3
v2 headers to recognize
PAYMENT-REQUIRED, PAYMENT-SIGNATURE, and PAYMENT-RESPONSE are the names new x402 implementations should understand.
HTTP 402 payment protocol field guide
Build with x402 without copying stale payment examples.
x402docs.com explains the current v2 flow, headers, server setup, buyer policy, facilitators, networks, schemes, migration traps, and agent-commerce patterns with primary sources attached.
402 challenge
signed retry
settlement
CAIP-2
network identifiers
v2 uses identifiers such as eip155:8453 rather than ambiguous local aliases such as "Base mainnet."
0
standard browser payment UI
HTTP 402 is reserved in HTTP semantics, but generic browser behavior is still not a universal payment experience.
Free tools
Generate, price, and decode x402 flows
PAYMENT-REQUIREDx402 example
402 Response Builder
Generate a client-side x402 v2 exact-scheme PAYMENT-REQUIRED response, Base64 header value, decoded JSON object, and retry curl shape.
pricingpay per use API
x402 Pricing Calculator
Compare pay-per-use x402 API revenue against subscription or API-key pricing with dated fee assumptions you can edit.
PAYMENT-SIGNATUREX-PAYMENT
Payment Header Decoder
Decode Base64 x402 PAYMENT-SIGNATURE, PAYMENT-REQUIRED, PAYMENT-RESPONSE, or legacy X-PAYMENT values entirely in the browser.
Protocol map
The shortest correct mental model
1
Request
The client asks for a paid HTTP resource.
2
402
The server returns payment requirements.
3
Sign
The client signs a scheme-specific payload.
4
Verify
The server or facilitator checks the payload.
5
Serve
The server returns content plus payment response.
Read first
Core guides
HTTP 402stablecoins
What Is x402?
A plain-English but implementation-aware explanation of x402, the HTTP 402 payment protocol for paid APIs, content, and AI agents.
headersv2
The HTTP 402 Flow
A step-by-step map of the x402 v2 request flow, including PAYMENT-REQUIRED, PAYMENT-SIGNATURE, and PAYMENT-RESPONSE.
sellermiddleware
Server Integration Guide
How to decide what to protect with x402, configure route requirements, choose a facilitator, and test paid API endpoints.
buyeragents
Client and Agent Guide
How x402 buyers, HTTP clients, and AI agents should evaluate payment requirements, sign payloads, and retry safely.
facilitatorverify
Facilitators: Verify and Settle
What x402 facilitators do, when to use one, and how to evaluate facilitator trust, uptime, and settlement behavior.
CAIP-2USDC
Networks and Assets
A practical guide to x402 network identifiers, default assets, stablecoin choices, and chain-specific implementation tradeoffs.
Implementation depth
Migration, agents, security, and sources
Payment Schemes
A scheme defines what the client authorizes and what the server or facilitator can settle. Pick the scheme based on pricing semantics, not just SDK availability.
Security and Reliability Checklist
x402 makes payment easy to attach to HTTP. That does not make payment easy to secure. The main risks are replay, wrong destination, wrong amount, unlimited wallet authority, unclear refunds, and retry ambiguity.
v1 to v2 Migration
Many search results and older examples still show v1 patterns. New integrations should recognize the differences before copying code.
MCP and Agent Commerce
x402 is compelling for agents because it gives tools a way to ask for payment in the same protocol the agent already uses to fetch resources. The hard part is giving the agent safe authority.
x402 vs Subscriptions and API Keys
x402 is not a universal replacement for subscriptions. It is a different commercial primitive: a resource can quote a price inside HTTP, a client can authorize one purchase, and the seller can serve the result after verification.
x402 Header Format
Most x402 debugging starts with the headers. v2 makes the payment handshake explicit: the server advertises requirements, the client retries with a signed payload, and the server returns settlement feedback.
Primary Source Map
This site is intentionally citation-heavy. x402 is moving quickly, so source selection matters: protocol docs and specs first, provider docs second, commentary last.