TOON vs JSON / YAML / XML / TOML / CSV: The Ultimate Comparison

In software engineering, we often treat data serialization formats as a matter of taste. Python developers like YAML. Rust developers like TOML. Web developers like JSON. But in the era of Generative AI, format choice is no longer just about preference—it is about Mathematics.

Every bracket, every quote, and every trailing comma consumes a token. When you are paying $10/million tokens, or trying to fit a 200,000-word codebase into a context window, "Verbose" means "Expensive" and "Inefficient."

This guide provides a definitive, unbiased comparison of the six major data formats: JSON, YAML, XML, TOML, CSV, and TOON.

The Decision Matrix

Deep Dive: The Formats

1. JSON (JavaScript Object Notation)

The King of the Web. JSON won the API wars because it map 1:1 to JavaScript objects. It is simple, predictable, and supported by every language on Earth.

The LLM Problem: JSON is repetitive.

[
  {"id": 1, "name": "Alice"},
  {"id": 2, "name": "Bob"}
]

The strings `"id"` and `"name"` are repeated for every single row. For a list of 1,000 users, you pay for those keys 1,000 times. That is wasted money.

2. YAML (YAML Ain't Markup Language)

The King of Config. YAML is beautiful to read. It uses indentation (like Python) instead of brackets.

- id: 1
  name: Alice
- id: 2
  name: Bob

The LLM Problem: While cleaner than JSON, it still repeats the keys (`id`, `name`) for every item. Also, YAML specification is famously complex (the "Norway Problem" where `NO` is parsed as `false` for boolean No). This ambiguity confuses smaller LLMs.

3. XML (eXtensible Markup Language)

The King of Enterprise. XML is rigorous. It supports schemas, attributes, namespaces, and validation.

<user>
  <id>1</id>
  <name>Alice</name>
</user>

The LLM Problem: Closing tags. Writing </name> conveys zero new information but costs tokens. XML is usually 2-3x larger than JSON. Use it only if you are forced to.

4. TOML (Tom's Obvious, Minimal Language)

The King of Rust/Python. TOML is designed to be mapped to a Hash Table. It is great for setting files (`Cargo.toml`, `pyproject.toml`).

[[user]]
id = 1
name = "Alice"

[[user]]
id = 2
name = "Bob"

The LLM Problem: The "Array of Tables" syntax (`[[user]]`) is incredibly verbose if you have a list of small objects. It repeats the header for every item.

5. CSV (Comma-Separated Values)

The King of Data. CSV is brutally efficient.

id,name
1,Alice
2,Bob

The LLM Problem: No structure. You can't nest an address inside a user. You can't have a list of roles. It is purely 2D. Great for dataframes, bad for trees.

6. TOON (Token-Oriented Object Notation)

The Native Format for AI. TOON was designed in 2024 to solve exactly these problems.

• It uses Indentation (like YAML) to avoid brackets.
• It uses Headers (like CSV) to avoid repeated keys.
• It supports Nesting (like JSON) to represent complex data.

users[2]{id, name}:
  1, Alice
  2, Bob

It is as compact as CSV, but as flexible as JSON.

Cost Comparison: The Million Request Scenario

Let's assume you are building a RAG application. You fetch 50 products from your database and send them to GPT-4o for ranking.

Payload: 50 products, 10 fields each.

Result: Switching to TOON saves 52% on your API bill instantly.

Conclusion: Choose the Right Tool

Format wars are over. The answer is "It depends."

• For Humans: Use YAML. It is easy to write.
• For Browsers: Use JSON. It is native.
• For LLMs: Use TOON. It is efficient.

The modern AI stack is hybrid. You store data in a database (SQL), you configure your app in YAML, your frontend speaks JSON, and your backend converts that JSON to TOON before talking to OpenAI.