True, the 123 Agreement nowhere says, as has been pointed out by Mr Pranab Mukherjee, that India cannot explode a nuclear device. But it does say that both the US and India shall implement the Agreement in accordance with its ‘respective applicable’ national laws.
The Hyde Act being a US national law, the US is duty-bound to hold India accountable for adhering to its prescriptions. Which means that India can only explode a nuclear device on pain of being subjected to the sanctions under the Act. Yes, India can explode, but in the process, the US-India relations also will explode!