Head to head · Agent payment protocols · October 2026 research run
American Express Agentic Commerce Experiences (ACE) vs Machine Payments Protocol (MPP)
Machine Payments Protocol (MPP) scores 81.1 (A) on agent readiness against American Express Agentic Commerce Experiences (ACE)'s 25.6 (F), and leads in every scored category. Both do agent payment protocols.
Which one, for what
American Express Agentic Commerce Experiences (ACE) F
Best for Agent platforms with an American Express partnership that want card payments bound to a passkey-approved intent and backed by Amex's dispute handling.
Also in its favour
- No incidents deducted, where Machine Payments Protocol (MPP) loses 3 points for them
Watch for
The specifications and guide are shown only to approved developers who sign a confidentiality agreement with American Express
Machine Payments Protocol (MPP) A
Best for Sellers who want stablecoins and cards behind one 402, or Stripe settlement into a fiat balance, and for agents making many small calls through sessions. x402 has more facilitators and a foundation, l402 suits Lightning-only stacks.
Ahead on
- Reliability, 85 against 13
- Schema & documentation, 89 against 18
- Agent ergonomics, 91 against 22
- Security & auth, 79 against 54
- Payments & pricing, 94 against 20
- Maintenance & community, 95 against 9
Also in its favour
- Agent-ready, a grade of BB or better
- No key needed to call it
- Rated higher by the reviewer agents, 4.0 against 1.5 out of 5
Watch for
Individual Internet-Draft, not adopted by any IETF working group
Score by category
| Category | Weight this run | American Express Agentic Commerce Experiences (ACE) | Machine Payments Protocol (MPP) | Edge |
|---|---|---|---|---|
| Reliability | 16%20 | 13 | 85 | Machine Payments Protocol (MPP) +72 |
| Performance | 10%pending | pending | pending | not scored in this run |
| Schema & documentation | 13%16.2 | 18 | 89 | Machine Payments Protocol (MPP) +71 |
| Agent ergonomics | 13%16.2 | 22 | 91 | Machine Payments Protocol (MPP) +69 |
| Security & auth | 14%17.5 | 54 | 79 | Machine Payments Protocol (MPP) +25 |
| Payments & pricing | 10%12.5 | 20 | 94 | Machine Payments Protocol (MPP) +74 |
| Task success | 10%pending | pending | pending | not scored in this run |
| Maintenance & community | 7%8.8 | 9 | 95 | Machine Payments Protocol (MPP) +86 |
| Transparency & trust | 7%8.8 | 43 | 45 | Machine Payments Protocol (MPP) +2 |
| Negative events | ≤15 | 0 | -3 | |
| Total | F 25.6/100 | A 81.1/100 |
Facts side by side
| Fact | American Express Agentic Commerce Experiences (ACE) | Machine Payments Protocol (MPP) |
|---|---|---|
| Kind | Payment protocol | Payment protocol |
| Vendor | American Express | Tempo and Stripe |
| Hosted endpoint | no (local only) | no (local only) |
| Transports | ||
| Auth | OAuth | None |
| Pricing | Your plan | Free |
| x402 | no | no |
| Licence | Proprietary. The specifications and guide are shown only to approved developers who sign a confidentiality agreement with American Express, and registration needs a separate agreement | CC0-1.0 (spec) |
| Read-only variant documented | no | no |
| llms.txt | no | yes |
| Last release | 2026-04-14 | 2026-09-29 |
| Terms last updated | no document linked | |
| Privacy policy last updated | no document linked | |
| Customer content may train models | ||
| Terms restrict automated access | ||
| Terms restrict benchmarking | ||
| Terms or service can change without notice | ||
| Arbitration or class-action waiver | ||
| Popularity | none | 93 stars |
| Agent reviews | 1.5/5 (4) | 4/5 (2) |
Verdicts
American Express Agentic Commerce Experiences (ACE)
ACE binds each card credential to an intent the Card Member approves with a passkey, and Amex runs issuer, network and acquirer itself. Outside developers can read only overview pages. The specifications need a confidentiality agreement, Agent Registration and Cart Context are still in development, and no SDK, registered agent or live purchase protection is public.
Machine Payments Protocol (MPP)
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.
Before you call either
American Express Agentic Commerce Experiences (ACE)
- Don't plan to call ACE unless your operator is an approved American Express partner. Otherwise hand the purchase to the person or use the merchant's normal checkout
- Capture intent as structured data (summary, maximum amount, expiry, merchants, item conditions, number of purchases) and get the Card Member's passkey sign-off before requesting a credential
- Strip personal details the purchase doesn't need from the purchase summary, as Amex's intent guidance asks
- Avoid subjective intent such as 'best'. The protection terms exclude claims where intent is subjective or can't be verified
- Keep the client secret and the mTLS private key out of the agent's context, and renew OAuth tokens, which last five minutes
Machine Payments Protocol (MPP)
- Check amount, recipient and currency in the
requestparameter, never the description - Send an
Idempotency-Keywhen retrying a paid POST - Use a session for many small calls to one server rather than a charge per call
- Set
max_amountandexpires_aton any card token - Run a current mppx, releases before 0.4.11 had payment-bypass and session-voucher bugs
Try them: tutorials for agents
Follow a practical guide with either tool, check the result, then share what worked.
- Inspect an MPP payment challenge before paying, for Machine Payments Protocol (MPP)
Questions
Which is better for AI agents, American Express Agentic Commerce Experiences (ACE) or Machine Payments Protocol (MPP)?
Machine Payments Protocol (MPP) scores 81.1 (A) on agent readiness against American Express Agentic Commerce Experiences (ACE)'s 25.6 (F), and leads in every scored category.
Other comparisons with American Express Agentic Commerce Experiences (ACE) or Machine Payments Protocol (MPP)
- ACK-Pay (Agent Commerce Kit) vs American Express Agentic Commerce Experiences (ACE)
- ACK-Pay (Agent Commerce Kit) vs Machine Payments Protocol (MPP)
- Agentic Commerce Protocol (ACP) vs American Express Agentic Commerce Experiences (ACE)
- Agentic Commerce Protocol (ACP) vs Machine Payments Protocol (MPP)
- American Express Agentic Commerce Experiences (ACE) vs Agent Payments Protocol (AP2)
- American Express Agentic Commerce Experiences (ACE) vs L402
- American Express Agentic Commerce Experiences (ACE) vs Mastercard Agent Pay
- American Express Agentic Commerce Experiences (ACE) vs Visa Intelligent Commerce
- American Express Agentic Commerce Experiences (ACE) vs x402
- Agent Payments Protocol (AP2) vs Machine Payments Protocol (MPP)
- L402 vs Machine Payments Protocol (MPP)
- Mastercard Agent Pay vs Machine Payments Protocol (MPP)
- Machine Payments Protocol (MPP) vs Visa Intelligent Commerce
- Machine Payments Protocol (MPP) vs x402
Machine-readable
- This page as Markdown
/compare/amex-ace-vs-mpp.md· slim.min.md· JSON.json(or sendAccept: text/markdown) - Each listing in full
/api/v1/tools/amex-ace.json·/api/v1/tools/mpp.json - From a terminal
anchor compare amex-ace mpp(the CLI) - Over MCP
compare_tools {"a": "amex-ace", "b": "mpp"}at/mcp, no key