When Let's Encrypt Is Valid but GitHub Actions Still Rejects It

A few days ago I ran into a surprisingly tricky TLS issue while setting up automated tests for a service running behind Let’s Encrypt certificates.

The symptoms were confusing at first:

• Browsers accepted the certificate.

• curl --insecure worked.

• The certificate looked perfectly valid.

• Yet GitHub Actions consistently failed with:

  SSL certificate problem: unable to get local issuer certificate

At first glance this looked like a classic missing intermediate certificate problem.