diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ad485a7..48b6624 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,6 +25,10 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  # The self-hosted act_runner network can terminate index.crates.io with a
+  # certificate that does not match the sparse-index hostname. Use Cargo's git
+  # index path in CI; GitHub access is already required for the actions above.
+  CARGO_REGISTRIES_CRATES_IO_PROTOCOL: git
 
 jobs:
   build-test: