YAML vs TOON: Human-Readable Format Battle for LLM Optimization

YAML and TOON are both human-readable, indentation-based formats—but they serve different purposes. YAML prioritizes human editability with powerful features, while TOON prioritizes token efficiency for LLMs. Let's see how they compare for AI applications.

The Contenders

Syntax Comparison

Both formats use indentation, but with different philosophies.

YAML Example (156 bytes, ~80 tokens):

users:
  - id: 1
    name: Alice
    role: admin
  - id: 2
    name: Bob
    role: user
  - id: 3
    name: Charlie
    role: user

TOON Example (72 bytes, ~50 tokens - 38% reduction):

users[3]{id,name,role}:
  1, Alice, admin
  2, Bob, user
  3, Charlie, user

YAML repeats field names for each array item. TOON declares them once in the header.

The YAML Gotchas Problem

YAML is infamous for parsing surprises. Here are common issues that TOON avoids:

TOON has simpler, more predictable parsing rules—important for LLM reliability.

Feature Comparison

Array Handling: The Key Difference

For arrays of similar objects, TOON's tabular format provides massive savings.

YAML (100 products = ~3,200 tokens):

products:
  - id: 1
    name: Widget
    price: 29.99
    category: Electronics
  - id: 2
    name: Gadget
    price: 49.99
    category: Electronics
  # ... 98 more items with repeated keys

TOON (100 products = ~1,985 tokens - 38% savings):

products[100]{id,name,price,category}:
  1, Widget, 29.99, Electronics
  2, Gadget, 49.99, Electronics
  # ... compact rows continue

Performance Benchmarks

LLM Comprehension

Both formats are well-understood by modern LLMs:

• YAML: 99% accuracy on analytical tasks. LLMs are extensively trained on YAML.
• TOON: 98.5% accuracy. Slightly newer but equally understood for structured data.

The difference is negligible, but YAML's edge cases occasionally cause LLM confusion (e.g., the "Norway problem" where country code "NO" becomes false).

When to Use Which?

Stick with YAML if:

• You're working with Kubernetes, Docker Compose, or CI/CD configs.
• You need anchors and aliases for DRY configuration.
• Comments are essential for documentation.
• Multi-document support is required.
• Human editing is the primary use case.

Switch to TOON if:

• You're sending structured data to LLMs.
• Your data includes arrays of similar objects.
• Token costs are a significant concern.
• You want predictable, safe parsing.
• You're optimizing API response times.

Conversion Example

// YAML to TOON
const yamlData = yaml.parse(yamlString);
const toonString = toToon(yamlData);

// TOON to YAML
const toonData = fromToon(toonString);
const yamlString = yaml.stringify(toonData);

Final Verdict

YAML excels for DevOps configuration where human editing, comments, and anchors are essential. For LLM data interchange, TOON offers 38-51% token savings with simpler, more predictable parsing.

For advanced features like queries and schema validation, see our YAML vs TONL comparison. You can also explore API cost optimization or check our complete format comparison guide.