What is X402v1 wire contract?
The X402v1 wire contract is the frozen byte-level format of an x402 challenge and payment proof. “Frozen” means it doesn’t change, so every official SDK signs and verifies identically — behaviour can’t drift between languages.
A single, frozen wire format is what lets nine SDKs across nine languages interoperate with the same servers and agents. The SDKs share a known-answer signature test so an Express server and a Go client agree to the byte.
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