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TOML vs YAML: Which Config Format Should You Use?
Rust chose TOML. Kubernetes chose YAML. Python packaging chose TOML. Both formats are winning — just in different worlds. Here's an honest comparison and how to move between them.
The short answer
| TOML | YAML | |
|---|---|---|
| Spec size | Small, unambiguous | Huge, surprising corners |
| Deep nesting | Gets verbose fast | Clean indentation |
| Copy-paste safety | High | Indentation errors are silent bugs |
| Comments | Yes | Yes |
| Ecosystem home | Rust, Python packaging, Cargo | Kubernetes, CI/CD, Ansible, Docker Compose |
Where TOML wins
- No surprise semantics. In YAML, the "Norway problem" is famous:
no,offand country codes silently become booleans in some parsers. TOML has none of that. - Flat configs read beautifully.
pyproject.toml,Cargo.toml— key/value pairs with sections are what most config actually is.
Where YAML wins
- Deeply nested structures. A Kubernetes Deployment is 60+ lines in YAML; in TOML it becomes a wall of repeated bracket-sections.
- Everyone in DevOps knows it. Network effect matters more than elegance.
Converting between them
You rarely get to choose the format — you inherit it. When you need one in the shape of the other:
$ cat pyproject.toml | transmute --output yaml project: name: my-app version: "2.0"
Install the free CLI with npm i -g @mahope/transmute — it detects the input format automatically and converts between JSON, YAML, CSV and XML. Or paste into the in-browser demo on the Transmute page.
Recommendation
- New app config, shallow structure: TOML. Fewer foot-guns.
- Kubernetes-style manifests, deep nesting, ecosystem pressure: YAML. Fight the battle you're in.
- Need both? Keep sources in whichever, convert with Transmute when a consumer demands the other format.
Convert between JSON, YAML, CSV and XML
Transmute is a free, open-source CLI. Filter, sort, map and convert — nothing leaves your machine.
Get Transmute →