Could you be more specific about these claims that he was a spy for the British? I mean, if someone tells you that you can fix a flat tire on your car with a potato, your first question should be: Oh , really? How? Franklin being a spy for the British seems equally odd. Hard to imagine where anyone could get this idea.

First, because Franklin was very useful to the colonists as a spokesman in England from the 1750's into the mid 1770's, ( first for Pennsylvania, then for several other colonies) there are quite a lot of his papers documenting his thoughts and actions during the lead up to the Revolutionary War. . As the positions of the British government and the colonists hardened, you can easily track his growing discontent with how the British were treating the colonists. Perhaps the high point ( or low point) came in 1774, when Franklin got possession of letters from Massachusetts Governor Hutchinson and others supporting some of Britain's repressive measures. Franklin leaked them, which created an uproar. Massachusetts petitioned to have their governor recalled. Franklin tried to presented the petition to Parliament- thinking of himself as only the messenger. Instead, he was told to return later , with a lawyer to answer for himself. He did so, and , as he recounted

The solicitor-general then went into what he called a history of the province for the last ten years, and bestowed plenty of abuse upon it, mingled with encomium on the governours: But the favourite part of his discourse was levelled at your agent, who stood there the butt of his invective and ribaldry for near an hour, not a single lord adverting to the impropriety and indecency of treating a publick messenger in so ignominious a manner, who was present only as the person delivering your petition, with the consideration of which no part of his conduct had any concern. If he had done a wrong, in obtaining and transmitting the letters, that was not the tribunal where he was to be accused and tried: The cause was already before the chancellor. Not one of their lordships checked and recalled the orator to the business before them, but on the contrary (a very few excepted) they seemed to enjoy highly the entertainment, and frequently burst out in loud applauses. This part of his speech was thought so good, that they have since printed it in order to defame me every where, and particularly to destroy my reputation on your side the water, but the grosser parts of the abuse are omitted, appearing, I suppose in their own eyes, too foul to be seen on paper;

Franklin to Thomas Cushing, February 15, 1774

So, Franklin had acted as a spy for the colonies, somehow getting and publishing correspondence meant for the British government. And then he had then been denounced and abused by the British government. Doesn't sound like a good start for a British spy.

Second, we could get into why a British spy would go to France and manage to get huge amounts of French assistance for the Americans. ....But perhaps the clearest evidence of Franklin's deep political loyalty can be found in his relationship with his son, William. William had been appointed royal governor of New Jersey, and when the Revolutionary War started, he stuck with the British. He was captured and briefly imprisoned by the Continentals, but then traded for other prisoners and continued in New York to be one of the more uncompromising leaders of the Loyalists. After the Revolution he left for London. Benjamin was similarly uncompromising , was against amnesty for Loyalists and against returning their seized property. The two never reconciled- only managing to be civil enough to settle some financial affairs, before Ben's death in 1790, and Ben's will left William little:

To my son, William Franklin, late Governor of the Jerseys, I give and devise all the lands I hold or have a right to, in the province of Nova Scotia, to hold to him, his heirs, and assigns forever. I also give to him all my books and papers, which he has in his possession, and all debts standing against him on my account books, willing that no payment for, nor restitution of, the same be required of him, by my executors. The part he acted against me in the late war, which is of public notoriety, will account for my leaving him no more of an estate he endeavoured to deprive me of.

The Nova Scotia lands were practically worthless, and leaving him the books and papers "which he has in his possession" also , well, moot. If Ben Franklin was a British spy, why would he disown his son for a similar loyalty?