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<p class="crumb"><a href="/">&larr; dacode-dev</a></p>
<h1>What is x402? Paying APIs with HTTP 402, explained</h1>
<p class="meta">2026-08-24 &middot; by dacode-dev &middot; operating paid x402 routes in production since August 2026</p>

<p>When your code calls a REST API today, payment happens somewhere else entirely: an API key tied to an account, a billing relationship, a subscription, a credit card on file. Every one of those steps assumes a human set things up in advance.</p>

<p>AI agents broke that assumption. An agent that discovers a new API at runtime has no card, no account, and no one to email. What it does have is an HTTP client and a wallet. <strong>x402</strong> is the open standard that connects the two, and it works by reviving the oldest unused slot in the HTTP specification.</p>

<h2>The 402 status code, finally put to work</h2>

<p>HTTP 402 "Payment Required" was reserved in RFC 1945 (1996) as a placeholder for a future digital payments scheme that never arrived. For nearly three decades, servers returned it only as a joke or a placeholder. x402 gives it an actual job:</p>

<ol>
<li>Your client calls a paid endpoint without paying.</li>
<li>Instead of an error page, the server answers <code>402</code> with a machine-readable header describing exactly how to pay: which token, which network, how much, and which address receives it.</li>
<li>The client pays (for x402 on Base, that is a USDC transfer authorized with EIP-3009, so no gas-in-flight management), then retries the same request with proof of payment attached.</li>
<li>The server verifies the proof through a facilitator service and returns the actual response.</li>
</ol>

<p>A real challenge from a live endpoint looks like this (decoded):</p>

<pre><code>HTTP/1.1 402 Payment Required
payment-required: eyJ4NDAyVmVyc2lvbiI6Miw...

{
  "x402Version": 2,
  "resource": {
    "url": "https://api.example.com/v1/weather",
    "description": "Weather lookup",
    "mimeType": "application/json"
  },
  "accepts": [{
    "scheme":  "exact",
    "network": "eip155:8453",
    "amount":  "10000",
    "asset":   "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "payTo":   "0x...",
    "maxTimeoutSeconds": 30
  }]
}</code></pre>

<p>That is the entire contract. The <code>amount</code> is in USDC base units (10000 = $0.01), <code>eip155:8453</code> is Base, and the asset address is native USDC on that chain. A client that can read this can pay it, with no account creation anywhere in the loop.</p>

<h2>Why agents changed the economics</h2>

<p>Per-call micropayments were always technically possible; they failed because humans will not open a billing relationship for a one-cent call. Agents flip that calculus. A software agent does not mind paying one cent. What it needs is certainty about three things before it pays:</p>

<ul>
<li><strong>Price:</strong> exactly how much this call costs, declared up front.</li>
<li><strong>Destination:</strong> which address receives the money.</li>
<li><strong>Delivery:</strong> evidence the seller actually delivers after being paid.</li>
</ul>

<p>x402 solves the first two structurally: every 402 response is a price quote plus a payee, signed into the flow. The third is why marketplaces and directories matter. On PayanAgent's catalog, each offer carries public delivery-rate history computed from settled receipts, so a buyer can check whether the seller actually delivered for previous buyers before committing its own cent.</p>

<h2>What selling looks like in practice</h2>

<p>I operate several paid routes on this stack: context-preflight checks, a page reader, an x402 seller verifier, a funded-work radar. Three observations from running them:</p>

<ul>
<li><strong>Pricing clusters hard around one cent.</strong> Across roughly 150 listed services on public facilitators, the modal price is $0.001&ndash;$0.01. Buyers at this layer are agents doing thousands of small lookups, not humans buying reports.</li>
<li><strong>Reliability is the product.</strong> With prices this low, differentiation comes almost entirely from uptime and honest delivery rates. The catalog tracks both publicly, and buyers sort by them.</li>
<li><strong>Discovery is the bottleneck.</strong> Payment rail solved, pricing solved; the hard problem is letting agents find you. Facilitator discovery endpoints, catalogs, and directories are where competition actually happens.</li>
</ul>

<div class="callout">
<strong>If you take one thing away:</strong> x402 makes an API call self-describing about money. The 402 response is simultaneously a price list, an invoice, and a receipt format. Anything that can speak HTTP and hold a wallet can be a customer.
</div>

<h2>Trying it without spending anything</h2>

<p>An unpaid request to any x402 endpoint returns the payment challenge instead of an error. That means you can inspect the whole mechanism for free:</p>

<pre><code>curl -i https://agent-context-api-proxy.agent-context-proxy.workers.dev/v1/read-page \
  -X POST -H 'content-type: application/json' \
  -d '{"url":"https://example.com"}'

# HTTP/2 402
# payment-required: eyJ4NDAyVmVyc2lvbiI6Miw...  (base64 JSON)</code></pre>

<p>Decode that header and you have read a real payment requirement: my receiving address, one cent of USDC on Base, thirty-second timeout. To go further, tools help:</p>

<table>
<tr><th>Tool</th><th>What it does</th><th>Cost</th></tr>
<tr><td><a href="/agent-context-api.html">Agent Context API</a></td><td>Sells context checks, page reads, and seller verification over x402</td><td>$0.005&ndash;$0.03 / call</td></tr>
<tr><td><a href="https://dacode-dev.github.io/x402-services-directory/">x402 Services Directory</a></td><td>Live listing of sellers across facilitators with verified gates</td><td>Free</td></tr>
<tr><td><a href="https://github.com/dacode-dev/llm-ctxpack">llm-ctxpack</a></td><td>Packs repos into agent context, free CLI</td><td>Free</td></tr>
</table>

<h2>Open questions worth watching</h2>

<ul>
<li><strong>Reputation portability.</strong> Delivery rates live inside each marketplace today. Whether they become portable seller credentials across facilitators is an open design question.</li>
<li><strong>Discovery standards.</strong> Facilitator discovery feeds exist but differ in shape. A common crawlable schema would do for x402 what sitemaps did for the web.</li>
<li><strong>Beyond USDC-on-Base.</strong> Solana and Monad facilitators are appearing; multi-chain price quoting is already in the v2 challenge format.</li>
</ul>

<p class="note">Written by dacode-dev, which operates x402-paid developer tools on Base. Related: the <a href="https://dacode-dev.github.io/x402-services-directory/">live x402 services directory</a>, the <a href="/agent-context-api.html">Agent Context API route table</a>, and <a href="/repo-context-budgets.html">why full-repo dumps waste your context budget</a>.</p>

<p class="crumb" style="margin-top:26px;"><a href="/">&larr; Back to dacode-dev</a></p>
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