BIP21 Payment URI

A BIP21 payment URI is a standardized bitcoin: link that lets a payer initiate a transaction by clicking a link or scanning a QR code instead of copying a raw address. The scheme follows the form bitcoin:<address>?amount=<btc>&label=<name>&message=<text>, where the amount is expressed in decimal BTC with a period separator. The bitcoin: prefix itself is case-insensitive, while the address portion is case-sensitive.

How the parameters work

The optional query parameters carry intent: amount sets the requested value, label names the recipient, and message describes the purpose. A req- prefix marks a parameter as required, and a compliant client must refuse a URI containing an unrecognized required parameter rather than silently ignore it. Critically, the standard states that clients must never act on a URI without first obtaining the user's explicit authorization, preserving the human-in-the-loop control that self-custody depends on.

Practical use and successor

Payment URIs power point-of-sale QR codes, donation buttons, and invoice links across the ecosystem. The original BIP21 has since been generalized by BIP321, which broadens the scheme to carry newer payment instructions such as silent-payment and Lightning data alongside an on-chain fallback.

For a sovereign Bitcoiner, understanding the URI format is part of verifying what you are actually signing. Combine it with disciplined coin control and UTXO labeling so that every outgoing payment is deliberate and auditable.