diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d10d01c..029ca04 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -216,7 +216,15 @@ jobs:
       # ─────────────────────────────────────────────────────────────────────────
       - name: Install cosign
         if: steps.vercheck.outputs.bumped == 'true' && steps.releasecheck.outputs.exists == 'false'
-        uses: sigstore/cosign-installer@398d4b0eeef1380460a10c8013a76f728fb906ac # v3
+        uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
+        with:
+          # Pin the cosign CLI to v2.x. The signing step below uses
+          # sign-blob --output-signature / --output-certificate to emit the
+          # detached .sig + .pem alongside each artifact. cosign v3 removed
+          # those flags in favour of --bundle, so an unpinned installer would
+          # silently break release signing. Keep the installer action current
+          # while holding the CLI on the flags this workflow relies on.
+          cosign-release: 'v2.5.2'
 
       - name: Sign release artifacts with cosign (keyless)
         if: steps.vercheck.outputs.bumped == 'true' && steps.releasecheck.outputs.exists == 'false'