How to Format, Validate, and Debug JSON on a Mac (Without Online Tools)

Format, validate, and debug JSON on Mac without online tools

1. Open JSON payload Load local file or paste JSON into native editor.
2. Pretty-print AST Format single-line JSON into structured indented tree.
3. Fix line/col errors Resolve RFC 8259 syntax violations using offset markers.
4. Compare AST diffs Review structural changes side-by-side.
5. Query with JSONPath Target specific values via JSONPath expressions.
6. Generate native DTOs Export validated AST into Swift or TypeScript models.

The example: a tiny JSON payload with annoying problems

Consider a minified API response containing common syntax violations:

{
  "user": {
    "id": 42,
    "name": "Maya",
    "email": "maya@example.com",
    "roles": ["admin", "editor",]
  },
  "active": true
  "lastLogin": "2026-07-18T09:30:00Z"
}

This payload contains two structural RFC 8259 violations:

• Trailing Comma: The roles array contains a trailing comma after "editor".
• Missing Key Separator: Missing comma after "active": true.

Step 1: Format minified JSON into readable AST structures

Pretty-printing formats minified single-line byte streams into structured token trees.

JsonXmlEditor for macOS detects JSON structures automatically and formats payloads with 2, 4, or 8 space indentation without sending data to web workers.

Step 2: Validate syntax errors down to line and column offsets

Native AST validators parse byte streams and jump directly to error line/column offsets.

Fixing reported trailing commas and missing key separators turns malformed text into valid RFC 8259 JSON:

{
  "user": {
    "id": 42,
    "name": "Maya",
    "email": "maya@example.com",
    "roles": [
      "admin",
      "editor"
    ]
  },
  "active": true,
  "lastLogin": "2026-07-18T09:30:00Z"
}

Step 3: Compare source with validated formatted output

Side-by-side diff views compare raw source bytes with pretty-printed AST output, highlighting structural changes instantly.

Step 4: Recursively unescape nested stringified JSON

Webhook payloads and message queues often embed JSON as escaped string primitives.

JsonXmlEditor auto-detects escaped JSON strings and unescapes nested objects into dedicated tabs for isolated AST inspection.

Step 5: Query values using JSONPath expressions

Instead of manually expanding large object trees, JSONPath expressions extract specific fields:

$.user.email

Or target array elements:

$.user.roles[0]

Step 6: Test API endpoints locally with integrated REST inspector

Test HTTP status codes, headers, and response payloads directly within one sandboxed macOS workspace.

Step 7: Generate native DTO models for Swift, TypeScript, and Python

Convert validated JSON AST trees into strongly typed DTO interfaces for Swift, TypeScript, Go, Java, and Python.