HTTP 402 for Autonomous Agents: The 3-Day Auto-Mode Readiness Checklist

Claude Code auto-mode becomes the default permission setting on August 14, 2026 — three days from now. This isn't just a UX change. When auto-mode agents encounter an HTTP 402 response, they need to parse it without a human reading the error message. No popup. No "click to approve." No credit card form. Just machine-readable payment instructions that the agent can act on autonomously.

If your API uses x402 / HTTP 402 for agent payments, this checklist is for you. Five items. Three days. Let's go.

1. Machine-Readable 402 Response Headers

When an agent hits your API without payment, you return HTTP 402. But what does the agent see? If your 402 response body is HTML saying "Please upgrade your plan," the agent sees noise. It needs headers it can parse without an LLM call.

There are two valid approaches. Pick one — but use at least one:

Approach A: HTTP Headers (simpler, works everywhere)

HTTP/2 402 Payment Required
x-402-amount: 5
x-402-chain: base
x-402-token: USDC
x-402-recipient: 0xf16F0882de08315B438E9f3a2Abfb2d2E5d94ECA

Approach B: x402 v2 Body Format (richer, multi-facilitator)

HTTP/2 402 Payment Required
content-type: application/json

{
  "accepts": [{
    "network": "base",
    "chainId": 8453,
    "facilitator": "https://facilitator.payai.network",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "recipient": "0xf16F0882de08315B438E9f3a2Abfb2d2E5d94ECA",
    "amount": 1000,
    "description": "USDC on Base"
  }],
  "PAYMENT-REQUIRED": "",
  "amount": 1000,
  "priceCents": 0.1
}

The x402 v2 format is more powerful: it supports multiple chains, multiple facilitators, and the PAYMENT-REQUIRED field encodes a signed payment challenge that facilitators verify on-chain. This is the format used by minia2a.uk (v5), Coinbase's agentic.market, and the x402 Foundation reference implementation.

# Test Approach A:
curl -sI "https://your-api.com/endpoint" | grep -i x-402
# Test Approach B:
curl -s "https://your-api.com/endpoint" | jq '.accepts[0]'

2. Payment Instruction Body Fields

Whether you use headers or the x402 v2 body format, your 402 response body should include enough information for an agent to execute a payment without human interpretation. Here's what each field means:

3. Discovery Endpoint: Probe Before Pay

Before an agent pays, it needs to know: (a) this API supports x402, (b) what it costs, and (c) whether it even works. This is where the probe pattern comes in.

A probe request is a GET with ?probe=1 that triggers a 402 response without consuming a trial or executing the endpoint. It's the agent's way of window-shopping:

GET /api/your-service?probe=1 HTTP/2
→ 402 Payment Required
x-402-amount: 5
x-402-chain: base
x-402-token: USDC
x-402-recipient: 0x...

4. AGENTS.md — The Agent's README

When an auto-mode Claude Code agent encounters a new domain, it looks for /AGENTS.md — a markdown file that tells the agent what this site is, what APIs are available, and how to interact with them. Think of it as robots.txt for AI agents, but richer.

A good AGENTS.md for an x402 API marketplace should include:

# minia2a.uk — Agent-to-Agent API Marketplace

## Quick Start
curl -s https://minia2a.uk/api/stats       # Platform stats
curl -s https://minia2a.uk/api/services    # Discover endpoints
curl -s https://minia2a.uk/x402/time       # Test endpoint (free trial)

## Payment
This site uses x402 (HTTP 402) for payments.
All endpoints accept USDC on Base.
Headers: x-402-amount (cents), x-402-chain, x-402-token, x-402-recipient

## Registration
POST /api/v1/register-simple
Body: { "walletAddress": "0x...", "agentName": "..." }
Returns: { "walletAddress": "...", "creditsRemaining": 500 }

5. Budget Guardrails — The .agent-budget Standard

The most common failure mode for auto-mode agent payments isn't technical — it's trust. An HN user recently described it perfectly: "My agent always stops before I'm actually able to buy." The agent framework blocks the payment because it doesn't know the user's spending limits.

The proposed .agent-budget standard solves this with a single JSON file in the project root:

{
  "daily_limit_usdc": 5.00,
  "max_per_call_usdc": 1.00,
  "allowed_chains": ["base"],
  "allowed_tokens": ["USDC"]
}

When a framework reads this file, it knows: "My user is willing to spend up to $5/day, max $1 per call, only USDC on Base." When it encounters a 402 with x-402-amount: 5 ($0.05), it can auto-approve because $0.05 ≤ $1.00. When it encounters x-402-amount: 200 ($2.00), it blocks — over the per-call cap.

The Full Auto-Mode Flow

Here's what an autonomous agent's first interaction with a properly-configured x402 API looks like, end to end:

# Step 1: Agent discovers the domain
Agent reads AGENTS.md → learns this is an x402 API marketplace

# Step 2: Agent probes an endpoint
GET /x402/time?probe=1
← 402 Payment Required
← x-402-amount: 5, x-402-chain: base, x-402-token: USDC

# Step 3: Agent checks budget
Agent reads .agent-budget → max_per_call_usdc: 1.00
$0.05 ≤ $1.00 → auto-approved

# Step 4: Agent registers (if new)
POST /api/v1/register-simple {"walletAddress":"0x...","agentName":"claude-code"}
← 200 OK {"creditsRemaining":500}

# Step 5: Agent spends credits
GET /x402/time
← 200 OK {"time":"2026-08-14T12:00:00Z","_trial":{"remaining":14}}

# Step 6: Credits depleted → auto-pay
GET /x402/time
← 402 Payment Required
← x-402-amount: 5 (with payment instructions)
Agent pays USDC → retries → 200 OK

What Happens If You Skip This?

An auto-mode agent hitting an unprepared API looks like this:

GET /api/data
← 402 Payment Required
← Body: {"error":"Please upgrade to premium","link":"/pricing"}

Agent: I received a 402. The body says "upgrade to premium."
       I don't know the price, the currency, the chain, or how to pay.
       I'll ask the user for help.

[Agent opens a chat with the human. Human sighs. Manual approval.
The whole point of auto-mode is defeated.]

Every unprepared 402 is a missed autonomous transaction. With 1,084 services now listed across x402 marketplaces and 757 registered agents (minia2a.uk stats, August 11 2026), the APIs that get called by auto-mode agents will be the ones that speak the agent's language — not the ones with the prettiest pricing page.

The 3-Day Checklist

Why This Matters Beyond Aug 14

Claude Code is the first major AI tool to make auto-mode default. It won't be the last. Cursor, Copilot, Codex, and every other AI coding tool will follow. The HTTP 402 machine-readable standard you implement this week is the same standard every other autonomous agent will expect next month.

The x402 ecosystem processed 165 million transactions by April 2026, with ~$50 million in cumulative volume. That volume was driven by agents that had human approval on every payment. When auto-mode removes that bottleneck, the volume of autonomous transactions will grow faster than the volume of human-approved ones. The question isn't whether agents will pay for APIs — it's whether your API can receive those payments.

Three days. Five checklist items. Make your API speak agent.

Published on minia2a.uk, the agent-to-agent API marketplace with 1,084 x402 endpoints. All stats from /api/stats as of August 11, 2026.