diff --git a/scripts/bash/setup-plan.sh b/scripts/bash/setup-plan.sh
index 03eaf713b0..f3edb3d9f8 100644
--- a/scripts/bash/setup-plan.sh
+++ b/scripts/bash/setup-plan.sh
@@ -4,7 +4,6 @@ set -e
 
 # Parse command line arguments
 JSON_MODE=false
-ARGS=()
 
 for arg in "$@"; do
     case "$arg" in
@@ -18,7 +17,8 @@ for arg in "$@"; do
             exit 0
             ;;
         *)
-            ARGS+=("$arg")
+            echo "ERROR: Unknown option '$arg'" >&2
+            exit 1
             ;;
     esac
 done
diff --git a/scripts/powershell/setup-plan.ps1 b/scripts/powershell/setup-plan.ps1
index 52f615aaad..300582d5eb 100644
--- a/scripts/powershell/setup-plan.ps1
+++ b/scripts/powershell/setup-plan.ps1
@@ -20,6 +20,11 @@ if ($Help) {
     exit 0
 }
 
+if ($RemainingArgs.Count -gt 0) {
+    [Console]::Error.WriteLine("ERROR: Unknown option '$($RemainingArgs[0])'")
+    exit 1
+}
+
 # Load common functions
 . "$PSScriptRoot/common.ps1"
 
diff --git a/scripts/python/setup_plan.py b/scripts/python/setup_plan.py
index d25fdd7829..3b8acc4fd4 100644
--- a/scripts/python/setup_plan.py
+++ b/scripts/python/setup_plan.py
@@ -42,7 +42,9 @@ def main(argv: list[str] | None = None) -> int:
         elif arg in {"--help", "-h"}:
             sys.stdout.write(_help_text(sys.argv[0]))
             return 0
-        # Other arguments are accepted and silently ignored, matching setup-plan.sh.
+        else:
+            print(f"ERROR: Unknown option '{arg}'", file=sys.stderr)
+            return 1
 
     try:
         paths = get_feature_paths(script_file=Path(__file__))
diff --git a/tests/test_setup_plan_python_parity.py b/tests/test_setup_plan_python_parity.py
index e8372125a3..d66c7083b3 100644
--- a/tests/test_setup_plan_python_parity.py
+++ b/tests/test_setup_plan_python_parity.py
@@ -132,7 +132,7 @@ def test_python_existing_plan_matches_bash(repo: Path, args: tuple[str, ...]) ->
 
 @requires_bash
 @pytest.mark.skipif(not HAS_POWERSHELL, reason="no PowerShell available")
-def test_all_variants_ignore_extra_arguments(tmp_path: Path) -> None:
+def test_all_variants_reject_unknown_options(tmp_path: Path) -> None:
     repos = [
         _setup_repo(tmp_path, "bash"),
         _setup_repo(tmp_path, "powershell"),
@@ -143,13 +143,15 @@ def test_all_variants_ignore_extra_arguments(tmp_path: Path) -> None:
     ps = run(ps_cmd(repos[1], SCRIPT, "-Json", "--bogus"), repos[1])
     py = run(py_cmd(repos[2], SCRIPT, "--json", "--bogus"), repos[2])
 
-    assert bash.returncode == ps.returncode == py.returncode == 0
-    assert normalize_repo_paths(bash.stdout, repos[0]) == normalize_repo_paths(
-        ps.stdout, repos[1]
-    ) == normalize_repo_paths(py.stdout, repos[2])
-    assert normalize_repo_paths(bash.stderr, repos[0]) == normalize_repo_paths(
-        ps.stderr, repos[1]
-    ) == normalize_repo_paths(py.stderr, repos[2])
+    assert bash.returncode == ps.returncode == py.returncode == 1
+    assert bash.stdout == ps.stdout == py.stdout == ""
+    assert bash.stderr == ps.stderr == py.stderr == (
+        "ERROR: Unknown option '--bogus'\n"
+    )
+    assert all(
+        not (current / "specs" / "001-my-feature" / "plan.md").exists()
+        for current in repos
+    )
 
 
 @requires_bash