diff --git a/src/components/settings/integrations/__tests__/garmin-card.test.tsx b/src/components/settings/integrations/__tests__/garmin-card.test.tsx
new file mode 100644
index 000000000..c9beb65c5
--- /dev/null
+++ b/src/components/settings/integrations/__tests__/garmin-card.test.tsx
@@ -0,0 +1,165 @@
+/**
+ * Garmin on the shared integration card.
+ *
+ *   1. Not connected: the key form, the link to Garmin Connect's key page,
+ *      the validity choice and Connect, inside the shared card.
+ *   2. Connected: the same menu as Nightscout, a refused key opens the form,
+ *      and disconnect asks first and invalidates the envelope.
+ *   3. The key's end: a sentence in the body while it is close, past or
+ *      unknown, and the date in the details while it is comfortably valid.
+ */
+import { describe, expect, it, vi } from "vitest";
+import { createElement } from "react";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { queryKeys } from "@/lib/query-keys";
+
+const invalidateSpy = vi.fn();
+const capturedDisconnectOnSuccess: { fn: (() => void) | null } = { fn: null };
+
+vi.mock("@tanstack/react-query", () => ({
+  useMutation: ({ onSuccess }: { onSuccess?: () => void }) => {
+    capturedDisconnectOnSuccess.fn = onSuccess ?? null;
+    return { mutate: vi.fn(), isPending: false };
+  },
+  useQueryClient: () => ({ invalidateQueries: invalidateSpy }),
+}));
+
+let lastCardProps: IntegrationCardProps | null = null;
+vi.mock("../integration-card", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../integration-card")>();
+  return {
+    ...actual,
+    IntegrationCard: (props: IntegrationCardProps) => {
+      lastCardProps = props;
+      return createElement(actual.IntegrationCard, props);
+    },
+  };
+});
+
+import { I18nProvider } from "@/lib/i18n/context";
+import type { GarminKeyExpiry } from "@/lib/garmin/key-expiry";
+import type { IntegrationCardProps } from "../integration-card";
+import { GarminCard, GARMIN_KEY_SETTINGS_URL } from "../garmin-card";
+import type { IntegrationStatusViewModel } from "../shared";
+import { healthFixture } from "./health-fixture";
+
+function render(viewModel?: Partial<IntegrationStatusViewModel>) {
+  return renderToStaticMarkup(
+    <I18nProvider initialLocale="en">
+      <GarminCard
+        viewModel={
+          viewModel
+            ? ({
+                integration: "garmin",
+                ...viewModel,
+              } as IntegrationStatusViewModel)
+            : undefined
+        }
+      />
+    </I18nProvider>,
+  );
+}
+
+function connected(keyExpiry: GarminKeyExpiry | null) {
+  return render({
+    connected: true,
+    configured: true,
+    health: healthFixture(),
+    keyExpiry,
+    metricFreshness: [],
+  });
+}
+
+describe("GarminCard on the shared card", () => {
+  it("shows the key form inside the shared card while not connected", () => {
+    const html = render({ connected: false, configured: false, health: null });
+    expect(html).toContain('data-slot="integration-card"');
+    expect(html).toContain('data-state="not_connected"');
+    expect(html).toContain('id="garmin-api-key"');
+    expect(html).toContain('id="garmin-key-validity"');
+    expect(html).toContain(`href="${GARMIN_KEY_SETTINGS_URL}"`);
+    expect(html).toContain('data-testid="garmin-connect"');
+    expect(html).toContain(
+      'href="https://docs.healthlog.dev/guides/connect-garmin/"',
+    );
+    // 30, 90, 180 and 365 days, plus "I don't know".
+    expect(html.match(/<option /g)).toHaveLength(5);
+  });
+
+  it("keeps the sibling menu and opens the form for a new key", () => {
+    render({
+      connected: true,
+      configured: true,
+      health: healthFixture({
+        state: "action_required",
+        reason: "reauth_required",
+        action: "reconnect",
+      }),
+    });
+    expect((lastCardProps?.menu ?? []).map((i) => i.id)).toEqual([
+      "sync",
+      "test",
+      "view-data",
+      "credentials",
+      "setup-guide",
+      "disconnect",
+    ]);
+    expect(lastCardProps?.primary?.onReconnect).toBeTypeOf("function");
+    const disconnect = lastCardProps?.menu?.find((i) => i.id === "disconnect");
+    expect(disconnect?.destructive).toBe(true);
+    expect(disconnect?.confirm?.title).toBe("Disconnect Garmin?");
+    invalidateSpy.mockClear();
+    capturedDisconnectOnSuccess.fn?.();
+    expect(invalidateSpy).toHaveBeenCalledWith({
+      queryKey: queryKeys.integrationsStatus(),
+    });
+  });
+
+  it("warns in the body when the key is about to expire", () => {
+    const html = connected({
+      state: "expiring",
+      expiresAt: "2026-11-08T00:00:00.000Z",
+      daysLeft: 3,
+    });
+    expect(html).toMatch(
+      /data-testid="garmin-key-expiry"[^>]*data-state="expiring"/,
+    );
+    expect(html).toContain("Your Garmin key expires on");
+    expect(html).not.toContain('data-testid="garmin-key-valid-until"');
+  });
+
+  it("says when the key expires today and when it has expired", () => {
+    expect(
+      connected({
+        state: "expiring",
+        expiresAt: "2026-11-04T20:00:00.000Z",
+        daysLeft: 0,
+      }),
+    ).toContain("Your Garmin key expires today.");
+    expect(
+      connected({
+        state: "expired",
+        expiresAt: "2026-11-01T00:00:00.000Z",
+        daysLeft: null,
+      }),
+    ).toContain("Your Garmin key expired on");
+  });
+
+  it("asks for the validity when the end is unknown", () => {
+    expect(
+      connected({ state: "unknown", expiresAt: null, daysLeft: null }),
+    ).toContain("Garmin does not say when this key expires.");
+  });
+
+  it("keeps a valid key's date in the details, not the body", () => {
+    const html = connected({
+      state: "ok",
+      expiresAt: "2027-01-30T00:00:00.000Z",
+      daysLeft: 87,
+    });
+    expect(html).not.toContain('data-testid="garmin-key-expiry"');
+    const details = html.slice(html.indexOf('data-slot="integration-details"'));
+    expect(details).toContain('data-testid="garmin-key-valid-until"');
+  });
+});
diff --git a/src/lib/garmin/__tests__/client.test.ts b/src/lib/garmin/__tests__/client.test.ts
new file mode 100644
index 000000000..2c70ff4dc
--- /dev/null
+++ b/src/lib/garmin/__tests__/client.test.ts
@@ -0,0 +1,319 @@
+import { afterEach, describe, expect, it } from "vitest";
+
+import {
+  chunkGarminRange,
+  clearGarminTokenCache,
+  exchangeGarminKey,
+  extractGarminToken,
+  GarminApiError,
+  GarminSession,
+  garminTokenExpiry,
+  GARMIN_TOKEN_PATH,
+  isGarminKeyRefusal,
+  isGarminRateLimit,
+} from "../client";
+
+const NOW = Date.parse("2026-11-04T08:00:00Z");
+const HOUR = 60 * 60 * 1000;
+
+/** A made-up unsigned JWT with the given `exp` (epoch seconds). */
+function jwt(expSeconds: number, nonce = "a"): string {
+  const enc = (o: object) =>
+    Buffer.from(JSON.stringify(o)).toString("base64url");
+  return `${enc({ alg: "none" })}.${enc({ exp: expSeconds, n: nonce })}.sig`;
+}
+
+interface Call {
+  url: string;
+  auth: string | null;
+}
+
+/** A scripted Garmin: each request takes the next answer for its path kind. */
+function fakeGarmin(script: {
+  token?: Array<() => Response>;
+  data?: Array<() => Response>;
+}) {
+  const calls: Call[] = [];
+  const tokens = [...(script.token ?? [])];
+  const data = [...(script.data ?? [])];
+  const fetch = (async (url: string | URL, init?: RequestInit) => {
+    const u = url.toString();
+    const headers = new Headers(init?.headers);
+    calls.push({ url: u, auth: headers.get("authorization") });
+    const next = u.includes(GARMIN_TOKEN_PATH) ? tokens.shift() : data.shift();
+    if (!next) throw new Error(`unexpected request ${u}`);
+    return next();
+  }) as unknown as typeof import("@/lib/safe-fetch").safeFetch;
+  return { fetch, calls };
+}
+
+const tokenResponse = (value: string) => () =>
+  new Response(value, { status: 200 });
+const json =
+  (body: unknown, status = 200) =>
+  () =>
+    new Response(JSON.stringify(body), {
+      status,
+      headers: { "content-type": "application/json" },
+    });
+
+afterEach(() => clearGarminTokenCache());
+
+describe("extractGarminToken", () => {
+  it("reads a bare token, a quoted one and a JSON wrapper", () => {
+    expect(extractGarminToken("  abc.def.ghi\n")).toBe("abc.def.ghi");
+    expect(extractGarminToken('"abc.def.ghi"')).toBe("abc.def.ghi");
+    expect(extractGarminToken('{"access_token":"abc.def.ghi"}')).toBe(
+      "abc.def.ghi",
+    );
+    expect(extractGarminToken("{not json")).toBe("");
+  });
+});
+
+describe("garminTokenExpiry", () => {
+  it("reads exp in ms and ignores tokens without one", () => {
+    expect(garminTokenExpiry(jwt(1_800_000_000))).toBe(1_800_000_000_000);
+    expect(garminTokenExpiry("opaque")).toBeUndefined();
+  });
+});
+
+describe("exchangeGarminKey", () => {
+  it("sends the key as ApiKey and returns a token with its refresh point", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({ token: [tokenResponse(jwt(exp))] });
+    const token = await exchangeGarminKey("k".repeat(20), "global", {
+      fetch: g.fetch,
+      now: () => NOW,
+    });
+    expect(g.calls[0]!.auth).toBe(`ApiKey ${"k".repeat(20)}`);
+    expect(g.calls[0]!.url).toBe(
+      `https://personal.connectapi.garmin.com${GARMIN_TOKEN_PATH}`,
+    );
+    expect(token.expiresAt).toBe(exp * 1000);
+    // An hour before expiry for a regular day-long token.
+    expect(token.refreshAt).toBe(exp * 1000 - HOUR);
+    // A regular token says nothing about the key's end.
+    expect(token.keyExpiresAt).toBeNull();
+  });
+
+  it("learns the key's end from a token capped short of a day", async () => {
+    const exp = Math.floor((NOW + 5 * HOUR) / 1000);
+    const g = fakeGarmin({ token: [tokenResponse(jwt(exp))] });
+    const token = await exchangeGarminKey("k".repeat(20), "global", {
+      fetch: g.fetch,
+      now: () => NOW,
+    });
+    expect(token.keyExpiresAt).toBe(exp * 1000);
+  });
+
+  it.each([401, 403, 404])(
+    "treats %s at the exchange as a refused key",
+    async (status) => {
+      const g = fakeGarmin({
+        token: [() => new Response("", { status })],
+      });
+      const err = await exchangeGarminKey("k".repeat(20), "china", {
+        fetch: g.fetch,
+      }).catch((e) => e);
+      expect(err).toBeInstanceOf(GarminApiError);
+      expect(isGarminKeyRefusal(err)).toBe(true);
+      expect(err.classification).toBe("reauth_required");
+      expect(g.calls[0]!.url).toContain("connectapi.garmin.cn");
+    },
+  );
+
+  it("carries Retry-After on a rate-limited exchange", async () => {
+    const g = fakeGarmin({
+      token: [
+        () =>
+          new Response("", { status: 429, headers: { "retry-after": "120" } }),
+      ],
+    });
+    const err = await exchangeGarminKey("k".repeat(20), "global", {
+      fetch: g.fetch,
+    }).catch((e) => e);
+    expect(isGarminRateLimit(err)).toBe(true);
+    expect(err.retryAfterMs).toBe(120_000);
+    expect(isGarminKeyRefusal(err)).toBe(false);
+  });
+
+  it("refuses a 200 that is not a token, without echoing the body", async () => {
+    const g = fakeGarmin({
+      token: [() => new Response("<html>maintenance</html>", { status: 200 })],
+    });
+    const err = await exchangeGarminKey("k".repeat(20), "global", {
+      fetch: g.fetch,
+    }).catch((e) => e);
+    expect(err).toBeInstanceOf(GarminApiError);
+    expect(err.reason).toBe("token_not_jwt");
+    expect(String(err.message)).not.toContain("maintenance");
+  });
+});
+
+describe("GarminSession", () => {
+  const key = "secret-key-0123456789";
+
+  function session(fetch: ReturnType<typeof fakeGarmin>["fetch"], now = NOW) {
+    return new GarminSession({
+      userId: "u1",
+      apiKey: key,
+      region: "global",
+      transport: { fetch, now: () => now },
+    });
+  }
+
+  it("caches the token across calls and sessions", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp))],
+      data: [json({ a: 1 }), json({ a: 2 })],
+    });
+    expect(await session(g.fetch).get("x", "/p")).toEqual({ a: 1 });
+    expect(await session(g.fetch).get("x", "/p")).toEqual({ a: 2 });
+    const exchanges = g.calls.filter((c) => c.url.includes(GARMIN_TOKEN_PATH));
+    expect(exchanges).toHaveLength(1);
+    expect(g.calls[1]!.auth).toBe(`Bearer ${jwt(exp)}`);
+  });
+
+  it("buys a new token once the cached one is near its end", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp, "a")), tokenResponse(jwt(exp + 10, "b"))],
+      data: [json({}), json({})],
+    });
+    await session(g.fetch).get("x", "/p");
+    await session(g.fetch, NOW + 23.5 * HOUR).get("x", "/p");
+    expect(
+      g.calls.filter((c) => c.url.includes(GARMIN_TOKEN_PATH)),
+    ).toHaveLength(2);
+  });
+
+  it("does not reuse another key's token", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp, "a")), tokenResponse(jwt(exp, "b"))],
+      data: [json({}), json({})],
+    });
+    await session(g.fetch).get("x", "/p");
+    await new GarminSession({
+      userId: "u1",
+      apiKey: "another-key-0123456789",
+      region: "global",
+      transport: { fetch: g.fetch, now: () => NOW },
+    }).get("x", "/p");
+    expect(
+      g.calls.filter((c) => c.url.includes(GARMIN_TOKEN_PATH)),
+    ).toHaveLength(2);
+  });
+
+  it("re-exchanges once on a 401 and retries", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp, "a")), tokenResponse(jwt(exp, "b"))],
+      data: [() => new Response("", { status: 401 }), json({ ok: true })],
+    });
+    const s = session(g.fetch);
+    expect(await s.get("x", "/p")).toEqual({ ok: true });
+    expect(s.exchanges).toBe(2);
+    expect(g.calls.at(-1)!.auth).toBe(`Bearer ${jwt(exp, "b")}`);
+  });
+
+  it("treats a second 401 as the key's end and stops", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp, "a")), tokenResponse(jwt(exp, "b"))],
+      data: [
+        () => new Response("", { status: 401 }),
+        () => new Response("", { status: 401 }),
+      ],
+    });
+    const err = await session(g.fetch)
+      .get("x", "/p")
+      .catch((e) => e);
+    expect(isGarminKeyRefusal(err)).toBe(true);
+    // Exactly one retry: two data calls, two exchanges, nothing more.
+    expect(g.calls).toHaveLength(4);
+  });
+
+  it("surfaces a 429 with its wait and does not retry", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp))],
+      data: [
+        () =>
+          new Response("", { status: 429, headers: { "retry-after": "30" } }),
+      ],
+    });
+    const err = (await session(g.fetch)
+      .get("x", "/p")
+      .catch((e) => e)) as GarminApiError;
+    expect(isGarminRateLimit(err)).toBe(true);
+    expect(err.retryAfterMs).toBe(30_000);
+    expect(g.calls).toHaveLength(2);
+  });
+
+  it("answers null for 204 and an empty body", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp))],
+      data: [
+        () => new Response(null, { status: 204 }),
+        () => new Response("", { status: 200 }),
+      ],
+    });
+    const s = session(g.fetch);
+    expect(await s.get("x", "/p")).toBeNull();
+    expect(await s.get("x", "/p")).toBeNull();
+  });
+
+  it("puts the query on the URL and drops empty values", async () => {
+    const exp = Math.floor((NOW + 24 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp))],
+      data: [json([])],
+    });
+    await session(g.fetch).get("x", "/p", {
+      startDate: "2026-10-01",
+      none: undefined,
+      flag: true,
+    });
+    expect(g.calls[1]!.url).toBe(
+      "https://personal.connectapi.garmin.com/p?startDate=2026-10-01&flag=true",
+    );
+  });
+
+  it("remembers the key's end when a token reveals it", async () => {
+    const exp = Math.floor((NOW + 3 * HOUR) / 1000);
+    const g = fakeGarmin({
+      token: [tokenResponse(jwt(exp))],
+      data: [json({})],
+    });
+    const s = session(g.fetch);
+    await s.get("x", "/p");
+    expect(s.learnedKeyExpiresAt).toBe(exp * 1000);
+  });
+});
+
+describe("chunkGarminRange", () => {
+  it("splits newest first into windows of at most maxDays, both ends counted", () => {
+    expect(chunkGarminRange("2026-01-01", "2026-03-01", 28)).toEqual([
+      { startDate: "2026-02-02", endDate: "2026-03-01" },
+      { startDate: "2026-01-05", endDate: "2026-02-01" },
+      { startDate: "2026-01-01", endDate: "2026-01-04" },
+    ]);
+  });
+
+  it("keeps a year of daily summaries in one 365-day request", () => {
+    const chunks = chunkGarminRange("2025-11-05", "2026-11-04", 365);
+    expect(chunks).toEqual([
+      { startDate: "2025-11-05", endDate: "2026-11-04" },
+    ]);
+  });
+
+  it("returns one single-day window and nothing for an inverted range", () => {
+    expect(chunkGarminRange("2026-05-05", "2026-05-05", 31)).toEqual([
+      { startDate: "2026-05-05", endDate: "2026-05-05" },
+    ]);
+    expect(chunkGarminRange("2026-05-06", "2026-05-05", 31)).toEqual([]);
+  });
+});
diff --git a/src/lib/garmin/__tests__/map.test.ts b/src/lib/garmin/__tests__/map.test.ts
new file mode 100644
index 000000000..4f36a37fa
--- /dev/null
+++ b/src/lib/garmin/__tests__/map.test.ts
@@ -0,0 +1,380 @@
+/**
+ * Mapping per Garmin resource, over made-up records shaped from Garmin's
+ * OpenAPI document (Personal API 1.0.0). No value here comes from a real
+ * account.
+ */
+import { describe, expect, it } from "vitest";
+
+import {
+  mapGarminDailySummary,
+  mapGarminHrv,
+  mapGarminSleep,
+  mapGarminWeightRange,
+  parseGarminInstant,
+} from "../map";
+import { mapGarminActivity } from "../sync-workouts";
+import { mapGarminSportType } from "../sport-map";
+import { garminCycleDay, periodFromGarminCycleSummary } from "../cycle-sync";
+import { estimateGarminKeyExpiry, resolveGarminKeyExpiry } from "../key-expiry";
+
+const TZ = "Europe/Berlin";
+
+function byType(rows: Array<{ type: string; value: number }>) {
+  return Object.fromEntries(rows.map((r) => [r.type, r.value]));
+}
+
+describe("parseGarminInstant", () => {
+  it("reads epoch ms and zone-less GMT strings", () => {
+    expect(parseGarminInstant(1_792_560_600_000)?.toISOString()).toBe(
+      "2026-10-21T05:30:00.000Z",
+    );
+    expect(parseGarminInstant("2026-10-22 05:15:02")?.toISOString()).toBe(
+      "2026-10-22T05:15:02.000Z",
+    );
+    expect(parseGarminInstant("2026-10-22T05:40:00.0")?.toISOString()).toBe(
+      "2026-10-22T05:40:00.000Z",
+    );
+    expect(parseGarminInstant(null)).toBeNull();
+    expect(parseGarminInstant("")).toBeNull();
+  });
+});
+
+describe("mapGarminDailySummary", () => {
+  const day = {
+    calendarDate: "2026-10-22",
+    includesWellnessData: true,
+    movement: {
+      steps: { value: 8421, distanceInMeters: 6410.5 },
+      floorsAscended: { value: 7 },
+    },
+    calories: { burnedActive: 512 },
+    heartRate: { restingValue: 55 },
+    stress: { avgLevel: 31 },
+    bodyBattery: { maxValue: 88 },
+  };
+
+  it("maps every daily field to its type and unit, keyed on the day", () => {
+    const rows = mapGarminDailySummary(day, TZ);
+    expect(byType(rows)).toEqual({
+      ACTIVITY_STEPS: 8421,
+      WALKING_RUNNING_DISTANCE: 6410.5,
+      FLIGHTS_CLIMBED: 7,
+      ACTIVE_ENERGY_BURNED: 512,
+      RESTING_HEART_RATE: 55,
+      STRESS_LEVEL: 31,
+      BODY_BATTERY: 88,
+    });
+    const units = Object.fromEntries(rows.map((r) => [r.type, r.unit]));
+    expect(units).toMatchObject({
+      ACTIVITY_STEPS: "steps",
+      WALKING_RUNNING_DISTANCE: "m",
+      FLIGHTS_CLIMBED: "flights",
+      ACTIVE_ENERGY_BURNED: "kcal",
+      RESTING_HEART_RATE: "bpm",
+      STRESS_LEVEL: "score",
+      BODY_BATTERY: "score",
+    });
+    expect(rows.map((r) => r.externalId)).toContain(
+      "daily:2026-10-22:body_battery",
+    );
+    // Local noon of the date in the user's zone (CEST, UTC+2).
+    expect(rows[0]!.measuredAt.toISOString()).toBe("2026-10-22T10:00:00.000Z");
+  });
+
+  it("writes nothing for a day the watch recorded nothing", () => {
+    expect(
+      mapGarminDailySummary({ ...day, includesWellnessData: false }, TZ),
+    ).toEqual([]);
+    expect(
+      mapGarminDailySummary({ ...day, includesWellnessData: null }, TZ),
+    ).toEqual([]);
+  });
+
+  it("drops Garmin's negative not-enough-data sentinels", () => {
+    const rows = mapGarminDailySummary(
+      { ...day, stress: { avgLevel: -1 }, bodyBattery: { maxValue: -2 } },
+      TZ,
+    );
+    expect(rows.map((r) => r.type)).not.toContain("STRESS_LEVEL");
+    expect(rows.map((r) => r.type)).not.toContain("BODY_BATTERY");
+  });
+});
+
+describe("mapGarminSleep", () => {
+  const night = {
+    calendarDate: "2026-10-22",
+    sleepTimeSeconds: 25_200,
+    sleepEndTimestampGMT: 1_792_647_000_000,
+    deepSleepSeconds: 5_400,
+    lightSleepSeconds: 13_800,
+    remSleepSeconds: 6_000,
+    awakeSleepSeconds: 900,
+    averageSpO2Value: 95,
+    averageRespirationValue: 14.5,
+    sleepScores: { overall: { value: 79, qualifierKey: "FAIR" } },
+  };
+
+  it("maps stage totals onto the wake instant, light as CORE", () => {
+    const rows = mapGarminSleep(night, TZ);
+    const stages = rows
+      .filter((r) => r.type === "SLEEP_DURATION")
+      .map((r) => [r.sleepStage, r.value]);
+    expect(stages).toEqual([
+      ["CORE", 230],
+      ["DEEP", 90],
+      ["REM", 100],
+      ["AWAKE", 15],
+    ]);
+    for (const r of rows) {
+      expect(r.measuredAt.getTime()).toBe(1_792_647_000_000);
+    }
+    expect(byType(rows.filter((r) => r.type !== "SLEEP_DURATION"))).toEqual({
+      SLEEP_SCORE: 79,
+      OXYGEN_SATURATION: 95,
+      RESPIRATORY_RATE: 14.5,
+    });
+    expect(new Set(rows.map((r) => r.externalId)).size).toBe(rows.length);
+  });
+
+  it("maps a no-sleep day (null date, all-null fields) to nothing", () => {
+    expect(
+      mapGarminSleep(
+        {
+          calendarDate: null,
+          sleepTimeSeconds: null,
+          deepSleepSeconds: null,
+        },
+        TZ,
+      ),
+    ).toEqual([]);
+    expect(mapGarminSleep({ ...night, sleepTimeSeconds: 0 }, TZ)).toEqual([]);
+  });
+});
+
+describe("mapGarminHrv", () => {
+  it("maps the overnight average to HRV_RMSSD and skips empty nights", () => {
+    const rows = mapGarminHrv(
+      {
+        hrvSummaries: [
+          { calendarDate: "2026-10-21", lastNightAvg: 41 },
+          { calendarDate: "2026-10-22", lastNightAvg: null },
+          { calendarDate: null, lastNightAvg: 50 },
+        ],
+      },
+      TZ,
+    );
+    expect(rows).toHaveLength(1);
+    expect(rows[0]).toMatchObject({
+      type: "HRV_RMSSD",
+      value: 41,
+      unit: "ms",
+      externalId: "hrv:2026-10-21:hrv_rmssd",
+    });
+    expect(mapGarminHrv(null, TZ)).toEqual([]);
+  });
+});
+
+describe("mapGarminWeightRange", () => {
+  it("converts grams to kilograms and body water from percent", () => {
+    const rows = mapGarminWeightRange({
+      dailyWeightSummaries: [
+        {
+          summaryDate: "2026-10-20",
+          allWeightMetrics: [
+            {
+              samplePk: 11,
+              timestampGMT: 1_792_474_200_000,
+              weight: 80_000,
+              bmi: 24.7,
+              bodyFat: 18.5,
+              bodyWater: 55,
+              muscleMass: 36_400,
+              boneMass: 3_200,
+              visceralFat: 7,
+              sourceType: "INDEX_SCALE",
+            },
+          ],
+        },
+        {
+          summaryDate: "2026-10-21",
+          latestWeight: {
+            samplePk: 12,
+            date: 1_792_560_000_000,
+            weight: 79_600,
+          },
+        },
+      ],
+    });
+    const first = rows.filter((r) => r.externalId.startsWith("weight:11:"));
+    expect(byType(first)).toEqual({
+      WEIGHT: 80,
+      BODY_MASS_INDEX: 24.7,
+      BODY_FAT: 18.5,
+      TOTAL_BODY_WATER: 44,
+      MUSCLE_MASS: 36.4,
+      BONE_MASS: 3.2,
+      VISCERAL_FAT: 7,
+    });
+    expect(first.every((r) => r.deviceType === "scale")).toBe(true);
+    const second = rows.filter((r) => r.externalId.startsWith("weight:12:"));
+    expect(byType(second)).toEqual({ WEIGHT: 79.6 });
+    expect(second[0]!.deviceType).toBeUndefined();
+  });
+
+  it("does not write a weigh-in twice when it is both latest and listed", () => {
+    const w = { samplePk: 5, date: 1_792_560_000_000, weight: 70_000 };
+    const rows = mapGarminWeightRange({
+      dailyWeightSummaries: [{ latestWeight: w, allWeightMetrics: [w] }],
+    });
+    expect(rows).toHaveLength(1);
+  });
+});
+
+describe("mapGarminActivity", () => {
+  it("maps an activity to a workout row", () => {
+    const row = mapGarminActivity({
+      activityId: 10_000_000_001,
+      startTimeGMT: "2026-10-22 05:15:02",
+      activityType: { typeKey: "trail_running" },
+      distance: 8012.4,
+      duration: 2701.3,
+      elapsedDuration: 2750.5,
+      elevationGain: 54,
+      calories: 610.4,
+      averageHR: 148,
+      maxHR: 171,
+      steps: 7400,
+      aerobicTrainingEffect: 3.2,
+      activityTrainingLoad: 112.5,
+    });
+    expect(row).toMatchObject({
+      externalId: "10000000001",
+      sportType: "running",
+      durationSec: 2701,
+      totalEnergyKcal: 610,
+      totalDistanceM: 8012.4,
+      avgHeartRate: 148,
+      maxHeartRate: 171,
+      stepCount: 7400,
+      elevationM: 54,
+      metadata: {
+        garminActivity: "trail_running",
+        garminAerobicEffect: 3.2,
+        garminTrainingLoad: 112.5,
+      },
+    });
+    expect(row!.startedAt.toISOString()).toBe("2026-10-22T05:15:02.000Z");
+    expect(row!.endedAt.getTime() - row!.startedAt.getTime()).toBe(2_750_500);
+  });
+
+  it("drops an activity without id, start or duration", () => {
+    expect(
+      mapGarminActivity({ startTimeGMT: "2026-10-22 05:15:02" }),
+    ).toBeNull();
+    expect(mapGarminActivity({ activityId: 1, duration: 60 })).toBeNull();
+    expect(
+      mapGarminActivity({
+        activityId: 1,
+        beginTimestamp: 1_792_560_000_000,
+        duration: 0,
+      }),
+    ).toBeNull();
+  });
+});
+
+describe("mapGarminSportType", () => {
+  it.each([
+    ["running", "running"],
+    ["road_biking", "cycling"],
+    ["lap_swimming", "swimming"],
+    ["kayaking_v2", "rowing"],
+    ["strength_training", "strength"],
+    ["yoga", "yoga"],
+    ["resort_skiing", "other"],
+    ["something_new", "other"],
+    [null, "other"],
+  ])("%s → %s", (key, expected) => {
+    expect(mapGarminSportType(key)).toBe(expected);
+  });
+});
+
+describe("cycle summary", () => {
+  it("reads a cycle date bare or wrapped", () => {
+    expect(garminCycleDay("2026-10-10")).toBe("2026-10-10");
+    expect(garminCycleDay({ localDate: "2026-10-10" })).toBe("2026-10-10");
+    expect(garminCycleDay({ date: "2026-10-10T00:00:00Z" })).toBe("2026-10-10");
+    expect(garminCycleDay(null)).toBeNull();
+  });
+
+  it("derives the logged period days, never in the future", () => {
+    expect(
+      periodFromGarminCycleSummary(
+        { startDate: "2026-10-10", periodLength: 5, predictedCycle: false },
+        "2026-11-04",
+      ),
+    ).toEqual({
+      start: "2026-10-10",
+      days: [
+        "2026-10-10",
+        "2026-10-11",
+        "2026-10-12",
+        "2026-10-13",
+        "2026-10-14",
+      ],
+    });
+    expect(
+      periodFromGarminCycleSummary(
+        { startDate: "2026-11-03", periodLength: 5 },
+        "2026-11-04",
+      )?.days,
+    ).toEqual(["2026-11-03", "2026-11-04"]);
+  });
+
+  it("imports nothing from a predicted cycle or a pregnancy", () => {
+    expect(
+      periodFromGarminCycleSummary(
+        { startDate: "2026-10-10", periodLength: 5, predictedCycle: true },
+        "2026-11-04",
+      ),
+    ).toBeNull();
+    expect(
+      periodFromGarminCycleSummary(
+        { startDate: "2026-10-10", periodLength: 5, pregnancyCycle: true },
+        "2026-11-04",
+      ),
+    ).toBeNull();
+    expect(periodFromGarminCycleSummary({}, "2026-11-04")).toBeNull();
+  });
+});
+
+describe("key expiry", () => {
+  const now = new Date("2026-11-04T12:00:00Z");
+
+  it("estimates the end from the picked validity", () => {
+    expect(
+      estimateGarminKeyExpiry(
+        new Date("2026-11-01T00:00:00Z"),
+        90,
+      ).toISOString(),
+    ).toBe("2027-01-30T00:00:00.000Z");
+  });
+
+  it("is ok well ahead, expiring inside the last week, expired after", () => {
+    expect(
+      resolveGarminKeyExpiry(new Date("2026-12-01T00:00:00Z"), now).state,
+    ).toBe("ok");
+    const soon = resolveGarminKeyExpiry(new Date("2026-11-08T00:00:00Z"), now);
+    expect(soon).toMatchObject({ state: "expiring", daysLeft: 3 });
+    expect(
+      resolveGarminKeyExpiry(new Date("2026-11-04T18:00:00Z"), now).daysLeft,
+    ).toBe(0);
+    expect(
+      resolveGarminKeyExpiry(new Date("2026-11-01T00:00:00Z"), now),
+    ).toMatchObject({ state: "expired", daysLeft: null });
+    expect(resolveGarminKeyExpiry(null, now)).toEqual({
+      state: "unknown",
+      expiresAt: null,
+      daysLeft: null,
+    });
+  });
+});
diff --git a/tests/integration/garmin-sync.test.ts b/tests/integration/garmin-sync.test.ts
new file mode 100644
index 000000000..ba265bb84
--- /dev/null
+++ b/tests/integration/garmin-sync.test.ts
@@ -0,0 +1,646 @@
+/**
+ * The Garmin Personal API integration against the real schema and a fake
+ * Garmin served over HTTP.
+ *
+ * `safeFetch` is wrapped so a request to either Garmin host is sent to a
+ * local `http.Server` instead; everything else about the request (method,
+ * headers, body reading, the byte cap) is the production path. The fake
+ * answers the token exchange (`ApiKey <key>` → a JWT), and the daily
+ * summaries, sleep, HRV, weigh-ins, activity search and cycle summary from a
+ * made-up history generated on request. No value is from a real account.
+ *
+ * What is pinned:
+ *   - a sync writes each resource's rows under `source = GARMIN`, a workout,
+ *     the cycle's period days, and stamps the `garmin` ledger;
+ *   - a second sync writes no second row (idempotent re-read);
+ *   - a key Garmin stops accepting is re-exchanged exactly once, then the
+ *     connection is parked at `reauth_required`;
+ *   - a 429 ends the run without reading further resources;
+ *   - the history backfill walks back to the start of the history in slices
+ *     and stamps its marker; a 429 in the middle sends the next slice after
+ *     the wait Garmin named;
+ *   - connect validates and stores the key sealed, disconnect purges it and
+ *     everything stored about it, while the imported history stays.
+ */
+import { createServer, type IncomingMessage, type Server } from "node:http";
+import type { AddressInfo } from "node:net";
+
+import { NextRequest } from "next/server";
+import {
+  afterAll,
+  beforeAll,
+  beforeEach,
+  describe,
+  expect,
+  it,
+  vi,
+} from "vitest";
+
+process.env.ENCRYPTION_KEY ??=
+  "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
+
+import { cookieJar } from "./mock-next-headers";
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+vi.mock("next/headers", async () => {
+  const { cookieJar, headerJar } = await import("./mock-next-headers");
+  return {
+    headers: vi.fn(async () => ({
+      get: (name: string) => headerJar.get(name.toLowerCase()) ?? null,
+    })),
+    cookies: vi.fn(async () => ({
+      get: (name: string) => {
+        const value = cookieJar.get(name);
+        return value ? { name, value } : undefined;
+      },
+      set: (name: string, value: string) => cookieJar.set(name, value),
+      delete: (name: string) => cookieJar.delete(name),
+    })),
+  };
+});
+
+const fake = vi.hoisted(() => ({
+  base: "",
+  key: "made-up-garmin-key-0001",
+  /** Days with wellness data: from this day to `today`. */
+  historyFrom: "2026-10-01",
+  today: "2026-11-04",
+  requests: [] as string[],
+  /** Next data calls answered 401 (count down). */
+  dataUnauthorized: 0,
+  /** Answer the n-th data request with a 429 (0-based), -1 for never. */
+  rateLimitAt: -1,
+  dataCalls: 0,
+  /** Index in `requests` of the request the 429 answered. */
+  rateLimitHit: -1,
+  /** First day of the one logged cycle, relative to today. */
+  cycleStart: "",
+  sends: [] as Array<{ queue: string; payload: unknown }>,
+}));
+
+vi.mock("@/lib/jobs/boss-instance", () => ({
+  getGlobalBoss: () => ({
+    send: async (queue: string, payload: unknown) => {
+      fake.sends.push({ queue, payload });
+      return `job-${fake.sends.length}`;
+    },
+  }),
+}));
+
+vi.mock("@/lib/safe-fetch", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/safe-fetch")>();
+  return {
+    ...actual,
+    safeFetch: async (
+      url: string | URL,
+      init?: RequestInit,
+      opts?: Parameters<typeof actual.safeFetch>[2],
+    ) => {
+      const target = url
+        .toString()
+        .replace(
+          /^https:\/\/personal\.connectapi\.garmin\.(com|cn)/,
+          fake.base,
+        );
+      return actual.safeFetch(target, init, opts);
+    },
+  };
+});
+
+const USER_ID = "user-garmin";
+const DAY_MS = 86_400_000;
+
+function addDays(day: string, n: number): string {
+  return new Date(Date.parse(`${day}T00:00:00Z`) + n * DAY_MS)
+    .toISOString()
+    .slice(0, 10);
+}
+
+function daysBetween(from: string, to: string): string[] {
+  const out: string[] = [];
+  for (let d = from; d <= to; d = addDays(d, 1)) out.push(d);
+  return out;
+}
+
+function jwt(): string {
+  const enc = (o: object) =>
+    Buffer.from(JSON.stringify(o)).toString("base64url");
+  const exp = Math.floor(Date.now() / 1000) + 24 * 3600;
+  return `${enc({ alg: "none" })}.${enc({ exp })}.sig`;
+}
+
+const TOKEN = jwt();
+
+/** The made-up history, generated per request. */
+function answer(path: string, q: URLSearchParams): unknown {
+  const inHistory = (d: string) => d >= fake.historyFrom && d <= fake.today;
+  if (path === "/usersummary-service/v2/dailysummaries") {
+    return {
+      data: daysBetween(q.get("startDate")!, q.get("endDate")!).map((d) =>
+        inHistory(d)
+          ? {
+              calendarDate: d,
+              includesWellnessData: true,
+              movement: {
+                steps: { value: 9000, distanceInMeters: 7000 },
+                floorsAscended: { value: 5 },
+              },
+              calories: { burnedActive: 450 },
+              heartRate: { restingValue: 54 },
+              stress: { avgLevel: 28 },
+              bodyBattery: { maxValue: 82 },
+            }
+          : { calendarDate: d, includesWellnessData: false },
+      ),
+    };
+  }
+  if (path === "/sleep-service/sleep/dailySleepSummariesByDate") {
+    return daysBetween(q.get("startDate")!, q.get("endDate")!).map((d) =>
+      inHistory(d)
+        ? {
+            calendarDate: d,
+            sleepTimeSeconds: 25_200,
+            sleepEndTimestampGMT: Date.parse(`${d}T05:30:00Z`),
+            deepSleepSeconds: 5_400,
+            lightSleepSeconds: 13_800,
+            remSleepSeconds: 6_000,
+            awakeSleepSeconds: 900,
+            averageSpO2Value: 95,
+            averageRespirationValue: 14,
+            sleepScores: { overall: { value: 77 } },
+          }
+        : { calendarDate: null, sleepTimeSeconds: null },
+    );
+  }
+  if (path.startsWith("/hrv-service/hrv/daily/")) {
+    const [, , , , from, to] = path.split("/");
+    return {
+      hrvSummaries: daysBetween(from!, to!)
+        .filter(inHistory)
+        .map((d) => ({ calendarDate: d, lastNightAvg: 44 })),
+    };
+  }
+  if (path.startsWith("/weight-service/weight/range/")) {
+    const [, , , , from, to] = path.split("/");
+    const weighDay = fake.today;
+    return {
+      dailyWeightSummaries:
+        weighDay >= from! && weighDay <= to!
+          ? [
+              {
+                summaryDate: weighDay,
+                allWeightMetrics: [
+                  {
+                    samplePk: 501,
+                    timestampGMT: Date.parse(`${weighDay}T06:00:00Z`),
+                    weight: 75_500,
+                    bodyFat: 19,
+                  },
+                ],
+              },
+            ]
+          : [],
+    };
+  }
+  if (path === "/activitylist-service/activities/search/activities") {
+    const runDay = addDays(fake.today, -2);
+    const start = q.get("startDate")!;
+    const end = q.get("endDate")!;
+    if (Number(q.get("start")) > 0) return [];
+    return runDay >= start && runDay <= end
+      ? [
+          {
+            activityId: 777,
+            startTimeGMT: `${runDay} 06:00:00`,
+            activityType: { typeKey: "running" },
+            duration: 1800,
+            distance: 5000,
+            calories: 350,
+          },
+        ]
+      : [];
+  }
+  if (path.startsWith("/periodichealth-service/menstrualcycle/summary/")) {
+    const day = path.split("/").pop()!;
+    const cycleStart = fake.cycleStart;
+    return day >= cycleStart
+      ? { startDate: cycleStart, periodLength: 4, predictedCycle: false }
+      : {};
+  }
+  return null;
+}
+
+let server: Server;
+
+function readBody(req: IncomingMessage): Promise<void> {
+  return new Promise((resolve) => {
+    req.on("data", () => {});
+    req.on("end", () => resolve());
+  });
+}
+
+beforeAll(async () => {
+  server = createServer(async (req, res) => {
+    await readBody(req);
+    const u = new URL(req.url!, "http://fake");
+    fake.requests.push(`${req.method} ${u.pathname}${u.search}`);
+    const auth = req.headers.authorization ?? "";
+    if (u.pathname.startsWith("/personalapitoken-service/")) {
+      if (auth !== `ApiKey ${fake.key}`) {
+        res.writeHead(401).end();
+        return;
+      }
+      res.writeHead(200, { "content-type": "text/plain" }).end(TOKEN);
+      return;
+    }
+    const n = fake.dataCalls++;
+    if (fake.dataUnauthorized > 0) {
+      fake.dataUnauthorized--;
+      res.writeHead(401).end();
+      return;
+    }
+    if (auth !== `Bearer ${TOKEN}`) {
+      res.writeHead(401).end();
+      return;
+    }
+    if (n === fake.rateLimitAt) {
+      fake.rateLimitHit =
+        fake.requests.filter((r) =>
+          r.includes("/usersummary-service/v2/dailysummaries"),
+        ).length - 1;
+      res.writeHead(429, { "retry-after": "900" }).end();
+      return;
+    }
+    const body = answer(u.pathname, u.searchParams);
+    if (body === null) {
+      res.writeHead(404).end();
+      return;
+    }
+    res
+      .writeHead(200, { "content-type": "application/json" })
+      .end(JSON.stringify(body));
+  });
+  await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+  fake.base = `http://127.0.0.1:${(server.address() as AddressInfo).port}`;
+});
+
+afterAll(async () => {
+  await new Promise<void>((resolve) => server.close(() => resolve()));
+});
+
+async function connectUser() {
+  const { encrypt } = await import("@/lib/crypto");
+  await getPrismaClient().user.update({
+    where: { id: USER_ID },
+    data: {
+      garminApiKeyEncrypted: encrypt(fake.key),
+      garminRegion: "global",
+      garminConnectedAt: new Date(),
+    },
+  });
+}
+
+beforeEach(async () => {
+  const prisma = getPrismaClient();
+  await truncateAllTables(prisma);
+  cookieJar.clear();
+  fake.requests.length = 0;
+  fake.sends.length = 0;
+  fake.dataUnauthorized = 0;
+  fake.rateLimitAt = -1;
+  fake.dataCalls = 0;
+  fake.rateLimitHit = -1;
+  fake.key = "made-up-garmin-key-0001";
+  fake.today = new Date().toISOString().slice(0, 10);
+  fake.historyFrom = addDays(fake.today, -20);
+  fake.cycleStart = addDays(fake.today, -15);
+  const { clearGarminTokenCache } = await import("@/lib/garmin/client");
+  clearGarminTokenCache();
+  await prisma.user.create({
+    data: {
+      id: USER_ID,
+      username: "garmin-user",
+      email: "garmin@example.test",
+      timezone: "UTC",
+      gender: "FEMALE",
+    },
+  });
+  const session = await prisma.session.create({
+    data: { userId: USER_ID, expiresAt: new Date(Date.now() + 3_600_000) },
+  });
+  cookieJar.set("healthlog_session", session.id);
+});
+
+async function garminRowCounts() {
+  const rows = await getPrismaClient().measurement.groupBy({
+    by: ["type"],
+    where: { userId: USER_ID, source: "GARMIN", deletedAt: null },
+    _count: { _all: true },
+  });
+  return Object.fromEntries(rows.map((r) => [r.type, r._count._all]));
+}
+
+describe("Garmin sync", () => {
+  it("writes every resource under source GARMIN and stamps the ledger", async () => {
+    await connectUser();
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    const result = await syncUserGarmin(USER_ID, { lookbackDays: 6 });
+    expect(result.failed).toBe(false);
+
+    const counts = await garminRowCounts();
+    // Seven days (today and six back), each with its daily and nightly rows.
+    expect(counts).toMatchObject({
+      ACTIVITY_STEPS: 7,
+      WALKING_RUNNING_DISTANCE: 7,
+      FLIGHTS_CLIMBED: 7,
+      ACTIVE_ENERGY_BURNED: 7,
+      RESTING_HEART_RATE: 7,
+      STRESS_LEVEL: 7,
+      BODY_BATTERY: 7,
+      SLEEP_DURATION: 28,
+      SLEEP_SCORE: 7,
+      OXYGEN_SATURATION: 7,
+      RESPIRATORY_RATE: 7,
+      HRV_RMSSD: 7,
+      WEIGHT: 1,
+      BODY_FAT: 1,
+    });
+    const prisma = getPrismaClient();
+    const weight = await prisma.measurement.findFirst({
+      where: { userId: USER_ID, type: "WEIGHT" },
+    });
+    expect(weight).toMatchObject({ source: "GARMIN", value: 75.5, unit: "kg" });
+    const stages = await prisma.measurement.findMany({
+      where: { userId: USER_ID, type: "SLEEP_DURATION" },
+      select: { sleepStage: true },
+      distinct: ["sleepStage"],
+    });
+    expect(stages.map((s) => s.sleepStage).sort()).toEqual([
+      "AWAKE",
+      "CORE",
+      "DEEP",
+      "REM",
+    ]);
+
+    const workouts = await prisma.workout.findMany({
+      where: { userId: USER_ID },
+    });
+    expect(workouts).toHaveLength(1);
+    expect(workouts[0]).toMatchObject({
+      source: "GARMIN",
+      externalId: "777",
+      sportType: "running",
+      durationSec: 1800,
+    });
+
+    const cycleDays = await prisma.cycleDayLog.findMany({
+      where: { userId: USER_ID },
+      orderBy: { date: "asc" },
+    });
+    expect(cycleDays.map((d) => [d.date, d.source, d.flow])).toEqual(
+      [0, 1, 2, 3].map((i) => [addDays(fake.cycleStart, i), "GARMIN", "LIGHT"]),
+    );
+
+    const ledger = await prisma.integrationStatus.findFirst({
+      where: { userId: USER_ID, integration: "garmin" },
+    });
+    expect(ledger?.state).toBe("connected");
+    // One exchange for the whole run.
+    expect(
+      fake.requests.filter((r) => r.includes("personalapitoken")),
+    ).toHaveLength(1);
+  });
+
+  it("writes no second row on a re-sync", async () => {
+    await connectUser();
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    await syncUserGarmin(USER_ID, { lookbackDays: 6 });
+    const before = await getPrismaClient().measurement.count({
+      where: { userId: USER_ID },
+    });
+    await syncUserGarmin(USER_ID, { lookbackDays: 6 });
+    const after = await getPrismaClient().measurement.count({
+      where: { userId: USER_ID },
+    });
+    expect(before).toBeGreaterThan(0);
+    expect(after).toBe(before);
+    expect(
+      await getPrismaClient().workout.count({ where: { userId: USER_ID } }),
+    ).toBe(1);
+    expect(
+      await getPrismaClient().cycleDayLog.count({ where: { userId: USER_ID } }),
+    ).toBe(4);
+  });
+
+  it("never overwrites a cycle day the person logged", async () => {
+    await connectUser();
+    await getPrismaClient().cycleDayLog.create({
+      data: {
+        userId: USER_ID,
+        date: addDays(fake.cycleStart, 1),
+        flow: "HEAVY",
+      },
+    });
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    await syncUserGarmin(USER_ID, { lookbackDays: 2 });
+    const own = await getPrismaClient().cycleDayLog.findFirst({
+      where: { userId: USER_ID, date: addDays(fake.cycleStart, 1) },
+    });
+    expect(own).toMatchObject({ source: "MANUAL", flow: "HEAVY" });
+  });
+
+  it("re-exchanges once on a refused token, then parks the key", async () => {
+    await connectUser();
+    fake.dataUnauthorized = 2;
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    await expect(
+      syncUserGarmin(USER_ID, { lookbackDays: 2 }),
+    ).rejects.toThrow();
+    expect(
+      fake.requests.filter((r) => r.includes("personalapitoken")),
+    ).toHaveLength(2);
+    // The two refused data calls and nothing after them.
+    expect(fake.dataCalls).toBe(2);
+    const ledger = await getPrismaClient().integrationStatus.findFirst({
+      where: { userId: USER_ID, integration: "garmin" },
+    });
+    expect(ledger?.state).toBe("error_reauth");
+    // The hourly cohort skips a parked key instead of asking Garmin again.
+    const before = fake.requests.length;
+    const skipped = await syncUserGarmin(USER_ID, { skipWhenParked: true });
+    expect(skipped).toEqual({ imported: 0, failed: false });
+    expect(fake.requests.length).toBe(before);
+  });
+
+  it("stops at a 429 and records a transient failure", async () => {
+    await connectUser();
+    fake.rateLimitAt = 1; // the sleep summaries, after the daily summaries
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    const result = await syncUserGarmin(USER_ID, { lookbackDays: 2 });
+    expect(result.failed).toBe(true);
+    // Daily summaries, then the refused sleep read; no HRV, weight or
+    // activity request, no cycle request.
+    expect(fake.dataCalls).toBe(2);
+    expect(fake.requests.some((r) => r.includes("/hrv-service/"))).toBe(false);
+    const ledger = await getPrismaClient().integrationStatus.findFirst({
+      where: { userId: USER_ID, integration: "garmin" },
+    });
+    expect(ledger?.state).toBe("error_transient");
+    expect((await garminRowCounts()).ACTIVITY_STEPS).toBe(3);
+  });
+});
+
+describe("Garmin history backfill", () => {
+  it("walks the history in slices to its start and stamps the marker", async () => {
+    await connectUser();
+    fake.historyFrom = addDays(fake.today, -400);
+    fake.rateLimitAt = 3;
+    const { runGarminBackfillForUser } =
+      await import("@/lib/jobs/garmin-backfill");
+    const prisma = getPrismaClient();
+
+    // Slice 0 hits the 429 and sends the next slice after Garmin's wait.
+    const first = await runGarminBackfillForUser(USER_ID, () => false, 0);
+    expect(first.continued).toBe(true);
+    expect(fake.sends.at(-1)).toMatchObject({
+      queue: "integration-backfill-admission",
+      payload: { kind: "garmin-backfill", data: { slice: 1 } },
+    });
+
+    let slice = 1;
+    for (; slice < 20; slice++) {
+      const user = await prisma.user.findUnique({
+        where: { id: USER_ID },
+        select: { garminBackfillCompletedAt: true },
+      });
+      if (user?.garminBackfillCompletedAt) break;
+      await runGarminBackfillForUser(USER_ID, () => false, slice);
+    }
+    const user = await prisma.user.findUnique({
+      where: { id: USER_ID },
+      select: { garminBackfillCompletedAt: true, garminBackfillProgress: true },
+    });
+    expect(user?.garminBackfillCompletedAt).not.toBeNull();
+    expect(
+      (user?.garminBackfillProgress as { historyStart?: string }).historyStart,
+    ).toBe(fake.historyFrom);
+
+    // Every day of the history has its steps row, once.
+    const counts = await garminRowCounts();
+    expect(counts.ACTIVITY_STEPS).toBe(401);
+    expect(counts.HRV_RMSSD).toBe(401);
+    expect(counts.SLEEP_DURATION).toBe(401 * 4);
+    // No daily-summary window was read twice, apart from the one the 429
+    // interrupted: the next slice resumed exactly there.
+    const dailyReads = fake.requests.filter((r) =>
+      r.includes("/usersummary-service/v2/dailysummaries"),
+    );
+    expect(dailyReads.length - new Set(dailyReads).size).toBe(1);
+    const repeated = dailyReads.find((r, i) => dailyReads.indexOf(r) !== i);
+    expect(dailyReads.indexOf(repeated!)).toBe(fake.rateLimitHit);
+  });
+});
+
+describe("Garmin connect and disconnect", () => {
+  function post(url: string, body?: unknown) {
+    return new NextRequest(`http://localhost${url}`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: body === undefined ? undefined : JSON.stringify(body),
+    });
+  }
+
+  it("refuses a key Garmin does not accept and stores nothing", async () => {
+    const { POST } = await import("@/app/api/garmin/connect/route");
+    const res = await POST(
+      post("/api/garmin/connect", { apiKey: "w".repeat(24) }),
+    );
+    expect(res.status).toBe(422);
+    const body = await res.json();
+    expect(body.meta?.errorCode).toBe("garmin.key_refused");
+    expect(JSON.stringify(body)).not.toContain("w".repeat(24));
+    // Tried the global service, then the China one.
+    expect(
+      fake.requests.filter((r) => r.includes("personalapitoken")),
+    ).toHaveLength(2);
+    const user = await getPrismaClient().user.findUnique({
+      where: { id: USER_ID },
+      select: { garminApiKeyEncrypted: true },
+    });
+    expect(user?.garminApiKeyEncrypted).toBeNull();
+  });
+
+  it("stores the key sealed, then disconnect purges it and keeps the history", async () => {
+    const prisma = getPrismaClient();
+    const { POST: connect } = await import("@/app/api/garmin/connect/route");
+    const res = await connect(
+      post("/api/garmin/connect", { apiKey: fake.key, validityDays: 90 }),
+    );
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.data).toMatchObject({
+      connected: true,
+      region: "global",
+      keyExpiry: { state: "ok" },
+    });
+    const stored = await prisma.user.findUnique({
+      where: { id: USER_ID },
+      select: {
+        garminApiKeyEncrypted: true,
+        garminKeyExpiresAt: true,
+        garminRegion: true,
+      },
+    });
+    expect(stored?.garminApiKeyEncrypted).toBeTruthy();
+    expect(stored?.garminApiKeyEncrypted).not.toContain(fake.key);
+    expect(stored?.garminRegion).toBe("global");
+    const days = (stored!.garminKeyExpiresAt!.getTime() - Date.now()) / DAY_MS;
+    expect(Math.round(days)).toBe(90);
+    expect(fake.sends.map((s) => s.queue)).toEqual(
+      expect.arrayContaining(["garmin-sync", "garmin-backfill"]),
+    );
+
+    const { syncUserGarmin } = await import("@/lib/garmin/sync");
+    await syncUserGarmin(USER_ID, { lookbackDays: 1 });
+    const rows = await prisma.measurement.count({ where: { userId: USER_ID } });
+    expect(rows).toBeGreaterThan(0);
+
+    const { POST: disconnect } =
+      await import("@/app/api/garmin/disconnect/route");
+    const out = await disconnect();
+    expect(out.status).toBe(200);
+    const after = await prisma.user.findUnique({
+      where: { id: USER_ID },
+      select: {
+        garminApiKeyEncrypted: true,
+        garminRegion: true,
+        garminConnectedAt: true,
+        garminKeyExpiresAt: true,
+        garminBackfillProgress: true,
+        garminBackfillCompletedAt: true,
+      },
+    });
+    expect(after).toEqual({
+      garminApiKeyEncrypted: null,
+      garminRegion: null,
+      garminConnectedAt: null,
+      garminKeyExpiresAt: null,
+      garminBackfillProgress: null,
+      garminBackfillCompletedAt: null,
+    });
+    expect(await prisma.measurement.count({ where: { userId: USER_ID } })).toBe(
+      rows,
+    );
+    const ledger = await prisma.integrationStatus.findFirst({
+      where: { userId: USER_ID, integration: "garmin" },
+    });
+    expect(ledger?.state).toBe("disconnected");
+    // With no key, a sync is a clean no-op that asks Garmin nothing.
+    const before = fake.requests.length;
+    expect(await syncUserGarmin(USER_ID)).toEqual({
+      imported: 0,
+      failed: false,
+    });
+    expect(fake.requests.length).toBe(before);
+  });
+});