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The agent payment standards, counted: x402, AP2, ACK and what is actually implemented

Answer first. A measurement sweep of 617,850 search results across 14,850 queries, run on 18 and 19 September 2026, found 326 IETF internet-drafts whose own name or title concerns AI agents, MCP, A2A or x402. The median number of distinct implementing repositories per draft is 1. Of 158 drafts probed for implementations, 72 of them (46%) have zero, and the maximum for any single draft is 3. The named specifications that developers actually build against sit far above that floor: MCP at 168 implementing repositories, AP2 at 63, x402 at 27. Drafting activity in agentic payments is enormous. Implemented surface area is small, and it clusters on a handful of names. (Neuronto's own measurement, September 2026. Every count below is a floor, not a census; the method and its limits are stated in full at the end.)

How many agent specifications are there

417 candidate internet-drafts surfaced in the sweep. 326 survive a strict check that the draft's own name or title, not its body text, concerns AI agents, MCP, A2A or x402. The rest mention agents in passing and were dropped.

Sorted by area:

The 326 agent-related IETF internet-drafts by area Segmented bar. Transport/routing 89, Identity/auth 66, Discovery 32, Provenance/receipts 15, Delegation 12, Payments 7, Safety/policy 6, Other 101. Area tallies sum to 328. 326 agent-related IETF internet-drafts, by area Name-or-title match on AI agents, MCP, A2A or x402. Neuronto measurement, September 2026. Transport/routing 89 Identity/auth 66 32 Other 101 Transport/routing 89 Identity/auth 66 Discovery 32 Provenance/receipts 15 Delegation 12 Payments 7 Safety/policy 6 Other 101 Area tallies sum to 328, two more than 326, so at least two drafts carry two area labels.

Two things there need a plain reading. First, the area tallies sum to 328, two more than 326, so the areas are overlapping labels and not a partition: at least two drafts carry two labels. Treat the eight numbers as labels applied to drafts, not as slices of a pie.

Second, payments is the smallest named area in the whole set, with 7 drafts, against 89 for transport and routing and 66 for identity and authentication. The 101 drafts in "other" are a reminder that most of the agent-standards output is not about money at all.

The payment-specific drafts, by exact datatracker name and title:

Draft nameTitle
draft-vauban-x402-consolidatedx402 Cryptographic Receipts: Format, Post-Quantum Discipline, and Starknet Anchor
draft-vauban-x402-stark-receiptsx402 STARK Receipt Format Extension
draft-hawkins-x402-dns-discoveryDiscovering x402 Payment Capability via DNS and a Well-Known URI
draft-king-yew-choo-agentic-paymentsA Server-Side Model for Gating Agent-Initiated Human-Sourced Asynchronous HTTP Tasks on Payment or Entitlement
draft-sharif-agent-payment-trustTrust Scoring and Identity Verification for Autonomous AI Agent Payment Transactions
draft-agentir-aeppAgent Execution and Payment Protocol (AEPP)
draft-blake-bmwg-agent-payment-measurementOut-of-Path Measurement Methodology for Agent-Initiated Payment Rails

Standards work is not confined to the IETF. The sweep also identified nine W3C community groups in the agent space, among them agent-conformance and agentic-arbitration, and the Agentic AI Foundation under the Linux Foundation, at 247 member organizations when measured. Neither body has yet published a recommendation a payment implementer can conform to.

Implementing repositories, by specification

One capped search method was applied to every named specification, so the numbers compare fairly even though none is complete.

Implementing repositories per agent specification Horizontal bar chart. MCP 168, AGENTS.md 94, Web Bot Auth 85, A2A 65, AP2 63, llms.txt 54, OpenTelemetry GenAI 48, C2PA 47, NLWeb 47, Biscuit 43, ARD 42, UCAN 36, x402 27, ACP 25, ACK 19, AGNTCY 18, SPIFFE 15, ANP 14, KERI 12, RSL 11. Implementing repositories per specification Floor counts from one capped search method applied identically to every specification. Neuronto measurement, September 2026. MCP 168 AGENTS.md 94 Web Bot Auth 85 A2A 65 AP2 63 llms.txt 54 OpenTelemetry GenAI 48 C2PA 47 NLWeb 47 Biscuit 43 ARD 42 UCAN 36 x402 27 ACP 25 ACK 19 AGNTCY 18 SPIFFE 15 ANP 14 KERI 12 RSL 11
RankSpecificationImplementing repositoriesWhat it governs
1MCP168Tool and context exchange between an AI application and external systems
2AGENTS.md94A repository file describing a codebase to coding agents
3Web Bot Auth85Cryptographic identification of automated HTTP clients
4A2A65Agent to agent task delegation and messaging
5AP263Mandates, authorization and settlement for agent purchases
6llms.txt54A site file pointing models at canonical documentation
7OpenTelemetry GenAI48Trace and metric conventions for model calls
8C2PA47Content provenance and signed media assertions
9NLWeb47Natural language query endpoints over site content
10Biscuit43Attenuable authorization tokens
11ARD42Machine readable resource description and discovery
12UCAN36Capability based delegation between keys
13x40227HTTP native payment over status code 402
14ACP25Agent initiated checkout against a merchant of record
15ACK19Verifiable agent identity plus payment receipts
16AGNTCY18Multi agent interoperability components
17SPIFFE15Workload identity issuance
18ANP14Decentralized identity, discovery and messaging for agents
19KERI12Key event receipt infrastructure for portable identifiers
20RSL11Machine readable content licensing terms

The inconvenient line, for anyone whose work sits on x402, is line 13. x402 has 27 implementing repositories. AP2 has 63, better than twice as many. The stablecoin-native HTTP payment standard is not the most implemented payment specification in this set; the mandate protocol published by Google and its payment partners is, by a wide margin. Anyone claiming x402 has already won the agent payment question is not reading the repositories.

A second reading is less flattering to the payments field as a whole. Every payment specification in the set sits below every general agent-plumbing specification in it, and MCP alone carries more implementing repositories than x402, ACP, ACK and ANP put together, 168 against 85. Agents got tools before they got wallets.

What each of these standards actually says about itself

Each quotation below is verbatim from that specification's own primary source.

x402 is a payment scheme carried inside ordinary HTTP. Its documentation states: "x402 is the open payment standard that enables services to charge for access to their APIs and content directly over HTTP." The specification site describes the exchange in one line: "If a request arrives without payment, the server responds with HTTP 402, prompting the client to pay and retry." Coinbase, which published the standard, wrote in its launch note: "Coinbase is launching x402, a payment protocol that enables instant stablecoin payments directly over HTTP."

The status code it uses is still formally unassigned. RFC 9110, section 15.5.3, says in full: "The 402 (Payment Required) status code is reserved for future use."

AP2, the Agent Payments Protocol, describes itself as "an open protocol for the emerging Agent Economy." Google Cloud's announcement states that "AP2 is designed as a universal protocol, providing security and trust for a variety of payments like stablecoins and cryptocurrencies," and that "in collaboration with Coinbase, Ethereum Foundation, MetaMask and other leading organizations, we have extended the core constructs of AP2 and launched the A2A x402 extension, a production-ready solution for agent-based crypto payments." AP2 and x402 are not competing answers to one question. One governs who authorized a purchase and under what limits; the other governs how a single HTTP request gets paid for.

ACK, the Agent Commerce Kit, says of itself: "The Agent Commerce Kit (ACK) proposes solutions built on open standard protocols and patterns designed to bridge the gap between AI agents and financial systems." Its emphasis is verifiable agent identity and receipts rather than a wire format for one paid request.

ACP, the Agentic Commerce Protocol, describes itself on its own site as "An open standard for programmatic commerce flows between buyers, AI agents, and businesses," and states that "ACP is open source and community-designed under the Apache 2.0 license." Its subject is retail checkout, with the business remaining the merchant of record.

ANP, the Agent Network Protocol, presents itself as an "Open protocol stack for the Agentic Web" offering "Decentralized identity, discovery, messaging, and payment protocols for interoperable AI agents." Its own payment layer is published as an "ANP adaptation of AP2 for payment mandates, receipts, DID-based signing keys, and auditable agent commerce," which is a second reason the AP2 repository count understates AP2's reach.

For the layer underneath all of them, the Model Context Protocol documentation states: "MCP (Model Context Protocol) is an open-source standard for connecting AI applications to external systems."

What the counts mean

A median of one implementing repository per draft, with 46% of probed drafts at zero, is not a scandal. Internet-drafts are cheap to publish by design, and several of them say outright that they are not implementable artifacts. draft-king-yew-choo-agentic-payments states: "It defines no protocol, wire format, or conformance requirements," and, later, "It defines no new protocol elements and requests no registrations."

What the counts do mean is that the drafting volume cannot be used as evidence of adoption. A reader who finds 326 agent-related drafts and concludes that the agent payment question is being settled at the IETF has the direction backwards. The implementation is happening around a small number of vendor-published specifications, some of which have no internet-draft at all, and the IETF material is largely downstream commentary and extension work.

The drafts that do carry engineering detail name the problems a payment implementer actually hits. draft-vauban-x402-consolidated summarizes the wire flow as: "The x402 protocol [X402-V2] defines HTTP-native payment flows using three messages: PAYMENT-REQUIRED (402 response), PAYMENT-SIGNATURE (client request), and PAYMENT-RESPONSE (facilitator confirmation)." It then names the failure mode that off-chain receipts create: "On-chain finality gap: an off-chain receipt does not confirm that a settlement was committed to a public ledger." draft-sharif-agent-payment-trust sets a timing requirement most identity systems get wrong: "Revocation MUST take effect within 1 second and MUST be reflected in all subsequent trust queries."

Choosing between them today

These specifications answer different questions, so the choice is usually determined by what is being sold.

Selling machine access to an API or an MCP tool, priced per call and with no account to open, points to x402. The buyer is a program and the amount is small, so payment riding in the same request as the work is the whole point. Its 27 implementing repositories are a real constraint: tooling is thinner than MCP's, and an implementer should expect to write more plumbing than the documentation implies.

Selling goods or services to a human whose agent is shopping points to AP2 or ACP. AP2's subject is the mandate: a record of what the human authorized and at what price ceiling. ACP's subject is the checkout, with the merchant keeping the customer relationship. Their higher counts reflect that retail has more integrators than machine-to-machine API billing does.

Needing to prove which agent acted and on whose authority, with a receipt that survives an audit, points to ACK, KERI or UCAN. None of them is an alternative to x402 or AP2; they sit beside it.

The honest summary for a team choosing in September 2026: no single standard covers identity, authorization and settlement well enough to be adopted alone, and the combination most visible in public code is AP2 for authorization with x402 as the settlement leg, which is what Google's own A2A x402 extension formalizes.

Method, and what it cannot tell you

The sweep ran on 18 and 19 September 2026 against a self-hosted metasearch instance, covering 14,850 queries and 617,850 returned results. Draft identification used the IETF datatracker; each draft counted here was checked so that its own name or title, not its body, matches the agent subject. Implementation counts come from repository searches capped at a fixed number of results per query, with the identical query template and cap applied to every specification.

The limits, stated plainly:

  • Every count is a floor, not a census. A capped search returns at most the cap. A specification with 400 implementing repositories and a specification with 170 can both report as the cap allows, so large numbers compress more than small ones.
  • The comparison is valid; the absolute values are not. One method with one cap was applied to all 20 specifications and all 158 probed drafts, so the ordering and the ratios are meaningful. The individual numbers understate reality by an unknown and uneven factor.
  • "Implementing repository" means a distinct public repository that a search for that specification's name and artifacts returns. It does not measure quality, completeness, production use, or whether the code runs.
  • Private and internal implementations are invisible. The specifications with the largest commercial backers are likely to be the most undercounted, because their adopters ship closed code.
  • 158 of the 326 drafts were probed for implementations, not all 326. The median of 1 and the 46% zero rate describe that probed subset.
  • The counts are a snapshot dated 18 and 19 September 2026 and will drift.

FAQ

What is a payment agent? A software agent that can complete a purchase or pay for a resource without a human at the keyboard for each transaction. The standards above split that job into authorization, which is AP2 and ACP territory, and settlement, where x402 sits.

What is the x402 protocol? An open payment standard that carries payment inside an ordinary HTTP exchange, using the 402 Payment Required status code that RFC 9110 reserves for future use. Its documentation calls it "the open payment standard that enables services to charge for access to their APIs and content directly over HTTP."

What is Google's agent payment protocol? AP2, the Agent Payments Protocol, published with payment and commerce partners. It describes itself as "an open protocol for the emerging Agent Economy" and covers mandates and authorization across card, bank and stablecoin payment types.

x402 vs AP2: which one do I need? They answer different questions and are frequently used together. AP2 records what a user authorized. x402 makes one HTTP request payable. In this measurement AP2 has 63 implementing repositories to x402's 27, and Google's A2A x402 extension exists precisely to join them.

How many agent payment protocols are there? In the measured set, four named specifications address agent payment directly: AP2 at 63 implementing repositories, x402 at 27, ACP at 25 and ACK at 19. Seven IETF internet-drafts are payment-specific out of 326 agent-related drafts.

What is an agentic wallet? A key and a spending policy held on behalf of an agent, so it can sign a payment without a human present. The specifications differ on where the policy lives: AP2 puts it in a mandate, while draft-sharif-agent-payment-trust puts it in a trust score with spend limit tiers.

What is an API payment? A payment made by the calling program as part of the API request, rather than through an account opened in advance of it. x402's documentation states that it "allows clients to programmatically pay for resources without accounts, sessions, or credential management."

How do I monetize an MCP server? MCP defines no payment. Charging per tool call means putting a payment step in front of the tool, which is what an HTTP-native scheme such as x402 is for. MCP's 168 implementing repositories against x402's 27 is the practical gap: there are far more tools than paid tools.

Sources

Original measurement: Neuronto's own sweep of 617,850 search results across 14,850 queries, 18 and 19 September 2026.