diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 3b6ddd195..b632c1c2b 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -5866,6 +5866,33 @@ paths:
         "404": *a7
         "422": *a3
         "429": *a4
+  /api/measurement-reminders/{id}/complete:
+    post:
+      tags:
+        - MeasurementReminders
+      summary: Complete a Vorsorge reminder (v1.18.6)
+      description: "Explicit user-action completion (iOS #23): the app marks a reminder done server-side instead of only
+        dismissing it locally. Routes through the same satisfaction primitive as the cron auto-resolve — stamps
+        lastSatisfiedAt = now, re-anchors nextDueAt, fires no notification. Idempotent: completing an already-completed
+        / auto-satisfied reminder returns 200 with completed=false. Owner-scoped."
+      parameters:
+        - in: path
+          name: id
+          schema:
+            type: string
+          required: true
+      responses:
+        "200":
+          description: Completion applied (completed=true) or already-satisfied no-op (completed=false); reminder carries the
+            canonical post-completion DTO.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/MeasurementReminderCompleteEnvelope"
+        "401": *a1
+        "404": *a7
+        "422": *a3
+        "429": *a4
   /api/labs:
     get:
       tags:
@@ -18498,6 +18525,37 @@ components:
         - data
         - error
       additionalProperties: false
+    MeasurementReminderCompleteEnvelope:
+      type: object
+      properties:
+        data:
+          $ref: "#/components/schemas/MeasurementReminderCompletion"
+        error:
+          type: "null"
+        meta:
+          type: object
+          properties:
+            requestId:
+              type: string
+          additionalProperties: false
+      required:
+        - data
+        - error
+      additionalProperties: false
+    MeasurementReminderCompletion:
+      type: object
+      properties:
+        completed:
+          type: boolean
+        reminder:
+          $ref: "#/components/schemas/MeasurementReminderDTO"
+      required:
+        - completed
+        - reminder
+      additionalProperties: false
+      description: Result of an explicit reminder completion. completed=true when this call advanced lastSatisfiedAt;
+        completed=false when an earlier satisfy or a matching reading had already fulfilled the current cycle
+        (idempotent no-op). reminder is the canonical post-completion DTO.
     ListLabResultsEnvelope:
       type: object
       properties:
diff --git a/src/app/api/measurement-reminders/[id]/complete/__tests__/route.test.ts b/src/app/api/measurement-reminders/[id]/complete/__tests__/route.test.ts
new file mode 100644
index 000000000..78778d154
--- /dev/null
+++ b/src/app/api/measurement-reminders/[id]/complete/__tests__/route.test.ts
@@ -0,0 +1,133 @@
+/**
+ * v1.18.6 — explicit-completion route (iOS #23 follow-up).
+ *
+ * Covers: marks done via the shared primitive, idempotent no-op surfaces
+ * completed=false, owner-scoped 404 on a cross-user / tombstoned id, and the
+ * structural no-double-notify guarantee (the route delegates to
+ * `satisfyReminder` and never reaches the notification dispatcher).
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { NextRequest } from "next/server";
+
+vi.mock("@/lib/api-handler", () => ({
+  apiHandler: <T extends (...args: unknown[]) => unknown>(fn: T) => fn,
+  requireAuth: vi.fn(async () => ({ user: { id: "u1", locale: "en" } })),
+}));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/logging/context", () => ({ annotate: vi.fn() }));
+
+const satisfyReminderMock = vi.fn();
+vi.mock("@/lib/measurement-reminders/satisfy", () => ({
+  satisfyReminder: (...args: unknown[]) => satisfyReminderMock(...args),
+}));
+
+const findFirstMock = vi.fn();
+const findUserMock = vi.fn();
+const findUniqueOrThrowMock = vi.fn();
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurementReminder: {
+      findFirst: (...a: unknown[]) => findFirstMock(...a),
+      findUniqueOrThrow: (...a: unknown[]) => findUniqueOrThrowMock(...a),
+    },
+    user: { findUnique: (...a: unknown[]) => findUserMock(...a) },
+  },
+}));
+
+import { POST } from "../route";
+
+const ROW = {
+  id: "r1",
+  userId: "u1",
+  label: "Blutdruck messen",
+  measurementType: "BLOOD_PRESSURE_SYS",
+  intervalDays: 7,
+  rrule: null,
+  anchorDate: null,
+  endsOn: null,
+  origin: "VORSORGE",
+  notifyHour: 9,
+  location: null,
+  nextDueAt: new Date("2026-06-25T07:00:00Z"),
+  lastSatisfiedAt: null,
+  enabled: true,
+  createdAt: new Date("2026-06-01T00:00:00Z"),
+  updatedAt: new Date("2026-06-18T00:00:00Z"),
+  deletedAt: null,
+};
+
+function makeRequest(): NextRequest {
+  return new NextRequest("http://localhost/api/measurement-reminders/r1/complete", {
+    method: "POST",
+  });
+}
+
+const params = { params: Promise.resolve({ id: "r1" }) };
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  findUserMock.mockResolvedValue({ timezone: "Europe/Berlin" });
+});
+
+describe("POST /api/measurement-reminders/[id]/complete", () => {
+  it("marks the reminder done via the shared primitive and returns completed=true", async () => {
+    findFirstMock.mockResolvedValue(ROW);
+    satisfyReminderMock.mockResolvedValue({
+      satisfied: true,
+      nextDueAt: new Date("2026-06-25T07:00:00Z"),
+    });
+    findUniqueOrThrowMock.mockResolvedValue({
+      ...ROW,
+      lastSatisfiedAt: new Date("2026-06-18T08:00:00Z"),
+    });
+
+    const res = await POST(makeRequest(), params);
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.error).toBeNull();
+    expect(body.data.completed).toBe(true);
+    expect(body.data.reminder.id).toBe("r1");
+    expect(body.data.reminder.lastSatisfiedAt).toBe("2026-06-18T08:00:00.000Z");
+    // The route delegates to the ONE shared satisfaction primitive.
+    expect(satisfyReminderMock).toHaveBeenCalledTimes(1);
+  });
+
+  it("is idempotent: an already-satisfied reminder is a 200 no-op with completed=false", async () => {
+    findFirstMock.mockResolvedValue({
+      ...ROW,
+      lastSatisfiedAt: new Date("2026-06-18T08:00:00Z"),
+    });
+    // Forward-only guard inside the primitive returns satisfied=false.
+    satisfyReminderMock.mockResolvedValue({ satisfied: false, nextDueAt: null });
+    findUniqueOrThrowMock.mockResolvedValue({
+      ...ROW,
+      lastSatisfiedAt: new Date("2026-06-18T08:00:00Z"),
+    });
+
+    const res = await POST(makeRequest(), params);
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.data.completed).toBe(false);
+    expect(body.data.reminder.id).toBe("r1");
+  });
+
+  it("is owner-scoped: a cross-user reminder 404s and never satisfies", async () => {
+    findFirstMock.mockResolvedValue({ ...ROW, userId: "someone-else" });
+
+    const res = await POST(makeRequest(), params);
+    expect(res.status).toBe(404);
+    expect(satisfyReminderMock).not.toHaveBeenCalled();
+  });
+
+  it("404s a tombstoned / missing reminder", async () => {
+    findFirstMock.mockResolvedValue(null);
+
+    const res = await POST(makeRequest(), params);
+    expect(res.status).toBe(404);
+    expect(satisfyReminderMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/app/api/measurement-reminders/[id]/complete/route.ts b/src/app/api/measurement-reminders/[id]/complete/route.ts
new file mode 100644
index 000000000..408bf12c8
--- /dev/null
+++ b/src/app/api/measurement-reminders/[id]/complete/route.ts
@@ -0,0 +1,81 @@
+/**
+ * v1.18.6 — explicit "complete" for a Vorsorge reminder (iOS #23 follow-up).
+ *
+ * The user-action equivalent of the cron auto-satisfy: the iOS app (and web)
+ * call this when the user actively marks a measurement reminder done, rather
+ * than only dismissing the notification locally. It routes through the ONE
+ * shared `satisfyReminder` primitive — the same code the cron auto-resolve
+ * and the eventful ingest worker use — so it:
+ *
+ *   - stamps `lastSatisfiedAt = now` and re-anchors the server-authoritative
+ *     `nextDueAt` past now (one engine, no duplicated reschedule logic);
+ *   - fires NO notification of its own (the primitive only writes the row;
+ *     the dispatcher is never invoked on this path);
+ *   - is idempotent via the primitive's forward-only guard — completing an
+ *     already-completed / auto-satisfied reminder is a no-op that still
+ *     returns 200, with `completed: false` so the client can tell the second
+ *     tap apart from the first.
+ *
+ * Distinct from `satisfy` only in that the response surfaces the `completed`
+ * flag the explicit-action flow benefits from; the underlying effect is the
+ * shared satisfaction primitive either way.
+ */
+import { NextRequest } from "next/server";
+
+import { prisma } from "@/lib/db";
+import { auditLog } from "@/lib/auth/audit";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { apiSuccess, apiError, getClientIp } from "@/lib/api-response";
+import { annotate } from "@/lib/logging/context";
+import { satisfyReminder } from "@/lib/measurement-reminders/satisfy";
+import { toMeasurementReminderDto } from "@/lib/measurement-reminders/dto";
+
+type RouteParams = { params: Promise<{ id: string }> };
+
+const DEFAULT_TIMEZONE = "Europe/Berlin";
+
+export const POST = apiHandler(
+  async (request: NextRequest, { params }: RouteParams) => {
+    const { user } = await requireAuth();
+    const { id } = await params;
+
+    const existing = await prisma.measurementReminder.findFirst({
+      where: { id, deletedAt: null },
+    });
+    if (!existing || existing.userId !== user.id) {
+      return apiError("Measurement reminder not found", 404);
+    }
+
+    const userRow = await prisma.user.findUnique({
+      where: { id: user.id },
+      select: { timezone: true },
+    });
+    const timezone = userRow?.timezone || DEFAULT_TIMEZONE;
+
+    const now = new Date();
+    // Forward-only: a completion strictly after any prior satisfy advances
+    // the row; a completion at/behind the last satisfy is a no-op. Either
+    // way no notification is emitted — the primitive only stamps + reschedules.
+    const result = await satisfyReminder(prisma, existing, timezone, now);
+
+    const updated = await prisma.measurementReminder.findUniqueOrThrow({
+      where: { id },
+    });
+
+    await auditLog("measurementReminder.complete", {
+      userId: user.id,
+      ipAddress: getClientIp(request),
+      details: { reminderId: id, completed: result.satisfied },
+    });
+
+    annotate({
+      action: { name: "measurement-reminders.complete" },
+      meta: { reminderId: id, completed: result.satisfied },
+    });
+
+    return apiSuccess({
+      completed: result.satisfied,
+      reminder: toMeasurementReminderDto(updated),
+    });
+  },
+);
diff --git a/src/lib/openapi/routes/measurement-reminders.ts b/src/lib/openapi/routes/measurement-reminders.ts
index 47952eec3..796c87fd5 100644
--- a/src/lib/openapi/routes/measurement-reminders.ts
+++ b/src/lib/openapi/routes/measurement-reminders.ts
@@ -12,6 +12,7 @@ import {
   createMeasurementReminderSchema,
   updateMeasurementReminderSchema,
   measurementReminderDto,
+  measurementReminderCompletionDto,
 } from "@/lib/validations/measurement-reminders";
 import { dataEnvelope, errorEnvelope, stdResponses } from "./shared";
 
@@ -169,4 +170,29 @@ export const measurementReminderPaths: NonNullable<
       },
     },
   },
+  "/api/measurement-reminders/{id}/complete": {
+    post: {
+      tags: ["MeasurementReminders"],
+      summary: "Complete a Vorsorge reminder (v1.18.6)",
+      description:
+        "Explicit user-action completion (iOS #23): the app marks a reminder done server-side instead of only dismissing it locally. Routes through the same satisfaction primitive as the cron auto-resolve — stamps lastSatisfiedAt = now, re-anchors nextDueAt, fires no notification. Idempotent: completing an already-completed / auto-satisfied reminder returns 200 with completed=false. Owner-scoped.",
+      requestParams: { path: z.object({ id: z.string() }) },
+      responses: {
+        "200": {
+          description:
+            "Completion applied (completed=true) or already-satisfied no-op (completed=false); reminder carries the canonical post-completion DTO.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(
+                measurementReminderCompletionDto,
+                "MeasurementReminderCompleteEnvelope",
+              ),
+            },
+          },
+        },
+        ...reminderNotFound,
+        ...stdResponses,
+      },
+    },
+  },
 };
diff --git a/src/lib/validations/measurement-reminders.ts b/src/lib/validations/measurement-reminders.ts
index fc4f55b50..3c17f2ea4 100644
--- a/src/lib/validations/measurement-reminders.ts
+++ b/src/lib/validations/measurement-reminders.ts
@@ -184,3 +184,26 @@ export const measurementReminderDto = z
     description:
       "A Vorsorge reminder. nextDueAt is server-computed (server-authoritative). A free-text reminder carries measurementType=null and resolves only on a manual satisfy. origin distinguishes a user-created (VORSORGE) reminder from one minted by a Coach cadence suggestion (COACH); endsOn bounds a finite course window (null = open-ended).",
   });
+
+/**
+ * v1.18.6 — explicit-completion result. The `complete` route is the
+ * user-action equivalent of the cron auto-satisfy: it stamps
+ * `lastSatisfiedAt` and re-anchors `nextDueAt` through the SAME shared
+ * primitive, so it fires no notification of its own and is idempotent.
+ *
+ * `completed` reports whether THIS call advanced the reminder (true) or was
+ * a forward-only no-op because a prior satisfy / matching reading already
+ * fulfilled the current cycle (false). Either way the response is a 200 so
+ * a client that completes an already-completed reminder need not special-case
+ * the second tap. `reminder` carries the canonical post-completion DTO.
+ */
+export const measurementReminderCompletionDto = z
+  .object({
+    completed: z.boolean(),
+    reminder: measurementReminderDto,
+  })
+  .meta({
+    id: "MeasurementReminderCompletion",
+    description:
+      "Result of an explicit reminder completion. completed=true when this call advanced lastSatisfiedAt; completed=false when an earlier satisfy or a matching reading had already fulfilled the current cycle (idempotent no-op). reminder is the canonical post-completion DTO.",
+  });