HTTP 402 (Payment Required)
An HTTP status code reserved for payment in the original specification and left unused for about thirty years, now given a concrete meaning by x402.
HTTP 402 Payment Required is a status code that has sat in the specification since the early 1990s marked, in effect, as reserved for future use. Browsers know it, servers can send it, and until recently almost nothing did — there was no agreed way to say what payment was wanted or how to supply it.
x402 is the first widely adopted attempt to fill that in. A server answers a request with 402 and a machine-readable body describing the amount, the asset, the destination and the scheme it accepts. The client pays, retries the same request carrying proof of payment, and receives the resource.
The reason it matters now and not in 1995 is that the caller is no longer a person. A human hitting a paywall can open a checkout page and type a card number. An agent deciding at runtime which API it needs cannot, and 402 is the only part of HTTP that was ever meant for this.