🐛 FIX: Don't crash on nested YAML dates in front matter

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### 🐛 Bug Fixes
+
+- 🐛 Don't crash when a YAML date/datetime is nested in front matter by <gh-user:00200200>
+
 ## 5.1.0 - 2026-05-13
 
 ### ✨ New Features

myst_parser/mdit_to_docutils/base.py

@@ -1387,8 +1387,11 @@ def dict_to_fm_field_list(
                     )
                     continue
                 try:
-                    value = json.dumps(value)
-                except (ValueError, RecursionError):
+                    # Nested date/datetime values (allowed at top level) are
+                    # not JSON-serializable; stringify them like scalars.
+                    # TypeError also covers non-string keys such as a YAML date.
+                    value = json.dumps(value, default=str)
+                except (ValueError, TypeError, RecursionError):
                     # e.g. a self-referential structure via a YAML alias
                     self.create_warning(
                         f"Front matter field {key!r} could not be serialized",

tests/test_docutils.py

@@ -334,6 +334,48 @@ def test_topmatter_deeply_nested_yaml_warns():
     assert "content" in doctree.pformat()
 
 
+@pytest.mark.parametrize(
+    "yaml_block",
+    [
+        "review:\n  date: 2026-08-03",  # date nested in a mapping
+        "reviewed: [2026-08-03]",  # date nested in a sequence
+        "review:\n  at: 2026-08-03T12:00:00",  # datetime nested in a mapping
+    ],
+)
+def test_topmatter_nested_dates_do_not_crash(yaml_block):
+    """YAML dates nested in dicts/lists must not abort the build.
+
+    Top-level ``date``/``datetime`` scalars skip JSON encoding, but a nested
+    date used to reach ``json.dumps`` and raise ``TypeError`` (#1203).
+    """
+    stream = io.StringIO()
+    doctree = publish_doctree(
+        source=f"---\n{yaml_block}\n---\n\ncontent\n",
+        parser=Parser(),
+        settings_overrides={"warning_stream": stream},
+    )
+    assert "content" in doctree.pformat()
+    assert "2026-08-03" in doctree.astext()
+    assert "[myst.topmatter]" not in stream.getvalue()
+
+
+def test_topmatter_date_mapping_key_warns():
+    """A YAML date used as a mapping *key* cannot be JSON-serialized.
+
+    ``json.dumps(..., default=str)`` does not convert keys, so this still
+    cannot be rendered as a field, but it must warn rather than crash.
+    """
+    stream = io.StringIO()
+    doctree = publish_doctree(
+        source="---\nreview:\n  2026-08-03: done\n---\n\ncontent\n",
+        parser=Parser(),
+        settings_overrides={"warning_stream": stream},
+    )
+    assert "content" in doctree.pformat()
+    assert "could not be serialized" in stream.getvalue()
+    assert "[myst.topmatter]" in stream.getvalue()
+
+
 def test_topmatter_alias_expansion_bomb_warns():
     """A YAML alias-expansion ("billion laughs") bomb warns, not hangs.