The Short Answer
Use JSON when
- arrow_rightData is consumed by machines (APIs, SDKs)
- arrow_rightYou need strict type safety
- arrow_rightThe payload is programmatically generated
- arrow_rightComments are not needed
- arrow_rightInteroperability across languages is critical
Use YAML when
- arrow_rightHumans write and maintain the file
- arrow_rightComments are essential (CI/CD configs)
- arrow_rightMulti-line strings appear often
- arrow_rightThe file is a config, not a data exchange
- arrow_rightReadability trumps strict parsing
The Readability Argument
YAML's syntax is designed for human authors. No curly braces, no required quotes around strings, no trailing commas to worry about. A Kubernetes deployment manifest in YAML reads almost like documentation. The equivalent in JSON requires significantly more syntactic noise.
YAML
name: web-server replicas: 3 image: nginx:1.25 ports: - 80 - 443 # Production only env: prod
JSON
{
"name": "web-server",
"replicas": 3,
"image": "nginx:1.25",
"ports": [80, 443],
"env": "prod"
}The YAML version is shorter and supports comments. The JSON version is unambiguous and requires no special parser configuration.
Where YAML Gets Dangerous
YAML's flexibility is also its biggest liability. Several behaviors routinely cause production incidents:
JSON's Constraints Are Features
JSON's lack of comments, its strict quoting requirements, and its limited type system are often cited as weaknesses. In practice, they're often strengths:
No comments means no documentation drift — comments in config files get stale and mislead. Strict quoting means no implicit type coercion. The limited type system (string, number, boolean, null, array, object) maps cleanly to every programming language without surprises.
JSON is also the lingua franca of the modern web. Every language has a JSON parser. Every API returns JSON. Every database can serialize to JSON. The ubiquity is a real advantage.
See What is JSON? for the fuller history of why it overtook XML for most APIs in the first place.
The 2024 Landscape
In 2024, the default choice has crystallized:
CI/CD and infrastructure config (GitHub Actions, Kubernetes, Helm, Docker Compose, Ansible) is overwhelmingly YAML. The tooling ecosystem expects it.
API design, data exchange, and SDK payloads are overwhelmingly JSON. OpenAPI/Swagger supports both but most teams default to JSON. gRPC uses Protocol Buffers, not either.
Application config varies by ecosystem: Node.js uses JSON and JSONC, Python prefers TOML (via pyproject.toml) or YAML, Go often uses YAML or TOML.
Practical rule: If the file is written by a human and lives in a repository, YAML is fine. If the file is generated or consumed by code, use JSON.
Convert YAML ↔ JSON
Instantly convert between formats — preserves types, handles anchors.