TOML Validator Online — Free TOML Syntax Checker

Validate Cargo.toml, pyproject.toml, and any TOML config file. Catch unterminated strings, malformed sections, and invalid values — line numbers included.

Input TOML

Validation Result

What is a TOML validator?

A TOML validator parses a TOML document and reports any errors that would prevent a real TOML parser — Cargo, Poetry, Hugo, or your own runtime library — from accepting the file. It catches unterminated strings, missing equals signs, malformed sections, invalid value forms, and unbalanced inline tables, with the exact line number for each issue.

TOML is intentionally strict: there's no ambiguity, no whitespace-sensitive structure, and no implicit type coercion. That strictness makes errors precise and easy to fix, but it also means a single typo (an unclosed quote, a missing bracket) takes down the whole file. The OpenFormatter validator runs entirely in your browser — no upload, no rate limits — and is built for engineers who edit Cargo.toml, pyproject.toml, and Hugo config every day.

How to validate TOML online — 4 steps

Paste your TOML. Copy a Cargo.toml, pyproject.toml, or any TOML config file into the Input panel. Click Load Sample to try a clean example.
Click Validate. The validator scans every line, balances brackets, and confirms each value matches a recognised TOML type.
Read the result. A green badge with section and key counts means well-formed TOML. A red error badge surfaces the offending line number and reason.
Fix and re-validate. Edit the line in your editor, paste back, and re-run until the file is clean. No upload, no waiting.

TOML errors the validator catches

1. Unterminated strings

A missing closing quote is the single most common TOML error.

# Invalid — missing closing quote
name = "OpenFormatter

# Valid
name = "OpenFormatter"

2. Malformed table headers

A table header must close its bracket on the same line.

# Invalid — missing ]
[package
name = "demo"

# Valid
[package]
name = "demo"

3. Invalid value types

Bare unquoted text isn't a TOML value.

# Invalid — bare word isn't a TOML value
license = MIT

# Valid
license = "MIT"

Spec-Aware Validation

Strings (basic and literal), integers (decimal/hex/octal/binary), floats, booleans, RFC3339 datetimes, arrays, and inline tables — every value is checked.

Precise Line Numbers

Each error reports the offending line and a one-line reason, so you can jump straight back to your editor and fix it without guessing.

Client-Side Only

Validation runs in JavaScript on your machine. Cargo.toml registry tokens and pyproject.toml secrets never leave the browser.

Common use cases

check_circleValidating Cargo.toml before cargo build to catch typos in dependencies
check_circleChecking pyproject.toml after manual edits (Poetry, Hatch, PDM, setuptools)
check_circleLinting Hugo config.toml or Zola config.toml before deploy
check_circleVerifying GitHub Actions toolchain pinning files like rust-toolchain.toml
check_circleCatching syntax errors in netlify.toml or wrangler.toml deploy configs
check_circleValidating Helix or Neovim plugin manager TOML files
check_circleChecking generated TOML from JSON (round-trip verification)
check_circlePre-commit hook replacement for projects that lack a TOML linter

Why client-side validation matters

TOML files frequently contain registry credentials (private crates.io tokens, JFrog Artifactory keys), private dependency Git URLs with embedded auth, deploy tokens for netlify.toml or wrangler.toml, and other secrets you really don't want sitting on a third-party server. The OpenFormatter validator never sends a single byte across the network — open DevTools → Network and confirm. The validator is a static page; the parsing happens entirely in JavaScript on your machine.

Need to do more with TOML?

Convert, format, and inspect TOML alongside the rest of the OpenFormatter toolkit.

TOML to JSON JSON to TOML YAML Validator

Frequently Asked Questions

What is TOML 1.0?

TOML 1.0.0 is the stable specification of Tom’s Obvious Minimal Language released in January 2021. It defines an unambiguous mapping from configuration text to a hash table, with explicit types for strings, integers, floats, booleans, datetimes, arrays, and tables. Cargo, Poetry, Hugo, and many other tools target TOML 1.0.

What does the validator check?

It checks well-formedness: each line is either blank, a comment, a [table] or [[array of tables]] header, or a key = value pair. Strings must be terminated, arrays must close, inline tables must balance braces, and values must match a recognised TOML type (string, number, datetime, boolean, array, inline table).

What are common TOML errors?

Top causes of TOML parse failures: (1) missing closing quote on a string, (2) trailing comma allowed inside arrays but causing odd parses elsewhere, (3) duplicate table definitions across the same path, (4) bare keys with disallowed characters, (5) array-of-table headers ([[ x ]]) confused with arrays of arrays inside a value.

How is dotted notation parsed?

A key like a.b.c = 1 creates implicit tables: { "a": { "b": { "c": 1 } } }. A header like [a.b] makes a, b explicit tables. The validator confirms that nothing redefines an already-existing table at a given path.

Why does my Cargo.toml fail validation?

The two most frequent culprits in Cargo.toml: an unquoted version string with characters Cargo accepts but TOML doesn’t, and an inline table like { version = "1", features = ["a", "b"], } where a stray newline appears inside the braces. TOML inline tables must be on a single line.

Does the validator check semantic correctness?

No — only well-formedness. Whether your Cargo.toml has the right [package] keys, whether dependency versions exist on crates.io, whether pyproject.toml conforms to PEP 621 — those are semantic checks beyond TOML syntax. Use cargo verify-project or a build-system-specific tool for that.

Can it handle pyproject.toml?

Yes. pyproject.toml files used by Poetry, PDM, Hatch, setuptools, and the PEP 518/621 specs all parse cleanly. Headers like [tool.poetry], [tool.poetry.dependencies], and [build-system] are all recognised correctly.

Is my TOML uploaded?

No. The validator runs entirely in JavaScript inside your browser. TOML files often contain registry tokens, deploy keys, or proprietary settings — open DevTools → Network and verify no request is sent when you click Validate.