The API

# 1 · A decision for the product page
curl -s -X POST "$CROSSUP_BASE_URL/v1/recommendations" \
  -H "Authorization: Bearer $CROSSUP_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "connection_id": "cn_…", "surface": "pdp", "placement_key": "pdp-related", "context_item_id": "itm_…" }'

# 2 · The signal that it was shown, tied to that decision
curl -s -X POST "$CROSSUP_BASE_URL/v1/connections/cn_…/storefront-signals" \
  -H "Authorization: Bearer $CROSSUP_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "signals": [ { "signal_id": "sig_…", "type": "impression", "occurred_at": "2026-09-18T12:00:00Z", "decision_id": "dec_…" } ] }'

const res = await fetch(`${base}/v1/recommendations`, {
  method: "POST",
  headers: { Authorization: `Bearer ${apiKey}`, "Content-Type": "application/json" },
  body: JSON.stringify({ connection_id, surface: "pdp", placement_key: "pdp-related", context_item_id }),
});
const decision = await res.json(); // decision_id, results[], reason_codes[]

// The signal, with a stable id in case you retry.
await fetch(`${base}/v1/connections/${connection_id}/storefront-signals`, {
  method: "POST",
  headers: { Authorization: `Bearer ${apiKey}`, "Content-Type": "application/json" },
  body: JSON.stringify({ signals: [{ signal_id, type: "impression", occurred_at, decision_id: decision.decision_id }] }),
});

req, _ := http.NewRequest("POST", base+"/v1/recommendations", toJSON(map[string]any{
    "connection_id":   connectionID,
    "surface":         "pdp",           // the moment
    "placement_key":   "pdp-related",   // the placement name
    "context_item_id": itemID,          // the product the shopper is viewing
}))
req.Header.Set("Authorization", "Bearer "+apiKey)
req.Header.Set("Content-Type", "application/json")
resp, err := http.DefaultClient.Do(req) // 200 with the decision; empty is also valid

// The signal that it was shown goes next, to the same host, tied to decision_id.