diff --git a/src/lib/measurements/__tests__/rollups.test.ts b/src/lib/measurements/__tests__/rollups.test.ts
index 104376630..a761b92ef 100644
--- a/src/lib/measurements/__tests__/rollups.test.ts
+++ b/src/lib/measurements/__tests__/rollups.test.ts
@@ -83,6 +83,7 @@ void queryRaw;
 import {
   ROLLUP_FULL_BACKFILL_QUEUE,
   ROLLUP_RECOMPUTE_QUEUE,
+  _resetEnsureUserRollupsFreshInFlightForTests,
   collapseToTypeDayKeys,
   enqueueBootTimeRollupBackfill,
   enqueueRollupRecompute,
@@ -101,6 +102,10 @@ beforeEach(() => {
   findFirstMeasurement.mockReset();
   bossSend.mockReset();
   getGlobalBossMock.mockReset();
+  // v1.4.38 — clear the per-userId in-flight map so a previous test's
+  // resolved promise does not short-circuit the next test's recompute
+  // assertion.
+  _resetEnsureUserRollupsFreshInFlightForTests();
   // Default: $transaction takes an array of pre-built promises (the
   // populator passes `slice.map(prisma.measurementRollup.upsert(...))`)
   // and returns them awaited.
@@ -291,6 +296,76 @@ describe("ensureUserRollupsFresh", () => {
     expect(result.recomputed).toBe(false);
   });
 
+  // v1.4.38 — concurrent callers for the same userId share one
+  // in-flight promise so a cold fan-out can never queue N parallel
+  // recompute runs against the same 90-day window.
+  it("dedups concurrent callers for the same userId onto one in-flight promise", async () => {
+    const rollupAt = new Date("2026-05-10T10:00:00.000Z");
+    const measurementAt = new Date("2026-05-10T12:00:00.000Z");
+    // Both round-trips inside the helper run twice (the helper issues
+    // a parallel `Promise.all` for the rollup-most-recent + measurement
+    // -most-recent). With dedup, those should fire ONCE across two
+    // concurrent callers.
+    findFirst.mockResolvedValue({ computedAt: rollupAt });
+    findFirstMeasurement.mockResolvedValue({
+      updatedAt: measurementAt,
+      measuredAt: measurementAt,
+    });
+    queryRawUnsafe.mockResolvedValue([]);
+
+    const [a, b, c] = await Promise.all([
+      ensureUserRollupsFresh("user-1"),
+      ensureUserRollupsFresh("user-1"),
+      ensureUserRollupsFresh("user-1"),
+    ]);
+    expect(a.recomputed).toBe(true);
+    expect(b.recomputed).toBe(true);
+    expect(c.recomputed).toBe(true);
+    // The Postgres-side recompute fires exactly once across the three
+    // concurrent callers — proof the dedup map collapsed the fan-out.
+    expect(queryRawUnsafe).toHaveBeenCalledTimes(1);
+    // The findFirst probe fires exactly once per call type (rollup,
+    // measurement) across all three concurrent callers.
+    expect(findFirst).toHaveBeenCalledTimes(1);
+    expect(findFirstMeasurement).toHaveBeenCalledTimes(1);
+  });
+
+  it("clears the in-flight slot after resolution so the next call runs fresh", async () => {
+    const rollupAt = new Date("2026-05-10T10:00:00.000Z");
+    const measurementAt = new Date("2026-05-10T12:00:00.000Z");
+    findFirst.mockResolvedValue({ computedAt: rollupAt });
+    findFirstMeasurement.mockResolvedValue({
+      updatedAt: measurementAt,
+      measuredAt: measurementAt,
+    });
+    queryRawUnsafe.mockResolvedValue([]);
+
+    await ensureUserRollupsFresh("user-1");
+    await ensureUserRollupsFresh("user-1");
+    // Two serial calls => two recompute runs (no dedup short-circuit
+    // because the first call already resolved and dropped its slot).
+    expect(queryRawUnsafe).toHaveBeenCalledTimes(2);
+  });
+
+  it("clears the in-flight slot on rejection so the next call retries", async () => {
+    findFirst.mockRejectedValueOnce(new Error("pool exhausted"));
+    findFirstMeasurement.mockRejectedValueOnce(new Error("pool exhausted"));
+    const first = await ensureUserRollupsFresh("user-1");
+    expect(first.recomputed).toBe(false);
+
+    // Second call: probe succeeds, stale watermark triggers a recompute.
+    const rollupAt = new Date("2026-05-10T10:00:00.000Z");
+    const measurementAt = new Date("2026-05-10T12:00:00.000Z");
+    findFirst.mockResolvedValueOnce({ computedAt: rollupAt });
+    findFirstMeasurement.mockResolvedValueOnce({
+      updatedAt: measurementAt,
+      measuredAt: measurementAt,
+    });
+    queryRawUnsafe.mockResolvedValueOnce([]);
+    const second = await ensureUserRollupsFresh("user-1");
+    expect(second.recomputed).toBe(true);
+  });
+
   it("annotates the failure when the inner recompute throws (H3)", async () => {
     // Stale watermark — rollup older than the newest measurement so
     // the inner recompute branch fires.
diff --git a/src/lib/measurements/rollups.ts b/src/lib/measurements/rollups.ts
index 66f73aa3a..faee36a83 100644
--- a/src/lib/measurements/rollups.ts
+++ b/src/lib/measurements/rollups.ts
@@ -555,6 +555,26 @@ function bucketSpan(
   }
 }
 
+/**
+ * v1.4.38 — per-userId in-flight dedup. v1.4.37.1 fire-and-forgets the
+ * `ensureUserRollupsFresh` call from the read path so a stale 90-day
+ * recompute can no longer stall the response, but if a request DOES
+ * end up awaiting a synchronous recompute (worker code path, future
+ * caller without the fire-and-forget) and the same user has a cold
+ * fan-out hitting analytics + comprehensive + summaries within the
+ * same tick, the legacy shape would queue N parallel
+ * `recomputeUserRollups` invocations against the same 90-day window.
+ * The map below collapses those into one shared promise — every
+ * caller after the first re-uses the in-flight resolution and only
+ * one Postgres aggregate runs. Mirrors the `buildCoachSnapshot` dedup
+ * pattern that lives one layer up. Cleared in `finally` so a slow
+ * promise that eventually rejects does not poison the next request.
+ */
+const ensureUserRollupsFreshInFlight = new Map<
+  string,
+  Promise<{ recomputed: boolean }>
+>();
+
 /**
  * Cheap read-side warm-up. Called from the analytics + comprehensive
  * read paths so the rollup table stays current for downstream
@@ -572,9 +592,39 @@ function bucketSpan(
  *     here.
  *   - Best-effort: thrown errors are swallowed so the read path
  *     never fails because of a populator hiccup.
+ *   - v1.4.38: concurrent callers for the same `userId` share one
+ *     in-flight promise so a cold fan-out can never queue N parallel
+ *     `recomputeUserRollups` runs against the same 90-day window.
  */
 export async function ensureUserRollupsFresh(
   userId: string,
+): Promise<{ recomputed: boolean }> {
+  // v1.4.38 — share the in-flight promise across concurrent callers
+  // for the same userId. The map is keyed on `userId` because the
+  // helper has no other parameters; the next caller hits the
+  // resolved promise as soon as it lands. We delete the slot in
+  // `finally` so a slow rejected promise does not stay cached and
+  // poison the next request.
+  const existing = ensureUserRollupsFreshInFlight.get(userId);
+  if (existing) return existing;
+  const inflight = ensureUserRollupsFreshImpl(userId).finally(() => {
+    ensureUserRollupsFreshInFlight.delete(userId);
+  });
+  ensureUserRollupsFreshInFlight.set(userId, inflight);
+  return inflight;
+}
+
+/**
+ * Test-only handle on the in-flight map. Mirrors the
+ * `_resetTrustViolationWarningForTests` pattern in api-response.ts —
+ * production code never calls this.
+ */
+export function _resetEnsureUserRollupsFreshInFlightForTests(): void {
+  ensureUserRollupsFreshInFlight.clear();
+}
+
+async function ensureUserRollupsFreshImpl(
+  userId: string,
 ): Promise<{ recomputed: boolean }> {
   try {
     // Single round-trip — the most-recent DAY rollup for the user