diff --git a/src/lib/measurements/__tests__/cumulative-day-sum.test.ts b/src/lib/measurements/__tests__/cumulative-day-sum.test.ts
index 9ad9f52db..62f4c8c6b 100644
--- a/src/lib/measurements/__tests__/cumulative-day-sum.test.ts
+++ b/src/lib/measurements/__tests__/cumulative-day-sum.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 
+import { CUMULATIVE_HK_TYPES } from "../apple-health-mapping";
 import {
   CUMULATIVE_DAY_SUM_TYPES,
   isCumulativeDaySumType,
@@ -105,3 +106,16 @@ describe("isCumulativeDaySumType", () => {
     expect(isCumulativeDaySumType("MOOD")).toBe(false);
   });
 });
+
+describe("CUMULATIVE_DAY_SUM_TYPES / CUMULATIVE_HK_TYPES parity", () => {
+  // v1.4.37 W10 — single source of truth. The day-sum array is derived
+  // from `CUMULATIVE_HK_TYPES` so adding a sixth cumulative HK type to
+  // the mapping module automatically flows through every downstream
+  // consumer (analytics route, exports, chart). The set equivalence
+  // guard pins the contract — divergence is impossible at runtime, but
+  // a future refactor that flips the derivation back to a literal
+  // would trip this assertion before shipping.
+  it("matches CUMULATIVE_HK_TYPES membership exactly", () => {
+    expect(new Set(CUMULATIVE_DAY_SUM_TYPES)).toEqual(CUMULATIVE_HK_TYPES);
+  });
+});
diff --git a/src/lib/measurements/cumulative-day-sum.ts b/src/lib/measurements/cumulative-day-sum.ts
index 3648d3a9f..f09ac657e 100644
--- a/src/lib/measurements/cumulative-day-sum.ts
+++ b/src/lib/measurements/cumulative-day-sum.ts
@@ -62,25 +62,32 @@ export function pickCumulativeDaySum<T extends CumulativeRow>(
   return out;
 }
 
+import type { MeasurementType } from "@/generated/prisma/client";
+import { CUMULATIVE_HK_TYPES } from "./apple-health-mapping";
+
 /**
- * v1.4.36 W4c — Canonical set of cumulative metric types whose
- * dashboard tile should read the *day's sum* and not the latest
- * slice. Kept here so the analytics route and any other consumer
- * (Coach snapshot, exports) share the same source of truth.
+ * v1.4.36 W4c / v1.4.37 W10 — Canonical list of cumulative metric
+ * types whose dashboard tile should read the *day's sum* and not the
+ * latest slice.
+ *
+ * The set lives in `apple-health-mapping.ts` as `CUMULATIVE_HK_TYPES`
+ * (used by the measurements route + nightly drain + chart). This
+ * array is the same membership in stable alphabetical order so
+ * downstream consumers (analytics route, exports) can iterate
+ * deterministically. The W10 reconcile pass collapses the previously
+ * duplicated literal to a single derivation so adding a sixth type
+ * in `apple-health-mapping.ts` automatically flows through every
+ * cumulative-day-sum consumer.
  *
  * Note: SLEEP_DURATION is also cumulative but the route handles
  * it inline because the picker is the canonical sleep source ladder
  * ("sleep" metricKey vs "default"). Once Apple-Health passthrough
  * lands and the sleep ladder needs the same shared treatment it can
- * fold into this list.
+ * fold into `CUMULATIVE_HK_TYPES`.
  */
-export const CUMULATIVE_DAY_SUM_TYPES = [
-  "ACTIVITY_STEPS",
-  "ACTIVE_ENERGY_BURNED",
-  "WALKING_RUNNING_DISTANCE",
-  "FLIGHTS_CLIMBED",
-  "TIME_IN_DAYLIGHT",
-] as const;
+export const CUMULATIVE_DAY_SUM_TYPES = Array.from(
+  CUMULATIVE_HK_TYPES,
+).sort() as readonly MeasurementType[];
 
 export type CumulativeDaySumType = (typeof CUMULATIVE_DAY_SUM_TYPES)[number];