TOML Formatter

What Is a TOML Formatter?

A TOML formatter parses valid TOML and rewrites it with consistent spacing, indentation, table layout, and line breaks while keeping the configuration's meaning intact. It is a TOML beautifier and pretty printer for configuration files, not a general-purpose text indent tool.

TOML is commonly used for files such as Cargo.toml and pyproject.toml. A dedicated formatter is needed because TOML is not JSON and is not the same as INI: it has typed values, tables, arrays of tables, and comments.

How to Use the TOML Formatter

1. Paste or upload TOML.
2. The tool checks syntax and shows Valid TOML or Invalid TOML.
3. Choose 2-space or 4-space indentation if needed.
4. Click Format to beautify valid input.
5. Review the formatted output.
6. Use Diff to see exactly what changed.
7. Copy or download the result.

If the input is invalid, fix the reported line and column, then validate again. The formatter does not invent a valid document from broken syntax.

Format vs Validate

Formatting and validating are separate actions. A TOML linter or syntax checker reports problems. A pretty printer rewrites valid source.

Format Cargo.toml Files

Cargo.toml is the TOML manifest used for Rust package and workspace configuration. A Cargo.toml formatter here checks TOML syntax and improves readability for tables such as [package], [dependencies], [[bin]], and [workspace].

This is not Cargo package validation. It does not verify crate names, feature flags, edition values, or workspace membership the way cargo does. Use it to format Cargo.toml and catch TOML grammar mistakes before you run Cargo.

Format pyproject.toml Files

pyproject.toml is a TOML configuration file used in modern Python project tooling. Typical tables include [build-system], [project], and [tool] namespaces.

A pyproject.toml validator on this page means TOML syntax validation. It does not replace Poetry, PDM, Hatch, Ruff, or pytest semantic checks. If those tools reject a file that parses as TOML, the problem is in the ecosystem schema, not the TOML grammar.

What Is TOML?

TOML means Tom's Obvious Minimal Language. It is a human-readable, case-sensitive, UTF-8 configuration format. A TOML parser reads key/value pairs, tables, arrays, inline tables, arrays of tables, strings, numbers, booleans, and date/time values.

• Tables use [table]. Nested tables stay nested; they are not flattened.
• Arrays of tables use [[products]]. That is not the same as [products].
• Inline tables use { key = "value" }.
• Dotted keys such as physical.color express nested structure.
• Comments begin with # outside strings. # inside a URL string is not a comment.

TOML Formatting Examples

# Input
[database]
host="localhost"
port=5432
enabled=true

# Formatted
[database]
host = "localhost"
port = 5432
enabled = true

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

[[products]]
name = "Hammer"
price = 19.99

[[products]]
name = "Nail"
price = 3.49

# Production API
url = "https://api.example.com"

TOML vs JSON and YAML

Use a TOML-specific formatter when the file is TOML. JSON and YAML tools will misread comments, dates, and table syntax.

If the file is JSON-shaped data, continue with the JSON Formatter. Use the YAML Formatter when the configuration is YAML rather than TOML. To compare two revisions as plain text, open the Text Diff Tool.

Common TOML Formatting Use Cases

• Rust Cargo projects and workspaces
• Python projects that use pyproject.toml
• CI and development-tooling configuration written in TOML
• Static-site configuration such as Hugo config.toml
• CLI and application configuration
• Cleaning generated TOML before a pull request
• Reviewing formatting changes before commit

Common TOML Formatting and Syntax Mistakes

• Duplicate keys — the same key cannot appear twice in one table.
• Duplicate table definitions — [server] cannot be opened twice as a standard table.
• Invalid table headers — missing ] or mixing [table] with [[table]].
• Malformed arrays — missing commas or an unclosed [.
• Unclosed strings — basic, literal, and multiline strings must terminate.
• Inline tables vs tables — point = { x = 1 } is not [point].
• Comments mistaken for values — # starts a comment only outside strings.
• Invalid date/time syntax — offset and local date-times follow the TOML 1.0 forms.
• Quoted dotted keys — site."google.com" is one dotted path, not a hostname rewrite.
• Array-of-table syntax — changing [[products]] to [products] changes meaning.
• Assuming Cargo or Poetry validation — TOML syntax is not package metadata validation.

TOML Formatter Limitations

This tool validates TOML syntax. It does not automatically validate Cargo semantics, Python package metadata, Poetry or PDM rules, Ruff settings, pytest options, Deno configuration, or Hugo-specific keys unless those validators are implemented separately. They are not implemented here.

JSON conversion drops comments because JSON has no standard comment syntax. Date values are serialized as ISO-8601 strings. For JSON documents that fail to parse, use the JSON Validator.

Formatter, Beautifier, Validator, and Parser

People search for a TOML formatter online, a TOML beautifier, a pretty printer, a TOML validator, a linter, or a syntax checker. On this page those names describe one browser workflow: parse the document, report grammar errors with line and column, then optionally emit formatted TOML. Format TOML online here without uploading the file. The same parser is used for Cargo.toml and pyproject.toml because both are TOML documents.

TOML Formatter FAQ