diff --git a/server/src/__tests__/access-service.test.ts b/server/src/__tests__/access-service.test.ts
new file mode 100644
index 00000000000..de839370cb4
--- /dev/null
+++ b/server/src/__tests__/access-service.test.ts
@@ -0,0 +1,99 @@
+import { randomUUID } from "node:crypto";
+import { eq } from "drizzle-orm";
+import { afterAll, afterEach, beforeAll, describe, expect, it } from "vitest";
+import {
+  companies,
+  companyMemberships,
+  createDb,
+  principalPermissionGrants,
+} from "@paperclipai/db";
+import {
+  getEmbeddedPostgresTestSupport,
+  startEmbeddedPostgresTestDatabase,
+} from "./helpers/embedded-postgres.js";
+import { accessService } from "../services/access.js";
+
+const embeddedPostgresSupport = await getEmbeddedPostgresTestSupport();
+const describeEmbeddedPostgres = embeddedPostgresSupport.supported ? describe : describe.skip;
+
+async function createCompanyWithOwner(db: ReturnType<typeof createDb>) {
+  const company = await db
+    .insert(companies)
+    .values({
+      name: `Access Service ${randomUUID()}`,
+      issuePrefix: `AS${randomUUID().slice(0, 6).toUpperCase()}`,
+    })
+    .returning()
+    .then((rows) => rows[0]!);
+
+  const owner = await db
+    .insert(companyMemberships)
+    .values({
+      companyId: company.id,
+      principalType: "user",
+      principalId: `owner-${randomUUID()}`,
+      status: "active",
+      membershipRole: "owner",
+    })
+    .returning()
+    .then((rows) => rows[0]!);
+
+  return { company, owner };
+}
+
+describeEmbeddedPostgres("access service", () => {
+  let db!: ReturnType<typeof createDb>;
+  let tempDb: Awaited<ReturnType<typeof startEmbeddedPostgresTestDatabase>> | null = null;
+
+  beforeAll(async () => {
+    tempDb = await startEmbeddedPostgresTestDatabase("paperclip-access-service-");
+    db = createDb(tempDb.connectionString);
+  }, 20_000);
+
+  afterEach(async () => {
+    await db.delete(principalPermissionGrants);
+    await db.delete(companyMemberships);
+    await db.delete(companies);
+  });
+
+  afterAll(async () => {
+    await tempDb?.cleanup();
+  });
+
+  it("rejects combined access updates that would demote the last active owner", async () => {
+    const { company, owner } = await createCompanyWithOwner(db);
+    const access = accessService(db);
+
+    await expect(
+      access.updateMemberAndPermissions(
+        company.id,
+        owner.id,
+        { membershipRole: "admin", grants: [] },
+        "admin-user",
+      ),
+    ).rejects.toThrow("Cannot remove the last active owner");
+
+    const unchanged = await db
+      .select()
+      .from(companyMemberships)
+      .where(eq(companyMemberships.id, owner.id))
+      .then((rows) => rows[0]!);
+    expect(unchanged.membershipRole).toBe("owner");
+  });
+
+  it("rejects role-only updates that would suspend the last active owner", async () => {
+    const { company, owner } = await createCompanyWithOwner(db);
+    const access = accessService(db);
+
+    await expect(
+      access.updateMember(company.id, owner.id, { status: "suspended" }),
+    ).rejects.toThrow("Cannot remove the last active owner");
+
+    const unchanged = await db
+      .select()
+      .from(companyMemberships)
+      .where(eq(companyMemberships.id, owner.id))
+      .then((rows) => rows[0]!);
+    expect(unchanged.status).toBe("active");
+  });
+});
diff --git a/server/src/services/access.ts b/server/src/services/access.ts
index dd5f74af2b3..792c28f4606 100644
--- a/server/src/services/access.ts
+++ b/server/src/services/access.ts
@@ -6,6 +6,7 @@ import {
   principalPermissionGrants,
 } from "@paperclipai/db";
 import type { PermissionKey, PrincipalType } from "@paperclipai/shared";
+import { conflict } from "../errors.js";
 
 type MembershipRow = typeof companyMemberships.$inferSelect;
 type GrantInput = {
@@ -153,15 +154,52 @@ export function accessService(db: Db) {
     },
     grantedByUserId: string | null,
   ) {
-    const existing = await getMemberById(companyId, memberId);
-    if (!existing) return null;
-
-    const nextMembershipRole =
-      data.membershipRole !== undefined ? data.membershipRole : existing.membershipRole;
-    const nextStatus = data.status ?? existing.status;
-    const now = new Date();
-
     return db.transaction(async (tx) => {
+      await tx.execute(sql`
+        select ${companyMemberships.id}
+        from ${companyMemberships}
+        where ${companyMemberships.companyId} = ${companyId}
+          and ${companyMemberships.principalType} = 'user'
+          and ${companyMemberships.status} = 'active'
+          and ${companyMemberships.membershipRole} = 'owner'
+        for update
+      `);
+
+      const existing = await tx
+        .select()
+        .from(companyMemberships)
+        .where(and(eq(companyMemberships.companyId, companyId), eq(companyMemberships.id, memberId)))
+        .then((rows) => rows[0] ?? null);
+      if (!existing) return null;
+
+      const nextMembershipRole =
+        data.membershipRole !== undefined ? data.membershipRole : existing.membershipRole;
+      const nextStatus = data.status ?? existing.status;
+
+      if (
+        existing.principalType === "user" &&
+        existing.status === "active" &&
+        existing.membershipRole === "owner" &&
+        (nextStatus !== "active" || nextMembershipRole !== "owner")
+      ) {
+        const activeOwnerCount = await tx
+          .select({ id: companyMemberships.id })
+          .from(companyMemberships)
+          .where(
+            and(
+              eq(companyMemberships.companyId, companyId),
+              eq(companyMemberships.principalType, "user"),
+              eq(companyMemberships.status, "active"),
+              eq(companyMemberships.membershipRole, "owner"),
+            ),
+          )
+          .then((rows) => rows.length);
+        if (activeOwnerCount <= 1) {
+          throw conflict("Cannot remove the last active owner");
+        }
+      }
+
+      const now = new Date();
       const updated = await tx
         .update(companyMemberships)
         .set({
@@ -429,21 +467,62 @@ export function accessService(db: Db) {
       status?: "pending" | "active" | "suspended";
     },
   ) {
-    const existing = await getMemberById(companyId, memberId);
-    if (!existing) return null;
-    const nextMembershipRole =
-      data.membershipRole !== undefined ? data.membershipRole : existing.membershipRole;
-    const nextStatus = data.status ?? existing.status;
-    return db
-      .update(companyMemberships)
-      .set({
-        membershipRole: nextMembershipRole,
-        status: nextStatus,
-        updatedAt: new Date(),
-      })
-      .where(eq(companyMemberships.id, existing.id))
-      .returning()
-      .then((rows) => rows[0] ?? existing);
+    return db.transaction(async (tx) => {
+      await tx.execute(sql`
+        select ${companyMemberships.id}
+        from ${companyMemberships}
+        where ${companyMemberships.companyId} = ${companyId}
+          and ${companyMemberships.principalType} = 'user'
+          and ${companyMemberships.status} = 'active'
+          and ${companyMemberships.membershipRole} = 'owner'
+        for update
+      `);
+
+      const existing = await tx
+        .select()
+        .from(companyMemberships)
+        .where(and(eq(companyMemberships.companyId, companyId), eq(companyMemberships.id, memberId)))
+        .then((rows) => rows[0] ?? null);
+      if (!existing) return null;
+
+      const nextMembershipRole =
+        data.membershipRole !== undefined ? data.membershipRole : existing.membershipRole;
+      const nextStatus = data.status ?? existing.status;
+
+      if (
+        existing.principalType === "user" &&
+        existing.status === "active" &&
+        existing.membershipRole === "owner" &&
+        (nextStatus !== "active" || nextMembershipRole !== "owner")
+      ) {
+        const activeOwnerCount = await tx
+          .select({ id: companyMemberships.id })
+          .from(companyMemberships)
+          .where(
+            and(
+              eq(companyMemberships.companyId, companyId),
+              eq(companyMemberships.principalType, "user"),
+              eq(companyMemberships.status, "active"),
+              eq(companyMemberships.membershipRole, "owner"),
+            ),
+          )
+          .then((rows) => rows.length);
+        if (activeOwnerCount <= 1) {
+          throw conflict("Cannot remove the last active owner");
+        }
+      }
+
+      return tx
+        .update(companyMemberships)
+        .set({
+          membershipRole: nextMembershipRole,
+          status: nextStatus,
+          updatedAt: new Date(),
+        })
+        .where(eq(companyMemberships.id, existing.id))
+        .returning()
+        .then((rows) => rows[0] ?? existing);
+    });
   }
 
   return {