About this tool
Paste YAML and this tool walks the indentation structure, reporting the line and column where it breaks. When nothing is broken it does more than say "valid": it collects the places where YAML quietly bites. Tabs mixed into indentation, a key written twice in the same mapping, unquoted yes, no, on and off, numbers with a leading zero, and clock-like values such as 12:30.
Tabs are an error because the YAML specification forbids them in indentation. Duplicate keys are reported with both the first line and the repeated line, since parsers usually keep only the last value without complaining. yes and no are booleans under YAML 1.1 and strings under YAML 1.2, the current specification as of October 2026, so you can pick which reading to apply. Aliases with no matching anchor are flagged, and block scalars are counted as literal or folded.
The checked scope covers indentation-based block mappings and sequences. Flow collections written inline with braces or brackets, complex keys, tags and quoted strings spanning several lines are not inspected, so this is not a full specification parser. It does not convert values to real types and does not turn YAML into JSON.
Frequently asked questions
The YAML specification does not allow tabs for indentation. In an editor a tab looks the same as spaces, so it is hard to spot by eye, yet a parser fails the moment it reads one. That is why it is reported as an error with the exact column.
YAML 1.1 reads yes, no, on and off as booleans while YAML 1.2 reads them as strings. The same file can therefore mean different things to different libraries, so quote the value when you want the literal word.
It does not convert to JSON or validate against a schema, and it does not look inside flow collections. It checks indentation structure and common traps only.