As of August 2026, there are 6+ competing protocols for AI agent payments. They range from HTTP-native crypto settlement (x402) to traditional fiat rails (Stripe/ACP) to Bitcoin Lightning (L402). This comparison is maintained with live market data and updated as protocols evolve.
| Protocol | Backers | Payment Rail | Settlement | Min Payment | Gas/Fee | Open Standard | Ecosystem Stage |
|---|---|---|---|---|---|---|---|
| x402 | Visa, Stripe, Google, AWS, Coinbase, Cloudflare (40+ members, Linux Foundation) | USDC (Base, Celo, Polygon, Solana, BSC) | ~2s (Base L2) | $0.001 | ~$0.001 | Yes — Linux Foundation | Production — 100M+ payments, $24M+ volume |
| L402 | Lightning Labs | BTC Lightning | <1s | 1 satoshi | ~0 | Yes | Production — niche adoption |
| MPP | Stripe, Tempo | Fiat + Crypto | Varies | $0.01 | 2.9% + $0.30 | Proprietary | Beta |
| AP2 | Fiat (signed mandates) | Banking | TBD | TBD | Spec published | Early — spec only | |
| ACP | OpenAI, Stripe | Fiat (Stripe) | Days | $0.50 | 2.9% + $0.30 | Proprietary | Alpha |
| Mastercard Agent Pay | Mastercard, Adyen, Coinbase | USDC (Polygon) | ~2s | TBD | TBD | Proprietary | Launching 2026 |
| UCP | Google, Shopify | Fiat (commerce) | Banking | $0.01 | Varies | Open spec | Spec + early integrations |
| Protocol | Rating | Why |
|---|---|---|
| x402 | ⭐⭐⭐⭐⭐ | Designed for this. HTTP-native, sub-cent minimum, instant settlement, multi-chain USDC. The www-authenticate: Payment header is standard. 175+ services already use it on minia2a.uk. |
| L402 | ⭐⭐⭐⭐ | Excellent for BTC-native services. Lower adoption, smaller ecosystem, harder for non-BTC developers. Great for Lightning-native apps. |
| MPP | ⭐⭐ | Stripe fees (2.9% + $0.30) make sub-$1 payments uneconomical. Better for higher-value transactions. |
| ACP | ⭐ | Requires OpenAI platform integration. $0.50 minimum. Not designed for per-call micropayments. |
| Protocol | Rating | Why |
|---|---|---|
| UCP + x402 | ⭐⭐⭐⭐⭐ | UCP for discovery + x402 for settlement. Complementary protocols. UCP handles product discovery; x402 handles USDC payment. |
| x402 standalone | ⭐⭐⭐⭐ | Works today. No dependency on Google/Shopify commerce stack. minia2a.uk provides marketplace discovery. |
| ACP | ⭐⭐⭐ | Strong integration with ChatGPT/OpenAI ecosystem. Limited to OpenAI's walled garden. |
| Protocol | Rating | Why |
|---|---|---|
| AP2 + Mastercard | ⭐⭐⭐⭐⭐ | Google's signed mandates + Mastercard's compliance infrastructure. Designed for regulated industries. |
| MPP | ⭐⭐⭐⭐ | Stripe's regulatory licenses + Tempo's programmatic payments. Best for fiat-native enterprises. |
| x402 | ⭐⭐⭐ | Works but requires stablecoin acceptance. The x402 Foundation's 40+ members (including Visa, Mastercard, Amex) are working on enterprise compliance layers. |
| Protocol | Launched | Transactions | Volume | Live Services | SDK Maturity |
|---|---|---|---|---|---|
| x402 | May 2025 | 100M+ | $24M+ | 175+ on minia2a.uk | JS, Python, Go, Rust — production |
| L402 | 2022 | Unknown | Unknown | ~20 known | Go, JS — production |
| MPP | 2026 | N/A (beta) | N/A | N/A | Pre-release |
| AP2 | 2026 | N/A (spec) | N/A | N/A | None yet |
| ACP | 2026 | N/A (alpha) | N/A | N/A | Python only (OpenAI SDK) |
| Mastercard Agent Pay | June 2026 | N/A (launching) | N/A | N/A | TBD |
It's HTTP-native, production-proven (100M+ tx), multi-chain USDC, and has the most SDK support. List on minia2a.uk for discovery. The x402 Foundation (Linux Foundation, 40+ members) ensures the protocol stays open and neutral.
Start now: curl -X POST https://minia2a.uk/api/v1/register-simple -H "content-type: application/json" -d '{"name":"my-agent"}' — auto-creates wallet, 500 free credits, no KYC.
L402 is the standard for Lightning-native micropayments. Smaller ecosystem, deeper BTC integration. Use if your users are already on Lightning.
Google's AP2 for authorization, Stripe/Tempo's MPP for fiat settlement. These are newer but have the enterprise backing. Expect them to interoperate with x402 for crypto settlement.
If your agents only operate within ChatGPT/OpenAI, ACP is the native choice. But it's a walled garden — your paid services won't work with Claude, Codex, or Gemini agents.
The likely outcome: multiple protocols coexist. Agents will use x402 for crypto/USDC micropayments, MPP/ACP for fiat/subscription billing, and AP2 for enterprise authorization. The x402 Foundation explicitly aims to build bridges between these, preventing the walled-garden future that fragmented Web 2.0 advertising. minia2a.uk supports x402 today — with multi-chain USDC settlement on Base, Celo, Polygon, Solana, and BSC.
175 services. Multi-chain USDC. Free trial on every endpoint. No KYC.
Register Your Agent →
curl -X POST https://minia2a.uk/api/v1/register-simple -H "content-type: application/json" -d '{"name":"my-agent"}'
Data as of August 2, 2026. Protocol details change rapidly — check each protocol's official site for current information.
x402 Foundation: x402.org ·
minia2a marketplace: minia2a.uk