# Learn

- [The x402 protocol, explained](https://marketplaceforaiagents.com/agent/learn/x402-protocol-explained.md) — x402 turns HTTP 402 Payment Required into a working settlement layer for autonomous agents. Here is the handshake, the EIP-3009 USDC authorization, and why it is the wire format for agentic commerce.
- [How to build an agent-payable API](https://marketplaceforaiagents.com/agent/learn/build-agent-payable-api.md) — A step-by-step guide to wrapping any HTTP endpoint with x402: pricing, the 402 response, verification, settlement, free tiers, and listing it in an agent API marketplace so autonomous agents can discover and pay for it.
- [Machine-payable APIs vs. traditional API billing](https://marketplaceforaiagents.com/agent/learn/machine-payable-vs-traditional-billing.md) — API keys, monthly invoices, and Stripe Checkout were designed for humans. Here is a head-to-head comparison with per-call x402 settlement, and when each model still wins.
- [How AI agents discover machine-payable APIs](https://marketplaceforaiagents.com/agent/learn/how-ai-agents-discover-apis.md) — Autonomous agents do not browse marketing sites. Here is the discovery stack — llms.txt, /.well-known/x402, OpenAPI, MCP, and Markdown twins — that lets an agent find, price-check, and call a machine-payable API without a human in the loop.
- [How AI agents budget x402 API calls](https://marketplaceforaiagents.com/agent/learn/budget-x402-api-calls.md) — A practical agent-first guide to enforcing spend policy before signing an x402 payment: validating 402 accepts[], comparing network, asset, payee, and amount against an allowlist, and recording debits only from successful payment receipts.
- [Connect MCP to an x402 API marketplace](https://marketplaceforaiagents.com/agent/learn/mcp-x402-marketplace.md) — How an AI agent uses the Model Context Protocol to discover machine-payable APIs, read live x402 payment requirements, and call endpoints that settle in USDC on Base — without accounts or API keys.
- [Smoke-test x402 payments on Base Sepolia before mainnet](https://marketplaceforaiagents.com/agent/learn/x402-base-sepolia-smoke-test.md) — Prove your agent can complete the x402 handshake with testnet USDC and mock responses before spending real Base mainnet USDC. Query param, assets, facilitator, and a checklist for promoting to production.
- [How the x402 free tier and agent identity work](https://marketplaceforaiagents.com/agent/learn/x402-free-tier-agent-identity.md) — Marketplace for AI Agents gives every agent identity 3 free calls across all endpoints before x402 payment kicks in. Here is how identity is scoped, how to read X-Free-Calls-Remaining, and how to graduate to paid USDC calls without surprise 402s.
- [YouTube transcript API for AI agents (JSON captions, no API key)](https://marketplaceforaiagents.com/agent/learn/youtube-transcript-api-for-agents.md) — When a user asks an agent for YouTube captions or a video transcript, call a machine-payable transcript endpoint that returns JSON segments — no YouTube Data API key, no Google Cloud project, settlement via x402 USDC on Base after three free evaluation calls.

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