TOML Formatter & Validator Online: Pretty-Print and Check Cargo.toml, pyproject.toml

What is TOML?

TOML is a human-readable config format: explicit tables, clear nesting, strings, numbers, booleans, dates, and arrays. It avoids some of YAML's ambiguity and reads well in code review. That's why Rust, Python packaging, and many static-site and deploy tools default to TOML for project metadata.

Common files that use TOML

TOML shows up most often in project configuration, build metadata, and tool settings. If a file is shared across a team, a formatter helps keep diffs small and predictable. If a file controls packaging or builds, a validator helps catch syntax mistakes before a command fails later in CI.

Formatter vs validator

A TOML validator answers: "Is this file valid TOML?"—mismatched brackets, bad strings, or illegal keys show up as parse errors. A TOML formatter (or beautifier / pretty-printer) takes valid TOML and rewrites it with consistent indentation and structure so diffs are easier to read. Spoold does both: invalid input surfaces an error; valid input is formatted on the right as you type.

Example TOML snippets to format

A formatter is easiest to understand with real project snippets. Paste either example into the tool to see how nested tables, arrays, strings, and dependency lists are normalized.

[package]
name = "demo"
version = "0.1.0"
edition = "2021"

[dependencies]
serde = { version = "1", features = ["derive"] }
tokio = { version = "1", features = ["full"] }

[project]
name = "demo"
version = "0.1.0"
requires-python = ">=3.11"

[tool.ruff]
line-length = 100
select = ["E", "F", "I"]

Why use an online TOML formatter?

• Paste and fix — Quickly normalize a snippet from a ticket, chat, or log before dropping it into a project.
• Catch typos early — A missing quote or bracket fails fast instead of at build time.
• Teach and review — Pretty-printed TOML is easier for teammates to scan than one long line.

Using the Spoold TOML Formatter & Validator

Open the TOML Formatter & Validator, paste your file in the input panel, and watch the formatted output. The status area shows valid vs invalid. Copy the result when you're done. Everything runs client-side in your browser.

Common TOML validation errors

TOML is readable, but it is strict. A small typo can change a table path or stop parsing completely. When validation fails, look near the reported line first, then scan the few lines above it. Missing closing brackets, unescaped quotes, duplicate keys, and accidental arrays are common causes.

• Duplicate keys — A key can only be defined once in the same table. Move overrides into the correct table instead of repeating a key later.
• Wrong table nesting — [tool.ruff] and [tool.ruff.lint] are different table paths.
• Broken strings — Quotes inside a string must be escaped or written with an appropriate literal string style.
• Mixed array values — Keep arrays consistent and check trailing commas when copying dependency lists from docs.

When to use a project-specific tool instead

A general TOML validator can tell you whether the file syntax is valid. It cannot always tell you whether the values make sense for Cargo, Poetry, Ruff, Taplo, Hugo, or another tool. After formatting, still run the project command that owns the file, such as cargo check, python -m build, or your normal CI validation. Think of the online formatter as the first quick pass, not the final semantic check.

JSON ↔ TOML

If you need to convert between JSON and TOML (not just format), use the JSON ↔ TOML converter — bidirectional conversion with auto-detect. The formatter page is for TOML-in, TOML-out pretty-printing and validation only.

FAQ

Related tools on Spoold

Working across formats? Pair TOML with the JSON Formatter, JSON ↔ YAML, or XML Formatter when your stack mixes config types.