Architecture

One Node 22 / Express process serving everything; deliberately boring where possible.

                      ┌──────────────────────────────────────────────┐
   agent (buyer) ───▶ │  Express                                     │
                      │   free surfaces: /, /tools, /api/pricing,    │
                      │     /openapi.json, /llms.txt, /api/stats,    │
                      │     /api/pow*, /mcp, /privacy                │
                      │   gate (per request):                        │
                      │     marketplace token? ──▶ bypass            │
                      │     valid X-Pow-Solution? ──▶ bypass         │
                      │     else ──▶ x402 paywall (402 quote /       │
                      │              verify+settle via facilitator)  │
                      │   ~1,083 tool handlers (pure fns + kits)     │
                      └──────┬───────────────┬───────────────────────┘
                             │               │
                   Playwright Chromium   SQLite (WAL) on /data volume
                   ffmpeg (no shell)     (stats · memory · pow replay)

Key pieces

• Catalog as data. Every tool is an entry { route, slug, price, description, discovery: { inputSchema, example }, handler }. The paywall, docs pages, OpenAPI spec, llms.txt, sitemap, MCP servers, and CI tests are all generated from the same catalog — one source of truth, so a new tool is automatically priced, documented, discoverable, and tested.
• Payments (src/payments.js): @x402/express middleware quoting USDC on Base (eip155:8453), settled through the Coinbase CDP facilitator (CDP_API_KEY_ID/SECRET; FACILITATOR_URL overrides). Multi-chain USDC schemes are registered in code.
• Proof-of-work (src/pow.js): HMAC-signed challenges, difficulty 16 bits, single-use (replay table in SQLite), strictly slug-scoped. A WALLET_ONLY_SLUGS set keeps anything that costs real money out of the free tier.
• Browser tools (src/tools/render.js): a shared headless Chromium with max 3 concurrent contexts, self-healing relaunch on crash, and per-request SSRF re-validation of every subresource the page loads (see Security Model).
• Media tools: ffmpeg via execFile (no shell), 30 MB cap, 90 s timeout, max 2 concurrent with 429 + Retry-After.
• Remote MCP (src/mcp-http.js): stateless streamable-HTTP endpoint mounted before the paywall; it meters itself (free set + per-IP rate limit) and feeds the same stats counters.
• x402 Index + Router (src/x402-index.js): a free, in-memory aggregation layer. Crawls the local catalog + operator seeds + auto-discovered sellers (from public x402 registries, refreshed hourly) every 5 minutes via safeFetch. Every crawl outcome lands in a rolling 5-entry history per seller; the Smart Order Router (POST /api/route) skips sellers whose recent history shows errors, and tiebreaks on health then price. Public surfaces: /index (HTML), /api/index (JSON), /api/route (router). See x402-Index-and-Router.
• Marketplace bridge (/mkt/:token/:slug): agent402.app collects the buyer's USDC (settled directly to our wallet) and forwards the call with a token that bypasses our own paywall. The token in the URL is per-slug (HMAC(master, slug)), so the master secret never appears in a URL and a leaked endpoint exposes only its one tool — timing-safe comparison, global rate cap.
• State: SQLite (better-sqlite3, WAL) on a Railway persistent volume at /data — stats, memory namespaces, PoW replay protection all survive redeploys.
• Shutdown: SIGTERM drains in-flight requests before exit, because a hard kill would take an agent's money and return nothing.

Design positions

• No LLM in the serving path. Determinism is the product: schemas, flat prices, reproducible outputs.
• Payment is identity. No accounts means no credential database, no signup abuse surface, and memory ownership falls out of the payment protocol for free.
• Charge-then-fail is unacceptable. x402 settles before the handler runs, so anything that can't be served reliably (e.g. upstreams that block datacenter IPs) gets removed from the catalog rather than monetized.