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The building blocks every protocol reuses: the HTTP 402 status code, payment tokens, signed requests and mandates.

• HTTP 402 Payment Required and why it sat unused for decades Open: HTTP 402 Payment Required and why it sat unused for decades10 min
• Walk the Prehistory lane of the timeline, from micropayment schemes to Lightning Open: Walk the Prehistory lane of the timeline, from micropayment schemes to Lightning10 min
• Network tokens, vault tokens and cryptograms Open: Network tokens, vault tokens and cryptograms10 min
• Signed requests and signed data: RFC 9421 and EIP-712 Open: Signed requests and signed data: RFC 9421 and EIP-71215 min
• Mandates, intents and delegated authority Open: Mandates, intents and delegated authority10 min
• Human present and human not present Open: Human present and human not present5 min

Identity and trust, authorization and intent, checkout and orders, payment execution: what each family does and which protocols sit in it.

• Identity and trust: ERC-8004: Trustless Agents, Visa Trusted Agent Protocol Open: Identity and trust: ERC-8004: Trustless Agents, Visa Trusted Agent Protocol15 min
• Authorization and intent: AP2 Open: Authorization and intent: AP215 min
• Checkout and orders: Agentic Commerce Protocol (ACP), UCP Open: Checkout and orders: Agentic Commerce Protocol (ACP), UCP15 min
• Payment execution and credentials: x402, Stripe MPP (Machine Payments Protocol), Mastercard Agent Pay Open: Payment execution and credentials: x402, Stripe MPP (Machine Payments Protocol), Mastercard Agent Pay15 min
• Read the comparison matrix across all eight protocols Open: Read the comparison matrix across all eight protocols20 min
• See which controls each protocol provides natively Open: See which controls each protocol provides natively15 min

One stop per protocol. Each gate is the protocol's own ten-question self-test; 7 of 10 passes.

• ERC-8004: Trustless Agents: read the TL;DR and the detailed explanation Open: ERC-8004: Trustless Agents: read the TL;DR and the detailed explanation25 min
• ERC-8004: Trustless Agents: step through one flow in the diagrams Open: ERC-8004: Trustless Agents: step through one flow in the diagrams10 min
• x402: read the TL;DR and the detailed explanation Open: x402: read the TL;DR and the detailed explanation25 min
• x402: step through one flow in the diagrams Open: x402: step through one flow in the diagrams10 min
• Visa Trusted Agent Protocol: read the TL;DR and the detailed explanation Open: Visa Trusted Agent Protocol: read the TL;DR and the detailed explanation25 min
• Visa Trusted Agent Protocol: step through one flow in the diagrams Open: Visa Trusted Agent Protocol: step through one flow in the diagrams10 min
• Agentic Commerce Protocol (ACP): read the TL;DR and the detailed explanation Open: Agentic Commerce Protocol (ACP): read the TL;DR and the detailed explanation25 min
• Agentic Commerce Protocol (ACP): step through one flow in the diagrams Open: Agentic Commerce Protocol (ACP): step through one flow in the diagrams10 min
• UCP: read the TL;DR and the detailed explanation Open: UCP: read the TL;DR and the detailed explanation25 min
• UCP: step through one flow in the diagrams Open: UCP: step through one flow in the diagrams10 min
• Stripe MPP (Machine Payments Protocol): read the TL;DR and the detailed explanation Open: Stripe MPP (Machine Payments Protocol): read the TL;DR and the detailed explanation25 min
• Stripe MPP (Machine Payments Protocol): step through one flow in the diagrams Open: Stripe MPP (Machine Payments Protocol): step through one flow in the diagrams10 min
• AP2: read the TL;DR and the detailed explanation Open: AP2: read the TL;DR and the detailed explanation25 min
• AP2: step through one flow in the diagrams Open: AP2: step through one flow in the diagrams10 min
• Mastercard Agent Pay: read the TL;DR and the detailed explanation Open: Mastercard Agent Pay: read the TL;DR and the detailed explanation25 min
• Mastercard Agent Pay: step through one flow in the diagrams Open: Mastercard Agent Pay: step through one flow in the diagrams10 min

Quiz gates

• ERC-8004: Trustless AgentsNot passed yet. Pass mark 7 of 10.Take the quiz hereOn the ERC-8004: Trustless Agents page (self-test section)
• x402Not passed yet. Pass mark 7 of 10.Take the quiz hereOn the x402 page (self-test section)
• Visa Trusted Agent ProtocolNot passed yet. Pass mark 7 of 10.Take the quiz hereOn the Visa Trusted Agent Protocol page (self-test section)
• Agentic Commerce Protocol (ACP)Not passed yet. Pass mark 7 of 10.Take the quiz hereOn the Agentic Commerce Protocol (ACP) page (self-test section)
• UCPNot passed yet. Pass mark 7 of 10.Take the quiz hereOn the UCP page (self-test section)
• Stripe MPP (Machine Payments Protocol)Not passed yet. Pass mark 7 of 10.Take the quiz hereOn the Stripe MPP (Machine Payments Protocol) page (self-test section)
• AP2Not passed yet. Pass mark 7 of 10.Take the quiz hereOn the AP2 page (self-test section)
• Mastercard Agent PayNot passed yet. Pass mark 7 of 10.Take the quiz hereOn the Mastercard Agent Pay page (self-test section)

End-to-end walkthroughs where several protocols work together, with a legend of which protocol does what.

• An agent buys one API call Open: An agent buys one API call20 min
• Metered inference Open: Metered inference20 min
• A shopper buys running shoes in Gemini Open: A shopper buys running shoes in Gemini20 min
• Autonomous restock while the user is away Open: Autonomous restock while the user is away20 min
• An agent shops on a merchant website with a Visa card Open: An agent shops on a merchant website with a Visa card20 min
• From ChatGPT discovery to merchant checkout Open: From ChatGPT discovery to merchant checkout20 min
• A Mastercard Virtual Card Token in an existing checkout form Open: A Mastercard Virtual Card Token in an existing checkout form20 min
• A crawler pays per page Open: A crawler pays per page20 min
• Pick two protocols on the compare page and read the side-by-side diff Open: Pick two protocols on the compare page and read the side-by-side diff15 min

Open questions from every protocol page and the peer-reviewed and preprint literature.

• ERC-8004: Trustless Agents: 8 open questions Open: ERC-8004: Trustless Agents: 8 open questionsFor example: Will the Validation Registry be redesigned before an official deployment?10 min
• x402: 8 open questions Open: x402: 8 open questionsFor example: How much x402 activity is real commerce?10 min
• Visa Trusted Agent Protocol: 8 open questions Open: Visa Trusted Agent Protocol: 8 open questionsFor example: How do production agent keys reach merchants and CDNs?10 min
• Agentic Commerce Protocol (ACP): 8 open questions Open: Agentic Commerce Protocol (ACP): 8 open questionsFor example: Will OpenAI and Stripe move ACP to a neutral foundation?10 min
• UCP: 8 open questions Open: UCP: 8 open questionsFor example: Will the AP2 extension run in production, and where first?10 min
• Stripe MPP (Machine Payments Protocol): 8 open questions Open: Stripe MPP (Machine Payments Protocol): 8 open questionsFor example: Will the core reach an IETF working group?10 min
• AP2: 9 open questions Open: AP2: 9 open questionsFor example: Will FIDO publish AP2 as a standard, and when?10 min
• Mastercard Agent Pay: 8 open questions Open: Mastercard Agent Pay: 8 open questionsFor example: Does any live Agent Pay authorization check a Verifiable Intent chain today?10 min
• Browse the publications library by theme Open: Browse the publications library by theme20 min