TOML Validator: Check Syntax and See the Parsed Structure
TOML's rules are stricter than YAML's in a few key ways — every string must be quoted, and table nesting is explicit via [section.path] headers rather than indentation. This validator checks your TOML against those rules and shows you exactly how it resolves into a structured object, so you can confirm it means what you think it means.
Bare (unquoted) string values are invalid TOML — every string needs quotes.
TOML's Strictness Is the Point
Unlike YAML, where an unquoted 'yes' can silently become a boolean and trip up config parsers, TOML requires every string to be explicitly quoted and every value's type to be unambiguous from its syntax alone. That strictness is a deliberate design goal — TOML aims to eliminate an entire category of config-parsing surprises.
Table Headers vs. Indentation
Where YAML nests structure through indentation, TOML uses explicit [section] headers, and dotted paths like [servers.alpha] nest a table inside another table by name. This makes TOML files flatter to read but means getting the header path exactly right matters — a typo in [servers.alpah] silently creates a different, unintended table.
Arrays of Tables: [[ ]] vs. [ ]
A single [table] header can only be declared once per path. [[array.of.tables]] is different — each occurrence appends a new entry to an array at that path, which is how TOML represents a repeated structure, like multiple [[servers]] blocks each becoming one element in a servers array.
Practical Examples
Catching an Unquoted String
A common mistake porting config from another format.
- 1.Input: name = Tom
- 2.Error: Unrecognized or unquoted value: "Tom"
- 3.Fix: name = "Tom"
Building Nested Tables
Representing structured server configs.
- 1.[servers.alpha]
- 2.ip = "10.0.0.1"
- 3.Parses to: { servers: { alpha: { ip: "10.0.0.1" } } }
What Gets Validated
- Quoted strings: bare words are flagged
- [table] and [[array of tables]] headers
- Dotted keys: a.b.c = value
- Arrays and inline tables: { k = v }
- Numbers and booleans: type-checked
Good Use Cases
- Validating a Cargo.toml or pyproject.toml before committing
- Debugging a TOML config that a tool refuses to load
- Learning TOML syntax with an immediate JSON preview
- Converting a mental model from YAML/JSON to TOML's stricter rules
Frequently Asked Questions
Where is TOML commonly used?
Cargo.toml for Rust projects, pyproject.toml for Python packaging, and configuration files for tools like Hugo, Prettier alternatives, and various CLI apps — it's designed to be an easy-to-read config format that maps unambiguously to a hash table.
Why does the validator say a value is 'unquoted'?
TOML requires string values to be quoted — unlike YAML, a bare word like value = hello is invalid TOML syntax. It must be value = "hello". This is one of the most common mistakes when converting a YAML or JSON config to TOML by hand.
What's the difference between [table] and [[array of tables]]?
[table] declares (or re-opens) a single table at that path. [[array.of.tables]] appends a new table to an array at that path — using it twice with the same name creates two separate table entries in an array, which is how TOML represents repeated structured entries like multiple [[servers]] blocks.
Does this support multi-line strings?
This validator supports standard single-line quoted strings. Triple-quoted multi-line strings ("""...""") aren't parsed by this simplified validator — a known limitation for a best-effort browser-based tool.
How are dates handled?
TOML has native date/datetime types (like 1979-05-27). This validator recognizes the common ISO 8601 date pattern and keeps the value as a string in the JSON preview, rather than fully parsing it into a date object.
Is my TOML content sent anywhere?
No, parsing and validation happen entirely in your browser using a bundled TOML parser — nothing is uploaded.