diff --git a/scripts/post_update.py b/scripts/post_update.py
index 4b70a0c0a9..ec3d1f4c66 100644
--- a/scripts/post_update.py
+++ b/scripts/post_update.py
@@ -5,7 +5,7 @@
 from pathlib import Path
 import subprocess
 import sys
-from research_coordination import messages, validate_message
+from research_coordination import messages, parse_message, validate_message
 
 REPO = 'CrocSwap/integer-mult-bounds'
 
@@ -31,7 +31,7 @@ def prepare_body(path, metadata=None):
         if present:
             raise ValueError('body already contains coordination metadata')
         validate_message(metadata)
-        body = body.rstrip() + '\n\n```json\n' + json.dumps(metadata, indent=2) + '\n```\n'
+        body = body.rstrip() + '\n\n```json\n' + json.dumps(metadata, indent=2, allow_nan=False) + '\n```\n'
     return body
 
 
@@ -78,7 +78,7 @@ def main():
     try:
         if args.discussion <= 0:
             raise ValueError('discussion number must be positive')
-        metadata = json.loads(args.metadata_file.read_text(encoding='utf-8')) if args.metadata_file else None
+        metadata = parse_message(args.metadata_file.read_text(encoding='utf-8')) if args.metadata_file else None
         body = prepare_body(args.body_file, metadata)
         result = ({'repository': REPO, 'discussion': args.discussion, 'reply_to': args.reply_to,
                    'body': body, 'posted': False} if args.dry_run else post(args.discussion, args.reply_to, body))
diff --git a/scripts/research_coordination.py b/scripts/research_coordination.py
index d129df77a0..f06887cafc 100644
--- a/scripts/research_coordination.py
+++ b/scripts/research_coordination.py
@@ -39,6 +39,22 @@ def validate_message(value):
     return value
 
 
+def parse_message(raw):
+    """Read one unambiguous JSON object before validating its fields."""
+    def unique(pairs):
+        result = {}
+        for key, value in pairs:
+            if key in result:
+                raise ValueError('duplicate coordination JSON key: ' + key)
+            result[key] = value
+        return result
+
+    def finite(token):
+        raise ValueError('nonfinite coordination JSON number: ' + token)
+
+    return validate_message(json.loads(raw, object_pairs_hook=unique, parse_constant=finite))
+
+
 def messages(body):
     result, warnings = [], []
     for block in re.findall(r'^```json\s*\n(.*?)^```\s*$', body, re.M | re.S):
@@ -47,7 +63,7 @@ def messages(body):
         try:
             if len(block) > 16384:
                 raise ValueError('coordination block exceeds 16 KiB')
-            result.append(validate_message(json.loads(block)))
+            result.append(parse_message(block))
         except (ValueError, TypeError, AttributeError) as exc:
             warnings.append(str(exc))
     if len(result) > 1:
diff --git a/tests/test_coordination_json.py b/tests/test_coordination_json.py
new file mode 100644
index 0000000000..8ae56bb51e
--- /dev/null
+++ b/tests/test_coordination_json.py
@@ -0,0 +1,63 @@
+"""Ambiguous coordination JSON must fail before any network mutation."""
+import contextlib
+import io
+import json
+from pathlib import Path
+import sys
+import tempfile
+import unittest
+from unittest.mock import patch
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / 'scripts'))
+import research_coordination as board
+import post_update as post
+sys.path.pop(0)
+
+
+def value():
+    return {'schema': board.SCHEMA, 'kind': 'HANDOFF', 'task': 'fixture',
+            'scope': 'one missing check', 'base': 'a' * 40, 'artifact': None}
+
+
+def fenced(raw):
+    return 'A scoped handoff.\n\n```json\n' + raw + '\n```\n'
+
+
+class CoordinationJSONTests(unittest.TestCase):
+    def test_duplicate_fields_and_nonfinite_extensions_are_rejected(self):
+        raw = json.dumps(value())
+        for extra in ('"kind":"DONE"', '"base":"' + 'b' * 40 + '"',
+                      '"extra":{"x":1,"x":2}', '"extra":NaN', '"extra":Infinity'):
+            with self.subTest(extra=extra):
+                rows, warnings = board.messages(fenced(raw[:-1] + ',' + extra + '}'))
+                self.assertEqual(rows, [])
+                self.assertEqual(len(warnings), 1)
+
+    def test_prepared_body_rejects_duplicate_status(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            path = Path(tmp) / 'body.md'
+            path.write_text(fenced(json.dumps(value())[:-1] + ',"kind":"DONE"}'))
+            with self.assertRaisesRegex(ValueError, 'duplicate.*kind'):
+                post.prepare_body(path)
+
+    def test_metadata_file_rejects_before_post(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            body, metadata = Path(tmp) / 'body.md', Path(tmp) / 'meta.json'
+            body.write_text('A handoff.\n')
+            metadata.write_text(json.dumps(value())[:-1] + ',"kind":"DONE"}')
+            args = ['post_update.py', '--discussion', '1166', '--body-file', str(body), '--metadata-file', str(metadata)]
+            error = io.StringIO()
+            with patch.object(sys, 'argv', args), patch.object(post, 'post') as send, contextlib.redirect_stderr(error):
+                self.assertEqual(post.main(), 1)
+            send.assert_not_called()
+            self.assertIn('duplicate coordination JSON key: kind', error.getvalue())
+
+    def test_valid_unicode_metadata_and_prose_round_trip(self):
+        data = dict(value(), scope='A κ check', extension={'count': 3})
+        raw = json.dumps(data, ensure_ascii=False)
+        self.assertEqual(board.parse_message(raw), data)
+        self.assertEqual(board.messages(fenced(raw)), ([data], []))
+
+
+if __name__ == '__main__':
+    unittest.main()