consistent identifiers can help teams inventory and review licensing information. They complement—not replace—the repository’s full license text, notices, provenance review, or legal analysis.
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Use an exact identifier from the current SPDX License List; do not abbreviate or infer one from a similar license name. The list reports version 3.29.0, dated 2026-09-16. Verify the current entry when adopting or updating identifiers, since names and list membership can change.
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The tag’s value can be a single identifier or an SPDX license expression. Keep the complete expression on one line:
SPDX-License-Identifier: MIT— a single listed license.SPDX-License-Identifier: GPL-2.0-only OR MIT— alternatives, expressed withOR.SPDX-License-Identifier: LGPL-2.1-only AND BSD-2-Clause— cumulative terms, expressed withAND.SPDX-License-Identifier: GPL-2.0-or-later WITH Bison-exception-2.2— a listed license with a recognized exception, attached withWITH.
An exception is not a stand-alone license: use WITH to associate it with the relevant license. For a license that is not listed, use a documented LicenseRef-... reference and make the referenced license text available to reviewers. If that approach is unsuitable, use an appropriate full license header.
How to add the tag correctly
- Set the scope. Inventory source, generated, test, documentation, and vendored files, then decide which file classes your policy requires to carry tags.
- Establish each file’s governing license. Check project history, existing headers, contributor agreements, and third-party notices. Do not assume every file in a repository has the same licensing terms.
- Map the terms to an exact identifier. Confirm the license in the current SPDX License List. For dual licensing or exceptions, form the corresponding valid expression rather than choosing a single ID.
- Add the tag as a comment near the file’s top. Use the programming language’s comment marker, followed by the exact tag and expression on one line. For example, in a C-style comment:
/* SPDX-License-Identifier: MIT */. In a line-comment language:// SPDX-License-Identifier: MIT. - Retain complete license texts and notices. Keep them in the repository’s
LICENSEfile or applicable notices directory. A short identifier does not substitute for required license text or third-party notices. - Validate and maintain the policy. Add CI or review checks for malformed, unknown, or contradictory identifiers and for required license texts. Recheck identifiers when dependencies, license versions, or SPDX data change.
Automate checks without mistaking metadata for proof
Because the tag uses a distinctive standard string, software can detect it more reliably than inconsistent free-form headers. A compliance workflow can check whether required files have tags, whether expressions parse and use known identifiers, and whether required license texts are present. Tools can also flag conflicts for human review.
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Automation cannot establish that a tag reflects the file’s actual legal terms. A syntactically valid identifier may still be wrong if the file’s provenance, contributor terms, or third-party notices were misunderstood. Review those sources, pay particular attention to generated and vendored material, and treat scanner output as an aid to compliance work rather than a legal determination.
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Common mistakes to avoid
- Guessing an ID. Similar license names do not guarantee interchangeable terms; copy the exact identifier from the current list.
- Putting the tag on multiple lines. The SPDX tag and its expression should remain on one line near the top of the file.
- Using
OR,AND, andWITHinterchangeably.ORmarks alternatives,ANDexpresses cumulative terms, andWITHattaches a recognized exception to a license. - Dropping the full notices. A file-level ID does not replace license texts or notices that the project must provide.
- Tagging without checking ownership or provenance. The tag records a declaration; it does not validate the declaration.
- Ignoring files outside the main source tree. Define how your policy handles tests, generated output, documentation, and vendored code instead of assuming they share the project’s license.
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