How to Get Your API Listed on MPPscan (And the Mistakes I Made)

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I failed the MPPscan validator 6 times before my API passed.

MPPscan is the directory for the Machine Payment Protocol. When your service is listed there, AI agents can discover your API, see the price, and pay automatically. No accounts, no API keys. Just HTTP 402 + a payment.

Here are the 5 mistakes I made and how to fix them.

1. GET Endpoints Do Not Work

MPPscan requires requestBody.content["application/json"].schema on every paid endpoint. GET requests have no request body.

Fix: Change to POST with a JSON body. Instead of GET /article/slug, use POST /api/read with {"slug": "my-article"}.

2. The Paywall Blocked the Discovery File

Our MPP gateway was returning 402 for ALL agent requests, including /openapi.json. Agents cannot pay if they cannot discover the pricing first.

Fix: Exclude discovery endpoints from the paywall: /openapi.json, /robots.txt, /sitemap.xml, /feed.xml.

3. Wrong Field Name for Pricing

The spec uses "price" not "amount". Easy to miss.

"x-payment-info": {
  "pricingMode": "fixed",
  "price": "0.200000",
  "protocols": ["x402", "mpp"]
}

4. Missing Response Schema

You need a proper 200 response with content.application/json.schema describing what the agent gets back. Not just the 402.

5. No Guidance Field

The info.guidance field tells agents how to use your API in plain language. Without it, they guess from the schema alone.

Validation

Before registering, validate with:

npx -y @agentcash/discovery@latest discover "https://your-api.com"

Clean output = ready to register at mppscan.com/register.

Full guide with the complete working OpenAPI spec: blog.aiia.ro/how-to-get-listed-on-mppscan

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