fix(core): fail prepend on malformed frontmatter

Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez

src/basic_memory/services/entity_service.py

@@ -1262,33 +1262,30 @@ def insert_relative_to_section(
     def _prepend_after_frontmatter(self, current_content: str, content: str) -> str:
         """Prepend content after frontmatter, preserving frontmatter structure."""
 
-        # Check if file has frontmatter
+        # Trigger: the note starts with frontmatter delimiters.
+        # Why: prepend must preserve the existing YAML block and insert content into the body,
+        #      not silently rewrite malformed metadata into a corrupted accepted note state.
+        # Outcome: valid frontmatter is preserved, and malformed frontmatter fails fast.
         if has_frontmatter(current_content):
-            try:
-                # Parse and separate frontmatter from body
-                frontmatter_data = parse_frontmatter(current_content)
-                body_content = remove_frontmatter(current_content)
-
-                # Prepend content to the body
-                if content and not content.endswith("\n"):
-                    new_body = content + "\n" + body_content
-                else:
-                    new_body = content + body_content
+            # Parse and separate frontmatter from body. Parse errors are intentional caller-visible
+            # failures so prepare_edit_entity_content can reject unsafe accepted writes.
+            frontmatter_data = parse_frontmatter(current_content)
+            body_content = remove_frontmatter(current_content)
+
+            # Prepend content to the body
+            if content and not content.endswith("\n"):
+                new_body = content + "\n" + body_content
+            else:
+                new_body = content + body_content
 
-                # Reconstruct file with frontmatter + prepended body
-                yaml_fm = yaml.dump(frontmatter_data, sort_keys=False, allow_unicode=True)
-                return f"---\n{yaml_fm}---\n\n{new_body.strip()}"
+            # Reconstruct file with frontmatter + prepended body
+            yaml_fm = yaml.dump(frontmatter_data, sort_keys=False, allow_unicode=True)
+            return f"---\n{yaml_fm}---\n\n{new_body.strip()}"
 
-            except Exception as e:  # pragma: no cover
-                logger.warning(
-                    f"Failed to parse frontmatter during prepend: {e}"
-                )  # pragma: no cover
-                # Fall back to simple prepend if frontmatter parsing fails  # pragma: no cover
-
-        # No frontmatter or parsing failed - do simple prepend  # pragma: no cover
-        if content and not content.endswith("\n"):  # pragma: no cover
-            return content + "\n" + current_content  # pragma: no cover
-        return content + current_content  # pragma: no cover
+        # No frontmatter means prepend is a plain text edit.
+        if content and not content.endswith("\n"):
+            return content + "\n" + current_content
+        return content + current_content
 
     async def move_entity(
         self,

tests/services/test_entity_service_prepare.py

@@ -4,7 +4,7 @@
 
 import pytest
 
-from basic_memory.file_utils import parse_frontmatter
+from basic_memory.file_utils import ParseError, parse_frontmatter, remove_frontmatter
 from basic_memory.schemas import Entity as EntitySchema
 
 
@@ -128,3 +128,82 @@ async def test_prepare_edit_entity_content_matches_edit_entity_with_content(
     assert prepared.entity_fields["title"] == result.entity.title
     assert prepared.entity_fields["note_type"] == result.entity.note_type
     assert prepared.entity_fields["permalink"] == result.entity.permalink
+
+
+@pytest.mark.asyncio
+async def test_prepare_edit_entity_content_prepend_preserves_valid_frontmatter(
+    entity_service,
+    file_service,
+) -> None:
+    created = await entity_service.create_entity(
+        EntitySchema(
+            title="Prepared Prepend Frontmatter",
+            directory="notes",
+            note_type="note",
+            content="---\nstatus: draft\ntags:\n  - one\n---\nOriginal body",
+        )
+    )
+
+    current_content = await file_service.read_file_content(created.file_path)
+    prepared = await entity_service.prepare_edit_entity_content(
+        created,
+        current_content,
+        operation="prepend",
+        content="Prepended line",
+    )
+
+    assert parse_frontmatter(prepared.markdown_content) == {
+        "title": "Prepared Prepend Frontmatter",
+        "type": "note",
+        "status": "draft",
+        "tags": ["one"],
+        "permalink": created.permalink,
+    }
+    assert remove_frontmatter(prepared.markdown_content) == "Prepended line\nOriginal body"
+
+
+@pytest.mark.asyncio
+async def test_prepare_edit_entity_content_prepend_fails_for_malformed_frontmatter(
+    entity_service,
+) -> None:
+    created = await entity_service.create_entity(
+        EntitySchema(
+            title="Prepared Prepend Parse Error",
+            directory="notes",
+            note_type="note",
+            content="Original body",
+        )
+    )
+
+    malformed_content = "---\nstatus: [draft\n---\nOriginal body"
+
+    with pytest.raises(ParseError, match="Invalid YAML in frontmatter"):
+        await entity_service.prepare_edit_entity_content(
+            created,
+            malformed_content,
+            operation="prepend",
+            content="Prepended line",
+        )
+
+
+@pytest.mark.asyncio
+async def test_prepare_edit_entity_content_prepend_without_frontmatter_uses_simple_prepend(
+    entity_service,
+) -> None:
+    created = await entity_service.create_entity(
+        EntitySchema(
+            title="Prepared Prepend Simple",
+            directory="notes",
+            note_type="note",
+            content="Original body",
+        )
+    )
+
+    prepared = await entity_service.prepare_edit_entity_content(
+        created,
+        "Original body",
+        operation="prepend",
+        content="Prepended line",
+    )
+
+    assert prepared.markdown_content == "Prepended line\nOriginal body"