How the decoder chooses the text
2026-01-15 · Captchas
Options constrain the reading: digits only, an exact length, or a sum.
You do not name a model in the request. The options choose the engine. math: true selects arithmetic. math plus hindi: true selects Hindi number words. Anything else uses the text engine.
The text engine then folds characters to match the options. numeric: true by itself keeps digits and turns look-alikes into digits, so O becomes 0 and S becomes 5. uppercase or lowercase alone folds the other case in. With none of those three, both cases are kept as read.
length asks for an exact length from 1 to 32. The decoder returns the best reading of that length, not a shorter string that scored well.
Unknown JSON fields are a 422. A batch can set options once and override them on a single item.
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