diff --git a/scripts/validate_pr.py b/scripts/validate_pr.py
index 32ab9a08..82aadc72 100644
--- a/scripts/validate_pr.py
+++ b/scripts/validate_pr.py
@@ -307,6 +307,10 @@ def _files_to_validate(
     args: argparse.Namespace, parser: argparse.ArgumentParser
 ) -> list:
     """The files named on the command line, or the changed ones."""
+    if args.changed and args.files:
+        # Named files were dropped for the git diff, and the run reported
+        # "No changed BIOS files detected" about files it never looked at.
+        parser.error("--changed reads the git diff; name files or pass --changed, not both")
     if not args.changed:
         if not args.files:
             parser.error("No files specified. Use --changed or provide file paths.")
@@ -316,7 +320,7 @@ def _files_to_validate(
     except (RuntimeError, OSError) as exc:
         print(f"Error: {exc}", file=sys.stderr)
         sys.exit(2)
-    if not files:
+    if not files and not args.json:
         print("No changed BIOS files detected")
     return files
 
@@ -340,14 +344,19 @@ def main() -> None:
     parser.add_argument("--db", default=DEFAULT_DB, help="Path to database.json")
     parser.add_argument("--platforms-dir", default=DEFAULT_PLATFORMS_DIR)
     parser.add_argument("--emulators-dir", default=DEFAULT_EMULATORS_DIR)
-    parser.add_argument(
+    # One output format: --json beside --markdown printed JSON and dropped
+    # the markdown without a word.
+    output = parser.add_mutually_exclusive_group()
+    output.add_argument(
         "--markdown", action="store_true", help="Output as markdown (for PR comments)"
     )
-    parser.add_argument("--json", action="store_true", help="Output as JSON")
+    output.add_argument("--json", action="store_true", help="Output as JSON")
     args = parser.parse_args()
 
     files = _files_to_validate(args, parser)
     if not files:
+        if args.json:
+            print("[]")
         return
 
     db = _load_database_if_any(args.db)
diff --git a/tests/test_validate_pr.py b/tests/test_validate_pr.py
index 378d7d7e..43127578 100644
--- a/tests/test_validate_pr.py
+++ b/tests/test_validate_pr.py
@@ -247,5 +247,29 @@ class PlatformHashLoading(_Fixture):
         self.assertIn("boot.bin", known["names"])
 
 
+class FlagsAreAppliedOrRefused(unittest.TestCase):
+    """Files named beside --changed were dropped for the git diff and the run
+    said "No changed BIOS files detected" about files it never read; --json
+    beside --markdown printed JSON and dropped the markdown."""
+
+    def _run(self, *argv: str):
+        import subprocess  # noqa: PLC0415
+
+        return subprocess.run(
+            [sys.executable, "scripts/validate_pr.py", *argv], cwd=REPO_ROOT,
+            capture_output=True, text=True, timeout=300, check=False,
+        )
+
+    def test_files_beside_changed_are_refused(self):
+        result = self._run("--changed", "bios/Sony/PlayStation/scph5501.bin", "--json")
+        self.assertEqual(result.returncode, 2)
+        self.assertIn("--changed", result.stderr)
+
+    def test_two_output_formats_are_refused(self):
+        result = self._run("--json", "--markdown", "bios/Sony/PlayStation/scph5501.bin")
+        self.assertEqual(result.returncode, 2)
+        self.assertIn("not allowed with", result.stderr)
+
+
 if __name__ == "__main__":
     unittest.main()