Validate TOML Syntax & Get Error Line Numbers Instantly
Validate TOML configuration file syntax and report errors with line numbers. Paste any TOML content — Cargo.toml, pyproject.toml, config.toml — and instantly see a green checkmark with key counts and structure stats, or a precise error message pointing to the exact line. Includes a collapsible JSON structure preview to confirm what was parsed.
How to Use TOML Config Validator
How to Use the TOML Config Validator
Step 1: Paste Your TOML
Paste any TOML configuration content into the input panel. Validation runs instantly as you type.
Valid example:
[server]
host = "localhost"
port = 8080
debug = false
Result: green Valid TOML confirmation with key and section counts.
Invalid example:
[server]
host = "localhost"
port = 8080
host = "0.0.0.0" # duplicate key — error!
Result: red error panel showing Line 4 and the message "Duplicate key 'host'"
Step 2: Read the Error Message
When validation fails, the error panel shows:
- Line number — the exact line where parsing failed
- Error message — a human-readable description of the problem
Common errors and their causes:
| Error message | Cause |
|---|---|
| Duplicate key | Same key defined twice in the same table |
| Unterminated string | String opened with " but not closed |
| Newline inside string | Use """ for multi-line strings |
| Expected = after key | Missing equals sign after a key name |
| Expected ]] | Missing closing brackets on array-of-tables |
| Cannot redefine as array | Key already defined as table, not array |
Step 3: View Stats and Structure (on Valid TOML)
When validation passes, the tool shows:
- Total Keys — number of key-value pairs in the file
- Tables — number of
[table]sections - Array Tables — number of
[[array]]entries - Strings / Numbers / Booleans / Datetimes / Arrays — type breakdown
Click Show structure preview (JSON) to see the fully parsed TOML rendered as formatted JSON. This confirms that complex structures like dotted keys and array-of-tables were parsed as expected.
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