diff --git a/src/__tests__/managed-profile-contract.test.ts b/src/__tests__/managed-profile-contract.test.ts
index a925ecf84..eea6ec3d3 100644
--- a/src/__tests__/managed-profile-contract.test.ts
+++ b/src/__tests__/managed-profile-contract.test.ts
@@ -31,4 +31,25 @@ describe("managed-profile contract", () => {
       routes.some((entry) => entry.toLowerCase().includes("emancip")),
     ).toBe(false);
   });
+
+  it("uses the same record lock for every Guardian-reducing transition", async () => {
+    const lifecycle = await readFile(
+      path.join(root, "src/lib/managed-profiles/lifecycle.ts"),
+      "utf8",
+    );
+    const grants = await readFile(
+      path.join(root, "src/lib/sharing/grants.ts"),
+      "utf8",
+    );
+    const accountDeletion = await readFile(
+      path.join(root, "src/app/api/settings/account/route.ts"),
+      "utf8",
+    );
+
+    expect(lifecycle).toContain("pg_advisory_xact_lock");
+    expect(lifecycle).toContain("clearManagedProfileMarker");
+    expect(lifecycle).toContain("deleteManagedProfile");
+    expect(grants).toContain("reduceManagedProfileGuardian");
+    expect(accountDeletion).toContain("deleteGuardianAccountWithLifecycle");
+  });
 });
diff --git a/tests/integration/managed-profile-guardian-concurrency.test.ts b/tests/integration/managed-profile-guardian-concurrency.test.ts
new file mode 100644
index 000000000..274f943a8
--- /dev/null
+++ b/tests/integration/managed-profile-guardian-concurrency.test.ts
@@ -0,0 +1,82 @@
+import { beforeEach, describe, expect, it } from "vitest";
+
+import { createManagedProfile } from "@/lib/managed-profiles/create";
+import {
+  acceptGrant,
+  inviteGrant,
+  renounceGrant,
+  revokeGrant,
+} from "@/lib/sharing/grants";
+
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+let sequence = 0;
+
+async function makeAdult(label: string) {
+  const suffix = sequence++;
+  return getPrismaClient().user.create({
+    data: {
+      username: `${label}-${suffix}`,
+      email: `${label}-${suffix}@example.test`,
+    },
+  });
+}
+
+async function twoGuardians() {
+  const creator = await makeAdult("creator");
+  const secondGuardian = await makeAdult("second-guardian");
+  const { profile, creatorGrant } = await createManagedProfile({
+    creatorId: creator.id,
+    displayName: "Managed profile",
+    dateOfBirth: null,
+    locale: "en",
+    timezone: "UTC",
+  });
+  const invitation = await inviteGrant({
+    grantorId: profile.id,
+    granteeId: secondGuardian.id,
+    access: "MANAGE",
+    scope: null,
+  });
+  const secondGrant = await acceptGrant({
+    grantId: invitation.id,
+    granteeId: secondGuardian.id,
+  });
+  return { creator, secondGuardian, profile, creatorGrant, secondGrant };
+}
+
+beforeEach(async () => {
+  await truncateAllTables(getPrismaClient());
+});
+
+describe("managed profile Guardian concurrency (real Postgres)", () => {
+  it("allows only one concurrent reduction of the final two Guardians", async () => {
+    const { creatorGrant, secondGuardian, secondGrant, profile } =
+      await twoGuardians();
+
+    const results = await Promise.allSettled([
+      revokeGrant({ grantId: creatorGrant.id, grantorId: profile.id }),
+      renounceGrant({
+        grantId: secondGrant.id,
+        granteeId: secondGuardian.id,
+      }),
+    ]);
+
+    expect(
+      results.filter((result) => result.status === "fulfilled"),
+    ).toHaveLength(1);
+    expect(
+      results.filter((result) => result.status === "rejected"),
+    ).toHaveLength(1);
+    expect(
+      await getPrismaClient().accountGrant.count({
+        where: {
+          grantorId: profile.id,
+          access: "MANAGE",
+          acceptedAt: { not: null },
+          revokedAt: null,
+        },
+      }),
+    ).toBe(1);
+  });
+});
diff --git a/tests/integration/managed-profile-lifecycle.test.ts b/tests/integration/managed-profile-lifecycle.test.ts
index cd2f47ff1..f13d8670c 100644
--- a/tests/integration/managed-profile-lifecycle.test.ts
+++ b/tests/integration/managed-profile-lifecycle.test.ts
@@ -9,6 +9,11 @@ import {
   findActiveGrant,
   inviteGrant,
 } from "@/lib/sharing/grants";
+import {
+  LastManagedGuardianError,
+  clearManagedProfileMarker,
+  deleteManagedProfile,
+} from "@/lib/managed-profiles/lifecycle";
 
 vi.mock("next/headers", async () => {
   const { cookieJar, headerJar } = await import("./mock-next-headers");
@@ -210,4 +215,70 @@ describe("managed profile lifecycle (real Postgres)", () => {
       ),
     ).not.toBeNull();
   });
+
+  it("refuses to delete a Guardian account when it would leave a managed profile behind", async () => {
+    const guardian = await signInWithFreshMfa();
+    const { profile } = await createManagedProfile({
+      creatorId: guardian.id,
+      displayName: "Managed profile",
+      dateOfBirth: null,
+      locale: "en",
+      timezone: "UTC",
+    });
+    const { DELETE } = await import("@/app/api/settings/account/route");
+
+    const response = await DELETE(
+      new NextRequest("http://localhost/api/settings/account", {
+        method: "DELETE",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({ confirm: "DELETE_ACCOUNT" }),
+      }),
+    );
+    expect(response.status).toBe(409);
+    expect(
+      await getPrismaClient().user.findUnique({ where: { id: guardian.id } }),
+    ).not.toBeNull();
+    expect(
+      await getPrismaClient().user.findUnique({ where: { id: profile.id } }),
+    ).not.toBeNull();
+  });
+
+  it("deletes a managed profile only through its fresh-MFA Guardian path", async () => {
+    const guardian = await signInWithFreshMfa();
+    const { profile } = await createManagedProfile({
+      creatorId: guardian.id,
+      displayName: "Managed profile",
+      dateOfBirth: null,
+      locale: "en",
+      timezone: "UTC",
+    });
+
+    await deleteManagedProfile({
+      profileId: profile.id,
+      guardianId: guardian.id,
+    });
+    expect(
+      await getPrismaClient().user.findUnique({ where: { id: profile.id } }),
+    ).toBeNull();
+  });
+
+  it("keeps marker clearing internal while stopping managed behavior", async () => {
+    const guardian = await signInWithFreshMfa();
+    const { profile } = await createManagedProfile({
+      creatorId: guardian.id,
+      displayName: "Managed profile",
+      dateOfBirth: null,
+      locale: "en",
+      timezone: "UTC",
+    });
+
+    await clearManagedProfileMarker({ profileId: profile.id });
+    const cleared = await getPrismaClient().user.findUniqueOrThrow({
+      where: { id: profile.id },
+    });
+    expect(cleared.managedProfileAt).toBeNull();
+    await expect(
+      clearManagedProfileMarker({ profileId: "missing-profile" }),
+    ).rejects.toBeInstanceOf(LastManagedGuardianError);
+  });
 });