diff --git a/tests/validation/multigrid/spacetime_reference.py b/tests/validation/multigrid/spacetime_reference.py
index 879aba6..2d3801e 100644
--- a/tests/validation/multigrid/spacetime_reference.py
+++ b/tests/validation/multigrid/spacetime_reference.py
@@ -229,6 +229,15 @@ def advance(spec, plan, state, mode="spacetime", mutation=None):
                        negative_boundary_count=negative_boundary, nonfinite_boundary_count=nonfinite_boundary)
 
 
+def write_receipt(path, result):
+    """Normalize NumPy scalars at the JSON boundary; reject other unknown types."""
+    def scalar(value):
+        if isinstance(value, np.generic):
+            return value.item()
+        raise TypeError(f"Object of type {type(value).__name__} is not JSON serializable")
+    path.write_text(json.dumps(result, indent=2, allow_nan=False, default=scalar)+"\n")
+
+
 def main():
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument("--n64", action="store_true", help="explicitly run the frozen 1024-interval experiment")
@@ -267,7 +276,7 @@ def main():
                   limits=dict(seconds=args.max_seconds,rss_mib=args.max_rss_mib),runs=[],completed=False)
     def save():
         result["resources"] = dict(seconds=time.monotonic()-started,peak_rss_mib=resource.getrusage(resource.RUSAGE_SELF).ru_maxrss/1024)
-        args.output.write_text(json.dumps(result,indent=2,allow_nan=False)+"\n")
+        write_receipt(args.output, result)
     save()
     try:
         for mode in ("constant_ivp", "spacetime"):
diff --git a/tests/validation/multigrid/test_spacetime_reference.py b/tests/validation/multigrid/test_spacetime_reference.py
index dd3387b..c18d181 100644
--- a/tests/validation/multigrid/test_spacetime_reference.py
+++ b/tests/validation/multigrid/test_spacetime_reference.py
@@ -1,5 +1,8 @@
 """Tiny CPU algebra/event tests; no full trajectory or GPU execution."""
 from dataclasses import replace
+import json
+from pathlib import Path
+import tempfile
 import unittest
 import numpy as np
 import spacetime_reference as st
@@ -117,5 +120,36 @@ class SpaceTime(unittest.TestCase):
         self.assertEqual(d['nonfinite_boundary_count'],0)
 
 
+class Receipts(unittest.TestCase):
+    def test_completed_and_failed_screen_receipts(self):
+        for passed in (True, False):
+            with self.subTest(passed=passed), tempfile.TemporaryDirectory() as directory:
+                # Same NumPy comparison type produced by the final metric screen.
+                checks = {'velocity_l2': np.float64(.5 if passed else 1.) <= .8,
+                          'mass_momentum': True}
+                self.assertIsInstance(checks['velocity_l2'], np.bool_)
+                result = dict(completed=True, checks=checks,
+                              experimental_pass=all(checks.values()),
+                              runs=[dict(rows=[dict(step=np.int64(1024),
+                                                   mass_relative=np.float32(0.))])])
+                path = Path(directory)/'receipt.json'
+                st.write_receipt(path, result)
+                saved = json.loads(path.read_text())
+                self.assertIs(saved['completed'], True)
+                self.assertIs(saved['experimental_pass'], passed)
+                self.assertIs(saved['checks']['velocity_l2'], passed)
+                self.assertEqual(saved['runs'][0]['rows'][0],
+                                 dict(step=1024, mass_relative=0.))
+
+    def test_receipts_still_reject_nonfinite_and_unknown_values(self):
+        with tempfile.TemporaryDirectory() as directory:
+            path = Path(directory)/'receipt.json'
+            with self.assertRaises(ValueError):
+                st.write_receipt(path, {'invalid': np.float32('nan')})
+            with self.assertRaises(TypeError):
+                st.write_receipt(path, {'invalid': object()})
+            self.assertFalse(path.exists())
+
+
 if __name__=='__main__':
     unittest.main(verbosity=2)