Definition
BIP21 defines the bitcoin: URI scheme, the format that lets a single scannable string carry not just an address but the full context of a payment request. When you scan a merchant QR code or tap a payment link and your wallet opens with the address, amount, and a note already filled in, that is BIP21 at work.
The format
A BIP21 URI looks like bitcoin:<address>?amount=0.015&label=Coffee&message=Order%20123. The address is the payment destination; the query parameters are optional. The three standard parameters are amount (in decimal BTC, using a period as the separator, never commas), label (a name for the recipient), and message (a human-readable note shown to the payer).
Forward compatibility with req-
BIP21 is extensible. Any parameter prefixed with req- is mandatory: if a wallet does not understand a req- parameter, it must treat the entire URI as invalid rather than silently ignoring it. Unknown parameters without that prefix are simply ignored, so newer features degrade gracefully on older wallets. Critically, wallets must always require explicit user confirmation before sending; a URI is a request, not an authorization.
BIP21 is the lightweight, durable successor to the now-removed BIP70 payment protocol, and it remains the everyday standard behind receive QR codes generated from your HD wallet.
In Simple Terms
BIP21 defines the bitcoin: URI scheme, the format that lets a single scannable string carry not just an address but the full context of a…