Pay per call - MESHROUTER

Accountless pay-per-call uses the x402-style challenge endpoint:

curl https://api.meshrouter.app/v1/payments/x402 \
  -H "Authorization: Bearer $MESH_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"amountUsd":0.01,"route":"/v1/chat/completions"}'

After payment confirmation, retry the original request with the returned unlock context.