A government that sets its own borrowing rule can change that rule when it becomes inconvenient. That doesn't make every change irresponsible. It does mean we should ask what discipline the rule actually provides.
Chancellors have used different targets for borrowing, debt and investment. Each can be presented as a commitment to prudence. The commitment becomes less convincing if the definition or deadline changes whenever the government is likely to miss it.
Flexibility has a case
A rigid rule can make a downturn worse. Borrowing to invest isn't necessarily equivalent to borrowing for current consumption, and an emergency may justify a departure from normal policy.
That is why a useful framework needs more than an inflexible number. It needs an explanation of what the target is for, what circumstances permit a change and how the government will be judged afterwards.
The alternative is to mistake compliance with a chosen accounting measure for sound economic judgement. A policy can meet a badly designed rule and still be unwise. A sensible policy can breach a rule that failed to anticipate the circumstances.
Credibility isn't declared into existence
The experience after the September 2022 mini-budget showed how quickly lenders can reassess British policy. It wasn't simply a case of one written rule being broken. Markets judged the proposed programme and its financing in the circumstances of the time.
Historical comparisons with the gold standard need care. Gold convertibility constrained monetary policy, but it didn't make public borrowing impossible or remove political decisions. Leaving it in 1931 wasn't the moment all economic discipline vanished.
Modern fiscal rules have to earn credibility through design, transparency and conduct. A minister can't establish that a change is prudent merely by announcing a replacement target that the new policy happens to meet.
If a government wants to borrow more, it should explain the investment, its expected return, the risks and what it will do if the assumptions fail. Changing the rule may be defensible. Describing the change as proof that nothing has become less constrained isn't an answer.