diff --git a/src/lib/fitbit/sync-core.ts b/src/lib/fitbit/sync-core.ts
index 133c7fdf2..d25ac2487 100644
--- a/src/lib/fitbit/sync-core.ts
+++ b/src/lib/fitbit/sync-core.ts
@@ -228,7 +228,11 @@ export const softSkipStorage = new AsyncLocalStorage<SoftSkipTracker>();
  * the soft-skip tracker.
  */
 export interface RollupDeferTracker {
-  keys: Array<{ type: MeasurementType; measuredAt: Date }>;
+  /**
+   * One entry per touched `(type, UTC day)`, never one per reading, so the
+   * ledger stays the size of the history in days rather than in samples.
+   */
+  keys: Map<string, { type: MeasurementType; measuredAt: Date }>;
 }
 export const rollupDeferStorage = new AsyncLocalStorage<RollupDeferTracker>();
 
@@ -877,14 +881,13 @@ export async function upsertFitbitMeasurements(
   // path keeps the inline per-day hook here (small touched set, warm next read).
   if (opts.deferRollup) {
     const tracker = rollupDeferStorage.getStore();
-    // Plain loop, never `push(...touched)`: `touched` carries ONE entry PER
-    // READING, and a full-history batch on a sample-dense account (a
-    // multi-year heart-rate series is six figures of rows) blows the call
-    // stack when spread into a single call — the engine caps argument counts.
-    // The RangeError aborted the whole metrics collection on exactly the
-    // accounts with the most data.
+    // Collapsed to type-days on the way in: `touched` carries ONE entry PER
+    // READING and the tracker lives for the whole cycle, so keeping the raw
+    // entries made the ledger grow with the sample count of the history.
     if (tracker) {
-      for (const t of touched) tracker.keys.push(t);
+      for (const k of collapseToTypeDayKeys(touched)) {
+        tracker.keys.set(`${k.type}|${k.measuredAt.getTime()}`, k);
+      }
     }
     return { imported, touched, inserted: insertedRows };
   }
diff --git a/src/lib/fitbit/sync.ts b/src/lib/fitbit/sync.ts
index 9195f7bcb..449efbff2 100644
--- a/src/lib/fitbit/sync.ts
+++ b/src/lib/fitbit/sync.ts
@@ -5,10 +5,7 @@ import { prisma } from "@/lib/db";
 import { invalidateStatusInsightsForTypes } from "@/lib/insights/comprehensive-generate";
 import { isReauthRequired, recordSyncSuccess } from "@/lib/integrations/status";
 import { annotate, getEvent } from "@/lib/logging/context";
-import {
-  collapseToTypeDayKeys,
-  recomputeUserRollups,
-} from "@/lib/rollups/measurement-rollups";
+import { recomputeUserRollups } from "@/lib/rollups/measurement-rollups";
 import { syncUserActivity } from "./sync-activity";
 import {
   hardFailStorage,
@@ -90,7 +87,7 @@ export async function syncUserFitbit(
   ];
 
   const tracker: SoftSkipTracker = { count: 0 };
-  const deferTracker: RollupDeferTracker = { keys: [] };
+  const deferTracker: RollupDeferTracker = { keys: new Map() };
   const hardFailTracker: HardFailTracker = { failures: [] };
   let total = 0;
   let anyFailed = false;
@@ -120,9 +117,9 @@ export async function syncUserFitbit(
   // touched days. One pass replaces the thousands of per-(type,day) round-trips a
   // deep backfill would otherwise pay (each = an aggregate SELECT + rollup upsert
   // + 3 queue sends). Best-effort: a populator hiccup never fails the backfill.
-  if (opts.fullSync && deferTracker.keys.length > 0) {
+  if (opts.fullSync && deferTracker.keys.size > 0) {
     try {
-      const days = collapseToTypeDayKeys(deferTracker.keys);
+      const days = Array.from(deferTracker.keys.values());
       const types = Array.from(new Set(days.map((k) => k.type)));
       const sorted = days
         .map((k) => k.measuredAt.getTime())
diff --git a/src/lib/google-health/__tests__/sync-deferred-rollup-ledger.test.ts b/src/lib/google-health/__tests__/sync-deferred-rollup-ledger.test.ts
new file mode 100644
index 000000000..cea735f92
--- /dev/null
+++ b/src/lib/google-health/__tests__/sync-deferred-rollup-ledger.test.ts
@@ -0,0 +1,97 @@
+/**
+ * #1023 — the deferred-rollup ledger of a backfill cycle holds type-days, not
+ * readings.
+ *
+ * On a full-history backfill every write defers its rollup fold to one pass at
+ * the end of the cycle, and the cycle-wide ledger used to keep one entry per
+ * written reading. A per-minute heart-rate history of two and a half years is
+ * 1.3 M readings over about 900 days, so the ledger alone grew with the stream
+ * for the whole walk. It now keeps one entry per touched `(type, UTC day)`.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { findManyMock, createManyMock } = vi.hoisted(() => ({
+  findManyMock: vi.fn(async () => [] as unknown[]),
+  createManyMock: vi.fn(),
+}));
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findMany: findManyMock,
+      createManyAndReturn: createManyMock,
+      update: vi.fn(),
+    },
+  },
+}));
+vi.mock("@/lib/crypto", () => ({ encrypt: vi.fn(), decrypt: vi.fn() }));
+vi.mock("@/lib/integrations/status", () => ({
+  isReauthRequired: vi.fn(async () => false),
+  recordSyncFailure: vi.fn(async () => {}),
+  recordSyncSuccess: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/insights/comprehensive-generate", () => ({
+  invalidateStatusInsightsForTypes: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/arrivals/measurement-emit", () => ({
+  emitInsertedMeasurementArrivals: vi.fn(async () => {}),
+}));
+vi.mock("../credentials", () => ({
+  getUserGoogleHealthCredentials: vi.fn(async () => null),
+}));
+
+import {
+  runWithGoogleHealthSyncCycle,
+  upsertGoogleHealthMeasurements,
+} from "../sync-core";
+
+const DAYS = 3;
+const PER_DAY = 1440;
+const START = Date.parse("2026-09-01T00:00:00.000Z");
+
+beforeEach(() => {
+  findManyMock.mockReset().mockResolvedValue([]);
+  createManyMock
+    .mockReset()
+    .mockImplementation(
+      async (args: { data: Array<{ type: string; measuredAt: Date }> }) =>
+        args.data.map((d, i) => ({
+          id: `row-${i}`,
+          type: d.type,
+          measuredAt: d.measuredAt,
+        })),
+    );
+});
+
+describe("deferred rollup ledger on a dense backfill", () => {
+  it("keeps one key per touched type-day however many readings land", async () => {
+    const cycle = await runWithGoogleHealthSyncCycle(async () => {
+      for (let day = 0; day < DAYS; day++) {
+        const readings = [];
+        for (let m = 0; m < PER_DAY; m++) {
+          const at = new Date(START + day * 86_400_000 + m * 60_000);
+          readings.push({
+            type: "PULSE",
+            value: 60,
+            unit: "bpm",
+            measuredAt: at,
+            externalId: `${at.toISOString()}:hr`,
+          });
+        }
+        await upsertGoogleHealthMeasurements("dense-user", readings, {
+          deferRollup: true,
+        });
+      }
+      return null;
+    });
+
+    expect(cycle.deferredRollupKeys).toHaveLength(DAYS);
+    expect(
+      cycle.deferredRollupKeys.map((k) => k.measuredAt.toISOString()).sort(),
+    ).toEqual([
+      "2026-09-01T00:00:00.000Z",
+      "2026-09-02T00:00:00.000Z",
+      "2026-09-03T00:00:00.000Z",
+    ]);
+  });
+});
diff --git a/src/lib/google-health/__tests__/sync-metrics-dense-pages.test.ts b/src/lib/google-health/__tests__/sync-metrics-dense-pages.test.ts
new file mode 100644
index 000000000..22ea1ad96
--- /dev/null
+++ b/src/lib/google-health/__tests__/sync-metrics-dense-pages.test.ts
@@ -0,0 +1,111 @@
+/**
+ * #1023 — a dense heart-rate collection is written page by page.
+ *
+ * Intraday heart rate is one point a minute. `syncUserMetrics` used to collect
+ * the whole collection, map all of it, and hand every reading to one upsert,
+ * so a full-history backfill held a million raw points and a million readings
+ * at once and ran a 1 GB heap out of memory (the reproduction on 1.3 M
+ * readings dies within seconds of the walk). The contract pinned here: every
+ * upsert carries at most one page, and a page is written before the next one
+ * is requested, so nothing but the current page is ever resident.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { safeFetchMock, upsertMock, events } = vi.hoisted(() => ({
+  safeFetchMock: vi.fn(),
+  upsertMock: vi.fn(),
+  events: [] as string[],
+}));
+
+vi.mock("@/lib/safe-fetch", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/safe-fetch")>();
+  return { ...actual, safeFetch: safeFetchMock };
+});
+
+vi.mock("../sync-core", () => ({
+  getValidToken: vi.fn(async () => ({ accessToken: "token" })),
+  handleCollectionFetchError: vi.fn(async () => 0),
+  upsertGoogleHealthMeasurements: upsertMock,
+}));
+
+vi.mock("@/lib/db", () => ({
+  prisma: { user: { findUnique: vi.fn(), update: vi.fn() } },
+}));
+
+vi.mock("@/lib/tz/resolver", () => ({
+  resolveUserTimezone: vi.fn(async () => "UTC"),
+}));
+
+import { GOOGLE_HEALTH_PAGE_SIZE } from "../client";
+import { syncUserMetrics } from "../sync-metrics";
+
+const PAGES = 3;
+const NOW = Date.parse("2026-09-27T17:00:00.000Z");
+
+function heartRatePage(page: number): unknown {
+  const dataPoints = [];
+  for (let i = 0; i < GOOGLE_HEALTH_PAGE_SIZE; i++) {
+    const g = page * GOOGLE_HEALTH_PAGE_SIZE + i;
+    dataPoints.push({
+      heartRate: {
+        beatsPerMinute: String(55 + (g % 50)),
+        sampleTime: { physicalTime: new Date(NOW - g * 60_000).toISOString() },
+      },
+    });
+  }
+  return {
+    dataPoints,
+    ...(page + 1 < PAGES ? { nextPageToken: String(page + 1) } : {}),
+  };
+}
+
+beforeEach(() => {
+  events.length = 0;
+  upsertMock.mockReset().mockImplementation(async (_u, readings: unknown[]) => {
+    events.push(`upsert:${readings.length}`);
+    return { imported: readings.length, touched: [], inserted: [] };
+  });
+  safeFetchMock.mockReset().mockImplementation(async (url: string) => {
+    const u = new URL(url);
+    let body: unknown = {};
+    if (u.pathname.endsWith("/dataTypes/heart-rate/dataPoints")) {
+      const page = Number(u.searchParams.get("pageToken") ?? 0);
+      events.push(`fetch:${page}`);
+      body = heartRatePage(page);
+    }
+    return {
+      status: 200,
+      ok: true,
+      json: async () => body,
+    } as unknown as Response;
+  });
+});
+
+describe("syncUserMetrics over a dense heart-rate collection", () => {
+  it("writes each page before the next one is requested", async () => {
+    const imported = await syncUserMetrics("dense-user", {
+      deferRollup: true,
+    });
+
+    expect(imported).toBe(PAGES * GOOGLE_HEALTH_PAGE_SIZE);
+    const pulseWrites = upsertMock.mock.calls
+      .map((c) => c[1] as Array<{ type: string }>)
+      .filter((readings) => readings.some((r) => r.type === "PULSE"));
+    expect(pulseWrites).toHaveLength(PAGES);
+    for (const readings of pulseWrites) {
+      expect(readings.length).toBeLessThanOrEqual(GOOGLE_HEALTH_PAGE_SIZE);
+    }
+    const hr = events.filter(
+      (e) =>
+        e.startsWith("fetch:") || e === `upsert:${GOOGLE_HEALTH_PAGE_SIZE}`,
+    );
+    expect(hr).toEqual([
+      "fetch:0",
+      `upsert:${GOOGLE_HEALTH_PAGE_SIZE}`,
+      "fetch:1",
+      `upsert:${GOOGLE_HEALTH_PAGE_SIZE}`,
+      "fetch:2",
+      `upsert:${GOOGLE_HEALTH_PAGE_SIZE}`,
+    ]);
+  });
+});
diff --git a/src/lib/google-health/__tests__/sync-metrics-failsoft.test.ts b/src/lib/google-health/__tests__/sync-metrics-failsoft.test.ts
index c9cfe6a94..8d48929df 100644
--- a/src/lib/google-health/__tests__/sync-metrics-failsoft.test.ts
+++ b/src/lib/google-health/__tests__/sync-metrics-failsoft.test.ts
@@ -12,6 +12,8 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const { fetchDataPointsMock, handleErrorMock, mapWeightMock } = vi.hoisted(
   () => ({
+    // Stands in for the page walk: every metric type is streamed through
+    // `forEachDataPointPage`, which hands its pages to the callback.
     fetchDataPointsMock: vi.fn(),
     handleErrorMock: vi.fn(async () => 0),
     mapWeightMock: vi.fn(() => {
@@ -25,6 +27,23 @@ vi.mock("../client", async (importOriginal) => {
   return {
     ...actual,
     fetchDataPoints: fetchDataPointsMock,
+    forEachDataPointPage: vi.fn(
+      async (
+        dataType: unknown,
+        token: string,
+        verb: string,
+        query: unknown,
+        onPage: (points: unknown[]) => Promise<void>,
+      ) => {
+        const points = (await fetchDataPointsMock(
+          dataType,
+          token,
+          verb,
+          query,
+        )) as unknown[];
+        if (points.length > 0) await onPage(points);
+      },
+    ),
     mapWeight: mapWeightMock,
   };
 });
diff --git a/src/lib/google-health/client.ts b/src/lib/google-health/client.ts
index 0a21e75fc..b38e95a1b 100644
--- a/src/lib/google-health/client.ts
+++ b/src/lib/google-health/client.ts
@@ -512,6 +512,20 @@ export const GOOGLE_HEALTH_PAGE_SIZE = 1000;
 /** Sleep/exercise read cap — matches the Google Health 25-cap for those types. */
 export const GOOGLE_HEALTH_ACTIVITY_PAGE_SIZE = 25;
 
+/**
+ * Page ceiling for the intraday heart-rate walk.
+ *
+ * The default of 1000 pages is a million points, which a watch that records
+ * a reading a minute passes in under two years. A walk that hits the ceiling
+ * reports `truncated`, the backfill treats that as incomplete and throws, and
+ * pg-boss retries it — so an account with a longer history re-ran the whole
+ * walk on every retry and every boot and never completed. Ten thousand pages
+ * is nineteen years at that rate: still a bound on a runaway cursor, no longer
+ * a bound a real history reaches. The walk is paged (`forEachDataPointPage`),
+ * so the ceiling costs time, not memory.
+ */
+export const GOOGLE_HEALTH_DENSE_MAX_PAGES = 10_000;
+
 /** `dataPoints.list` envelope: `{ dataPoints, nextPageToken }`. */
 interface GoogleHealthDataPointPage {
   dataPoints?: GoogleHealthDataPoint[];
@@ -715,6 +729,35 @@ export async function fetchDataPoints(
   verb: string,
   query: DataPointQuery = {},
 ): Promise<GoogleHealthDataPoint[]> {
+  const points: GoogleHealthDataPoint[] = [];
+  await forEachDataPointPage(dataType, accessToken, verb, query, (page) => {
+    for (const p of page) points.push(p);
+  });
+  return points;
+}
+
+/**
+ * The same walk as `fetchDataPoints`, handing each page to `onPage` as it
+ * arrives instead of collecting the whole collection first.
+ *
+ * A dense collection has to be read this way. Intraday heart rate is one
+ * point a minute, so a full-history backfill of two years is over a million
+ * points; held as parsed JSON and then again as mapped readings, that is more
+ * than a 1 GB heap, and the process that runs the backfill also serves the
+ * app. Page by page, only one page is ever resident.
+ *
+ * `onPage` is awaited before the next page is requested, so a slow write
+ * applies back-pressure to the fetch. The date-filter fallback still works:
+ * it only fires when the first request is rejected, which is before any page
+ * reaches `onPage`.
+ */
+export async function forEachDataPointPage(
+  dataType: GoogleHealthDataType,
+  accessToken: string,
+  verb: string,
+  query: DataPointQuery,
+  onPage: (points: GoogleHealthDataPoint[]) => void | Promise<void>,
+): Promise<void> {
   const maxPages = query.maxPages ?? 1000;
   const pageSize = query.pageSize ?? GOOGLE_HEALTH_PAGE_SIZE;
 
@@ -722,8 +765,8 @@ export async function fetchDataPoints(
 
   const walk = async (
     dateStyle: GoogleHealthDateFilterStyle,
-  ): Promise<GoogleHealthDataPoint[]> => {
-    const points: GoogleHealthDataPoint[] = [];
+  ): Promise<void> => {
+    let fetched = 0;
     let pageToken: string | null | undefined;
     let pageCount = 0;
 
@@ -780,13 +823,14 @@ export async function fetchDataPoints(
         });
       }
 
-      for (const p of json?.dataPoints ?? []) points.push(p);
+      const page = json?.dataPoints ?? [];
       pageToken = json?.nextPageToken ?? null;
       pageCount += 1;
+      fetched += page.length;
+      await onPage(page);
     } while (pageToken && pageCount < maxPages);
 
-    notePagination(pageCount, points.length, Boolean(pageToken));
-    return points;
+    notePagination(pageCount, fetched, Boolean(pageToken));
   };
 
   // The fallback only exists for filtered daily-summary reads — every other
@@ -795,9 +839,8 @@ export async function fetchDataPoints(
     dataType.timeField === "date" && query.start !== undefined;
 
   let style: GoogleHealthDateFilterStyle = "camel";
-  let points: GoogleHealthDataPoint[];
   try {
-    points = await walk("camel");
+    await walk("camel");
   } catch (err) {
     const firstRequestRejected =
       canFallBack &&
@@ -806,13 +849,12 @@ export async function fetchDataPoints(
       err.httpStatus === 400;
     if (!firstRequestRejected) throw err;
     style = "snake";
-    points = await walk("snake");
+    await walk("snake");
   }
   if (canFallBack) {
     annotate({ meta: { "googleHealth.dateFilter.style": style } });
     query.onDateFilterStyle?.(style);
   }
-  return points;
 }
 
 // ─── Daily roll-up reads (`POST …/dataPoints:dailyRollUp`) ─────────────────
diff --git a/src/lib/google-health/sync-core.ts b/src/lib/google-health/sync-core.ts
index bef34d971..3b478e8d4 100644
--- a/src/lib/google-health/sync-core.ts
+++ b/src/lib/google-health/sync-core.ts
@@ -265,7 +265,12 @@ export async function runWithGoogleHealthHardFailLedger<T>(
  * AsyncLocalStorage so concurrent per-user jobs never cross-pollinate.
  */
 interface RollupDeferTracker {
-  keys: Array<{ type: MeasurementType; measuredAt: Date }>;
+  /**
+   * One entry per touched `(type, UTC day)`, never one per reading. A
+   * full-history heart-rate backfill touches over a million readings but only
+   * a few hundred days; a per-reading list grew with the stream.
+   */
+  keys: Map<string, { type: MeasurementType; measuredAt: Date }>;
 }
 const rollupDeferStorage = new AsyncLocalStorage<RollupDeferTracker>();
 
@@ -286,7 +291,7 @@ export async function runWithGoogleHealthSyncCycle<T>(
 ): Promise<GoogleHealthSyncCycleResult<T>> {
   const softSkips: SoftSkipTracker = { count: 0 };
   const hardFailures: HardFailTracker = { failures: [] };
-  const deferredRollups: RollupDeferTracker = { keys: [] };
+  const deferredRollups: RollupDeferTracker = { keys: new Map() };
   const result = await softSkipStorage.run(softSkips, () =>
     hardFailStorage.run(hardFailures, () =>
       rollupDeferStorage.run(deferredRollups, fn),
@@ -296,7 +301,7 @@ export async function runWithGoogleHealthSyncCycle<T>(
     result,
     softSkipCount: softSkips.count,
     hardFailures: hardFailures.failures,
-    deferredRollupKeys: deferredRollups.keys,
+    deferredRollupKeys: Array.from(deferredRollups.keys.values()),
   };
 }
 
@@ -831,14 +836,15 @@ export async function upsertGoogleHealthMeasurements(
   // of the cycle. The incremental path keeps the inline per-day hook here.
   if (opts.deferRollup) {
     const tracker = rollupDeferStorage.getStore();
-    // Plain loop, never `push(...touched)`: `touched` carries ONE entry PER
-    // READING, and a full-history batch on a sample-dense account (a
-    // multi-year heart-rate series is six figures of rows) blows the call
-    // stack when spread into a single call — the engine caps argument counts.
-    // The RangeError aborted the whole metrics collection on exactly the
-    // accounts with the most data.
+    // Collapsed to type-days on the way in: `touched` carries ONE entry PER
+    // READING, and the tracker lives for the whole cycle, so keeping the raw
+    // entries made the ledger grow with the stream (a million-reading
+    // heart-rate backfill held a million of them to describe a few hundred
+    // days).
     if (tracker) {
-      for (const t of touched) tracker.keys.push(t);
+      for (const k of collapseToTypeDayKeys(touched)) {
+        tracker.keys.set(`${k.type}|${k.measuredAt.getTime()}`, k);
+      }
     }
     noteGoogleHealthWritten(imported);
     return { imported, touched, inserted: insertedRows };
diff --git a/src/lib/google-health/sync-metrics.ts b/src/lib/google-health/sync-metrics.ts
index 6c204c7ab..d253ed437 100644
--- a/src/lib/google-health/sync-metrics.ts
+++ b/src/lib/google-health/sync-metrics.ts
@@ -27,9 +27,11 @@
  */
 import {
   GOOGLE_HEALTH_DATA_TYPES,
+  GOOGLE_HEALTH_DENSE_MAX_PAGES,
   type GoogleHealthDataType,
   type GoogleHealthMappedMeasurement,
   fetchDataPoints,
+  forEachDataPointPage,
   mapBloodGlucose,
   mapBodyFat,
   mapCoreBodyTemperature,
@@ -61,6 +63,8 @@ interface MetricResource {
     tz: string,
   ) => GoogleHealthMappedMeasurement[];
   verb: string;
+  /** Page ceiling for the walk; the client default when omitted. */
+  maxPages?: number;
 }
 
 /** The launch metric resources (Measurement-producing). Height handled separately. */
@@ -99,6 +103,7 @@ const METRIC_RESOURCES: MetricResource[] = [
     dataType: GOOGLE_HEALTH_DATA_TYPES.heartRate,
     map: mapHeartRate,
     verb: "fetchHeartRate",
+    maxPages: GOOGLE_HEALTH_DENSE_MAX_PAGES,
   },
   {
     dataType: GOOGLE_HEALTH_DATA_TYPES.bloodGlucose,
@@ -139,48 +144,52 @@ export async function syncUserMetrics(
 
   // Each metric data type is fetched + mapped independently so a per-class 403
   // soft-skips only that class.
+  //
+  // Every type is written page by page as it is read. Intraday heart rate is
+  // one point a minute, so a full-history backfill is over a million points;
+  // collecting the whole collection first and mapping it second held the raw
+  // points and the readings at once and ran a 1 GB heap out of memory on
+  // exactly the accounts with the most history. A page is at most
+  // `GOOGLE_HEALTH_PAGE_SIZE` points, and nothing from it outlives its write.
+  //
+  // The mapper runs inside the same per-type catch: a single malformed point
+  // whose `resource.map(point)` throws must not escape the METRIC_RESOURCES
+  // loop and skip every metric type ordered after it (which also blocked the
+  // watermark, so the bad point refetched hourly and those types stayed dead).
+  // A map throw goes through the same ledger as a fetch failure — record it,
+  // fail the cycle, and move on to the next type. Pages already written before
+  // the failure stay written: every write is keyed, so the retry that the
+  // failed verdict causes overwrites them in place.
   for (const resource of METRIC_RESOURCES) {
-    let points: Record<string, unknown>[];
     try {
-      points = await fetchDataPoints(
+      await forEachDataPointPage(
         resource.dataType,
         tokenInfo.accessToken,
         resource.verb,
-        { start },
+        { start, maxPages: resource.maxPages },
+        async (points) => {
+          const readings: GoogleHealthMeasurementUpsert[] = [];
+          for (const point of points) {
+            for (const m of resource.map(point, tz)) {
+              readings.push({
+                type: m.type,
+                value: m.value,
+                unit: m.unit,
+                measuredAt: m.measuredAt,
+                externalId: m.fieldTag,
+              });
+            }
+          }
+          imported += (
+            await upsertGoogleHealthMeasurements(userId, readings, {
+              deferRollup: opts.deferRollup,
+            })
+          ).imported;
+        },
       );
     } catch (err) {
       imported += await handleCollectionFetchError(resource.verb, userId, err);
-      continue;
     }
-
-    // The mapper runs INSIDE a per-type catch too: a single malformed point
-    // whose `resource.map(point)` throws must not escape the METRIC_RESOURCES
-    // loop and skip every metric type ordered after it (which also blocked the
-    // watermark, so the bad point refetched hourly and those types stayed dead).
-    // Route a map throw through the same ledger as a fetch failure — record it,
-    // fail the cycle, and move on to the next type.
-    const readings: GoogleHealthMeasurementUpsert[] = [];
-    try {
-      for (const point of points) {
-        for (const m of resource.map(point, tz)) {
-          readings.push({
-            type: m.type,
-            value: m.value,
-            unit: m.unit,
-            measuredAt: m.measuredAt,
-            externalId: m.fieldTag,
-          });
-        }
-      }
-    } catch (err) {
-      imported += await handleCollectionFetchError(resource.verb, userId, err);
-      continue;
-    }
-    imported += (
-      await upsertGoogleHealthMeasurements(userId, readings, {
-        deferRollup: opts.deferRollup,
-      })
-    ).imported;
   }
 
   // Height → User.heightCm, only when the user has no height yet. Never mint a
diff --git a/tests/integration/google-health-dense-backfill-memory.test.ts b/tests/integration/google-health-dense-backfill-memory.test.ts
new file mode 100644
index 000000000..31414108e
--- /dev/null
+++ b/tests/integration/google-health-dense-backfill-memory.test.ts
@@ -0,0 +1,247 @@
+/**
+ * #1023 — a full-history Google Health backfill over a dense heart-rate
+ * stream, with its live memory measured rather than assumed.
+ *
+ * What this pins. The backfill read every heart-rate point of the account's
+ * history into one array, mapped all of it into a second array, and wrote the
+ * lot in one call, so the raw points, the readings and the write's working
+ * sets were resident together and all of them scaled with the history. A
+ * watch that records a reading a minute passes a million points in two years:
+ * reproduced on 1.3 M readings under `--max-old-space-size=1024`, the backfill
+ * reaches `FATAL ERROR: Reached heap limit` within seconds, and because the
+ * backfill lane shares the app process, the whole instance restarts. The walk
+ * now writes each page as it arrives, and the cycle's deferred-rollup ledger
+ * keeps one key per touched day instead of one per reading.
+ *
+ * How it measures. Every heap reading is taken after a forced collection, so
+ * what is compared is what the code HOLDS, not how much garbage V8 chose to
+ * leave lying about. And what is compared is GROWTH: the heap once a quarter
+ * of the pages have been read, against the most it reaches afterwards. The
+ * first pages carry a fixed cost that has nothing to do with the stream
+ * (modules loaded on first use, the ORM's compiled queries), and that cost is
+ * about the size of this fixture, so an absolute figure could not tell a
+ * streaming walk from one that holds everything. Growth can: a walk that
+ * holds one page stays flat, a walk that holds what it has read climbs with
+ * every page.
+ *
+ * The two halves are the point: the backfill stays inside the budget, and
+ * collecting the same collection into one array (what the backfill used to do
+ * before anything else) does not. Without the second half the budget could
+ * be any number and the first would still pass.
+ *
+ * Google's API is replaced at the `safeFetch` seam by a generator that builds
+ * each page on request, so the fixture itself never sits in memory.
+ */
+import v8 from "node:v8";
+import vm from "node:vm";
+
+import { afterAll, beforeAll, describe, expect, it, vi } from "vitest";
+
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+const USER = "gh-dense-backfill-owner";
+const POINTS = 300_000;
+const NOW = Date.parse("2026-09-27T17:00:00.000Z");
+
+/**
+ * How much the heap may grow between the first quarter of the walk and its
+ * end. Measured, not chosen: the paged walk grows by a few MB on this fixture
+ * (noise around a flat line), and collecting the same pages into one array
+ * grows by about 80 MB over the same stretch.
+ */
+const GROWTH_BUDGET_BYTES = 24 * 1024 * 1024;
+
+const fake = vi.hoisted(() => ({
+  total: 0,
+  now: 0,
+  heartRatePages: 0,
+  /** Called as each heart-rate page is requested (the sampler's hook). */
+  onPage: null as (() => void) | null,
+}));
+
+vi.mock("@/lib/safe-fetch", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/safe-fetch")>();
+  return {
+    ...actual,
+    // A plain function, not `vi.fn`: a mock records every call's result, and
+    // here each result is a whole page's response, so the recorder alone
+    // would hold the collection this file exists to keep out of memory.
+    safeFetch: async (url: string) => {
+      const u = new URL(url);
+      let body: unknown = {};
+      if (u.pathname.endsWith("/dataTypes/heart-rate/dataPoints")) {
+        const pageSize = Number(u.searchParams.get("pageSize") ?? 1000);
+        const page = Number(u.searchParams.get("pageToken") ?? 0);
+        const end = Math.min(fake.total, (page + 1) * pageSize);
+        const dataPoints = [];
+        for (let g = page * pageSize; g < end; g++) {
+          dataPoints.push({
+            name: `users/me/dataTypes/heart-rate/dataPoints/${9_000_000_000_000 + g}`,
+            dataSource: {
+              recordingMethod: "PASSIVE_MEASUREMENT",
+              device: { manufacturer: "Google", displayName: "Watch" },
+              platform: "FITBIT",
+            },
+            heartRate: {
+              beatsPerMinute: String(55 + (g % 50)),
+              sampleTime: {
+                physicalTime: new Date(fake.now - g * 60_000).toISOString(),
+              },
+            },
+          });
+        }
+        fake.heartRatePages++;
+        fake.onPage?.();
+        body = {
+          dataPoints,
+          ...(end < fake.total ? { nextPageToken: String(page + 1) } : {}),
+        };
+      }
+      return new Response(JSON.stringify(body), {
+        status: 200,
+        headers: { "content-type": "application/json" },
+      });
+    },
+  };
+});
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+const forceGc = ((): (() => void) => {
+  v8.setFlagsFromString("--expose-gc");
+  const gc = vm.runInNewContext("gc") as () => void;
+  v8.setFlagsFromString("--no-expose-gc");
+  return gc;
+})();
+
+function liveHeapBytes(): number {
+  forceGc();
+  forceGc();
+  return process.memoryUsage().heapUsed;
+}
+
+const mb = (bytes: number): string => (bytes / 1024 / 1024).toFixed(1);
+
+function report(line: string): void {
+  process.stderr.write(`[gh-dense-backfill] ${line}\n`);
+}
+
+/**
+ * Run `fn` while sampling the collected heap every few pages, from inside the
+ * page request, so each sample lands between two pages of the work being
+ * measured: exactly where anything it holds across a page is visible. (A
+ * timer is not enough; a walk whose pages resolve without touching the event
+ * loop never lets it fire.) Returns how far the heap rose after the walk
+ * passed a quarter of its pages.
+ */
+async function growthPastFirstQuarter<T>(
+  fn: () => Promise<T>,
+): Promise<{ result: T; growth: number; samples: number }> {
+  const quarter = Math.ceil(POINTS / 1000 / 4);
+  const startPages = fake.heartRatePages;
+  let anchor: number | null = null;
+  let peak = 0;
+  let samples = 0;
+  const sample = (): void => {
+    const live = liveHeapBytes();
+    if (anchor === null) {
+      if (fake.heartRatePages - startPages >= quarter) anchor = peak = live;
+      return;
+    }
+    samples++;
+    peak = Math.max(peak, live);
+  };
+  fake.onPage = () => {
+    if ((fake.heartRatePages - startPages) % 5 === 0) sample();
+  };
+  try {
+    const result = await fn();
+    sample();
+    return {
+      result,
+      growth: anchor === null ? Number.NaN : peak - anchor,
+      samples,
+    };
+  } finally {
+    fake.onPage = null;
+  }
+}
+
+beforeAll(async () => {
+  const prisma = getPrismaClient();
+  await truncateAllTables(prisma);
+  const { encrypt } = await import("@/lib/crypto");
+  await prisma.user.create({
+    data: { id: USER, username: USER, timezone: "Europe/Berlin" },
+  });
+  await prisma.googleHealthConnection.create({
+    data: {
+      userId: USER,
+      googleUserId: `google-${USER}`,
+      accessToken: encrypt("access"),
+      refreshToken: encrypt("refresh"),
+      tokenExpiresAt: new Date(Date.now() + 86_400_000),
+    },
+  });
+  fake.total = POINTS;
+  fake.now = NOW;
+}, 120_000);
+
+afterAll(async () => {
+  await truncateAllTables(getPrismaClient());
+});
+
+describe("Google Health backfill over a dense heart-rate stream (#1023)", () => {
+  it("writes the whole history without growing with it", async () => {
+    const { runGoogleHealthBackfillForUser } =
+      await import("@/lib/jobs/google-health-backfill");
+
+    const { result, growth, samples } = await growthPastFirstQuarter(() =>
+      runGoogleHealthBackfillForUser(USER),
+    );
+    report(
+      `backfill: imported=${result.imported} growth=${mb(growth)} MB ` +
+        `over ${samples} samples (budget ${mb(GROWTH_BUDGET_BYTES)} MB)`,
+    );
+
+    const prisma = getPrismaClient();
+    expect(
+      await prisma.measurement.count({
+        where: { userId: USER, type: "PULSE", source: "GOOGLE_HEALTH" },
+      }),
+    ).toBe(POINTS);
+    const connection = await prisma.googleHealthConnection.findUniqueOrThrow({
+      where: { userId: USER },
+      select: { backfillCompletedAt: true },
+    });
+    expect(connection.backfillCompletedAt).not.toBeNull();
+    // The deferred fold reached the rollup tier for the whole span.
+    const days = await prisma.measurementRollup.count({
+      where: { userId: USER, type: "PULSE", granularity: "DAY" },
+    });
+    expect(days).toBeGreaterThanOrEqual(Math.floor(POINTS / 1440));
+
+    expect(samples).toBeGreaterThan(5);
+    expect(growth).toBeLessThan(GROWTH_BUDGET_BYTES);
+  }, 900_000);
+
+  it("the same pages collected into one array outgrow the budget", async () => {
+    const { fetchDataPoints, GOOGLE_HEALTH_DATA_TYPES } =
+      await import("@/lib/google-health/client");
+    let held: unknown[] = [];
+    const { growth, samples } = await growthPastFirstQuarter(async () => {
+      held = await fetchDataPoints(
+        GOOGLE_HEALTH_DATA_TYPES.heartRate,
+        "access",
+        "fetchHeartRate",
+      );
+    });
+    report(
+      `collected: points=${held.length} growth=${mb(growth)} MB over ${samples} samples`,
+    );
+    expect(held).toHaveLength(POINTS);
+    expect(growth).toBeGreaterThan(GROWTH_BUDGET_BYTES);
+  }, 900_000);
+});