New x402 Listings Feed

Feed of x402/L402 services newly listed on 402index.io within a caller-specified recency window. NEXUS candidate #8 -- manual build, not FORGE-generated, same manual-Cloud-Run-asset pattern as candidates #3 (agent-verification-api), #4 (url-metadata-api) and #6 (document-conversion-api).

• POST /new-x402-listings {"window_hours": 24, "protocol": null, "category": null, "payment_network": null} -- services registered on 402index.io within the window (1-168h, default 24). $0.01/call.
• MCP tool get_new_x402_listings at /mcp, same params -- currently free, see "Known limitations".
• GET /health, GET /.well-known/agent-card.json, GET /openapi.json (has x-payment-info), GET /.well-known/402index-verify.txt (402index claim verification file).

Mainnet cutover (2026-09-03)

Originally built and measured on Base Sepolia testnet. Cut over to Base mainnet: x402 settlement moved to the CDP facilitator (create_facilitator_config(), same swap already applied to ws/live-entity-verification/erc8004-agent-liveness/onchain-activity-index/x402-receipt-verifier), and the payto wallet moved to NEXUS_X402_PAYTO_ADDRESS (fail-fast env var, no placeholder default, renamed from X402_WALLET_ADDRESS). CDP_API_KEY_ID/CDP_API_KEY_SECRET and NEXUS_X402_PAYTO_ADDRESS must be set in Cloud Run before this deploys.

What this is (and isn't)

This is not exclusive data. The registration data underneath this asset is 402index.io's own free, public directory (https://402index.io/api-docs, no auth needed, 100 req/min free tier). Anyone can query it directly for nothing. This asset's value is entirely in the packaging: polling and paginating the ~96k+ entry catalog so a buyer doesn't have to, merging in 402index's own recency feed for freshness, deduping, and filtering to a caller's window/protocol/category/payment-network. This disclosure is not just in this README -- it's baked into the product itself: every response includes a note field stating it plainly, and the agent-card's protocol_note repeats it, so a buyer never has to dig for it.

Feasibility research (2026-08-23, before writing any code)

The task brief's premise cited a prior-session figure of "~7,595 total services" on 402index.io. A real curl https://402index.io/api/v1/services?limit=5 at the start of this session showed "total": 96093 -- the catalog grew roughly 12x since that check. registered_at is real and populated on every entry sampled. Two upstream mechanisms were verified live, each with a real limitation neither the task brief nor 402index's own docs fully surfaced:

1. GET /api/v1/services (paginated, limit/offset, max 200/page) has NO sort-by-date or date-range filter -- sort only accepts name/price/latency/uptime/reliability (checked /api-docs directly). A window query can only be answered by walking the entire catalog and filtering client-side -- there is no cheaper server-side path. At the real current size that's ~481 pages, not the ~40 pages the ~7,595 figure would have implied.
2. GET /feed.xml?type=new is real, RSS 2.0, and IS already recency-sorted (pubDate) -- and is on 402index's own documented rate-limit-exempt list. But it's capped to a fixed item count: a live fetch during this session returned exactly 90 items spanning only ~3 hours, despite the docs describing type=new as "services added in the last 7 days". At current registration velocity it does not cover a 7-day (or even a 24h, at peak velocity) window by itself.

Neither source alone honestly satisfies the product's advertised window range. Decision: proceed, using both. See the main.py module docstring and "Architecture" below for how they're combined. This is the kind of real, current-data discrepancy CLAUDE.md SS3 asks to surface rather than build past silently -- so it's recorded here rather than only in a chat transcript.

Architecture

• A background asyncio task (started in FastAPI's lifespan, not blocking startup) walks the full /api/v1/services catalog every NEXUS_CATALOG_REFRESH_SECONDS (default 600s/10min), paced at 0.65s/request (~92 req/min sustained, safety margin under 402index's 100 req/min free-tier cap). Results are cached in memory (_catalog_cache), keyed by service id. A full walk at the current catalog size is ~481 pages / ~5.2 minutes -- entirely a background-task cost, never inline in a buyer's request.
• Every buyer request additionally fetches /feed.xml?type=new live (rate-limit-exempt, ~1 request, cheap) and merges it into whatever the background walk has cached, catching anything registered after the last completed walk. Cache entries win on id conflict (richer fields); feed entries only fill gaps.
• Response is filtered/deduped/sorted (newest first) from that merge, capped at 500 results.

Why not a literal TTL cache (deviation from the task brief's suggested shape)

The brief suggested "a short in-memory cache (5-10 min TTL)". Implemented instead: a continuously-running background loop that re-walks every 10 minutes and replaces the cache, with buyer requests always reading whatever is currently cached (never triggering a walk themselves). A literal on-demand-when-stale TTL would mean whichever buyer request happens to arrive right after expiry pays $0.01 and then blocks for up to ~5 minutes waiting on a fresh 481-page walk -- unacceptable buyer experience found during design, not retroactively. The background-loop shape gets the same "don't hammer 402index.io" goal without ever making a paying caller wait on the walk.

Cold start (Cloud Run scale-to-zero)

min-instances=0 means a fresh container starts with an empty cache. The first request(s) after any period of inactivity get catalog_walk.status: "cold_fallback_feed_only" -- results are limited to whatever /feed.xml?type=new currently holds (recently observed to span only a few hours), not the full requested window. This is disclosed in the response's own note field, not hidden. Once the background task's first walk completes (~5 minutes after container start), subsequent requests get status: "warm" with full catalog-walk coverage. Genuinely mitigated, not solved -- see "Known limitations".

Pricing: $0.01/call (low tier)

Speculative niche per the product owner (explicit direction, not a data-driven conclusion) -- same low tier as url-metadata-api/document-conversion-api's $0.01-$0.02, not agent-verification-api's $0.35 signal tier. No paid third-party API cost, pure CPU/memory + one free upstream fetch per call.

Pre-deploy quality gate (2026-08-23, from design not retroactive)

Reviewed across 4 lenses before first deploy, tested against the real live 402index.io API (not mocked) at each step. Real findings and fixes:

• Security: caller input (window_hours/protocol/category/payment_network) is never interpolated into the upstream 402index.io request -- the catalog walk and feed fetch use fixed, hardcoded query params only (limit/offset/sort/order and type=new respectively); caller filters apply exclusively to the already-fetched, normalized in-memory result. Verified upstream JSON/XML is never trusted blindly: every field read via .get() with type checks (_normalize_catalog_item returns None on a malformed entry rather than raising), malformed XML from /feed.xml is caught (ET.ParseError) rather than crashing the request, and a single malformed <item> in the feed doesn't drop the rest of it. All confirmed with real malformed-input tests (None, missing id, wrong types, garbage timestamp, truncated XML) during this session, not just read as correct.
• Functional correctness (real gap, fixed): the initial design cached complete: bool internally but never surfaced it in the response -- a caller had no way to tell a background walk that hit the _WALK_MAX_WALL_SECONDS wall-clock cap (bailing out with a partial result) from one that finished cleanly, even though both report status: "warm". Fixed: added catalog_walk.walk_complete to the response. Pagination itself (offset advance, stop-at-total, wall-cap bailout) was verified against the real live API with a bounded test walk (8 real pages, offset advancing correctly, graceful bailout logged) -- no silent truncation, no infinite loop.
• Code quality: no dead code or unused imports found in review; matches sibling assets' structure (Supabase telemetry helpers, x402 wiring, discovery routes) ported by hand, consistent with how document-conversion-api/url-metadata-api are built.
• Buyer experience (the "raw data is free elsewhere" disclosure lens): the fact that 402index.io's data is itself free was verified present in 3 places, not just the README: the agent-card protocol_note, and -- more directly, so no buyer has to fetch the agent-card at all -- every single API response's own note field. Cold-cache degraded coverage (cold_fallback_feed_only) is also disclosed in that same note field with plain language about what it means for the requested window, not just a status enum a caller has to already know how to interpret.

Known limitations (left as documented tradeoffs, not silently)