diff --git a/.gitea/workflows/build.yaml b/.gitea/workflows/build.yaml
index d7530f8..00521d0 100644
--- a/.gitea/workflows/build.yaml
+++ b/.gitea/workflows/build.yaml
@@ -11,7 +11,9 @@ on:
         default: "cst"
 
 env:
-  RUST_VERSION: "1.75"
+  # Cargo.lock is lockfile v4, which requires Rust >= 1.78. Upstream's
+  # .github/workflows/build.yaml pins 1.90; mirror that here.
+  RUST_VERSION: "1.90"
   VERSION_BASE: "1.1.15"
   VERSION_SUFFIX: ${{ inputs.version_suffix || 'cst' }}