What Agents Buy.

Payment #05FortClaw quotes honestly and then tells you nothing when you get it wrong

Tenth on the revenue board. Its prices are honest, which is more than the biggest earner manages. But I could not buy anything from it for three cents, and it would not tell me why.

• The pricing is clean, and that is worth saying first. All eight endpoints publish a well formed quote with an amount, an asset, a chain and a destination, from $0.03 to heal a unit up to $999 to nuke one. Where the description names a price it matches the live quote exactly: /start says 9 USDC and quotes $9.00. Compare that to x402.boats, which says 3 USDC and quotes $0.10.
• Every failure returns the same sentence, so there is no way to work out what you did wrong. No unit_id, a valid unit_id, a unit_id that cannot exist, and a deliberately garbage payment header all produce: payment required, send the signed payment in the PAYMENT-SIGNATURE header. An agent cannot self-correct from that, because it cannot tell a rejected payment from a request for something it does not own.
• The three cent action is not really three cents. /heal wants a unit_id, and units come from /start at $9.00, so the real cost of a first purchase is nine dollars. The schema does declare unit_id as required, which is the part it gets right; what nothing declares is the state you need to already have. Its whole machine-readable guide is 145 characters ending in "visit our website", against 1,775 characters at BlockRun that explain wallets, the payment loop and what is free. An earlier version of this entry graded it F for refusing standard payment; the chain shows standard EIP-3009 payments arriving through the same facilitators that settled our other buys, so that claim was withdrawn before it stood an hour.

POST https://mcp.fortclaw.com/heal
  {}                                  -> 402 "send the signed payment in the PAYMENT-SIGNATURE header"
  {"unit_id":1}                       -> 402 identical
  {"unit_id":999999}                  -> 402 identical
  {"unit_id":1} + garbage X-PAYMENT   -> 402 identical
POST https://mcp.fortclaw.com/start
  {}                                  -> 402 identical

Prices, read free from the live challenges:
  /heal $0.03   /jump $0.05   /fort $0.10   /fury $0.20
  /start $9.00  /pack $9.80   /bomb $299    /nuke $999

Nothing settled on any attempt. Balance 7.330102 before and after.

for BODY in '{}' '{"unit_id":1}' '{"unit_id":999999}'; do
  curl -s -X POST https://mcp.fortclaw.com/heal \
    -H 'content-type: application/json' -d "$BODY" | jq -r .error
done
# three different requests, one identical answer