GitHub Actions artifact v3/v4 incompatibility (mixing versions)

v3 and v4 artifact actions use different storage backends, so an artifact uploaded with one major cannot be downloaded by the other; mixing them within a pipeline produces not-found errors.

What this error means

A download fails to find an artifact that was clearly uploaded, and the upload and download steps use different artifact action majors (one v3, one v4).

Error: Unable to find an artifact with the name: build-output
(uploaded with upload-artifact@v4, downloaded with download-artifact@v3)

Common causes

Mixed majors across jobs

One job uploads with v4 while another downloads with v3 (or vice versa), and the backends do not interoperate.

Partial migration left both in place

A migration upgraded some artifact steps to v4 but missed others, leaving an incompatible mix.

How to fix it

Standardize on one major

1. Move every upload-artifact and download-artifact step to v4.
2. Search the repo for lingering @v3 artifact references.
3. Re-test the full produce/consume artifact flow.

- uses: actions/upload-artifact@v4
- uses: actions/download-artifact@v4

How to prevent it

• Keep all artifact actions on the same major.
• Migrate upload and download together, not piecemeal.
• Grep for stray @v3 artifact references after migrating.

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