fix: handle malformed YAML in update_frontmatter gracefully

Fixes #378

- Changed error handling in update_frontmatter() to match PR #368 pattern
- YAML parsing errors now logged as WARNING instead of ERROR
- Files with malformed frontmatter treated as plain markdown
- Added test for malformed YAML case (KB: title format)

This resolves 1,112 errors/3hrs in production logs from files with
colons in titles that confuse the YAML parser.

Co-authored-by: Paul Hernandez <phernandez@users.noreply.github.com>

src/basic_memory/file_utils.py

@@ -201,7 +201,6 @@ async def update_frontmatter(path: FilePath, updates: Dict[str, Any]) -> str:
 
     Raises:
         FileError: If file operations fail
-        ParseError: If frontmatter parsing fails
     """
     try:
         # Convert string to Path if needed
@@ -210,11 +209,20 @@ async def update_frontmatter(path: FilePath, updates: Dict[str, Any]) -> str:
         # Read current content
         content = path_obj.read_text(encoding="utf-8")
 
-        # Parse current frontmatter
+        # Parse current frontmatter with error handling for malformed YAML (issue #378)
         current_fm = {}
         if has_frontmatter(content):
-            current_fm = parse_frontmatter(content)
-            content = remove_frontmatter(content)
+            try:
+                current_fm = parse_frontmatter(content)
+                content = remove_frontmatter(content)
+            except (ParseError, yaml.YAMLError) as e:
+                # Log warning and treat as plain markdown (same pattern as PR #368)
+                logger.warning(
+                    f"Failed to parse YAML frontmatter in {path_obj}: {e}. "
+                    "Treating file as plain markdown without frontmatter."
+                )
+                # Keep full content, treat as having no frontmatter
+                current_fm = {}
 
         # Update frontmatter
         new_fm = {**current_fm, **updates}
@@ -229,11 +237,13 @@ async def update_frontmatter(path: FilePath, updates: Dict[str, Any]) -> str:
         return await compute_checksum(final_content)
 
     except Exception as e:  # pragma: no cover
-        logger.error(
-            "Failed to update frontmatter",
-            path=str(path) if isinstance(path, (str, Path)) else "<unknown>",
-            error=str(e),
-        )
+        # Only log real errors as ERROR (not YAML parsing errors which are already handled)
+        if not isinstance(e, (ParseError, yaml.YAMLError)):
+            logger.error(
+                "Failed to update frontmatter",
+                path=str(path) if isinstance(path, (str, Path)) else "<unknown>",
+                error=str(e),
+            )
         raise FileError(f"Failed to update frontmatter: {e}")
 
 

tests/utils/test_file_utils.py

@@ -259,6 +259,42 @@ async def test_update_frontmatter_errors(tmp_path: Path):
         await update_frontmatter(nonexistent, {"title": "Test"})
 
 
+@pytest.mark.asyncio
+async def test_update_frontmatter_malformed_yaml(tmp_path: Path):
+    """Test that malformed YAML frontmatter is handled gracefully (issue #378)."""
+    test_file = tmp_path / "test.md"
+
+    # Create file with malformed YAML frontmatter (colon in title breaks YAML)
+    malformed_content = """---
+title: KB: Something
+---
+# Test Content
+
+Some content here"""
+    test_file.write_text(malformed_content, encoding="utf-8")
+
+    # Update frontmatter should succeed by treating file as plain markdown
+    updates = {"type": "note", "tags": ["test"]}
+    checksum = await update_frontmatter(test_file, updates)
+
+    # Verify the update succeeded
+    assert checksum is not None
+    updated = test_file.read_text(encoding="utf-8")
+
+    # Should have new frontmatter
+    assert "type: note" in updated
+    assert "tags:" in updated
+    assert "- test" in updated
+
+    # Original content should be preserved
+    assert "Test Content" in updated
+    assert "Some content here" in updated
+
+    # Verify the new frontmatter is valid
+    fm = parse_frontmatter(updated)
+    assert fm == {"type": "note", "tags": ["test"]}
+
+
 @pytest.mark.asyncio
 def test_sanitize_for_filename_removes_invalid_characters():
     # Test all invalid characters listed in the regex