diff --git a/src/app/api/strava/credentials/__tests__/route.test.ts b/src/app/api/strava/credentials/__tests__/route.test.ts
index f57870da6..a2e6b53d3 100644
--- a/src/app/api/strava/credentials/__tests__/route.test.ts
+++ b/src/app/api/strava/credentials/__tests__/route.test.ts
@@ -27,7 +27,10 @@ const { storeMock, clearMock } = vi.hoisted(() => ({
   storeMock: vi.fn(),
   clearMock: vi.fn(),
 }));
-vi.mock("@/lib/strava/credentials", () => ({
+vi.mock("@/lib/strava/credentials", async (importOriginal) => ({
+  STRAVA_DISCONNECTED: (
+    await importOriginal<typeof import("@/lib/strava/credentials")>()
+  ).STRAVA_DISCONNECTED,
   storeStravaClientCredentials: storeMock,
   clearStravaClientCredentials: clearMock,
 }));
@@ -80,14 +83,18 @@ describe("DELETE /api/strava/credentials", () => {
     expect(markDisconnected).toHaveBeenCalledWith("u1", "strava");
   });
 
-  it("clears the athlete id alongside both tokens", async () => {
+  it("clears the athlete id and the backfill state alongside both tokens", async () => {
     await del();
+    // The backfill markers belong to the account that was connected; a later
+    // connection must start its own history walk from the newest activity.
     expect(userUpdate).toHaveBeenCalledWith(
       expect.objectContaining({
         data: {
           stravaAccessTokenEncrypted: null,
           stravaRefreshTokenEncrypted: null,
           stravaAthleteId: null,
+          stravaBackfillCompletedAt: null,
+          stravaBackfillResumeBefore: null,
         },
       }),
     );
diff --git a/src/app/api/strava/credentials/route.ts b/src/app/api/strava/credentials/route.ts
index 7605775ef..30607cc1a 100644
--- a/src/app/api/strava/credentials/route.ts
+++ b/src/app/api/strava/credentials/route.ts
@@ -12,6 +12,7 @@ import { markDisconnected } from "@/lib/integrations/status";
 import {
   storeStravaClientCredentials,
   clearStravaClientCredentials,
+  STRAVA_DISCONNECTED,
 } from "@/lib/strava/credentials";
 import { stravaCredentialsSchema } from "@/lib/validations/strava";
 import { NextRequest } from "next/server";
@@ -98,11 +99,7 @@ export const DELETE = apiHandler(async () => {
 
   await prisma.user.update({
     where: { id: user.id },
-    data: {
-      stravaAccessTokenEncrypted: null,
-      stravaRefreshTokenEncrypted: null,
-      stravaAthleteId: null,
-    },
+    data: STRAVA_DISCONNECTED,
   });
   await clearStravaClientCredentials(user.id);
 
diff --git a/src/app/api/strava/disconnect/__tests__/route.test.ts b/src/app/api/strava/disconnect/__tests__/route.test.ts
new file mode 100644
index 000000000..ee03d9dd0
--- /dev/null
+++ b/src/app/api/strava/disconnect/__tests__/route.test.ts
@@ -0,0 +1,63 @@
+/**
+ * `POST /api/strava/disconnect` leaves nothing of the connected account behind
+ * that a later connection would inherit: not the tokens, and not the backfill
+ * markers. A resume point left standing made the next account's first walk
+ * start below the old account's oldest activity, so its newer history was
+ * never asked for.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/api-handler", () => ({
+  apiHandler: <T extends (...args: unknown[]) => unknown>(fn: T) => fn,
+  requireAuth: vi.fn(async () => ({ user: { id: "u1" } })),
+}));
+vi.mock("@/lib/db", () => ({ prisma: { user: { update: vi.fn() } } }));
+vi.mock("@/lib/logging/context", () => ({
+  annotate: vi.fn(),
+  getEvent: () => null,
+}));
+vi.mock("@/lib/auth/audit", () => ({ auditLog: vi.fn() }));
+vi.mock("@/lib/integrations/status", () => ({ markDisconnected: vi.fn() }));
+vi.mock("@/lib/rate-limit", () => ({
+  checkRateLimit: vi.fn(async () => ({ allowed: true })),
+  rateLimitHeaders: vi.fn(() => ({})),
+}));
+vi.mock("@/lib/strava/client", () => ({ deauthorize: vi.fn() }));
+vi.mock("@/lib/strava/credentials", async (importOriginal) => ({
+  STRAVA_DISCONNECTED: (
+    await importOriginal<typeof import("@/lib/strava/credentials")>()
+  ).STRAVA_DISCONNECTED,
+  getStravaConnection: vi.fn(async () => ({ accessToken: "tok" })),
+}));
+vi.mock("@/lib/api-response", () => ({
+  apiSuccess: (data: unknown) => ({ data, error: null, status: 200 }),
+  apiError: (error: string, status: number) => ({ data: null, error, status }),
+}));
+
+import { POST } from "../route";
+import { prisma } from "@/lib/db";
+
+const userUpdate = prisma.user.update as ReturnType<typeof vi.fn>;
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  userUpdate.mockResolvedValue({});
+});
+
+describe("POST /api/strava/disconnect", () => {
+  it("clears the backfill markers with the tokens", async () => {
+    const res = (await (POST as unknown as () => Promise<{ data: unknown }>)())
+      .data;
+    expect(res).toEqual({ disconnected: true });
+    expect(userUpdate).toHaveBeenCalledWith({
+      where: { id: "u1" },
+      data: {
+        stravaAccessTokenEncrypted: null,
+        stravaRefreshTokenEncrypted: null,
+        stravaAthleteId: null,
+        stravaBackfillCompletedAt: null,
+        stravaBackfillResumeBefore: null,
+      },
+    });
+  });
+});
diff --git a/src/app/api/strava/disconnect/route.ts b/src/app/api/strava/disconnect/route.ts
index b41c4a75e..e3634b330 100644
--- a/src/app/api/strava/disconnect/route.ts
+++ b/src/app/api/strava/disconnect/route.ts
@@ -6,7 +6,10 @@ import { annotate, getEvent } from "@/lib/logging/context";
 import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
 import { markDisconnected } from "@/lib/integrations/status";
 import { deauthorize } from "@/lib/strava/client";
-import { getStravaConnection } from "@/lib/strava/credentials";
+import {
+  getStravaConnection,
+  STRAVA_DISCONNECTED,
+} from "@/lib/strava/credentials";
 
 /**
  * v1.28.x — disconnect Strava for the current user.
@@ -42,11 +45,7 @@ export const POST = apiHandler(async () => {
 
   await prisma.user.update({
     where: { id: user.id },
-    data: {
-      stravaAccessTokenEncrypted: null,
-      stravaRefreshTokenEncrypted: null,
-      stravaAthleteId: null,
-    },
+    data: STRAVA_DISCONNECTED,
   });
 
   await auditLog("strava.disconnect", { userId: user.id });
diff --git a/src/lib/strava/credentials.ts b/src/lib/strava/credentials.ts
index ff14695d6..768911c8e 100644
--- a/src/lib/strava/credentials.ts
+++ b/src/lib/strava/credentials.ts
@@ -70,6 +70,22 @@ export async function clearStravaClientCredentials(
   });
 }
 
+/**
+ * The `User` columns a removed Strava connection leaves behind: no tokens, no
+ * athlete, and no backfill state. The backfill markers belong to the account
+ * that was connected. Left standing, a later connection inherited them: a
+ * completed marker kept its history from ever being read, and a resume point
+ * made its first run start below the old account's oldest activity, so
+ * everything newer was never asked for.
+ */
+export const STRAVA_DISCONNECTED = {
+  stravaAccessTokenEncrypted: null,
+  stravaRefreshTokenEncrypted: null,
+  stravaAthleteId: null,
+  stravaBackfillCompletedAt: null,
+  stravaBackfillResumeBefore: null,
+} as const;
+
 export interface StravaConnection {
   accessToken: string;
   refreshToken: string;