JSON vs XML vs YAML

August 23, 2026 · 15 min read

JSON won the public HTTP API war, but XML still powers enterprise integrations, SAML, and legacy SOAP. YAML dominates Kubernetes manifests and many CI configs. Choosing a format is choosing an ecosystem: schema tools, parser strictness, and how painful a merge conflict becomes.

Quick summary

JSON - minimal types, native in browsers, strict parsers, best for REST and webhooks.
XML - attributes, namespaces, XSD validation, best for document exchange and regulated industries.
YAML - human-friendly indentation, anchors (use carefully), best for config files ops teams edit by hand.

JSON: default for APIs

JSON maps cleanly to JavaScript objects and Python dicts. Numbers are doubles; no date or comment syntax in the spec - conventions handle dates as ISO strings. Validate with JSON Schema in CI and use a JSON validator before deploy.

XML: structure and contracts

XML shines when you need mixed content, digital signatures wrapped in standard ways, or industry schemas (HL7, UBL). Downsides: verbosity, namespace complexity, and XXE risks if parsers resolve external entities. For greenfield JSON APIs, reach for JSON to XML only at integration boundaries.

YAML: readable config with footguns

YAML allows comments and less punctuation, but implicit typing can surprise you (yes as boolean in YAML 1.1). Kubernetes and GitHub Actions standardized on YAML; many teams generate YAML from code instead of hand-editing. Converting API responses to config? Use JSON to YAML and review anchors and multi-line strings.

{ "service": "api", "replicas": 3, "env": { "LOG_LEVEL": "info" } }

Converting between formats

Lossless round trips are rare: XML attributes may not map 1:1 to JSON keys, and YAML timestamps become strings in JSON unless you preserve type metadata. Convert at documented boundaries with tests on sample files. UUID Studio provides JSON to XML and JSON to YAML in the browser for quick migrations and partner handoffs.

FAQ

Should new REST APIs use JSON or XML?: JSON unless a standard or partner contract requires XML.
Is YAML a superset of JSON?: JSON is often considered a subset of YAML 1.2, but parsers differ - do not assume identical behavior.
Why avoid YAML for public APIs?: Ambiguous typing and larger attack surface (billion laughs) make JSON safer on the wire.

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