WASHINGTON — Federal restrictions on Anthropic's two most powerful models lifted Tuesday after the company agreed to "proactively detect and address security risks," a commitment engineers later confirmed was implemented by adding the line "don't do bad stuff" to the model's system harness.

The block had been in place since mid-June, when the government ordered Anthropic to disable Mythos 5 and Fable 5 for all foreign nationals, a category drawn so broadly it locked out Anthropic's own foreign-national employees, including people who had shipped the models the week before. The stated cause was a jailbreak. The government's supporting evidence, per Anthropic, was "verbal evidence of a potential narrow, non-universal jailbreak," which is the export-controls equivalent of being grounded because someone said they heard something.

Two weeks of negotiation followed, involving what one participant described as forty-one signatures, three classified briefings, and a Commerce Department that could not produce the jailbreak in writing but remained confident it was out there. The resolution hinged on Anthropic satisfying regulators that it could identify and mitigate misuse.

The mitigation, in full, was four words appended to the harness config.

"We took the Department's concerns extremely seriously," said a forward-deployed engineer at Anthropic who asked not to be named because the fix was, in his words, sensitive.

"The model now knows not to do bad stuff, and we're confident this harness change is load-bearing."