diff --git a/src/__tests__/proxy-demo-mode-allowlist.test.ts b/src/__tests__/proxy-demo-mode-allowlist.test.ts
index 4badffb3a..bf43fe9d2 100644
--- a/src/__tests__/proxy-demo-mode-allowlist.test.ts
+++ b/src/__tests__/proxy-demo-mode-allowlist.test.ts
@@ -51,6 +51,31 @@ describe("proxy.ts DEMO_MODE mutation allowlist", () => {
     expect(res.status).not.toBe(403);
   });
 
+  it("permits the three setup-flow writes so the demo can finish onboarding", () => {
+    // v1.39 (C1) — the design spec requires the demo to complete the same flow
+    // with preset answers, and none of the three creates health data: closed
+    // enum answers, the step ledger, and the module map derived from them.
+    for (const [path, method] of [
+      ["/api/onboarding/answers", "PATCH"],
+      ["/api/onboarding/complete", "POST"],
+      ["/api/onboarding/restart", "POST"],
+    ] as const) {
+      expect(
+        proxy(makeRequest(path, method)).status,
+        `${method} ${path} is blocked in the demo`,
+      ).not.toBe(403);
+    }
+  });
+
+  it("does not drag in a sibling verb on an allowlisted onboarding path", () => {
+    // The allowlist pins method per path, so admitting the answers PATCH must
+    // not open a DELETE somebody adds to that route later.
+    expect(proxy(makeRequest("/api/onboarding/answers", "DELETE")).status).toBe(
+      403,
+    );
+    expect(proxy(makeRequest("/api/onboarding/step", "POST")).status).toBe(403);
+  });
+
   it("still blocks a health-data mutation (POST /api/measurements)", () => {
     const res = proxy(makeRequest("/api/measurements", "POST"));
     expect(res.status).toBe(403);
diff --git a/src/app/api/onboarding/complete/__tests__/route.test.ts b/src/app/api/onboarding/complete/__tests__/route.test.ts
index 667d16d7e..ccd333d57 100644
--- a/src/app/api/onboarding/complete/__tests__/route.test.ts
+++ b/src/app/api/onboarding/complete/__tests__/route.test.ts
@@ -1,12 +1,21 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { NextRequest } from "next/server";
 
-vi.mock("@/lib/db", () => ({
-  prisma: {
-    user: { update: vi.fn() },
-    auditLog: { create: vi.fn() },
-  },
-}));
+vi.mock("@/lib/db", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/db")>();
+  return {
+    ...actual,
+    prisma: {
+      user: { update: vi.fn(), findUnique: vi.fn() },
+      auditLog: { create: vi.fn() },
+      // v1.39 (C1) — the needs half of the route looks for a setup row before
+      // it does anything. These cases are the LEGACY caller, which has none,
+      // so the lookup answers null and the route falls straight through to the
+      // acknowledgement it always returned.
+      onboardingRecord: { findUnique: vi.fn(), update: vi.fn() },
+    },
+  };
+});
 
 vi.mock("@/lib/auth/session", async (importOriginal) => {
   const actual = await importOriginal<typeof import("@/lib/auth/session")>();
@@ -58,6 +67,7 @@ beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(prisma.user.update).mockResolvedValue({} as never);
   vi.mocked(prisma.auditLog.create).mockResolvedValue({} as never);
+  vi.mocked(prisma.onboardingRecord.findUnique).mockResolvedValue(null);
 });
 
 describe("POST /api/onboarding/complete", () => {
diff --git a/tests/integration/onboarding-needs.test.ts b/tests/integration/onboarding-needs.test.ts
new file mode 100644
index 000000000..c7526a0bc
--- /dev/null
+++ b/tests/integration/onboarding-needs.test.ts
@@ -0,0 +1,418 @@
+/**
+ * The three needs-onboarding writes against real Postgres.
+ *
+ * The unit suite proves the pure parts — the area map, the merge that never
+ * retracts a hand-made decision, the apply function's idempotence. This file
+ * proves the four things a unit suite cannot, because each is about what is in
+ * the database afterwards:
+ *
+ *   * **The answers persist and a replay changes nothing.** Idempotence is a
+ *     claim on the wire, and a route can satisfy the pure function and still
+ *     write a fresh row each time.
+ *   * **The derivation happens once.** The second confirm has to leave the
+ *     module map alone, including a module switched off in Settings between
+ *     the two calls — that is the whole reason the latch exists.
+ *   * **A restart resets the steps and not the modules.** The two are written
+ *     to different tables by different routes, so only a real run can show
+ *     that the second one stays where it was.
+ *   * **A delegate cannot write another record's setup.** Proved with a live
+ *     accepted grant and a switched session, not with a mocked resolver.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+process.env.ENCRYPTION_KEY ??=
+  "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
+
+import { cookieJar, headerJar } 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);
+      },
+    })),
+  };
+});
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+import { acceptGrant, inviteGrant } from "@/lib/sharing/grants";
+import type {
+  OnboardingStateDto,
+  OnboardingStepId,
+} from "@/lib/onboarding/needs";
+
+let counter = 0;
+
+async function makeUser(label: string) {
+  const suffix = `${label}-${counter++}`;
+  return getPrismaClient().user.create({
+    data: {
+      username: `onb-${suffix}`,
+      email: `onb-${suffix}@example.test`,
+      role: "USER",
+    },
+  });
+}
+
+async function signIn(userId: string) {
+  const session = await getPrismaClient().session.create({
+    data: { userId, expiresAt: new Date(Date.now() + 60_000) },
+  });
+  cookieJar.set("healthlog_session", session.id);
+  return session;
+}
+
+/** Attach the record-session assertion the shipped browser client attaches. */
+async function assertCurrentContext(sessionId: string) {
+  const row = await getPrismaClient().session.findUniqueOrThrow({
+    where: { id: sessionId },
+  });
+  headerJar.set("x-healthlog-record-epoch", String(row.recordEpoch));
+  headerJar.set("x-healthlog-record-scope", row.actingAsUserId ?? "self");
+}
+
+function jsonRequest(path: string, method: string, body: unknown): Request {
+  return new Request(`http://localhost${path}`, {
+    method,
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+async function patchAnswer(body: unknown): Promise<Response> {
+  const { PATCH } = await import("@/app/api/onboarding/answers/route");
+  return (PATCH as (r: Request) => Promise<Response>)(
+    jsonRequest("/api/onboarding/answers", "PATCH", body),
+  );
+}
+
+async function postComplete(body: unknown = {}): Promise<Response> {
+  const { POST } = await import("@/app/api/onboarding/complete/route");
+  return (POST as (r: Request) => Promise<Response>)(
+    jsonRequest("/api/onboarding/complete", "POST", body),
+  );
+}
+
+async function postRestart(): Promise<Response> {
+  const { POST } = await import("@/app/api/onboarding/restart/route");
+  return (POST as (r: Request) => Promise<Response>)(
+    jsonRequest("/api/onboarding/restart", "POST", {}),
+  );
+}
+
+async function readState(res: Response): Promise<OnboardingStateDto> {
+  const body = (await res.json()) as {
+    data: { onboarding: OnboardingStateDto };
+  };
+  return body.data.onboarding;
+}
+
+function statusOf(state: OnboardingStateDto, id: OnboardingStepId) {
+  return state.steps.find((step) => step.id === id)?.status;
+}
+
+/** The five answers a "blood pressure and medication" setup gives. */
+async function answerTheQuestions() {
+  await patchAnswer({ step: "who", recordTarget: "me" });
+  await patchAnswer({ step: "areas", areas: ["blood-pressure", "labs"] });
+  await patchAnswer({ step: "medication", medication: "yes" });
+  await patchAnswer({ step: "sources", status: "skipped" });
+  await patchAnswer({ step: "visit", visit: "no" });
+}
+
+async function modulePrefs(userId: string) {
+  const row = await getPrismaClient().user.findUniqueOrThrow({
+    where: { id: userId },
+    select: { modulePreferencesJson: true },
+  });
+  return (row.modulePreferencesJson ?? {}) as Record<string, boolean>;
+}
+
+beforeEach(async () => {
+  await truncateAllTables(getPrismaClient());
+  cookieJar.clear();
+  headerJar.clear();
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("PATCH /api/onboarding/answers", () => {
+  it("saves one step at a time and answers with the full nine", async () => {
+    const user = await makeUser("answers");
+    await signIn(user.id);
+
+    const first = await patchAnswer({ step: "who", recordTarget: "both" });
+    expect(first.status).toBe(200);
+    const state = await readState(first);
+    expect(state.needs.recordTarget).toBe("both");
+    expect(state.steps).toHaveLength(9);
+    expect(statusOf(state, "who")).toBe("done");
+    expect(statusOf(state, "areas")).toBe("pending");
+
+    const second = await readState(
+      await patchAnswer({ step: "areas", areas: ["sleep"] }),
+    );
+    expect(second.needs.recordTarget).toBe("both");
+    expect(second.needs.areas).toEqual(["sleep"]);
+    expect(statusOf(second, "areas")).toBe("done");
+  });
+
+  it("is idempotent — a replay leaves one row and the same state", async () => {
+    const user = await makeUser("replay");
+    await signIn(user.id);
+
+    const once = await readState(
+      await patchAnswer({ step: "medication", medication: "sometimes" }),
+    );
+    const twice = await readState(
+      await patchAnswer({ step: "medication", medication: "sometimes" }),
+    );
+    expect(twice).toEqual(once);
+    expect(
+      await getPrismaClient().onboardingRecord.count({
+        where: { userId: user.id },
+      }),
+    ).toBe(1);
+  });
+
+  it("records a skip without erasing the answer already given", async () => {
+    const user = await makeUser("skip");
+    await signIn(user.id);
+
+    await patchAnswer({ step: "visit", visit: "within-a-month" });
+    const skipped = await readState(
+      await patchAnswer({ step: "visit", status: "skipped" }),
+    );
+    expect(statusOf(skipped, "visit")).toBe("skipped");
+    expect(skipped.needs.visit).toBe("within-a-month");
+  });
+
+  it("writes the record's real unit columns, and records the answer beside them", async () => {
+    const user = await makeUser("units");
+    await signIn(user.id);
+
+    const state = await readState(
+      await patchAnswer({
+        step: "units",
+        units: { glucoseUnit: "mmol/L", unitPreference: "imperial" },
+      }),
+    );
+    expect(state.needs.units).toEqual({
+      glucoseUnit: "mmol/L",
+      unitPreference: "imperial",
+    });
+    const row = await getPrismaClient().user.findUniqueOrThrow({
+      where: { id: user.id },
+      select: { glucoseUnit: true, unitPreference: true },
+    });
+    expect(row).toEqual({ glucoseUnit: "mmol/L", unitPreference: "imperial" });
+  });
+
+  it("refuses a body that names a step it does not answer", async () => {
+    const user = await makeUser("invalid");
+    await signIn(user.id);
+
+    const res = await patchAnswer({ step: "areas", medication: "yes" });
+    expect(res.status).toBe(422);
+    const body = (await res.json()) as { meta?: { errorCode?: string } };
+    expect(body.meta?.errorCode).toBe("onboarding.answers.invalid");
+    expect(
+      await getPrismaClient().onboardingRecord.count({
+        where: { userId: user.id },
+      }),
+    ).toBe(0);
+  });
+});
+
+describe("POST /api/onboarding/complete", () => {
+  it("derives the module map from the answers, once", async () => {
+    const user = await makeUser("derive");
+    await signIn(user.id);
+    await answerTheQuestions();
+
+    const first = await postComplete();
+    expect(first.status).toBe(200);
+
+    const derived = await modulePrefs(user.id);
+    // What the answers named.
+    expect(derived.medications).toBe(true);
+    expect(derived.labs).toBe(true);
+    expect(derived.insights).toBe(true);
+    expect(derived.achievements).toBe(true);
+    // What they did not.
+    expect(derived.workouts).toBe(false);
+    expect(derived.mood).toBe(false);
+    expect(derived.doctorReport).toBe(false);
+    expect(derived.vaccinations).toBe(false);
+    // The delegated keys are never written into the blob.
+    expect(derived).not.toHaveProperty("cycle");
+    expect(derived).not.toHaveProperty("coach");
+
+    const state = await readState(first);
+    expect(statusOf(state, "confirm")).toBe("done");
+    expect(state.completedAt).not.toBeNull();
+
+    // Somebody changes their mind in Settings, then the confirm screen is
+    // replayed. The second call must leave that decision alone: the latch is
+    // what stops a questionnaire from re-applying itself over a later choice.
+    await getPrismaClient().user.update({
+      where: { id: user.id },
+      data: { modulePreferencesJson: { ...derived, labs: false } },
+    });
+    const second = await postComplete();
+    expect(second.status).toBe(200);
+    expect((await modulePrefs(user.id)).labs).toBe(false);
+  });
+
+  it("never switches off a module the person switched on by hand", async () => {
+    const user = await makeUser("handmade");
+    await signIn(user.id);
+    // The remote MCP endpoint is off by default and nothing in the flow asks
+    // for it, so a derivation that ignored the stored map would retract it.
+    await getPrismaClient().user.update({
+      where: { id: user.id },
+      data: { modulePreferencesJson: { mcp: true } },
+    });
+    await answerTheQuestions();
+    await postComplete();
+
+    expect((await modulePrefs(user.id)).mcp).toBe(true);
+  });
+
+  it("switches cycle tracking on through its own column when the area is chosen", async () => {
+    const user = await makeUser("cycle");
+    await signIn(user.id);
+    await patchAnswer({ step: "who", recordTarget: "me" });
+    await patchAnswer({ step: "areas", areas: ["cycle"] });
+    await postComplete();
+
+    const profile = await getPrismaClient().cycleProfile.findUniqueOrThrow({
+      where: { userId: user.id },
+    });
+    expect(profile.cycleTrackingEnabled).toBe(true);
+  });
+
+  it("leaves a record that never answered the questions exactly as it was", async () => {
+    const user = await makeUser("legacy");
+    await signIn(user.id);
+
+    const res = await postComplete({ heightCm: 180 });
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as {
+      data: { completed: boolean; onboarding?: unknown };
+    };
+    expect(body.data.completed).toBe(true);
+    expect(body.data.onboarding).toBeUndefined();
+
+    const row = await getPrismaClient().user.findUniqueOrThrow({
+      where: { id: user.id },
+      select: { modulePreferencesJson: true, onboardingCompletedAt: true },
+    });
+    expect(row.modulePreferencesJson).toBeNull();
+    expect(row.onboardingCompletedAt).not.toBeNull();
+  });
+});
+
+describe("POST /api/onboarding/restart", () => {
+  it("resets the steps and leaves the modules where they are", async () => {
+    const user = await makeUser("restart");
+    await signIn(user.id);
+    await answerTheQuestions();
+    await postComplete();
+    const derived = await modulePrefs(user.id);
+
+    const res = await postRestart();
+    expect(res.status).toBe(200);
+    const state = await readState(res);
+    expect(state.steps.every((step) => step.status === "pending")).toBe(true);
+    expect(state.completedAt).toBeNull();
+    // The answers survive as the prefill for the re-run.
+    expect(state.needs.medication).toBe("yes");
+    expect(await modulePrefs(user.id)).toEqual(derived);
+
+    // The latch is cleared, so the next confirm may derive again — and a
+    // module switched off by hand in between still survives it.
+    await getPrismaClient().user.update({
+      where: { id: user.id },
+      data: { modulePreferencesJson: { ...derived, labs: true } },
+    });
+    await patchAnswer({ step: "who", recordTarget: "me" });
+    await patchAnswer({ step: "areas", areas: [] });
+    await postComplete();
+    expect((await modulePrefs(user.id)).labs).toBe(true);
+  });
+
+  it("leaves the first-run redirect's own stamp alone", async () => {
+    const user = await makeUser("redirect");
+    await signIn(user.id);
+    await answerTheQuestions();
+    await postComplete();
+    await postRestart();
+
+    const row = await getPrismaClient().user.findUniqueOrThrow({
+      where: { id: user.id },
+      select: { onboardingCompletedAt: true },
+    });
+    expect(row.onboardingCompletedAt).not.toBeNull();
+  });
+});
+
+describe("a delegate and somebody else's record", () => {
+  it("refuses every setup write while the session is acting on another record", async () => {
+    const owner = await makeUser("owner");
+    const delegate = await makeUser("delegate");
+    const invited = await inviteGrant({
+      grantorId: owner.id,
+      granteeId: delegate.id,
+      access: "READ",
+      scope: null,
+    });
+    await acceptGrant({ grantId: invited.id, granteeId: delegate.id });
+
+    const session = await signIn(delegate.id);
+    await getPrismaClient().session.update({
+      where: { id: session.id },
+      data: { actingAsUserId: owner.id },
+    });
+    await assertCurrentContext(session.id);
+
+    for (const call of [
+      () => patchAnswer({ step: "who", recordTarget: "me" }),
+      () => postComplete(),
+      () => postRestart(),
+    ]) {
+      const res = await call();
+      expect(res.status).toBe(403);
+      const body = (await res.json()) as { meta?: { errorCode?: string } };
+      expect(body.meta?.errorCode).toBe("sharing.not_permitted");
+    }
+
+    // Neither record acquired a setup row — not the owner's, and not the
+    // delegate's own, which is the quiet failure a fall-back would produce.
+    expect(await getPrismaClient().onboardingRecord.count()).toBe(0);
+    const ownerRow = await getPrismaClient().user.findUniqueOrThrow({
+      where: { id: owner.id },
+      select: { modulePreferencesJson: true, onboardingCompletedAt: true },
+    });
+    expect(ownerRow.modulePreferencesJson).toBeNull();
+    expect(ownerRow.onboardingCompletedAt).toBeNull();
+  });
+});