Offer the previous session of the same sport from the workout page

"How does this compare to last time?" is the most likely next question
after reading a workout, and answering it meant going back to the list and
scanning for the one before.

Resolve the previous session server-side, through the same canonical picker
the list uses, so the link can never land on a duplicate row recorded by a
second watch, and render it beside the own-history average line. Null when
nothing older exists, and then no link is shown.

The stats card's hand-rolled heading moves to the icon-less TileHeader
while the file is open.

docs/api/openapi.yaml

@@ -20073,6 +20073,10 @@ components:
             - type: "null"
         canonicalId:
           type: string
+        previousWorkoutId:
+          anyOf:
+            - type: string
+            - type: "null"
       required:
         - id
         - sportType
@@ -20099,6 +20103,7 @@ components:
         - sportContext
         - aiInsight
         - canonicalId
+        - previousWorkoutId
       additionalProperties: false
     WorkoutRouteGeometry:
       type: object

messages/de.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "An diesem Tag ist keine Stimmung erfasst",
         "sleepTrendsLink": "Schlaf-Trends",
         "moodHistoryLink": "Stimmungsverlauf",
+        "previousSessionLink": "Vorherige Einheit",
         "insightTitle": "Aktivitäts-Einblick"
       },
       "sport": {

messages/en.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "No mood logged that day",
         "sleepTrendsLink": "Sleep trends",
         "moodHistoryLink": "Mood history",
+        "previousSessionLink": "Previous session",
         "insightTitle": "Activity insight"
       },
       "sport": {

messages/es.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "No hay ánimo registrado ese día",
         "sleepTrendsLink": "Tendencias de sueño",
         "moodHistoryLink": "Historial de ánimo",
+        "previousSessionLink": "Sesión anterior",
         "insightTitle": "Análisis de la actividad"
       },
       "sport": {

messages/fr.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "Aucune humeur enregistrée ce jour-là",
         "sleepTrendsLink": "Tendances du sommeil",
         "moodHistoryLink": "Historique d'humeur",
+        "previousSessionLink": "Séance précédente",
         "insightTitle": "Analyse de l'activité"
       },
       "sport": {

messages/it.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "Nessun umore registrato quel giorno",
         "sleepTrendsLink": "Andamento del sonno",
         "moodHistoryLink": "Cronologia dell'umore",
+        "previousSessionLink": "Sessione precedente",
         "insightTitle": "Analisi dell'attività"
       },
       "sport": {

messages/pl.json

@@ -3110,6 +3110,7 @@
         "dayMoodNone": "Brak zapisanego nastroju tego dnia",
         "sleepTrendsLink": "Trendy snu",
         "moodHistoryLink": "Historia nastroju",
+        "previousSessionLink": "Poprzedni trening",
         "insightTitle": "Analiza aktywności"
       },
       "sport": {

src/app/api/workouts/[id]/route.ts

@@ -211,6 +211,47 @@ export const GET = apiHandler(
       row.id,
     );
 
+    // The session before this one in the same sport. "How does this compare
+    // to last time?" is the most likely next question after reading a
+    // workout, and today it costs a trip back to the list plus a scan.
+    // Scoped to rows that start before this cluster's slot, so a twin of THIS
+    // session can never be offered as the previous one, and run through the
+    // same canonical picker so the link lands on the row the list shows.
+    const previousRows = await prisma.workout.findMany({
+      where: {
+        userId: user.id,
+        sportType: row.sportType,
+        startedAt: { lt: slotStart },
+      },
+      orderBy: [{ startedAt: "desc" }, { id: "desc" }],
+      // Enough to survive a run of same-session twins and re-sends without
+      // reading the whole history; the picker collapses them to a handful.
+      take: 20,
+      select: {
+        id: true,
+        source: true,
+        startedAt: true,
+        sportType: true,
+        durationSec: true,
+        avgHeartRate: true,
+        maxHeartRate: true,
+        metadata: true,
+      },
+    });
+    // The picker takes rows in ascending order and does not promise to hand
+    // them back sorted, so pick the newest survivor explicitly.
+    const previousWorkoutId =
+      pickCanonicalWorkoutRows(
+        [...previousRows].reverse(),
+        userRow?.sourcePriorityJson ?? null,
+      ).reduce<{ id: string; startedAt: Date } | null>(
+        (newest, candidate) =>
+          newest === null || candidate.startedAt > newest.startedAt
+            ? candidate
+            : newest,
+        null,
+      )?.id ?? null;
+
     const route = row.route
       ? {
           geometry: row.route.geometry,
@@ -295,6 +336,10 @@ export const GET = apiHandler(
       // winner. `canonicalId === id` when the requested row already
       // is the winner.
       canonicalId: canonical?.id ?? row.id,
+      // The canonical previous session in this sport, or null when this is
+      // the first one on record. Additive; a client that does not read it is
+      // unaffected.
+      previousWorkoutId,
     });
   },
 );

src/app/api/workouts/__tests__/detail.test.ts

@@ -158,6 +158,67 @@ describe("GET /api/workouts/{id}", () => {
     expect(body.data.maxHr).toBe(170);
     expect(body.data.canonicalId).toBe("w-1");
     expect(body.data.route).toBeNull();
+    // Nothing older on record: absence, not a link to nowhere.
+    expect(body.data.previousWorkoutId).toBeNull();
+  });
+
+  it("points previousWorkoutId at the canonical session before this one", async () => {
+    vi.mocked(prisma.workout.findUnique).mockResolvedValueOnce(
+      BASE_ROW as never,
+    );
+    vi.mocked(prisma.workout.findMany)
+      // 1) the deep-link cluster
+      .mockResolvedValueOnce([
+        {
+          id: "w-1",
+          source: "APPLE_HEALTH",
+          startedAt: BASE_ROW.startedAt,
+          sportType: "RUNNING",
+        },
+      ] as never)
+      // 2) the sport-context baseline
+      .mockResolvedValueOnce([] as never)
+      // 3) the older sessions, newest first
+      .mockResolvedValueOnce([
+        {
+          id: "w-0",
+          source: "APPLE_HEALTH",
+          startedAt: new Date("2026-05-13T07:00:00Z"),
+          sportType: "RUNNING",
+          durationSec: 1800,
+          avgHeartRate: null,
+          maxHeartRate: null,
+          metadata: null,
+        },
+        // The same session as seen by the second watch. It must never win
+        // the link — the list would never show it.
+        {
+          id: "w-0-twin",
+          source: "WITHINGS",
+          startedAt: new Date("2026-05-13T07:01:00Z"),
+          sportType: "RUNNING",
+          durationSec: 1800,
+          avgHeartRate: null,
+          maxHeartRate: null,
+          metadata: null,
+        },
+        {
+          id: "w-older",
+          source: "APPLE_HEALTH",
+          startedAt: new Date("2026-05-10T07:00:00Z"),
+          sportType: "RUNNING",
+          durationSec: 1800,
+          avgHeartRate: null,
+          maxHeartRate: null,
+          metadata: null,
+        },
+      ] as never);
+
+    const res = await GET(makeRequest(), makeParams("w-1"));
+    const body = await res.json();
+
+    expect(res.status).toBe(200);
+    expect(body.data.previousWorkoutId).toBe("w-0");
   });
 
   it("dates the session by the user's timezone, not by UTC", async () => {

src/components/insights/__tests__/workout-detail.test.tsx

@@ -80,6 +80,7 @@ const FIXTURE: WorkoutDetailPayload = {
   sportContext: null,
   aiInsight: null,
   canonicalId: "w-1",
+  previousWorkoutId: null,
 };
 
 describe("<WorkoutDetailHeader>", () => {
@@ -104,6 +105,18 @@ describe("<WorkoutDetailStats>", () => {
     expect(html).toContain("Pace");
   });
 
+  it("links to the previous session only when the server resolved one", () => {
+    expect(render(<WorkoutDetailStats workout={FIXTURE} />)).not.toContain(
+      'data-slot="workout-detail-previous-link"',
+    );
+
+    const html = render(
+      <WorkoutDetailStats workout={{ ...FIXTURE, previousWorkoutId: "w-0" }} />,
+    );
+    expect(html).toContain('href="/insights/workouts/w-0"');
+    expect(html).toContain("Previous session");
+  });
+
   it("omits tiles for null fields", () => {
     const minimal: WorkoutDetailPayload = {
       ...FIXTURE,

src/components/insights/workout-detail/stats.tsx

@@ -1,7 +1,9 @@
 "use client";
 
 import type { ReactNode } from "react";
+import Link from "next/link";
 import {
+  ArrowRight,
   Flame,
   Footprints,
   HeartPulse,
@@ -11,6 +13,7 @@ import {
 } from "lucide-react";
 
 import { Card, CardContent, CardHeader } from "@/components/ui/card";
+import { TileHeader } from "@/components/insights/tile-header";
 import { cn } from "@/lib/utils";
 import { useTranslations } from "@/lib/i18n/context";
 import type { WorkoutDetailPayload } from "@/hooks/use-workouts";
@@ -157,12 +160,12 @@ export function WorkoutDetailStats({ workout }: WorkoutDetailStatsProps) {
   return (
     <Card data-slot="workout-detail-stats">
       <CardHeader>
-        <h2
-          data-slot="card-title"
-          className="text-base leading-none font-semibold"
-        >
-          {t("insights.workouts.detail.statsTitle")}
-        </h2>
+        {/* Icon-less `TileHeader`, not a hand-rolled h2 — the standards
+            allow exactly three header patterns and this card is a tile. */}
+        <TileHeader
+          title={t("insights.workouts.detail.statsTitle")}
+          titleAs="h2"
+        />
       </CardHeader>
       <CardContent className="space-y-3">
         <div
@@ -175,14 +178,32 @@ export function WorkoutDetailStats({ workout }: WorkoutDetailStatsProps) {
             <StatTile key={tile.label} {...tile} />
           ))}
         </div>
-        {averageLine ? (
-          <p
-            data-slot="workout-detail-sport-average"
-            className="text-muted-foreground text-xs"
-          >
-            {t("insights.workouts.detail.sportAverageLabel")}{" "}
-            <span className="tabular-nums">{averageLine}</span>
-          </p>
+        {averageLine || workout.previousWorkoutId ? (
+          // One row, per the inline-action shape: the sentence wraps
+          // internally, the link stays beside it.
+          <div className="flex items-start justify-between gap-3">
+            {averageLine ? (
+              <p
+                data-slot="workout-detail-sport-average"
+                className="text-muted-foreground min-w-0 flex-1 text-xs"
+              >
+                {t("insights.workouts.detail.sportAverageLabel")}{" "}
+                <span className="tabular-nums">{averageLine}</span>
+              </p>
+            ) : (
+              <span className="min-w-0 flex-1" />
+            )}
+            {workout.previousWorkoutId ? (
+              <Link
+                href={`/insights/workouts/${workout.previousWorkoutId}`}
+                data-slot="workout-detail-previous-link"
+                className="text-muted-foreground hover:text-foreground focus-visible:ring-ring/50 inline-flex shrink-0 items-center gap-1.5 text-xs underline-offset-4 transition-colors hover:underline focus-visible:ring-[3px] focus-visible:outline-none"
+              >
+                {t("insights.workouts.detail.previousSessionLink")}
+                <ArrowRight className="size-3.5" aria-hidden="true" />
+              </Link>
+            ) : null}
+          </div>
         ) : null}
       </CardContent>
     </Card>

src/hooks/use-workouts.ts

@@ -319,6 +319,12 @@ export interface WorkoutDetailPayload extends WorkoutListEntry {
   sportContext: WorkoutSportContextDto | null;
   aiInsight: WorkoutActivityInsight;
   canonicalId: string;
+  /**
+   * The canonical session before this one in the same sport, `null` when
+   * this is the first on record. Server-resolved through the same picker
+   * the list uses, so the link never lands on a duplicate row.
+   */
+  previousWorkoutId: string | null;
 }
 
 async function fetchWorkoutDetail(id: string): Promise<WorkoutDetailPayload> {

src/lib/openapi/routes/workouts.ts

@@ -199,6 +199,12 @@ const workoutDetailResponse = z
     // a provider reachable. Reading this endpoint never generates one.
     aiInsight: workoutActivityInsight.nullable(),
     canonicalId: z.string(),
+    /**
+     * The canonical session before this one in the same sport, or null when
+     * this is the first on record. Resolved through the same source-priority
+     * picker as the list, so the id always addresses a row the list shows.
+     */
+    previousWorkoutId: z.string().nullable(),
   })
   .meta({ id: "WorkoutDetailResponse" });