How to Convert JSON to TOML

Published 2026-09-072 min readCopy link

You're adopting Rust or Python packaging conventions and want your config in Cargo.toml/pyproject.toml style — but your data starts life as JSON from an API or an export. Here's how to convert it cleanly.

Method 1: CLI

Using transmute

$ cat config.json | transmute --to toml
name = "my-app"
version = "2.0"
retries = 3

The format is auto-detected on input, so there are no flags to argue with. Nested objects become TOML sections ([server.tls]), scalars keep their types, and strings are escaped per the TOML spec.

Using dasel

$ dasel convert -r json -w toml < config.json

dasel handles many formats both ways. It's a solid alternative if you already have Go installed; otherwise a single static binary like Transmute is quicker to set up.

Method 2: Python

import json, tomli_w
data = json.load(open("config.json"))
with open("config.toml", "wb") as f:
    tomli_w.dump(data, f)

tomli_w writes TOML fast and correctly. One caveat: Python dicts must be converted to TOML-compatible types first — tuples work as arrays, sets don't.

Gotchas: JSON and TOML Are Not Equivalent

• Mixed-type arrays: TOML allows them (items = [1, "two"]) but some round-trips through other tools will coerce types.
• null: TOML has no null. A JSON null must become an omitted key, an empty string, or a sentinel value — pick deliberately.
• Top-level nesting depth: deeply nested JSON produces long chains of bracket-sections in TOML. If your structure nests more than ~3 levels, YAML may be the better target.
• Keys with dots: {"a.b": 1} needs quoting in TOML ("a.b" = 1) or it silently becomes section [a]. Good generators quote; naive ones corrupt.

• Scripts/CI: Transmute CLI — JSON in, spec-correct TOML out.
• Python projects: tomli_w for direct file writing.
• Quick checks: free web converter handles JSON ⇄ TOML locally in the browser.