# JSON Mode Silently Stopped All 7 Open Models From Calling a Single Tool > Source: > Published: 2026-08-01 16:28:51+00:00 Member-only story # JSON Mode Silently Stopped All 7 Open Models From Calling a Single Tool Seven open-weight models. Seven suppression rates of exactly 100%. Tool invocation went from 100% to 0% the moment a JSON schema was attached to the same request, and not one line in any log said so. That is the finding in a June 2026 paper out of Focus Technology’s AI lab, and the first time I read it I assumed it was a prompt bug. It is not. It is arithmetic, and once you see the arithmetic you cannot unsee it: `response_format={"type":"json_schema"}` compiles your schema into a token bitmask, and the engine executes exactly one line that ends the argument. ``` logits[mask == 0] = -inf ``` Your model wanted to call a tool. The mask deleted the option before sampling ever ran. I spent this morning rebuilding the mask math from scratch to check whether the claim survives contact with a calculator. It does, and the numbers are worse than the paper’s headline. On a routine five-field extraction schema, **the grammar writes 72.4% of the response bytes** and the model does not get a single free choice until byte 17. We stopped calling this “generation” a while ago and nobody updated the mental model.