fix: preserve permalink when editing notes without frontmatter permalink

Fixes issue #170 where edit_note would fail with EntityResponse validation error
"Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]"
even though the edit operation succeeded.

Root cause: The entity_model_from_markdown function was unconditionally overwriting
the permalink field with markdown.frontmatter.permalink, even when it was None.
This caused existing entities to lose their permalinks when edited.

Changes:
- Only update permalink from frontmatter if it's not None, preserving existing permalink
- Add regression test to prevent this issue from recurring

The actual edit operation worked fine, but the API response validation failed because
EntityResponse.permalink is required but was being set to None.

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

Author: claude[bot]

src/basic_memory/markdown/utils.py

@@ -38,7 +38,9 @@ def entity_model_from_markdown(
     # Update basic fields
     model.title = markdown.frontmatter.title
     model.entity_type = markdown.frontmatter.type
-    model.permalink = markdown.frontmatter.permalink
+    # Only update permalink if it exists in frontmatter, otherwise preserve existing
+    if markdown.frontmatter.permalink is not None:
+        model.permalink = markdown.frontmatter.permalink
     model.file_path = str(file_path)
     model.content_type = "text/markdown"
     model.created_at = markdown.created

tests/mcp/test_tool_edit_note.py

@@ -359,3 +359,42 @@ async def test_edit_note_find_replace_empty_find_text(client):
     assert isinstance(result, str)
     assert "# Edit Failed" in result
     # Should contain helpful guidance about the error
+
+
+@pytest.mark.asyncio
+async def test_edit_note_preserves_permalink_when_frontmatter_missing(client):
+    """Test that editing a note preserves the permalink when frontmatter doesn't contain one.
+    
+    This is a regression test for issue #170 where edit_note would fail with a validation error
+    because the permalink was being set to None when the markdown file didn't have a permalink
+    in its frontmatter.
+    """
+    # Create initial note
+    await write_note.fn(
+        title="Test Note",
+        folder="test",
+        content="# Test Note\nOriginal content here.",
+    )
+
+    # Verify the note was created with a permalink
+    first_result = await edit_note.fn(
+        identifier="test/test-note",
+        operation="append",
+        content="\nFirst edit.",
+    )
+    
+    assert isinstance(first_result, str)
+    assert "permalink: test/test-note" in first_result
+    
+    # Perform another edit - this should preserve the permalink even if the 
+    # file doesn't have a permalink in its frontmatter
+    second_result = await edit_note.fn(
+        identifier="test/test-note",
+        operation="append", 
+        content="\nSecond edit.",
+    )
+
+    assert isinstance(second_result, str)
+    assert "Edited note (append)" in second_result
+    assert "permalink: test/test-note" in second_result
+    # The edit should succeed without validation errors