act "upload-artifact" fails without an artifact server locally

By Kaveh Alemi·Latchkey

actions/upload-artifact and download-artifact talk to a GitHub artifact service. act does not run one by default, so upload fails or is skipped and a later download finds nothing. act can start a local artifact server if you give it a path.

What this error means

Under act, upload-artifact errors with a missing runtime URL/token, or download-artifact finds no artifact because none was stored.

act

Error: Unable to get the ACTIONS_RUNTIME_URL env variable
Error: Create Artifact Container failed: the artifact server is not configured

Common causes

No artifact server is running

The runtime URL and token that upload-artifact needs come from the GitHub artifact service, which act does not provide by default.

Download has nothing to fetch

If upload was a no-op, a subsequent download-artifact in the same run finds no matching artifact.

How to fix it

Start act's artifact server

Give act an artifact server path so upload and download work locally.

Terminal

act --artifact-server-path /tmp/act-artifacts -j build

Gate artifact steps under act

Skip artifact upload/download locally when you only need to validate the build.

.github/workflows/ci.yml

- uses: actions/upload-artifact@v4
  if: ${{ !env.ACT }}

How to prevent it

Pass --artifact-server-path when a workflow uploads or downloads artifacts.
Gate artifact steps with if: !env.ACT when not testing them.
Validate full artifact retention and cross-job download on GitHub.

Frequently asked questions

What causes "upload-artifact has no server under act"?