diff --git a/librarian/storage/database.py b/librarian/storage/database.py
index e3a423f..c58e956 100644
--- a/librarian/storage/database.py
+++ b/librarian/storage/database.py
@@ -12,7 +12,7 @@
 import threading
 from collections.abc import Generator
 from contextlib import contextmanager
-from datetime import datetime
+from datetime import date, datetime
 from typing import Any
 
 import sqlite_vec
@@ -30,6 +30,19 @@
 logger = logging.getLogger(__name__)
 
 
+def _json_default(value: Any) -> str:
+    """JSON fallback for types YAML frontmatter emits but stdlib json can't encode.
+
+    Obsidian and other markdown frontmatter commonly contain `YYYY-MM-DD` values
+    which PyYAML parses into `datetime.date`. Stored as ISO strings; round-tripped
+    values come back as strings (acceptable because metadata is informational,
+    not queried as dates).
+    """
+    if isinstance(value, date | datetime):
+        return value.isoformat()
+    return str(value)
+
+
 def get_effective_embedding_dimension() -> int:
     """Get the embedding dimension based on configured provider."""
     if EMBEDDING_PROVIDER == "openai":
@@ -286,7 +299,9 @@ def insert_document(self, document: Document) -> int:
                     document.path,
                     document.title,
                     document.content,
-                    json.dumps(document.metadata) if document.metadata else None,
+                    json.dumps(document.metadata, default=_json_default)
+                    if document.metadata
+                    else None,
                     document.file_mtime,
                     document.asset_type.value,
                 ),
@@ -312,7 +327,9 @@ def update_document(self, document: Document) -> None:
                     document.path,
                     document.title,
                     document.content,
-                    json.dumps(document.metadata) if document.metadata else None,
+                    json.dumps(document.metadata, default=_json_default)
+                    if document.metadata
+                    else None,
                     document.file_mtime,
                     document.asset_type.value,
                     document.id,
diff --git a/tests/test_database.py b/tests/test_database.py
new file mode 100644
index 0000000..904c3ad
--- /dev/null
+++ b/tests/test_database.py
@@ -0,0 +1,69 @@
+"""Regression tests for librarian.storage.database."""
+
+from datetime import date, datetime
+from pathlib import Path
+
+import pytest
+
+from librarian.storage.database import Database
+from librarian.types import AssetType, Document
+
+
+@pytest.fixture
+def db(tmp_path: Path) -> Database:
+    """Fresh Database instance per test, isolated to a tmp file."""
+    return Database(db_path=str(tmp_path / "test.db"))
+
+
+class TestMetadataSerialization:
+    """JSON-serialize document metadata containing types that YAML frontmatter
+    emits but the stdlib json encoder doesn't know about."""
+
+    def test_insert_with_date_metadata(self, db: Database) -> None:
+        """A `date` in metadata (from `last_push: 2026-05-19` frontmatter) must
+        not crash on insert. Regression: previously raised
+        `TypeError: Object of type date is not JSON serializable`."""
+        doc = Document(
+            id=None,
+            path="/note-with-date.md",
+            title="Note",
+            content="body",
+            metadata={
+                "last_push": date(2026, 5, 19),
+                "updated_at": datetime(2026, 5, 19, 12, 30, 45),
+                "tags": ["repo", "MOC"],
+            },
+            file_mtime=0.0,
+            asset_type=AssetType.TEXT,
+        )
+        doc_id = db.insert_document(doc)
+        assert doc_id is not None
+
+        got = db.get_document_by_path("/note-with-date.md")
+        assert got is not None
+        # Dates round-trip as ISO strings; metadata is informational, not queried as dates.
+        assert got.metadata["last_push"] == "2026-05-19"
+        assert got.metadata["updated_at"].startswith("2026-05-19T12:30:45")
+        assert got.metadata["tags"] == ["repo", "MOC"]
+
+    def test_update_with_date_metadata(self, db: Database) -> None:
+        """update_document() must also handle date metadata (parallel call site)."""
+        doc = Document(
+            id=None,
+            path="/n.md",
+            title="N",
+            content="",
+            metadata={"tag": "a"},
+            file_mtime=0.0,
+            asset_type=AssetType.TEXT,
+        )
+        db.insert_document(doc)
+
+        loaded = db.get_document_by_path("/n.md")
+        assert loaded is not None
+        loaded.metadata = {"last_push": date(2026, 5, 19)}
+        db.update_document(loaded)
+
+        got = db.get_document_by_path("/n.md")
+        assert got is not None
+        assert got.metadata["last_push"] == "2026-05-19"