Quick context, in case you haven't run into this yet
AI agents are starting to need to pay for things on their own — call an API, get
charged a few cents in stablecoin, get the data back, no human clicking "buy." The
standard emerging for this reuses an old, mostly-unused HTTP status code: 402
Payment Required. An agent hits an endpoint, gets a 402 back with a price, pays in
USDC (or similar), retries the request, gets the real response. That's x402
(and its cousins MPP and ACP — same idea, different implementations).
Separately, MCP (Model Context Protocol) is the standard that lets AI assistants
(Claude, etc.) connect to external tools and data in a consistent way — instead of
every company inventing its own integration format.
We run Superstables: an index that sits at the intersection of both. It tracks
every service an agent can pay for via x402/MPP/ACP, and exposes that index through
an MCP server, so any MCP-compatible agent can query it directly.
The problem we ran into
Building the index was the easy part. The hard part: a lot of what gets "listed" as
payable across various sources doesn't actually work anymore — dead demos, abandoned
projects, services that changed their pricing or auth and never updated anywhere.
So we started probing. Every endpoint in the index gets a real HTTP request on a
rolling schedule, and only gets marked "live" if it responds the way a real payable
endpoint should (HTTP 402 with a valid payment-challenge, or an x402/MPP challenge
body). Right now: 2,328 services indexed, only 1,472 (~63%) actually live.
If your agent's only source of truth is a static list, roughly a third of its calls
are hitting something dead.
What it looks like in practice
Ask the MCP server directly (no auth, no key):
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