How agents find an API (and where FastAF is)
New “agent discovery” checkers test a site for the same few things: an AI catalog, an MCP server card, llms.txt and WebMCP tools. This page lists the discovery points for an API and the FastAF entry for each. Start at fastaf.co.
Answer first
| Discovery point | What an agent gets | FastAF |
|---|---|---|
| llms.txt | Short map of the docs | https://fastaf.co/llms.txt (full reference: https://fastaf.co/llms-full.txt) |
| MCP endpoint | Tools to call | https://api.fastaf.co/mcp (tools/list is public; tool calls need an API key or OAuth, not x402) |
| OpenAPI spec | Every REST route and parameter | https://api.fastaf.co/openapi.json |
| x402 402 response | Price and payment details, before paying | GET https://api.fastaf.co/x402/v1/<name>, 24 routes (23 endpoints plus the search alias of serp) |
| Public price list | Prices for humans and crawlers | x402 price list |
| MCP Registry | Listing MCP clients can search | co.fastaf/fastaf in the official registry (registry.modelcontextprotocol.io), remote https://api.fastaf.co/mcp |
| MCP server card / .well-known | Machine-readable server summary | https://api.fastaf.co/.well-known/mcp/server-card.json (server info, auth, tools), https://api.fastaf.co/.well-known/mcp (card built from the registry entry) and https://fastaf.co/.well-known/mcp.json (registry-format entry) |
| AI catalog | Index of the MCP server card and the agent skill | https://fastaf.co/.well-known/ai-catalog.json |
| API catalog | Links to the OpenAPI spec, docs and health check | https://fastaf.co/.well-known/api-catalog |
Check it yourself
curl -s https://fastaf.co/llms.txt | head
curl -s https://api.fastaf.co/openapi.json | head -c 400
curl -i "https://api.fastaf.co/x402/v1/serp?query=test" # 402 with price, no charge
curl -s https://api.fastaf.co/.well-known/mcp/server-card.json | head -c 400
curl -s "https://registry.modelcontextprotocol.io/v0/servers?search=fastaf" | head -c 400The 402 answers before any payment, so an agent can read the price before deciding. On 28 October 2026, eight unpaid requests got their 402 in 61-308 ms.
What actually makes an agent pick a tool
- It can find it (the files above).
- The tool description says exactly what comes back and what it costs.
- The first call works without a signup wall: 500 free credits every month after email verification, or x402 with no account.
For the x402-specific checks, see the x402 discovery checklist. To keep the MCP tool list small once an agent has found it, see too many MCP tools?.
Source of the advice: agent discovery checkers shared on X, October 2026.
Reference
- FastAF documentation: every parameter and response field
- Docs: authentication, errors, MCP and x402
- Pricing: credit packs from $10, without expiry
Other sources
- How to read public Amazon data when your requests fail
- How to read public Google Search data when your requests fail
- How to read public Walmart data when your requests fail
- How to read LinkedIn data with an API
- How to read public eBay data when your requests fail
- How to read public Booking.com data when your requests fail
- How to read X profile data with an API
- How to read public Zillow data when your requests fail
- Pay-per-call web scraping for AI agents with x402
- MCP servers for Amazon, SERP and web scraping
- Cost per agent: monthly SaaS vs x402 pay-per-call
- How FastAF handles heavy pages and timeouts
- x402 endpoint discovery checklist: how agents find a paid API (and how FastAF passes it)
- Add FastAF to Claude Code, Codex and Cursor
- How to detect poisoned or incomplete SERP results in AI agents
- Cap agent spend per call with fixed x402 prices
- Use FastAF from the Cloudflare Agents SDK
- FastAF as a fallback search provider
- Budget a scraping job before you run it
- Try FastAF for free, no card
- Too many MCP tools? Keep agent context small
- Why agent retries cost more than you think
- All how-to guides