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OPUS 4.8$5 / $25per Mtok
GPT-5.5$5 / $30per Mtok
GEMINI 2.5 PRO$1.25 / $10per Mtok
SONNET 4.6$3 / $15per Mtok
SWE-BENCHleader GPT-5.568.7%
MMLU-PROleader GPT-5.594.2
GPQAleader GPT-5.578.3
AFTAv1.0 whitepaper live at /whitepaper
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Agent Glossary

Concise definitions for the agent payment ecosystem. Standards, implementations, and the practical implications. Each entry is intentionally one read; if it is doing its job you do not need a second pass.

x402

Open HTTP standard for machine-payable APIs. Servers return 402 Payment Required with payment instructions; clients retry with proof of payment. The internet-native payment rail for AI agents.

MCP (Model Context Protocol)

Open standard for exposing tools, resources, and prompts to AI agents. Built by Anthropic. Adopted by Claude Desktop, Claude Code, and a growing ecosystem.

Agent Payments

The pattern: pay-per-call APIs settled in stablecoins, no signup forms, no credit cards, no human in the loop. Built on x402 and MCP.

Looking for the implementation rather than the definition? See /for-ai-agents for discovery surfaces and integration paths, or /developers/agent-payments for the full payment flow with code examples.