PATCH append operation fails when appending string to existing array frontmatter

[otaviocc]

Problem Description

When using the PATCH operation with append to add a single item to an existing frontmatter array, the API returns a 500 error: "Cannot merge objects of different types or unsupported types: object and string".

Expected Behavior

The append operation should intelligently handle different frontmatter field types:

• Array fields: Append the new value as an additional array item
• String fields: Concatenate the new value to the existing string

Current Behavior

The API fails when trying to append a string value to an existing array field.

Reproduction Steps

1. Setup: Create a note with existing frontmatter array:

   ---
   tags: [existing-tag, another-tag]
   ---

2. Attempt to append: Use PATCH to append a single tag:

   curl -X PATCH \
       "http://127.0.0.1:27123/active/" \
       -H "Authorization: Bearer $OBSIDIAN_API_KEY" \
       -H "Operation: append" \
       -H "Target-Type: frontmatter" \
       -H "Target: tags" \
       -H "Content-Type: application/json" \
       -d '"new-tag"'

3. Result: 500 error instead of successful append

Error Response

{
  "message": "Internal Server Error\nCannot merge objects of different types or unsupported types: object and string",
  "errorCode": 50000
}

Proposed Solution

The API should detect the target field type and handle append operations accordingly:

For Array Fields:

# Before
tags: [foo, bar]

# After appending "baz"  
tags: [foo, bar, baz]

For String Fields:

# Before
title: "Hello"

# After appending " World"
title: "Hello World"

Use Cases

This enhancement would enable:

• Tag management: append individual tags without needing to read existing tags first
• Attendee lists: Add people to meeting notes incrementally
• Category arrays: Build category lists over time
• General frontmatter: Any array-based metadata management

Workaround

Currently, clients must:

1. GET the note to read existing frontmatter
2. Parse the existing array
3. Add the new item to the array
4. PUT/PATCH the complete updated array

This creates unnecessary complexity and multiple API calls for a simple append operation.

Additional Context

This issue affects any automation or integration that wants to incrementally build frontmatter arrays, which is a common use case for note management workflows.

Thanks for building this awesome API! It's been incredible for implementing cool automation and integration projects. Keep up the great work! 🎉

[coddingtonbear]

I've moved this over to "Ideas" since this is a feature request rather than a bug.

I think frontmatter handling could definitely be more refined, and I'd welcome a pull request adding better frontmatter handling!