YAML vs TOML: Which Configuration Format Should You Choose?

Both YAML and TOML are human-readable configuration formats, but they take fundamentally different approaches. YAML uses indentation for structure; TOML uses INI-style headers and explicit key-value syntax. The right choice depends on your project, team, and ecosystem.

Syntax Comparison

The Same Configuration in Both Formats

YAML:

database:
  host: localhost
  port: 5432
  name: myapp
  ssl: true
  pool:
    min: 5
    max: 20
  
server:
  host: 0.0.0.0
  port: 8080
  workers: 4
  cors:
    origins:
      - https://example.com
      - https://api.example.com

TOML:

[database]
host = "localhost"
port = 5432
name = "myapp"
ssl = true

[database.pool]
min = 5
max = 20

[server]
host = "0.0.0.0"
port = 8080
workers = 4

[server.cors]
origins = [
  "https://example.com",
  "https://api.example.com"
]

Key Differences

Type Safety

YAML's Type Inference Problem

YAML aggressively infers types, causing subtle bugs:

version: 1.0     # Float, not string "1.0"
country: NO      # Boolean false, not string "NO"
port: '080'       # Octal number 64, not string "080"
on: true         # Boolean, not string "on"

TOML's Explicit Types

TOML requires explicit syntax, preventing type confusion:

version = "1.0"   # String (quotes required)
country = "NO"    # String (quotes required)
port = "080"      # String (quotes required)
on = true         # Boolean (unambiguous)

TOML has native types for dates and times:

created = 2024-01-15  # DateTime (native type)
date-only = 2024-01-15             # Date
time-only = 10-30-00               # Time

Ecosystem Support

YAML Dominates

• Docker Compose
• Kubernetes
• GitHub Actions
• Ansible
• CI/CD pipelines (CircleCI, GitLab CI, Travis CI)
• Cloud services (AWS CloudFormation, Azure)
• Most DevOps tools

TOML Is Growing

• Rust (Cargo.toml)
• Python (pyproject.toml, PEP 518)
• Go (go.mod uses custom format, but TOML for config)
• Hugo (site config)
• Netlify
• pip (pyproject.toml)

JSON Config (For Comparison)

• Node.js (package.json, tsconfig.json)
• VS Code settings
• ESLint, Prettier
• Most JavaScript tooling

When to Use YAML

• DevOps and infrastructure: Kubernetes, Docker, CI/CD — the ecosystem requires it
• Complex nested structures: YAML handles deep nesting more naturally
• Reuse with anchors: DRY configuration with anchors and aliases
• Team convention: If your team already uses YAML extensively

For YAML help, our YAML Formatter and YAML Validator are essential tools.

When to Use TOML

• Python projects: pyproject.toml is the standard
• Rust projects: Cargo.toml is the standard
• Simple configurations: Flat or shallow-nested settings
• Type safety matters: TOML's explicit quoting prevents type confusion
• New projects: If you are not constrained by ecosystem, TOML is safer for config files

When to Use Neither

• Machine-generated config: Use JSON (fastest to parse, strictest format)
• Application settings with schemas: Use JSON with JSON Schema validation
• Data exchange: Use JSON for APIs, CSV for tabular data

FAQ

Can I convert between YAML and TOML?

Yes, but with caveats. YAML features like anchors, aliases, and complex types have no TOML equivalent. Simple key-value and nested configurations convert cleanly. Tools like yj (YAML to JSON) combined with toml-cli handle basic conversions.

Will TOML replace YAML?

Unlikely. YAML is too deeply entrenched in the DevOps ecosystem (Kubernetes, Docker, CI/CD). TOML is gaining ground for application configuration (Rust, Python) where type safety matters more than deep nesting. The two formats will likely coexist, each dominating their respective ecosystems.

Related Resources

• YAML Formatter — Format YAML files
• YAML vs JSON — Another format comparison
• YAML Syntax Tutorial — Learn YAML fundamentals