diff --git a/scripts/diff_truth.py b/scripts/diff_truth.py
index b4a1ab2b..916e845e 100644
--- a/scripts/diff_truth.py
+++ b/scripts/diff_truth.py
@@ -168,12 +168,19 @@ def main() -> None:
         "--all", action="store_true", help="diff all registered platforms"
     )
     group.add_argument("--platform", help="diff a single platform")
-    parser.add_argument(
+    # One output form: --json beside --format markdown printed JSON and
+    # dropped the markdown without a word.
+    output = parser.add_mutually_exclusive_group()
+    output.add_argument(
         "--json", action="store_true", dest="json_output", help="JSON output"
     )
-    parser.add_argument(
+    output.add_argument(
         "--format", choices=["terminal", "markdown"], default="terminal"
     )
+    output.add_argument(
+        "--markdown", action="store_const", const="markdown", dest="format",
+        help="same as --format markdown, the spelling validate_pr uses",
+    )
     parser.add_argument("--truth-dir", default="dist/truth")
     parser.add_argument("--platforms-dir", default="platforms")
     parser.add_argument(
diff --git a/tests/test_audit_regressions.py b/tests/test_audit_regressions.py
index 44274d58..c50f892d 100644
--- a/tests/test_audit_regressions.py
+++ b/tests/test_audit_regressions.py
@@ -4401,5 +4401,25 @@ class VerifyAllSummaryNamesWhatIsWrong(unittest.TestCase):
         self.assertIn("0 critical,", summary)
 
 
+
+class OneOutputFormPerRun(unittest.TestCase):
+    """`diff_truth.py --json --format markdown` printed JSON with rc 0, the
+    markdown asked for dropped without a word."""
+
+    def test_json_and_markdown_are_refused_together(self):
+        for script, argv in (
+            ("diff_truth.py", ["--platform", "romm", "--json", "--format", "markdown"]),
+            ("diff_truth.py", ["--platform", "romm", "--json", "--markdown"]),
+            ("validate_pr.py", ["--json", "--markdown", "bios/Sony/PlayStation/scph5501.bin"]),
+        ):
+            with self.subTest(script=script, argv=argv):
+                proc = subprocess.run(
+                    [sys.executable, f"scripts/{script}", *argv], capture_output=True,
+                    text=True, cwd=str(ROOT), timeout=120, check=False,
+                )
+                self.assertEqual(proc.returncode, 2, proc.stdout[-300:])
+                self.assertIn("not allowed with", proc.stderr)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/wiki/tools.md b/wiki/tools.md
index 74c00b2a..e98651a0 100644
--- a/wiki/tools.md
+++ b/wiki/tools.md
@@ -597,7 +597,7 @@ Further options, by script:
 | `check_freshness.py` | `--cache-dir`, `--native-cache-dir` (originals of the native export) and `--truth-dir` move the caches it reads |
 | `check_release.py` | `--tag`, `--sums dist/SHA256SUMS.txt`, `--record` (default `release.json`), `--json` |
 | `check_release_assets.py` | `--gitignore` (the asset registry), `--db`, `--notes FILE`, `--json` |
-| `diff_truth.py` | `--format markdown`, `--truth-dir`, `--target` |
+| `diff_truth.py` | `--format markdown` (or `--markdown`), `--json`, one of them; `--truth-dir`, `--target` |
 | `download.py` | `--list`, `--info <platform>` shows a pack's file name, size and parts without downloading |
 | `export_native.py` | `--truth-dir`, `--upstream-dir` (where the platforms' own files are cached) |
 | `find_pinned_revision.py` | `--repo URL --path FILE --md5 HASH`, `--write PATH`, `--cache-dir` |