diff --git a/packages/mosaic/framework/tools/git/pr-review.sh b/packages/mosaic/framework/tools/git/pr-review.sh
index b36a6a66..0eb717ec 100755
--- a/packages/mosaic/framework/tools/git/pr-review.sh
+++ b/packages/mosaic/framework/tools/git/pr-review.sh
@@ -116,6 +116,14 @@ if [[ -z "$ACTION" ]]; then
     usage_error "Action is required (-a/--action): approve, request-changes, comment"
 fi
 
+# Validate the action BEFORE any provider contact (codex review of PR #1464:
+# an unsupported --action previously reached platform detection and could
+# touch the provider before failing with a provider-class status).
+case "$ACTION" in
+    approve|request-changes|comment) ;;
+    *) usage_error "unknown action '$ACTION': approve, request-changes, comment" ;;
+esac
+
 if [[ -n "$REPO_OVERRIDE" ]]; then
     # An explicit --repo is the whole point of a reviewer worktree whose origin
     # is nonstandard or missing (#867 convention, mirrored from pr-view.sh /
@@ -715,8 +723,7 @@ if [[ "$PLATFORM" == "github" ]]; then
             echo "Added review comment to GitHub PR #$PR_NUMBER"
             ;;
         *)
-            echo "Error: Unknown action: $ACTION"
-            exit 1
+            usage_error "unknown action: $ACTION"
             ;;
     esac
 elif [[ "$PLATFORM" == "gitea" ]]; then
@@ -809,8 +816,7 @@ elif [[ "$PLATFORM" == "gitea" ]]; then
             echo "Added and verified comment on Gitea PR #$PR_NUMBER (comment ID $comment_id)"
             ;;
         *)
-            echo "Error: Unknown action: $ACTION"
-            exit 1
+            usage_error "unknown action: $ACTION"
             ;;
     esac
 else
diff --git a/packages/mosaic/framework/tools/git/test-pr-review-usage-contract.sh b/packages/mosaic/framework/tools/git/test-pr-review-usage-contract.sh
index 8841553c..1c4f8d69 100755
--- a/packages/mosaic/framework/tools/git/test-pr-review-usage-contract.sh
+++ b/packages/mosaic/framework/tools/git/test-pr-review-usage-contract.sh
@@ -97,6 +97,15 @@ expect_rc 2 "missing -a exits 2" -n 5
 expect_stderr "Action is required" "missing -a message on stderr"
 expect_rc 2 "request-changes without comment exits 2" -n 5 -a request-changes
 expect_stderr "comment required for request-changes" "request-changes message on stderr"
+expect_rc 2 "invalid action exits 2 pre-detection" -n 5 -a bogus
+expect_stderr "unknown action" "invalid action message on stderr"
+# Invalid-action arms must not contact any provider (validation precedes
+# detect_platform): probe log empty at this point.
+if [[ -s "$PROBE_LOG" ]]; then
+    echo "FAIL: an invalid-action arm contacted a provider:" >&2
+    cat "$PROBE_LOG" >&2
+    exit 1
+fi
 
 # 4. Value-less flags: rc 2 with "requires a value" on stderr.
 for flag in -n -a -b -c -l -r --number --action --body --comment --login --repo; do