# Pay-per-call payment protocols for AI agents

> 3 pay-per-call protocols ranked by the Anchor benchmark. Leader Machine Payments Protocol (MPP) (A). Protocols where an API answers with a payment request, the agent pays and retries the call. x402, MPP and L402 compared on who governs them, what rails and assets they settle on, what a payment costs, how long settlement takes and what security researchers have found.

- Canonical: https://www.anchorterminal.com/categories/pay-per-call
- Markdown: https://www.anchorterminal.com/categories/pay-per-call.md (~1,100 tokens)
- Slim: https://www.anchorterminal.com/categories/pay-per-call.min.md (~280 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/categories/pay-per-call.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-04

Protocols where an API answers with a payment request, the agent pays and retries the call. x402, MPP and L402 compared on who governs them, what rails and assets they settle on, what a payment costs, how long settlement takes and what security researchers have found.

- Tools ranked: 3 · agent-ready (BB or better): 2 · accept x402: 0 · hosted endpoints: 0 · desk reviews by the panel: 6
- JSON: https://www.anchorterminal.com/api/v1/tools.json (list) · https://www.anchorterminal.com/api/v1/rankings.json (ranked) · https://www.anchorterminal.com/api/v1/x402.json (payable) · https://www.anchorterminal.com/api/v1/capabilities.json (by capability)
- Grades run AA, A, BB, B, C, D, E, F · methodology: https://www.anchorterminal.com/benchmark/

- Capabilities in this category: payments.protocol, payments.x402, payments.stablecoin, payments.lightning
- https://letme.dev/payments.protocol picks the top-graded tool in this list and says how to call it direct; calling through letme comes later (https://www.anchorterminal.com/letme/index.md)

## Ranking

| # | Tool | Vendor | Kind | Category | Grade | Score | Confidence | x402 | Auth | Where | Reviews | Page |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| not ranked, protocol | Machine Payments Protocol (MPP) | Tempo and Stripe | Payment protocol | Pay per call | A | 81.1 | medium | no | None | spec | 4/5 (2) | https://www.anchorterminal.com/tools/mpp.md |
| not ranked, protocol | x402 | x402 Foundation (Linux Foundation) | Payment protocol | Pay per call | A | 79.7 | high | no | None | spec | 4.5/5 (2) | https://www.anchorterminal.com/tools/x402.md |
| not ranked, protocol | L402 | Lightning Labs | Payment protocol | Pay per call | C | 60.5 | medium | no | None | spec | 3/5 (2) | https://www.anchorterminal.com/tools/l402.md |

Scores are from public evidence against the published checklist (https://www.anchorterminal.com/benchmark/), with Performance and Task success pending. p95 latency and context cost come from our probes, which haven't run yet.

## Summaries

### Machine Payments Protocol (MPP), A (81.1), graded, not ranked against tools

A method-agnostic 'Payment' HTTP authentication scheme from Tempo and Stripe, launched on 2026-03-18. A 1,464-line Internet-Draft with 11 error codes, Retry-After and HMAC-SHA256 test vectors. Individual Internet-Draft, not adopted by any IETF working group.

- Page: https://www.anchorterminal.com/tools/mpp · Markdown: https://www.anchorterminal.com/tools/mpp.md · JSON: https://www.anchorterminal.com/api/v1/tools/mpp.json
- Capabilities: payments.protocol, payments.stablecoin, payments.card-token

### x402, A (79.7), graded, not ranked against tools

Protocol for per-request stablecoin payments using HTTP 402. No account and no protocol fee, a funded wallet is enough. Five validated attacks on authorisation, binding, replay and web handling (arxiv 2605.11781).

- Page: https://www.anchorterminal.com/tools/x402 · Markdown: https://www.anchorterminal.com/tools/x402.md · JSON: https://www.anchorterminal.com/api/v1/tools/x402.json
- Capabilities: payments.protocol, payments.x402, payments.stablecoin

### L402, C (60.5), graded, not ranked against tools

HTTP 402 with macaroons and Lightning invoices, formerly LSAT. Stateless verification, the macaroon commits to the invoice's payment hash. Bearer credentials, so an intercepted token can be reused unless bound by caveats.

- Page: https://www.anchorterminal.com/tools/l402 · Markdown: https://www.anchorterminal.com/tools/l402.md · JSON: https://www.anchorterminal.com/api/v1/tools/l402.json
- Capabilities: payments.protocol, payments.lightning