diff --git a/.github/actions/publish-vps-mirror/action.yml b/.github/actions/publish-vps-mirror/action.yml
index 87ca8e6f..424a3fdb 100644
--- a/.github/actions/publish-vps-mirror/action.yml
+++ b/.github/actions/publish-vps-mirror/action.yml
@@ -115,7 +115,12 @@ runs:
         )
         remote="${MIRROR_SSH_USER}@${MIRROR_SSH_HOST}"
         payload_bytes="$(python3 -c 'import json,sys; print(json.load(open(sys.argv[1]))["payloadBytes"])' "${stage_dir}/deployment.json")"
-        available_bytes="$(ssh "${ssh_options[@]}" "${remote}" "df -PB1 --output=avail '${mirror_root}' | tail -n 1 | tr -d ' '")"
+        available_kib="$(ssh "${ssh_options[@]}" "${remote}" "LC_ALL=C df -Pk '${mirror_root}' | awk 'NR == 2 { print \$4 }'")"
+        [[ "${available_kib}" =~ ^[0-9]+$ ]] || {
+          echo "Mirror origin returned an invalid free space value: ${available_kib:-<empty>}" >&2
+          exit 1
+        }
+        available_bytes="$((available_kib * 1024))"
         required_bytes="$((payload_bytes + 2000000000))"
         if (( available_bytes < required_bytes )); then
           echo "Mirror origin has insufficient free space: available=${available_bytes} required=${required_bytes}" >&2
diff --git a/tools/prepare-vps-release-payload.test.py b/tools/prepare-vps-release-payload.test.py
index 69c9419d..aac3f7c5 100644
--- a/tools/prepare-vps-release-payload.test.py
+++ b/tools/prepare-vps-release-payload.test.py
@@ -12,6 +12,7 @@ from pathlib import Path
 
 
 SCRIPT = Path(__file__).with_name("prepare-vps-release-payload.py")
+PUBLISH_ACTION = SCRIPT.parents[1] / ".github/actions/publish-vps-mirror/action.yml"
 
 
 def sha256(value: bytes) -> str:
@@ -153,6 +154,14 @@ class PrepareVPSReleasePayloadTest(unittest.TestCase):
             self.assertNotEqual(result.returncode, 0)
             self.assertIn("invalid app asset name", result.stderr)
 
+    def test_publish_action_reads_remote_capacity_with_posix_df(self) -> None:
+        source = PUBLISH_ACTION.read_text(encoding="utf-8")
+
+        self.assertIn("df -Pk '${mirror_root}'", source)
+        self.assertNotIn("--output=avail", source)
+        self.assertIn("available_kib", source)
+        self.assertIn("available_kib * 1024", source)
+
 
 if __name__ == "__main__":
     unittest.main()