fix(workouts): match cross-source duplicates by a start window, not a clock slot

Two copies of one session were collapsed only when both starts fell in the
same fixed five-minute clock slot, so a pair thirty seconds apart that
straddled a slot boundary (08:04:50 and 08:05:20) stayed as two workouts in
the list, the batch write, the calendar, strain and the Coach.

Workouts of one user and sport are now clustered by start time: a workout
joins the open cluster while it starts less than five minutes after the
cluster's first start. Wherever no pair within five minutes straddles a slot
boundary the clusters are exactly the old slots, which a parity test checks
against the previous picker on thousands of generated inputs, strain impulses
included. A property test shows any offset under five minutes now collapses,
five minutes or more never does, and a uniform shift of every start keeps the
picked set.

src/lib/insights/strain-score.ts

@@ -74,7 +74,7 @@ import {
 import { shiftDateKey } from "@/lib/tz/format";
 import {
   pickCanonicalWorkoutRows,
-  workoutDuplicateSlotKey,
+  workoutDuplicateClusterKeys,
   workoutSourceLadder,
   type WorkoutPickerRow,
 } from "@/lib/measurements/pick-canonical-workout-rows";
@@ -591,7 +591,7 @@ export function canonicalDayActiveEnergy<
  * one source's rows count: the best-ranked on the ladder, a source off the
  * ladder ranking below every listed one, and among sources off the ladder
  * the one with the largest impulse. Rows of that one source still add up
- * (two distinct sessions of the same sport in one five-minute slot).
+ * (two distinct sessions of the same sport in one duplicate cluster).
  * Returns the positive impulses, one per session. Pure; exported for tests.
  */
 export function canonicalWorkoutTrimps<
@@ -615,9 +615,9 @@ export function canonicalWorkoutTrimps<
     string,
     { picked: Map<string, number>; dropped: number }
   >();
-  for (const row of rows) {
-    const key = workoutDuplicateSlotKey(row);
-    const slot = slots.get(key) ?? { picked: new Map(), dropped: 0 };
+  const keys = workoutDuplicateClusterKeys(rows);
+  for (const [index, row] of rows.entries()) {
+    const slot = slots.get(keys[index]!) ?? { picked: new Map(), dropped: 0 };
     const trimp = trimpOf(row);
     if (trimp > 0) {
       if (picked.has(row.id)) {
@@ -626,7 +626,7 @@ export function canonicalWorkoutTrimps<
         slot.dropped = Math.max(slot.dropped, trimp);
       }
     }
-    slots.set(key, slot);
+    slots.set(keys[index]!, slot);
   }
   const out: number[] = [];
   for (const slot of slots.values()) {

src/lib/measurements/__tests__/pick-canonical-workout-rows.legacy.ts

@@ -0,0 +1,329 @@
+/**
+ * The workout picker as it stood while duplicates were matched by fixed
+ * five-minute clock slots. Kept only as the reference the parity tests in
+ * `pick-canonical-workout-rows.property.test.ts` compare the duplicate window
+ * against: on every input where no two workouts of one sport within five
+ * minutes straddle a slot boundary, both must return the same rows.
+ */
+import type { MeasurementSource } from "@/generated/prisma/client";
+
+import {
+  getSourceLadder,
+  parseSourcePriority,
+} from "@/lib/validations/source-priority";
+
+/**
+ * Minimum row shape — same posture as `SourcePickerRow`. The helper
+ * only consults the three fields; downstream callers can read every
+ * other column they fetched alongside.
+ */
+export interface WorkoutPickerRow {
+  userId?: string;
+  startedAt: Date;
+  /**
+   * Session length. Together with `sportType` + `startedAt` this is the
+   * exact-session identity used to collapse repeated same-source sync rows.
+   * Optional only so fixtures can omit it; every production call site MUST
+   * select it, and `workout-canonical-identity-guard.test.ts` freezes that set.
+   */
+  durationSec?: number;
+  sportType: string;
+  source: MeasurementSource;
+  avgHeartRate?: number | null;
+  maxHeartRate?: number | null;
+  metadata?: unknown;
+}
+
+/**
+ * HealthKit can resend the same session with a new external UUID. The source
+ * ladder correctly chooses Apple over WHOOP, but previously kept every Apple
+ * row in that winning bucket, which made the web list show the same workout
+ * two or three times.
+ *
+ * The identity is `(sportType, startedAt, durationSec)`. `endedAt` was part of
+ * this key when the collapse first shipped and has been dropped deliberately:
+ *
+ *   - It is not a free extra check. On a workout with pauses, `endedAt` is the
+ *     wall-clock end while `durationSec` counts only moving time, so the two
+ *     legitimately disagree, and a stored row can carry either convention. Two
+ *     re-sends of one session can therefore differ in `endedAt` alone, and the
+ *     stricter key let that duplicate through.
+ *   - It bought nothing in exchange. Nobody starts a second workout of the
+ *     same sport in the same instant with the same duration, so start plus
+ *     duration already identifies the session uniquely.
+ *   - Requiring it split the callers. Four of the five call sites select
+ *     `durationSec` but not `endedAt`, so the collapse ran on the workout list
+ *     and silently did nothing for the Coach narration, the per-sport baseline
+ *     and the deep-link cluster. One list, one number, two answers.
+ *
+ * A row without `durationSec` cannot be identified and passes through
+ * uncollapsed. That is a fixture-only path: every production call site selects
+ * the field and `workout-canonical-identity-guard.test.ts` keeps it that way.
+ */
+function exactSessionKey(row: WorkoutPickerRow): string | null {
+  if (row.durationSec == null) return null;
+  return [row.sportType, row.startedAt.getTime(), row.durationSec].join(":");
+}
+
+function rowRichness(row: WorkoutPickerRow): number {
+  return (
+    (row.avgHeartRate != null ? 2 : 0) +
+    (row.maxHeartRate != null ? 2 : 0) +
+    (row.metadata != null ? 1 : 0)
+  );
+}
+
+function deduplicateWinningRows<T extends WorkoutPickerRow>(rows: T[]): T[] {
+  const bySession = new Map<string, T>();
+  const passthrough: T[] = [];
+  for (const row of rows) {
+    const key = exactSessionKey(row);
+    if (key == null) {
+      passthrough.push(row);
+      continue;
+    }
+    const existing = bySession.get(key);
+    if (!existing || rowRichness(row) > rowRichness(existing)) {
+      bySession.set(key, row);
+    }
+  }
+  return [...passthrough, ...bySession.values()];
+}
+
+/**
+ * Bucket-slot length. Two workouts whose `startedAt` falls in the
+ * same 5-minute slot AND share the same sportType are treated as the
+ * "same" workout from different sources. Tightens further than the
+ * 1-minute clock skew between HK + Withings would suggest because
+ * iOS posts HK `startDate` rounded to the second but Withings stamps
+ * its `startdate` independently — five minutes gives both heads
+ * enough slack on a heavy-traffic morning sync without crossing the
+ * "two back-to-back runs" boundary (a user does NOT do two ten-minute
+ * runs five minutes apart in practice).
+ */
+const BUCKET_SLOT_MS = 5 * 60 * 1000;
+
+/**
+ * The cross-source duplicate slot a workout falls in. Exported for the one
+ * reader that has to look past the picked row: the strain engine, whose winner
+ * may carry no heart-rate series while its dropped twin does.
+ */
+export function legacyWorkoutDuplicateSlotKey(row: WorkoutPickerRow): string {
+  return bucketKey(row);
+}
+
+function bucketKey(row: WorkoutPickerRow): string {
+  const slot = Math.floor(row.startedAt.getTime() / BUCKET_SLOT_MS);
+  return `${row.userId ?? ""}:${slot}:${row.sportType}`;
+}
+
+function jsonRecord(value: unknown): Record<string, unknown> | null {
+  return value !== null && typeof value === "object" && !Array.isArray(value)
+    ? (value as Record<string, unknown>)
+    : null;
+}
+
+const WHOOP_ZONE_DURATION_KEYS = [
+  "zone_zero_milli",
+  "zone_one_milli",
+  "zone_two_milli",
+  "zone_three_milli",
+  "zone_four_milli",
+  "zone_five_milli",
+] as const;
+
+function sanitizedWhoopZoneDurations(
+  metadata: unknown,
+): Record<string, number> | null {
+  const raw = jsonRecord(jsonRecord(metadata)?.zoneDurations);
+  if (!raw) return null;
+
+  const sanitized: Record<string, number> = {};
+  for (const key of WHOOP_ZONE_DURATION_KEYS) {
+    const value = raw[key];
+    if (value === undefined) continue;
+    if (typeof value !== "number" || !Number.isFinite(value) || value < 0) {
+      return null;
+    }
+    sanitized[key] = value;
+  }
+  return Object.keys(sanitized).length > 0 ? sanitized : null;
+}
+
+function enrichFromWhoopTwin<T extends WorkoutPickerRow>(
+  winner: T,
+  donor: T,
+): T {
+  const winnerMetadata = jsonRecord(winner.metadata);
+  const donorZones = sanitizedWhoopZoneDurations(donor.metadata);
+  const needsZones =
+    donorZones != null && winnerMetadata?.zoneDurations == null;
+  const needsAvg = winner.avgHeartRate == null && donor.avgHeartRate != null;
+  const needsMax = winner.maxHeartRate == null && donor.maxHeartRate != null;
+
+  if (!needsAvg && !needsMax && !needsZones) return winner;
+
+  return {
+    ...winner,
+    ...(needsAvg ? { avgHeartRate: donor.avgHeartRate } : {}),
+    ...(needsMax ? { maxHeartRate: donor.maxHeartRate } : {}),
+    ...(needsZones
+      ? {
+          metadata: {
+            ...(winnerMetadata ?? {}),
+            zoneDurations: donorZones,
+          },
+        }
+      : {}),
+  };
+}
+
+/**
+ * Pair winners and WHOOP donors one-to-one by nearest start time. A single
+ * donor can never enrich two same-source occurrences that happen to share a
+ * fixed slot. Stable input indexes break equal-distance ties.
+ */
+function enrichWinnersFromWhoop<T extends WorkoutPickerRow>(
+  winners: readonly T[],
+  donors: readonly T[],
+): T[] {
+  if (winners.length === 0 || donors.length === 0) return [...winners];
+
+  const candidates: Array<{
+    winnerIndex: number;
+    donorIndex: number;
+    deltaMs: number;
+  }> = [];
+  winners.forEach((winner, winnerIndex) => {
+    donors.forEach((donor, donorIndex) => {
+      candidates.push({
+        winnerIndex,
+        donorIndex,
+        deltaMs: Math.abs(
+          winner.startedAt.getTime() - donor.startedAt.getTime(),
+        ),
+      });
+    });
+  });
+  candidates.sort(
+    (left, right) =>
+      left.deltaMs - right.deltaMs ||
+      left.winnerIndex - right.winnerIndex ||
+      left.donorIndex - right.donorIndex,
+  );
+
+  const enriched = [...winners];
+  const matchedWinners = new Set<number>();
+  const matchedDonors = new Set<number>();
+  for (const candidate of candidates) {
+    if (
+      matchedWinners.has(candidate.winnerIndex) ||
+      matchedDonors.has(candidate.donorIndex)
+    ) {
+      continue;
+    }
+    enriched[candidate.winnerIndex] = enrichFromWhoopTwin(
+      winners[candidate.winnerIndex],
+      donors[candidate.donorIndex],
+    );
+    matchedWinners.add(candidate.winnerIndex);
+    matchedDonors.add(candidate.donorIndex);
+  }
+  return enriched;
+}
+
+/**
+ * The source order for workouts: the `steps` metric's ladder, the
+ * canonical-activity bucket the Sources settings configure, falling back to
+ * its default when the user saved none or it does not parse.
+ */
+function workoutSourceLadder(
+  userPriorityJson: unknown,
+): readonly MeasurementSource[] {
+  return getSourceLadder(parseSourcePriority(userPriorityJson), "steps");
+}
+
+/**
+ * Pick canonical rows across sources for a user's workouts.
+ *
+ * @param rows               input rows (deterministic order required)
+ * @param userPriorityJson   the user's `sourcePriorityJson` blob — same
+ *                           shape the measurement picker consults.
+ *                           Falls back to the canonical default ladder
+ *                           when null / unparseable.
+ * @returns the subset of input rows that survives the picker. Insertion
+ *          order preserved.
+ */
+export function legacyPickCanonicalWorkoutRows<T extends WorkoutPickerRow>(
+  rows: readonly T[],
+  userPriorityJson: unknown = null,
+): T[] {
+  if (rows.length === 0) return [];
+
+  const sourceLadder = workoutSourceLadder(userPriorityJson);
+
+  const buckets = new Map<
+    string,
+    {
+      rows: T[];
+      sources: Set<MeasurementSource>;
+    }
+  >();
+  for (const row of rows) {
+    const key = bucketKey(row);
+    const slot = buckets.get(key) ?? { rows: [], sources: new Set() };
+    slot.rows.push(row);
+    slot.sources.add(row.source);
+    buckets.set(key, slot);
+  }
+
+  const canonical: T[] = [];
+  for (const [, slot] of buckets) {
+    if (slot.rows.length === 1) {
+      // No cross-source overlap → keep the single row regardless of
+      // ladder membership.
+      canonical.push(slot.rows[0]);
+      continue;
+    }
+
+    let picked: MeasurementSource | undefined;
+    for (const source of sourceLadder) {
+      if (slot.sources.has(source)) {
+        picked = source;
+        break;
+      }
+    }
+
+    if (!picked) {
+      // None of the bucket's sources are listed in the ladder — never
+      // silently drop. Keep every row in the bucket; downstream
+      // callers can choose to surface a "duplicate from unrecognised
+      // sources" diagnostic.
+      canonical.push(...slot.rows);
+      continue;
+    }
+
+    const winners = deduplicateWinningRows(
+      slot.rows.filter((row) => row.source === picked),
+    );
+    const whoopDonors =
+      picked === "WHOOP"
+        ? []
+        : slot.rows.filter((row) => row.source === "WHOOP");
+    canonical.push(...enrichWinnersFromWhoop(winners, whoopDonors));
+  }
+
+  // Re-sort to preserve input order — the bucket map iterates in
+  // insertion order but the per-bucket fan-out can interleave rows
+  // from later buckets ahead of earlier-bucket survivors. Caller
+  // wants the same `startedAt ASC` order it passed in.
+  canonical.sort((a, b) => {
+    const delta = a.startedAt.getTime() - b.startedAt.getTime();
+    if (delta !== 0) return delta;
+    // Stable-tiebreak on sportType so the output is deterministic
+    // even when two surviving rows share startedAt (rare; manual
+    // entries on the same second).
+    return a.sportType.localeCompare(b.sportType);
+  });
+  return canonical;
+}

src/lib/measurements/__tests__/pick-canonical-workout-rows.property.test.ts

@@ -0,0 +1,300 @@
+import { describe, expect, it } from "vitest";
+
+import { canonicalWorkoutTrimps } from "@/lib/insights/strain-score";
+
+import {
+  pickCanonicalWorkoutRows,
+  workoutDuplicateClusterKeys,
+  workoutSourceLadder,
+} from "../pick-canonical-workout-rows";
+import {
+  legacyPickCanonicalWorkoutRows,
+  legacyWorkoutDuplicateSlotKey,
+} from "./pick-canonical-workout-rows.legacy";
+
+type Source = "APPLE_HEALTH" | "WHOOP" | "WITHINGS" | "MANUAL" | "STRAVA";
+
+interface Row {
+  id: string;
+  userId?: string;
+  startedAt: Date;
+  durationSec?: number;
+  sportType: string;
+  source: Source;
+  avgHeartRate: number | null;
+  maxHeartRate: number | null;
+  metadata: Record<string, unknown> | null;
+}
+
+const MIN = 60_000;
+const WINDOW = 5 * MIN;
+const SOURCES: Source[] = [
+  "APPLE_HEALTH",
+  "WHOOP",
+  "WITHINGS",
+  "MANUAL",
+  "STRAVA",
+];
+const SPORTS = ["running", "cycling", "walking"];
+
+/** Small seeded generator, so a failure names a reproducible case. */
+function rng(seed: number) {
+  let a = seed >>> 0;
+  const next = () => {
+    a = (a + 0x6d2b79f5) >>> 0;
+    let t = a;
+    t = Math.imul(t ^ (t >>> 15), t | 1);
+    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+  return {
+    next,
+    int: (lo: number, hi: number) => lo + Math.floor(next() * (hi - lo + 1)),
+    pick: <T>(list: readonly T[]) => list[Math.floor(next() * list.length)]!,
+  };
+}
+
+function randomRows(seed: number): Row[] {
+  const r = rng(seed);
+  const base = Date.UTC(2026, 4, 16, 6, 0, 0) + r.int(0, 86_400_000);
+  const count = r.int(1, 14);
+  const rows: Row[] = [];
+  for (let i = 0; i < count; i++) {
+    // Mostly near each other, so clusters form; sometimes a re-send of an
+    // earlier row (same start and duration) under another id.
+    const twin = i > 0 && r.next() < 0.2 ? rows[r.int(0, i - 1)]! : null;
+    const startedAt = twin
+      ? new Date(
+          twin.startedAt.getTime() + (r.next() < 0.5 ? 0 : r.int(1, 90) * 1000),
+        )
+      : new Date(base + r.int(0, 40 * MIN));
+    rows.push({
+      id: `w-${seed}-${i}`,
+      userId: r.next() < 0.15 ? "other" : "u1",
+      startedAt,
+      durationSec:
+        r.next() < 0.1 ? undefined : (twin?.durationSec ?? r.int(10, 90) * 60),
+      sportType: twin && r.next() < 0.8 ? twin.sportType : r.pick(SPORTS),
+      source: r.pick(SOURCES),
+      avgHeartRate: r.next() < 0.5 ? r.int(100, 160) : null,
+      maxHeartRate: r.next() < 0.5 ? r.int(160, 190) : null,
+      metadata:
+        r.next() < 0.3
+          ? { zoneDurations: { zone_two_milli: r.int(1, 60) * MIN } }
+          : null,
+    });
+  }
+  // Callers pass `startedAt ASC, id ASC`.
+  rows.sort(
+    (a, b) =>
+      a.startedAt.getTime() - b.startedAt.getTime() || a.id.localeCompare(b.id),
+  );
+  return rows;
+}
+
+/**
+ * True when some pair of one (user, sport) lies within the window but in two
+ * different fixed slots: the only inputs on which the old picker and the
+ * duplicate window may disagree.
+ */
+function straddlesASlot(rows: readonly Row[]): boolean {
+  for (let i = 0; i < rows.length; i++) {
+    for (let j = i + 1; j < rows.length; j++) {
+      const a = rows[i]!;
+      const b = rows[j]!;
+      if (a.userId !== b.userId || a.sportType !== b.sportType) continue;
+      if (Math.abs(a.startedAt.getTime() - b.startedAt.getTime()) >= WINDOW)
+        continue;
+      if (legacyWorkoutDuplicateSlotKey(a) !== legacyWorkoutDuplicateSlotKey(b))
+        return true;
+    }
+  }
+  return false;
+}
+
+const LADDERS: unknown[] = [
+  null,
+  { steps: ["WITHINGS", "APPLE_HEALTH"] },
+  { steps: ["WHOOP"] },
+];
+
+/** The strain fold as it stood on fixed slots, for the parity check. */
+function legacyTrimps(
+  rows: readonly Row[],
+  ladderJson: unknown,
+  trimpOf: (r: Row) => number,
+) {
+  const picked = new Set(
+    legacyPickCanonicalWorkoutRows(rows, ladderJson).map((r) => r.id),
+  );
+  const ladder = workoutSourceLadder(ladderJson);
+  const rankOf = (source: string) => {
+    const index = ladder.indexOf(source as (typeof ladder)[number]);
+    return index === -1 ? ladder.length : index;
+  };
+  const slots = new Map<
+    string,
+    { picked: Map<string, number>; dropped: number }
+  >();
+  for (const row of rows) {
+    const key = legacyWorkoutDuplicateSlotKey(row);
+    const slot = slots.get(key) ?? { picked: new Map(), dropped: 0 };
+    const t = trimpOf(row);
+    if (t > 0) {
+      if (picked.has(row.id))
+        slot.picked.set(row.source, (slot.picked.get(row.source) ?? 0) + t);
+      else slot.dropped = Math.max(slot.dropped, t);
+    }
+    slots.set(key, slot);
+  }
+  const out: number[] = [];
+  for (const slot of slots.values()) {
+    let best: { rank: number; trimp: number } | null = null;
+    for (const [source, trimp] of slot.picked) {
+      const rank = rankOf(source);
+      if (
+        best === null ||
+        rank < best.rank ||
+        (rank === best.rank && trimp > best.trimp)
+      ) {
+        best = { rank, trimp };
+      }
+    }
+    const trimp = best !== null ? best.trimp : slot.dropped;
+    if (trimp > 0) out.push(trimp);
+  }
+  return out;
+}
+
+describe("workout duplicate window", () => {
+  it("returns exactly the old rows wherever no pair straddles a slot boundary", () => {
+    let compared = 0;
+    for (let seed = 1; seed <= 4000; seed++) {
+      const rows = randomRows(seed);
+      if (straddlesASlot(rows)) continue;
+      compared++;
+      for (const ladder of LADDERS) {
+        expect(pickCanonicalWorkoutRows(rows, ladder), `seed ${seed}`).toEqual(
+          legacyPickCanonicalWorkoutRows(rows, ladder),
+        );
+      }
+    }
+    // The generator must actually produce comparable inputs, clusters
+    // included, or the loop above proves nothing.
+    expect(compared).toBeGreaterThan(1000);
+  });
+
+  it("gives the strain fold the same impulses wherever no pair straddles a slot", () => {
+    const trimpOf = (row: Row) => row.avgHeartRate ?? 0;
+    let compared = 0;
+    for (let seed = 1; seed <= 2000; seed++) {
+      const rows = randomRows(seed);
+      if (straddlesASlot(rows)) continue;
+      compared++;
+      for (const ladder of LADDERS) {
+        expect(
+          canonicalWorkoutTrimps(rows, ladder, trimpOf),
+          `seed ${seed}`,
+        ).toEqual(legacyTrimps(rows, ladder, trimpOf));
+      }
+    }
+    expect(compared).toBeGreaterThan(500);
+  });
+
+  it("collapses two sources' copies of one session at any offset under five minutes", () => {
+    const r = rng(7);
+    let legacyMissed = 0;
+    for (let i = 0; i < 3000; i++) {
+      const start = Date.UTC(2026, 4, 16) + r.int(0, 86_400_000);
+      const offset = r.int(0, WINDOW - 1);
+      const rows: Row[] = [
+        {
+          id: "apple",
+          userId: "u1",
+          startedAt: new Date(start),
+          durationSec: 1800,
+          sportType: "running",
+          source: "APPLE_HEALTH",
+          avgHeartRate: null,
+          maxHeartRate: null,
+          metadata: null,
+        },
+        {
+          id: "withings",
+          userId: "u1",
+          startedAt: new Date(start + offset),
+          durationSec: 1790,
+          sportType: "running",
+          source: "WITHINGS",
+          avgHeartRate: null,
+          maxHeartRate: null,
+          metadata: null,
+        },
+      ];
+      expect(pickCanonicalWorkoutRows(rows).map((row) => row.id)).toEqual([
+        "apple",
+      ]);
+      if (legacyPickCanonicalWorkoutRows(rows).length === 2) legacyMissed++;
+    }
+    // The fixed slots missed a share of these; the window misses none.
+    expect(legacyMissed).toBeGreaterThan(0);
+  });
+
+  it("never collapses two workouts five minutes or more apart", () => {
+    const r = rng(11);
+    for (let i = 0; i < 2000; i++) {
+      const start = Date.UTC(2026, 4, 16) + r.int(0, 86_400_000);
+      const gap = WINDOW + r.int(0, 30 * MIN);
+      const rows: Row[] = ["APPLE_HEALTH", "WITHINGS"].map((source, k) => ({
+        id: `w${k}`,
+        userId: "u1",
+        startedAt: new Date(start + k * gap),
+        durationSec: 1200,
+        sportType: "running",
+        source: source as Source,
+        avgHeartRate: null,
+        maxHeartRate: null,
+        metadata: null,
+      }));
+      expect(pickCanonicalWorkoutRows(rows)).toHaveLength(2);
+    }
+  });
+
+  it("does not depend on where the clock's five-minute marks fall", () => {
+    // Shifting every start by the same amount keeps every gap, so it must
+    // keep the picked set. The fixed slots broke this property.
+    for (let seed = 1; seed <= 1500; seed++) {
+      const rows = randomRows(seed);
+      const shift = rng(seed * 31).int(1, WINDOW - 1);
+      const shifted = rows.map((row) => ({
+        ...row,
+        startedAt: new Date(row.startedAt.getTime() + shift),
+      }));
+      const ids = (list: Row[]) => list.map((row) => row.id).sort();
+      expect(ids(pickCanonicalWorkoutRows(shifted)), `seed ${seed}`).toEqual(
+        ids(pickCanonicalWorkoutRows(rows)),
+      );
+    }
+  });
+
+  it("anchors on the first start, so a row of workouts does not chain", () => {
+    const at = (m: number) => new Date(Date.UTC(2026, 4, 16, 8, m, 0));
+    const rows: Row[] = [0, 4, 8, 12].map((m, k) => ({
+      id: `w${m}`,
+      userId: "u1",
+      startedAt: at(m),
+      durationSec: 600,
+      sportType: "running",
+      source: (k % 2 === 0 ? "APPLE_HEALTH" : "WITHINGS") as Source,
+      avgHeartRate: null,
+      maxHeartRate: null,
+      metadata: null,
+    }));
+    const keys = workoutDuplicateClusterKeys(rows);
+    expect(new Set(keys).size).toBe(2);
+    expect(pickCanonicalWorkoutRows(rows).map((row) => row.id)).toEqual([
+      "w0",
+      "w8",
+    ]);
+  });
+});

src/lib/measurements/__tests__/pick-canonical-workout-rows.test.ts

@@ -450,7 +450,12 @@ describe("pickCanonicalWorkoutRows", () => {
     ]);
   });
 
-  it("does not chain neighbouring workouts across fixed canonical slots", () => {
+  it("does not chain neighbouring workouts past the first start's window", () => {
+    // `middle` starts four minutes after `first`: inside the duplicate
+    // window, so it is the same run from a lower-ranked source. `last` is
+    // eight minutes after `first`, outside the window measured from the
+    // cluster's first start, so it stays even though it is only four minutes
+    // after `middle`.
     const rows: RowFixture[] = [
       {
         id: "first",
@@ -474,7 +479,6 @@ describe("pickCanonicalWorkoutRows", () => {
 
     expect(pickCanonicalWorkoutRows(rows).map((row) => row.id)).toEqual([
       "first",
-      "middle",
       "last",
     ]);
   });

src/lib/measurements/pick-canonical-workout-rows.ts

@@ -9,11 +9,14 @@
  *
  * Algorithm (v1.4.30 — matches the measurement ladder per R-F open
  * question #4):
- *   1. Bucket workouts by the rounded start timestamp + sport-type
- *      (`<5-minute slot>:<sport>`). Two HKWorkout entries for the same
- *      logical run start within a minute of each other on both
- *      sources; the rounding gives the cross-source matcher slack
- *      without collapsing distinct back-to-back workouts.
+ *   1. Cluster workouts per user and sport-type by start time: sorted by
+ *      start, a workout joins the open cluster when it starts less than
+ *      five minutes after the cluster's first workout, and opens a new
+ *      one otherwise. Two HKWorkout entries for the same logical run
+ *      start within a minute of each other on both sources; the window
+ *      gives the cross-source matcher slack without collapsing distinct
+ *      back-to-back workouts, and anchoring on the first start keeps a
+ *      row of workouts a few minutes apart from chaining into one.
  *   2. For each bucket, walk the source ladder (`workoutSourceLadder`)
  *      and pick the FIRST source that has any row in that bucket. Keep
  *      every row from the winning source; drop the rest.
@@ -115,30 +118,58 @@ function deduplicateWinningRows<T extends WorkoutPickerRow>(rows: T[]): T[] {
 }
 
 /**
- * Bucket-slot length. Two workouts whose `startedAt` falls in the
- * same 5-minute slot AND share the same sportType are treated as the
- * "same" workout from different sources. Tightens further than the
- * 1-minute clock skew between HK + Withings would suggest because
- * iOS posts HK `startDate` rounded to the second but Withings stamps
- * its `startdate` independently — five minutes gives both heads
- * enough slack on a heavy-traffic morning sync without crossing the
- * "two back-to-back runs" boundary (a user does NOT do two ten-minute
- * runs five minutes apart in practice).
+ * The cross-source duplicate window. Two workouts of the same sport whose
+ * starts lie less than five minutes apart are treated as the "same" workout
+ * from different sources. iOS posts the HealthKit `startDate` rounded to the
+ * second while Withings stamps its `startdate` independently; five minutes
+ * gives both heads enough slack on a heavy-traffic morning sync without
+ * crossing the "two back-to-back runs" boundary (a user does NOT do two
+ * ten-minute runs five minutes apart in practice).
+ *
+ * This used to be a fixed five-minute clock slot (`floor(start / 5 min)`),
+ * so two copies of one session thirty seconds apart that straddled a slot
+ * boundary (08:04:50 and 08:05:20) were never compared. The window is now
+ * measured from the cluster's first start. Where no two workouts of one sport
+ * within the window straddle a slot boundary, the clusters are exactly the
+ * old slots: every pair inside one slot is less than five minutes apart, and
+ * a cluster cannot reach past its slot without such a pair.
  */
-const BUCKET_SLOT_MS = 5 * 60 * 1000;
+const DUPLICATE_WINDOW_MS = 5 * 60 * 1000;
 
 /**
- * The cross-source duplicate slot a workout falls in. Exported for the one
- * reader that has to look past the picked row: the strain engine, whose winner
- * may carry no heart-rate series while its dropped twin does.
+ * The duplicate cluster of every row, aligned with `rows`. Rows of different
+ * users or sports never share a cluster. Within one (user, sport), rows are
+ * taken in start order (input order on equal starts) and a row joins the open
+ * cluster while it starts less than {@link DUPLICATE_WINDOW_MS} after the
+ * cluster's first row. The key names the cluster by that first row's start,
+ * so it is stable for a given set of rows. Exported for the one reader that
+ * has to look past the picked row: the strain engine, whose winner may carry
+ * no heart-rate series while its dropped twin does.
  */
-export function workoutDuplicateSlotKey(row: WorkoutPickerRow): string {
-  return bucketKey(row);
-}
-
-function bucketKey(row: WorkoutPickerRow): string {
-  const slot = Math.floor(row.startedAt.getTime() / BUCKET_SLOT_MS);
-  return `${row.userId ?? ""}:${slot}:${row.sportType}`;
+export function workoutDuplicateClusterKeys(
+  rows: readonly WorkoutPickerRow[],
+): string[] {
+  const groups = new Map<string, number[]>();
+  rows.forEach((row, index) => {
+    const group = `${row.userId ?? ""}:${row.sportType}`;
+    const list = groups.get(group);
+    if (list) list.push(index);
+    else groups.set(group, [index]);
+  });
+  const keys = new Array<string>(rows.length);
+  for (const [group, indexes] of groups) {
+    indexes.sort(
+      (a, b) =>
+        rows[a]!.startedAt.getTime() - rows[b]!.startedAt.getTime() || a - b,
+    );
+    let anchor = Number.NaN;
+    for (const index of indexes) {
+      const start = rows[index]!.startedAt.getTime();
+      if (!(start - anchor < DUPLICATE_WINDOW_MS)) anchor = start;
+      keys[index] = `${group}:${anchor}`;
+    }
+  }
+  return keys;
 }
 
 function jsonRecord(value: unknown): Record<string, unknown> | null {
@@ -205,7 +236,7 @@ function enrichFromWhoopTwin<T extends WorkoutPickerRow>(
 /**
  * Pair winners and WHOOP donors one-to-one by nearest start time. A single
  * donor can never enrich two same-source occurrences that happen to share a
- * fixed slot. Stable input indexes break equal-distance ties.
+ * duplicate cluster. Stable input indexes break equal-distance ties.
  */
 function enrichWinnersFromWhoop<T extends WorkoutPickerRow>(
   winners: readonly T[],
@@ -293,8 +324,9 @@ export function pickCanonicalWorkoutRows<T extends WorkoutPickerRow>(
       sources: Set<MeasurementSource>;
     }
   >();
-  for (const row of rows) {
-    const key = bucketKey(row);
+  const clusterKeys = workoutDuplicateClusterKeys(rows);
+  for (const [index, row] of rows.entries()) {
+    const key = clusterKeys[index]!;
     const slot = buckets.get(key) ?? { rows: [], sources: new Set() };
     slot.rows.push(row);
     slot.sources.add(row.source);