Supplier Stock & Price Monitor (Public Pages)

Watches a small list of your own suppliers' public product pages and reports price and stock/availability changes between runs. Built for dropshippers and small retailers whose suppliers have no API - the people currently hand-writing scraper scripts or catching supplier price rises days late. This is operational continuity ("did MY supplier's SKU go out of stock / did MY cost change"), not competitor price intelligence.

What it does

• One plain HTTP GET per watched URL per run. No crawling, no retries, no JavaScript rendering, no CAPTCHA or bot-check bypass.
• Fetch order: (1) Shopify storefronts - the public /products/<handle>.js endpoint (verified live 2026-10-31 on a public Shopify store: returns per-variant sku, available, and integer-cent price); (2) any other site - schema.org Product/Offer JSON-LD (price, priceCurrency, availability) embedded in the HTML (https://schema.org/Product, https://schema.org/Offer).
• Diffs against the previous run's snapshot and emits deterministic events: product_added, variant_added, variant_removed, price_changed (from/to in minor units + currency), out_of_stock, back_in_stock, status_changed.
• Blocked pages (401/403/429/503) are reported as status: "blocked" - never worked around. Verified live: repeated polling of one storefront within minutes returns HTTP 429, so schedule conservatively (hourly at most per URL; less for small watchlists) and expect some sites (e.g. gymshark.com) to refuse plain fetchers entirely.

Safety guards (v2, after review)

• SSRF protection: only http/https, no credentials in URLs, ports 80/443 only, and every hostname (including redirect targets, validated at redirect time) must resolve to public IPs - loopback/private/link-local/multicast/ reserved are refused before any request leaves the box. Residual limit: stdlib-only cannot pin DNS against rebinding between check and connect; documented, not hidden.
• Body cap: responses over 5MB are rejected.
• Baseline preservation: a failed run (429, timeout, parse failure) never overwrites the last-known-good snapshot for that URL, so transient failures cannot manufacture false change events on recovery (tested).
• Unknown stays unknown: a missing available field is unknown and fires no stock event - it is never treated as out-of-stock (tested).
• Currency precision: minor units per currency (JPY 0, BHD-type 3, else 2); prices with more precision than the currency supports are rejected, never rounded (tested).

Honest limits

• Availability is only what the page publicly exposes (usually a per-variant boolean). Stock quantities are typically not public and are not invented.
• Shopify .js prices carry no currency field; currency comes from the watchlist entry's optional currency hint, else null.
• Sites without Shopify .js or JSON-LD return parse_failed.
• No login-walled supplier portals. Public pages only.

Input

JSON watchlist on stdin / input field: [{"url": "https://supplier.example/products/some-item", "currency": "USD"}, ...]

CLI: python3 supplier_monitor.py --snapshot snap.json < watchlist.json (snapshot optional; without it every URL reports product_added). For the Apify wrapper: input schema {watchlist: [...]}; persist the snapshot in the Actor's default key-value store (STATE/snapshot.json), push results + events to the dataset, log counts only (never full URLs with customer context or variant detail).

Fixtures and tests

• fixtures/shopify_product.js - synthetic, modeled on the verified live Shopify .js shape (integer-cent prices)
• fixtures/product_page.html - synthetic JSON-LD page incl. a malformed block (skipped) and InStock/OutOfStock variants
• test_supplier_monitor.py - 38 deterministic tests, all passing (python3 test_supplier_monitor.py), incl. a two-pass run with snapshot proving price-change and restock events fire exactly once

No third-party dependencies. No code or data copied from external sources.

Demand evidence (gathered 2026-10-31)

• r/dropshipping "How do you track supplier stock and price changes?" (Jun 2026): users hand-write scraper scripts; "price increases can slip through for days... margin damage is already done." https://www.reddit.com/r/dropshipping/comments/1u9gt9c/
• r/dropshipping: "I lost two sales this week because an item was out of stock and I didn't catch it in time." https://www.reddit.com/r/dropshipping/comments/1my2wr4/

Competition and pricing (ESTIMATE ONLY, Tom decides)

• On-store: https://apify.com/zentrafoundry/supplier-stock-price-monitor - fetched 2026-10-31: "2 total users, 1 monthly users, no ratings", priced $43.00 per 1,000 change alerts, scoped to "approved public supplier product URLs". Same wedge, verified near-zero traction.
• Suggested event pricing: per change-detected event at ~$0.002-0.005 (i.e. $2-5 per 1,000 alerts vs competitor's $43), plus free first-run baseline (product_added events uncharged). Run cost is one GET per URL ($0.001 for a small watchlist), so margin is ample. Charge only real change events; status_changed to blocked stays free.

Privacy

Watchlists are user-provided public URLs. Runs process public page data only. When deployed on Apify, run inputs/datasets/logs persist on the platform by default - do not claim otherwise; log counts, not payloads.