diff --git a/src/app/api/ticket-categories/[id]/route.ts b/src/app/api/ticket-categories/[id]/route.ts
new file mode 100644
index 0000000..2bcba7c
--- /dev/null
+++ b/src/app/api/ticket-categories/[id]/route.ts
@@ -0,0 +1,60 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { updateCategory } from "@/lib/services/ticket-category-service";
+
+/**
+ * PUT /api/ticket-categories/[id]
+ *
+ * Update a ticket category (rename, change description, or deactivate).
+ * Accepts: { name?, description?, isActive? }
+ *
+ * Requires: manage on Ticket subject (admin only).
+ *
+ * Response:
+ *   200 OK — updated category
+ *   400 Bad Request — validation error
+ *   404 Not Found — category not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role (non-admin)
+ */
+export function PUT(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("manage", "Ticket")(
+    async (req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+
+      let body: unknown;
+      try {
+        body = await req.json();
+      } catch {
+        return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+      }
+
+      const { name, description, isActive } = body as Record<string, unknown>;
+
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      try {
+        const category = await updateCategory(tenantPrisma, id, {
+          name: name as string | undefined,
+          description: description as string | undefined,
+          isActive: isActive as boolean | undefined,
+        });
+        return NextResponse.json(category);
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to update category";
+        if (message.includes("not found")) {
+          return NextResponse.json({ error: message }, { status: 404 });
+        }
+        return NextResponse.json({ error: message }, { status: 400 });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/ticket-categories/route.ts b/src/app/api/ticket-categories/route.ts
new file mode 100644
index 0000000..4054d67
--- /dev/null
+++ b/src/app/api/ticket-categories/route.ts
@@ -0,0 +1,86 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { createCategory, listCategories } from "@/lib/services/ticket-category-service";
+
+/**
+ * GET /api/ticket-categories
+ *
+ * List all ticket categories for the authenticated tenant.
+ * Supports optional query param: activeOnly=true
+ *
+ * Requires: read on Ticket subject.
+ *
+ * Response:
+ *   200 OK — array of categories
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export const GET = withPermission("read", "Ticket")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    const { searchParams } = new URL(req.url);
+    const activeOnly = searchParams.get("activeOnly") === "true";
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+    const categories = await listCategories(tenantPrisma, { activeOnly });
+    return NextResponse.json(categories);
+  }
+);
+
+/**
+ * POST /api/ticket-categories
+ *
+ * Create a new ticket category (admin/staff only via manage permission).
+ * Accepts: { name, description? }
+ *
+ * Requires: manage on Ticket subject.
+ *
+ * Response:
+ *   201 Created — created category
+ *   400 Bad Request — validation error or duplicate name
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role (non-admin)
+ */
+export const POST = withPermission("manage", "Ticket")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    let body: unknown;
+    try {
+      body = await req.json();
+    } catch {
+      return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+    }
+
+    const { name, description } = body as Record<string, unknown>;
+
+    if (!name || typeof name !== "string") {
+      return NextResponse.json({ error: "name is required" }, { status: 400 });
+    }
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+
+    try {
+      const category = await createCategory(tenantPrisma, user.tenantId, {
+        name,
+        description: description as string | undefined,
+      });
+      return NextResponse.json(category, { status: 201 });
+    } catch (err) {
+      const message = err instanceof Error ? err.message : "Failed to create category";
+      return NextResponse.json({ error: message }, { status: 400 });
+    }
+  }
+);
diff --git a/src/app/api/tickets/[id]/route.ts b/src/app/api/tickets/[id]/route.ts
new file mode 100644
index 0000000..4239fc5
--- /dev/null
+++ b/src/app/api/tickets/[id]/route.ts
@@ -0,0 +1,102 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { getTicket, updateTicket } from "@/lib/services/ticket-service";
+import { TicketPriority } from "@prisma/client";
+
+/**
+ * GET /api/tickets/[id]
+ *
+ * Get a single ticket with its related data.
+ *
+ * Requires: read on Ticket subject.
+ *
+ * Response:
+ *   200 OK — ticket with category, subscriber, createdBy
+ *   404 Not Found — ticket not found in tenant scope
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function GET(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("read", "Ticket")(
+    async (_req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+
+      const tenantPrisma = withTenantContext(user.tenantId);
+      const ticket = await getTicket(tenantPrisma, id);
+
+      if (!ticket) {
+        return NextResponse.json({ error: "Ticket not found" }, { status: 404 });
+      }
+
+      return NextResponse.json(ticket);
+    }
+  )(req);
+}
+
+/**
+ * PUT /api/tickets/[id]
+ *
+ * Update ticket metadata (subject, description, category, priority, notes).
+ * Does NOT change ticket status — use POST /api/tickets/[id]/status for that.
+ *
+ * Accepts: { subject?, description?, categoryId?, priority?, notes? }
+ *
+ * Requires: update on Ticket subject.
+ *
+ * Response:
+ *   200 OK — updated ticket
+ *   400 Bad Request — validation error
+ *   404 Not Found — ticket not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function PUT(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("update", "Ticket")(
+    async (req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+
+      let body: unknown;
+      try {
+        body = await req.json();
+      } catch {
+        return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+      }
+
+      const { subject, description, categoryId, priority, notes } = body as Record<string, unknown>;
+
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      try {
+        const ticket = await updateTicket(tenantPrisma, id, {
+          subject: subject as string | undefined,
+          description: description as string | undefined,
+          categoryId: categoryId as string | undefined,
+          priority: priority as TicketPriority | undefined,
+          notes: notes as string | undefined,
+        });
+        return NextResponse.json(ticket);
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to update ticket";
+        if (message.includes("not found")) {
+          return NextResponse.json({ error: message }, { status: 404 });
+        }
+        return NextResponse.json({ error: message }, { status: 400 });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/tickets/[id]/status/route.ts b/src/app/api/tickets/[id]/status/route.ts
new file mode 100644
index 0000000..5b44eee
--- /dev/null
+++ b/src/app/api/tickets/[id]/status/route.ts
@@ -0,0 +1,76 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { transitionTicketStatus } from "@/lib/services/ticket-service";
+import { TicketStatus } from "@prisma/client";
+
+/**
+ * POST /api/tickets/[id]/status
+ *
+ * Transition a ticket to a new status.
+ * Enforces the guard map: OPEN->ASSIGNED|CLOSED, ASSIGNED->OPEN|RESOLVED,
+ * RESOLVED->CLOSED|OPEN, CLOSED->(terminal).
+ *
+ * Accepts: { status: TicketStatus }
+ *
+ * Requires: update on Ticket subject.
+ *
+ * Response:
+ *   200 OK — updated ticket with new status
+ *   400 Bad Request — invalid or disallowed transition
+ *   404 Not Found — ticket not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function POST(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("update", "Ticket")(
+    async (req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+
+      let body: unknown;
+      try {
+        body = await req.json();
+      } catch {
+        return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+      }
+
+      const { status } = body as Record<string, unknown>;
+
+      if (!status || typeof status !== "string") {
+        return NextResponse.json({ error: "status is required" }, { status: 400 });
+      }
+
+      const validStatuses = Object.values(TicketStatus) as string[];
+      if (!validStatuses.includes(status)) {
+        return NextResponse.json(
+          { error: `Invalid status. Must be one of: ${validStatuses.join(", ")}` },
+          { status: 400 }
+        );
+      }
+
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      try {
+        const ticket = await transitionTicketStatus(
+          tenantPrisma,
+          id,
+          status as TicketStatus
+        );
+        return NextResponse.json(ticket);
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to transition ticket status";
+        if (message.includes("not found")) {
+          return NextResponse.json({ error: message }, { status: 404 });
+        }
+        return NextResponse.json({ error: message }, { status: 400 });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/tickets/route.ts b/src/app/api/tickets/route.ts
new file mode 100644
index 0000000..8308448
--- /dev/null
+++ b/src/app/api/tickets/route.ts
@@ -0,0 +1,120 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { createTicket, listTickets } from "@/lib/services/ticket-service";
+import { TicketPriority, TicketSource, TicketStatus } from "@prisma/client";
+
+/**
+ * GET /api/tickets
+ *
+ * List tickets for the authenticated tenant.
+ * Supports optional query filters: status, categoryId, priority, subscriberId, page, limit.
+ *
+ * Requires: read on Ticket subject.
+ *
+ * Response:
+ *   200 OK — paginated ticket list
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export const GET = withPermission("read", "Ticket")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    const { searchParams } = new URL(req.url);
+    const status = searchParams.get("status") as TicketStatus | null;
+    const categoryId = searchParams.get("categoryId") ?? undefined;
+    const priority = searchParams.get("priority") as TicketPriority | null;
+    const subscriberId = searchParams.get("subscriberId") ?? undefined;
+    const page = parseInt(searchParams.get("page") ?? "1", 10);
+    const limit = parseInt(searchParams.get("limit") ?? "20", 10);
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+    const result = await listTickets(tenantPrisma, {
+      status: status ?? undefined,
+      categoryId,
+      priority: priority ?? undefined,
+      subscriberId,
+      page,
+      limit,
+    });
+
+    return NextResponse.json(result);
+  }
+);
+
+/**
+ * POST /api/tickets
+ *
+ * Create a new support ticket.
+ * Accepts: { subject, description, categoryId, priority?, subscriberId? }
+ *
+ * Requires: create on Ticket subject.
+ *
+ * Response:
+ *   201 Created — created ticket
+ *   400 Bad Request — validation error or deactivated category
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export const POST = withPermission("create", "Ticket")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    let body: unknown;
+    try {
+      body = await req.json();
+    } catch {
+      return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+    }
+
+    const {
+      subject,
+      description,
+      categoryId,
+      priority,
+      subscriberId,
+      source,
+    } = body as Record<string, unknown>;
+
+    if (!subject || typeof subject !== "string") {
+      return NextResponse.json({ error: "subject is required" }, { status: 400 });
+    }
+
+    if (!description || typeof description !== "string") {
+      return NextResponse.json({ error: "description is required" }, { status: 400 });
+    }
+
+    if (!categoryId || typeof categoryId !== "string") {
+      return NextResponse.json({ error: "categoryId is required" }, { status: 400 });
+    }
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+
+    try {
+      const ticket = await createTicket(tenantPrisma, user.tenantId, {
+        subject,
+        description,
+        categoryId,
+        priority: priority as TicketPriority | undefined,
+        subscriberId: subscriberId as string | undefined,
+        createdById: user.id,
+        source: source as TicketSource | undefined,
+      });
+      return NextResponse.json(ticket, { status: 201 });
+    } catch (err) {
+      const message = err instanceof Error ? err.message : "Failed to create ticket";
+      return NextResponse.json({ error: message }, { status: 400 });
+    }
+  }
+);
diff --git a/src/lib/__tests__/ticket-service.test.ts b/src/lib/__tests__/ticket-service.test.ts
new file mode 100644
index 0000000..2827029
--- /dev/null
+++ b/src/lib/__tests__/ticket-service.test.ts
@@ -0,0 +1,551 @@
+/**
+ * Ticket System Integration Tests
+ *
+ * Tests the full ticket lifecycle:
+ * - Default categories seeded on tenant creation (6 categories)
+ * - createCategory adds a new category
+ * - updateCategory deactivates a category
+ * - createTicket with valid category succeeds (TKT-0001)
+ * - createTicket with deactivated category throws
+ * - Second ticket gets TKT-0002
+ * - transitionTicketStatus OPEN -> CLOSED succeeds
+ * - transitionTicketStatus CLOSED -> OPEN throws (terminal state)
+ * - transitionTicketStatus OPEN -> RESOLVED throws (invalid)
+ * - resolveTicket is idempotent (calling on RESOLVED ticket does not throw)
+ * - listTickets with status filter returns correct subset
+ * - Cross-tenant isolation (Tenant B cannot see Tenant A tickets)
+ *
+ * These tests require a live PostgreSQL database connection.
+ *
+ * CLEANUP ORDER:
+ * tickets -> ticketCategories -> subscribers -> servicePlans ->
+ * tenantSettings -> accountingPeriods -> accounts -> users -> tenant
+ */
+
+import { prisma } from "@/lib/prisma";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { createTenant } from "@/lib/tenant";
+import {
+  createTicket,
+  updateTicket,
+  getTicket,
+  listTickets,
+  transitionTicketStatus,
+  resolveTicket,
+} from "@/lib/services/ticket-service";
+import {
+  createCategory,
+  updateCategory,
+  listCategories,
+} from "@/lib/services/ticket-category-service";
+import { TicketPriority, TicketSource, TicketStatus } from "@prisma/client";
+
+// ---------------------------------------------------------------------------
+// Shared test state
+// ---------------------------------------------------------------------------
+
+const TS = Date.now();
+
+let tenantAId: string;
+let tenantBId: string;
+let adminUserAId: string;
+let adminUserBId: string;
+
+// ---------------------------------------------------------------------------
+// Setup helpers
+// ---------------------------------------------------------------------------
+
+function tA() {
+  return withTenantContext(tenantAId);
+}
+
+function tB() {
+  return withTenantContext(tenantBId);
+}
+
+// ---------------------------------------------------------------------------
+// Before all: provision two tenants
+// ---------------------------------------------------------------------------
+
+beforeAll(async () => {
+  // Tenant A
+  const resultA = await createTenant({
+    businessName: `Ticket ISP A ${TS}`,
+    ownerFirstName: "Alice",
+    ownerLastName: "Admin",
+    ownerEmail: `ticket-admin-a-${TS}@test.com`,
+    password: "password123",
+  });
+  tenantAId = resultA.tenant.id;
+  adminUserAId = resultA.user.id;
+
+  // Tenant B
+  const resultB = await createTenant({
+    businessName: `Ticket ISP B ${TS}`,
+    ownerFirstName: "Bob",
+    ownerLastName: "Admin",
+    ownerEmail: `ticket-admin-b-${TS}@test.com`,
+    password: "password123",
+  });
+  tenantBId = resultB.tenant.id;
+  adminUserBId = resultB.user.id;
+});
+
+// ---------------------------------------------------------------------------
+// After all: cleanup in dependency order
+// ---------------------------------------------------------------------------
+
+afterAll(async () => {
+  // Delete tickets first
+  await prisma.ticket.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Then ticketCategories (seeded + any created in tests)
+  await prisma.ticketCategory.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Then subscribers (none created in these tests, but safe to call)
+  await prisma.subscriber.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Service plans
+  await prisma.servicePlan.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Tenant settings
+  await prisma.tenantSettings.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Accounting periods
+  await prisma.accountingPeriod.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Journal entry lines, then entries
+  await prisma.journalEntryLine.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  await prisma.journalEntry.updateMany({
+    where: { tenantId: { in: [tenantAId, tenantBId] } },
+    data: { reversesEntryId: null },
+  });
+  await prisma.journalEntry.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Chart of accounts
+  await prisma.account.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  // Users then tenants
+  await prisma.user.deleteMany({ where: { tenantId: { in: [tenantAId, tenantBId] } } });
+  await prisma.tenant.deleteMany({ where: { id: { in: [tenantAId, tenantBId] } } });
+});
+
+// ---------------------------------------------------------------------------
+// Category tests
+// ---------------------------------------------------------------------------
+
+describe("Default ticket categories", () => {
+  it("seeds 6 default categories on tenant creation", async () => {
+    const categories = await listCategories(tA());
+    expect(categories).toHaveLength(6);
+
+    const names = categories.map((c: { name: string }) => c.name);
+    expect(names).toContain("No Connection");
+    expect(names).toContain("Slow Speed");
+    expect(names).toContain("Billing Inquiry");
+    expect(names).toContain("New Installation");
+    expect(names).toContain("Equipment Issue");
+    expect(names).toContain("Other");
+  });
+
+  it("all default categories start as active", async () => {
+    const categories = await listCategories(tA());
+    const allActive = categories.every((c: { isActive: boolean }) => c.isActive);
+    expect(allActive).toBe(true);
+  });
+});
+
+describe("createCategory", () => {
+  it("creates a new category successfully", async () => {
+    const category = await createCategory(tA(), tenantAId, {
+      name: `Custom Category ${TS}`,
+      description: "A custom test category",
+    });
+
+    expect(category.id).toBeTruthy();
+    expect(category.name).toBe(`Custom Category ${TS}`);
+    expect(category.description).toBe("A custom test category");
+    expect(category.isActive).toBe(true);
+    expect(category.tenantId).toBe(tenantAId);
+  });
+
+  it("throws on empty name", async () => {
+    await expect(
+      createCategory(tA(), tenantAId, { name: "" })
+    ).rejects.toThrow("Category name is required");
+  });
+
+  it("throws on duplicate category name within tenant", async () => {
+    const name = `Duplicate Cat ${TS}`;
+    await createCategory(tA(), tenantAId, { name });
+    await expect(
+      createCategory(tA(), tenantAId, { name })
+    ).rejects.toThrow(/already exists/);
+  });
+});
+
+describe("updateCategory", () => {
+  it("deactivates a category", async () => {
+    const category = await createCategory(tA(), tenantAId, {
+      name: `To Deactivate ${TS}`,
+    });
+
+    const updated = await updateCategory(tA(), category.id, { isActive: false });
+    expect(updated.isActive).toBe(false);
+  });
+
+  it("renames a category", async () => {
+    const category = await createCategory(tA(), tenantAId, {
+      name: `Old Name ${TS}`,
+    });
+
+    const newName = `New Name ${TS}`;
+    const updated = await updateCategory(tA(), category.id, { name: newName });
+    expect(updated.name).toBe(newName);
+  });
+
+  it("throws if category not found", async () => {
+    await expect(
+      updateCategory(tA(), "non-existent-id", { isActive: false })
+    ).rejects.toThrow(/not found/);
+  });
+});
+
+describe("listCategories", () => {
+  it("returns all categories by default", async () => {
+    // Deactivate one category for this tenant to test filtering
+    const allCats = await listCategories(tA());
+    const firstActive = allCats.find((c: { isActive: boolean }) => c.isActive);
+    if (firstActive) {
+      await updateCategory(tA(), firstActive.id, { isActive: false });
+    }
+
+    const allAfter = await listCategories(tA());
+    const hasInactive = allAfter.some((c: { isActive: boolean }) => !c.isActive);
+    expect(hasInactive).toBe(true);
+  });
+
+  it("filters to active only when activeOnly=true", async () => {
+    const active = await listCategories(tA(), { activeOnly: true });
+    const allActive = active.every((c: { isActive: boolean }) => c.isActive);
+    expect(allActive).toBe(true);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Ticket creation tests
+// ---------------------------------------------------------------------------
+
+let activeCategoryId: string;
+
+describe("createTicket", () => {
+  beforeAll(async () => {
+    // Get or create a fresh active category for ticket creation tests
+    const cats = await listCategories(tA(), { activeOnly: true });
+    activeCategoryId = cats[0]?.id;
+  });
+
+  it("creates a ticket with TKT-0001 number", async () => {
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Internet is down",
+      description: "No connectivity since this morning",
+      categoryId: activeCategoryId,
+      createdById: adminUserAId,
+    });
+
+    expect(ticket.ticketNumber).toBe("TKT-0001");
+    expect(ticket.subject).toBe("Internet is down");
+    expect(ticket.status).toBe(TicketStatus.OPEN);
+    expect(ticket.priority).toBe(TicketPriority.MEDIUM);
+    expect(ticket.source).toBe(TicketSource.STAFF);
+    expect(ticket.tenantId).toBe(tenantAId);
+  });
+
+  it("creates a second ticket with TKT-0002", async () => {
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Slow speed",
+      description: "Getting 1 Mbps on 50 Mbps plan",
+      categoryId: activeCategoryId,
+      priority: TicketPriority.HIGH,
+      createdById: adminUserAId,
+    });
+
+    expect(ticket.ticketNumber).toBe("TKT-0002");
+    expect(ticket.priority).toBe(TicketPriority.HIGH);
+  });
+
+  it("throws when creating ticket with deactivated category", async () => {
+    // Create and deactivate a category
+    const cat = await createCategory(tA(), tenantAId, {
+      name: `Inactive Cat ${TS}`,
+    });
+    await updateCategory(tA(), cat.id, { isActive: false });
+
+    await expect(
+      createTicket(tA(), tenantAId, {
+        subject: "Test ticket",
+        description: "Should fail",
+        categoryId: cat.id,
+        createdById: adminUserAId,
+      })
+    ).rejects.toThrow(/deactivated/);
+  });
+
+  it("throws when creating ticket with non-existent category", async () => {
+    await expect(
+      createTicket(tA(), tenantAId, {
+        subject: "Test ticket",
+        description: "Should fail",
+        categoryId: "non-existent-category-id",
+        createdById: adminUserAId,
+      })
+    ).rejects.toThrow(/Category not found/);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Status transition tests
+// ---------------------------------------------------------------------------
+
+describe("transitionTicketStatus", () => {
+  let ticketId: string;
+
+  beforeAll(async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Transition test ticket",
+      description: "For testing status transitions",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+    ticketId = ticket.id;
+  });
+
+  it("transitions OPEN -> ASSIGNED", async () => {
+    const updated = await transitionTicketStatus(tA(), ticketId, TicketStatus.ASSIGNED);
+    expect(updated.status).toBe(TicketStatus.ASSIGNED);
+  });
+
+  it("transitions ASSIGNED -> RESOLVED and sets resolvedAt", async () => {
+    const updated = await transitionTicketStatus(tA(), ticketId, TicketStatus.RESOLVED);
+    expect(updated.status).toBe(TicketStatus.RESOLVED);
+    expect(updated.resolvedAt).toBeTruthy();
+  });
+
+  it("transitions RESOLVED -> CLOSED and sets closedAt", async () => {
+    const updated = await transitionTicketStatus(tA(), ticketId, TicketStatus.CLOSED);
+    expect(updated.status).toBe(TicketStatus.CLOSED);
+    expect(updated.closedAt).toBeTruthy();
+  });
+
+  it("throws when transitioning from CLOSED (terminal state)", async () => {
+    await expect(
+      transitionTicketStatus(tA(), ticketId, TicketStatus.OPEN)
+    ).rejects.toThrow(/terminal state/);
+  });
+});
+
+describe("transitionTicketStatus - invalid transitions", () => {
+  let ticketId: string;
+
+  beforeAll(async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Invalid transition test",
+      description: "Testing invalid transitions",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+    ticketId = ticket.id;
+  });
+
+  it("throws OPEN -> RESOLVED (must go through ASSIGNED first)", async () => {
+    await expect(
+      transitionTicketStatus(tA(), ticketId, TicketStatus.RESOLVED)
+    ).rejects.toThrow(/Invalid status transition/);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// resolveTicket idempotency test
+// ---------------------------------------------------------------------------
+
+describe("resolveTicket", () => {
+  it("is idempotent when called on an already-RESOLVED ticket", async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Idempotent resolve test",
+      description: "Testing idempotent resolve",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+
+    // First get to ASSIGNED, then RESOLVED
+    await transitionTicketStatus(tA(), ticket.id, TicketStatus.ASSIGNED);
+    await resolveTicket(tA(), ticket.id);
+
+    // Call resolveTicket again — should not throw
+    await expect(resolveTicket(tA(), ticket.id)).resolves.toBeUndefined();
+  });
+
+  it("throws when called on a CLOSED ticket (cannot resolve a closed ticket)", async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Closed ticket resolve test",
+      description: "Testing resolve on closed ticket",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+
+    // OPEN -> CLOSED
+    await transitionTicketStatus(tA(), ticket.id, TicketStatus.CLOSED);
+
+    // Try to resolve a CLOSED ticket — should throw (CLOSED -> RESOLVED is not valid)
+    await expect(resolveTicket(tA(), ticket.id)).rejects.toThrow(/Invalid status transition/);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// getTicket and updateTicket tests
+// ---------------------------------------------------------------------------
+
+describe("getTicket", () => {
+  it("returns ticket with related data", async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const created = await createTicket(tA(), tenantAId, {
+      subject: "Get ticket test",
+      description: "Testing getTicket",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+
+    const ticket = await getTicket(tA(), created.id);
+    expect(ticket).not.toBeNull();
+    expect(ticket.id).toBe(created.id);
+    expect(ticket.category).toBeTruthy();
+    expect(ticket.category.id).toBe(catId);
+    expect(ticket.createdBy).toBeTruthy();
+    expect(ticket.createdBy.id).toBe(adminUserAId);
+  });
+
+  it("returns null for non-existent ticket", async () => {
+    const ticket = await getTicket(tA(), "non-existent-id");
+    expect(ticket).toBeNull();
+  });
+});
+
+describe("updateTicket", () => {
+  it("updates ticket subject and notes", async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    const ticket = await createTicket(tA(), tenantAId, {
+      subject: "Original subject",
+      description: "Original description",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+
+    const updated = await updateTicket(tA(), ticket.id, {
+      subject: "Updated subject",
+      notes: "Staff added internal notes",
+    });
+
+    expect(updated.subject).toBe("Updated subject");
+    expect(updated.notes).toBe("Staff added internal notes");
+    expect(updated.status).toBe(TicketStatus.OPEN); // Status unchanged
+  });
+});
+
+// ---------------------------------------------------------------------------
+// listTickets with filter test
+// ---------------------------------------------------------------------------
+
+describe("listTickets", () => {
+  it("filters by status correctly", async () => {
+    const cats = await listCategories(tA(), { activeOnly: true });
+    const catId = cats[0]?.id;
+
+    // Create one OPEN and one CLOSED ticket
+    const openTicket = await createTicket(tA(), tenantAId, {
+      subject: "Filter test open",
+      description: "Open ticket for filter test",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+
+    const closedTicket = await createTicket(tA(), tenantAId, {
+      subject: "Filter test closed",
+      description: "Closed ticket for filter test",
+      categoryId: catId,
+      createdById: adminUserAId,
+    });
+    await transitionTicketStatus(tA(), closedTicket.id, TicketStatus.CLOSED);
+
+    // List only OPEN tickets
+    const openResult = await listTickets(tA(), { status: TicketStatus.OPEN });
+    const openIds = openResult.tickets.map((t: { id: string }) => t.id);
+    expect(openIds).toContain(openTicket.id);
+    expect(openIds).not.toContain(closedTicket.id);
+
+    // List only CLOSED tickets
+    const closedResult = await listTickets(tA(), { status: TicketStatus.CLOSED });
+    const closedIds = closedResult.tickets.map((t: { id: string }) => t.id);
+    expect(closedIds).toContain(closedTicket.id);
+    expect(closedIds).not.toContain(openTicket.id);
+  });
+
+  it("returns total count with pagination", async () => {
+    const result = await listTickets(tA(), { limit: 2, page: 1 });
+    expect(typeof result.total).toBe("number");
+    expect(result.tickets.length).toBeLessThanOrEqual(2);
+    expect(result.page).toBe(1);
+    expect(result.limit).toBe(2);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Cross-tenant isolation test
+// ---------------------------------------------------------------------------
+
+describe("Cross-tenant isolation", () => {
+  it("Tenant B cannot see Tenant A tickets", async () => {
+    const catsA = await listCategories(tA(), { activeOnly: true });
+    const catAId = catsA[0]?.id;
+
+    // Create a ticket in Tenant A
+    const ticketA = await createTicket(tA(), tenantAId, {
+      subject: "Isolation test ticket",
+      description: "Should not be visible to Tenant B",
+      categoryId: catAId,
+      createdById: adminUserAId,
+    });
+
+    // Tenant B tries to get the ticket (should return null — tenant-scoped)
+    const fromB = await getTicket(tB(), ticketA.id);
+    expect(fromB).toBeNull();
+
+    // Tenant B list should not include Tenant A's tickets
+    const listB = await listTickets(tB());
+    const bIds = listB.tickets.map((t: { id: string }) => t.id);
+    expect(bIds).not.toContain(ticketA.id);
+  });
+
+  it("Tenant B has its own seeded categories (separate from Tenant A)", async () => {
+    const catsA = await listCategories(tA());
+    const catsB = await listCategories(tB());
+
+    // Both have 6 default categories
+    // We add some extras in tests above to Tenant A, so just verify Tenant B has its own
+    expect(catsB.length).toBeGreaterThanOrEqual(6);
+
+    const bIds = catsB.map((c: { id: string }) => c.id);
+    const aIds = catsA.map((c: { id: string }) => c.id);
+
+    // No overlap between tenant category IDs (tenant isolation)
+    const overlap = bIds.filter((id: string) => aIds.includes(id));
+    expect(overlap).toHaveLength(0);
+  });
+});
diff --git a/src/lib/services/ticket-category-service.ts b/src/lib/services/ticket-category-service.ts
new file mode 100644
index 0000000..fc827ef
--- /dev/null
+++ b/src/lib/services/ticket-category-service.ts
@@ -0,0 +1,121 @@
+/**
+ * TicketCategoryService — Admin-configurable ticket category CRUD.
+ *
+ * ARCHITECTURE:
+ * - Categories are tenant-scoped and admin-configurable
+ * - Default ISP categories are seeded at tenant creation (see tenant.ts)
+ * - Deactivated categories (isActive=false) cannot be used for new tickets
+ * - Soft-deactivation preserves category history on existing tickets
+ */
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type TenantPrismaClient = any;
+
+// ---------------------------------------------------------------------------
+// Input/output types
+// ---------------------------------------------------------------------------
+
+export interface CreateCategoryInput {
+  name: string;
+  description?: string;
+}
+
+export interface UpdateCategoryInput {
+  name?: string;
+  description?: string;
+  isActive?: boolean;
+}
+
+// ---------------------------------------------------------------------------
+// createCategory
+// ---------------------------------------------------------------------------
+
+/**
+ * Create a new ticket category for this tenant.
+ *
+ * @throws Error if name is blank or already exists in tenant
+ */
+export async function createCategory(
+  tenantPrisma: TenantPrismaClient,
+  tenantId: string,
+  input: CreateCategoryInput
+) {
+  const { name, description } = input;
+
+  if (!name || !name.trim()) {
+    throw new Error("Category name is required");
+  }
+
+  try {
+    return await tenantPrisma.ticketCategory.create({
+      data: {
+        tenantId,
+        name: name.trim(),
+        description: description?.trim() ?? null,
+      },
+    });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    if (message.includes("Unique constraint") || message.includes("P2002")) {
+      throw new Error(`Category name "${name.trim()}" already exists`);
+    }
+    throw err;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// updateCategory
+// ---------------------------------------------------------------------------
+
+/**
+ * Update a category's name, description, or isActive flag.
+ * Used by admins to rename or deactivate categories.
+ *
+ * @throws Error if category not found
+ */
+export async function updateCategory(
+  tenantPrisma: TenantPrismaClient,
+  categoryId: string,
+  input: UpdateCategoryInput
+) {
+  const { name, description, isActive } = input;
+
+  const data: Record<string, unknown> = {};
+  if (name !== undefined) data.name = name.trim();
+  if (description !== undefined) data.description = description.trim() || null;
+  if (isActive !== undefined) data.isActive = isActive;
+
+  try {
+    return await tenantPrisma.ticketCategory.update({
+      where: { id: categoryId },
+      data,
+    });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    if (message.includes("Record to update not found") || message.includes("P2025")) {
+      throw new Error(`Category not found: ${categoryId}`);
+    }
+    throw err;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// listCategories
+// ---------------------------------------------------------------------------
+
+/**
+ * List all categories for this tenant.
+ *
+ * @param options.activeOnly - If true, only return isActive=true categories (default: false)
+ */
+export async function listCategories(
+  tenantPrisma: TenantPrismaClient,
+  options: { activeOnly?: boolean } = {}
+) {
+  const { activeOnly = false } = options;
+
+  return tenantPrisma.ticketCategory.findMany({
+    where: activeOnly ? { isActive: true } : undefined,
+    orderBy: { name: "asc" },
+  });
+}
diff --git a/src/lib/services/ticket-service.ts b/src/lib/services/ticket-service.ts
new file mode 100644
index 0000000..397666b
--- /dev/null
+++ b/src/lib/services/ticket-service.ts
@@ -0,0 +1,406 @@
+/**
+ * TicketService — Ticket CRUD, sequential numbering, and status lifecycle management.
+ *
+ * ARCHITECTURE:
+ * Tickets are the intake mechanism for subscriber issues. They follow a strict
+ * lifecycle enforced by the VALID_TICKET_TRANSITIONS guard map:
+ *
+ *   OPEN -> ASSIGNED (triggered by job order creation in 03-04, or manually)
+ *   OPEN -> CLOSED   (staff can close without assigning)
+ *   ASSIGNED -> OPEN (staff can un-assign / revert)
+ *   ASSIGNED -> RESOLVED (triggered by job order completion in 03-04, or manually)
+ *   RESOLVED -> CLOSED (final closure)
+ *   RESOLVED -> OPEN  (reopen if issue recurs)
+ *   CLOSED -> (terminal — no further transitions)
+ *
+ * resolveTicket() is idempotent: calling on an already-RESOLVED ticket is a no-op.
+ * This prevents race conditions when multiple job orders complete simultaneously
+ * (see RESEARCH.md pitfall 5).
+ *
+ * SEQUENTIAL NUMBERING:
+ * Ticket numbers are auto-generated as TKT-NNNN (e.g., TKT-0001, TKT-0042).
+ * Same pattern as invoiceNumber and entryNumber in earlier phases.
+ */
+
+import { TicketStatus, TicketPriority, TicketSource } from "@prisma/client";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type TenantPrismaClient = any;
+
+// ---------------------------------------------------------------------------
+// Status transition guard map
+// ---------------------------------------------------------------------------
+
+/**
+ * Valid transitions for each ticket status.
+ * CLOSED is terminal — no transitions allowed.
+ */
+export const VALID_TICKET_TRANSITIONS: Record<TicketStatus, TicketStatus[]> = {
+  [TicketStatus.OPEN]:     [TicketStatus.ASSIGNED, TicketStatus.CLOSED],
+  [TicketStatus.ASSIGNED]: [TicketStatus.OPEN, TicketStatus.RESOLVED],
+  [TicketStatus.RESOLVED]: [TicketStatus.CLOSED, TicketStatus.OPEN],
+  [TicketStatus.CLOSED]:   [],
+};
+
+// ---------------------------------------------------------------------------
+// Input/output types
+// ---------------------------------------------------------------------------
+
+export interface CreateTicketInput {
+  subject: string;
+  description: string;
+  categoryId: string;
+  priority?: TicketPriority;
+  subscriberId?: string;
+  createdById: string;
+  source?: TicketSource;
+}
+
+export interface UpdateTicketInput {
+  subject?: string;
+  description?: string;
+  categoryId?: string;
+  priority?: TicketPriority;
+  notes?: string;
+}
+
+export interface ListTicketsOptions {
+  status?: TicketStatus;
+  categoryId?: string;
+  priority?: TicketPriority;
+  subscriberId?: string;
+  page?: number;
+  limit?: number;
+}
+
+// ---------------------------------------------------------------------------
+// generateTicketNumber
+// ---------------------------------------------------------------------------
+
+/**
+ * Generate the next sequential ticket number for this tenant.
+ * Format: TKT-NNNN (e.g., TKT-0001, TKT-0042)
+ *
+ * Finds the most recently created ticket with a TKT- prefix,
+ * parses the last 4 digits, increments, and pads to 4 characters.
+ * Starts at TKT-0001 if no tickets exist.
+ */
+async function generateTicketNumber(tenantPrisma: TenantPrismaClient): Promise<string> {
+  const lastTicket = await tenantPrisma.ticket.findFirst({
+    where: { ticketNumber: { startsWith: "TKT-" } },
+    orderBy: { ticketNumber: "desc" },
+    select: { ticketNumber: true },
+  });
+
+  if (!lastTicket) {
+    return "TKT-0001";
+  }
+
+  const lastNum = parseInt(lastTicket.ticketNumber.replace("TKT-", ""), 10);
+  const nextNum = isNaN(lastNum) ? 1 : lastNum + 1;
+  return `TKT-${String(nextNum).padStart(4, "0")}`;
+}
+
+// ---------------------------------------------------------------------------
+// createTicket
+// ---------------------------------------------------------------------------
+
+/**
+ * Create a new support ticket.
+ *
+ * Validates that the category exists and is active before creation.
+ * Generates a sequential ticket number (TKT-NNNN).
+ * New tickets always start in OPEN status.
+ *
+ * @throws Error if category not found or deactivated
+ */
+export async function createTicket(
+  tenantPrisma: TenantPrismaClient,
+  tenantId: string,
+  input: CreateTicketInput
+) {
+  const {
+    subject,
+    description,
+    categoryId,
+    priority = TicketPriority.MEDIUM,
+    subscriberId,
+    createdById,
+    source = TicketSource.STAFF,
+  } = input;
+
+  if (!subject || !subject.trim()) {
+    throw new Error("Ticket subject is required");
+  }
+
+  if (!description || !description.trim()) {
+    throw new Error("Ticket description is required");
+  }
+
+  // Validate category exists and is active
+  const category = await tenantPrisma.ticketCategory.findFirst({
+    where: { id: categoryId },
+    select: { id: true, isActive: true, name: true },
+  });
+
+  if (!category) {
+    throw new Error(`Category not found: ${categoryId}`);
+  }
+
+  if (!category.isActive) {
+    throw new Error(
+      `Category "${category.name}" is deactivated and cannot be used for new tickets`
+    );
+  }
+
+  const ticketNumber = await generateTicketNumber(tenantPrisma);
+
+  return tenantPrisma.ticket.create({
+    data: {
+      tenantId,
+      ticketNumber,
+      subject: subject.trim(),
+      description: description.trim(),
+      categoryId,
+      priority,
+      status: TicketStatus.OPEN,
+      source,
+      subscriberId: subscriberId ?? null,
+      createdById,
+    },
+    include: {
+      category: true,
+      subscriber: true,
+      createdBy: {
+        select: { id: true, firstName: true, lastName: true, email: true },
+      },
+    },
+  });
+}
+
+// ---------------------------------------------------------------------------
+// updateTicket
+// ---------------------------------------------------------------------------
+
+/**
+ * Update ticket metadata (subject, description, category, priority, notes).
+ * Does NOT change ticket status — use transitionTicketStatus for that.
+ *
+ * @throws Error if ticket not found, or new category is deactivated
+ */
+export async function updateTicket(
+  tenantPrisma: TenantPrismaClient,
+  ticketId: string,
+  input: UpdateTicketInput
+) {
+  const { subject, description, categoryId, priority, notes } = input;
+
+  const data: Record<string, unknown> = {};
+  if (subject !== undefined) data.subject = subject.trim();
+  if (description !== undefined) data.description = description.trim();
+  if (priority !== undefined) data.priority = priority;
+  if (notes !== undefined) data.notes = notes.trim() || null;
+
+  if (categoryId !== undefined) {
+    const category = await tenantPrisma.ticketCategory.findFirst({
+      where: { id: categoryId },
+      select: { id: true, isActive: true, name: true },
+    });
+
+    if (!category) {
+      throw new Error(`Category not found: ${categoryId}`);
+    }
+
+    if (!category.isActive) {
+      throw new Error(
+        `Category "${category.name}" is deactivated and cannot be used for new tickets`
+      );
+    }
+
+    data.categoryId = categoryId;
+  }
+
+  try {
+    return await tenantPrisma.ticket.update({
+      where: { id: ticketId },
+      data,
+      include: {
+        category: true,
+        subscriber: true,
+        createdBy: {
+          select: { id: true, firstName: true, lastName: true, email: true },
+        },
+      },
+    });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    if (message.includes("Record to update not found") || message.includes("P2025")) {
+      throw new Error(`Ticket not found: ${ticketId}`);
+    }
+    throw err;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// getTicket
+// ---------------------------------------------------------------------------
+
+/**
+ * Get a single ticket by ID, including related data.
+ *
+ * Returns null if ticket not found in tenant scope.
+ */
+export async function getTicket(
+  tenantPrisma: TenantPrismaClient,
+  ticketId: string
+) {
+  return tenantPrisma.ticket.findFirst({
+    where: { id: ticketId },
+    include: {
+      category: true,
+      subscriber: true,
+      createdBy: {
+        select: { id: true, firstName: true, lastName: true, email: true },
+      },
+    },
+  });
+}
+
+// ---------------------------------------------------------------------------
+// listTickets
+// ---------------------------------------------------------------------------
+
+/**
+ * List tickets with optional filters and pagination.
+ *
+ * Includes category and subscriber for each ticket.
+ * Ordered by createdAt descending (newest first).
+ */
+export async function listTickets(
+  tenantPrisma: TenantPrismaClient,
+  options: ListTicketsOptions = {}
+) {
+  const { status, categoryId, priority, subscriberId, page = 1, limit = 20 } = options;
+  const skip = (page - 1) * limit;
+
+  const where: Record<string, unknown> = {};
+  if (status !== undefined) where.status = status;
+  if (categoryId !== undefined) where.categoryId = categoryId;
+  if (priority !== undefined) where.priority = priority;
+  if (subscriberId !== undefined) where.subscriberId = subscriberId;
+
+  const [tickets, total] = await Promise.all([
+    tenantPrisma.ticket.findMany({
+      where,
+      include: {
+        category: true,
+        subscriber: {
+          select: { id: true, accountNumber: true, firstName: true, lastName: true },
+        },
+      },
+      orderBy: { createdAt: "desc" },
+      skip,
+      take: limit,
+    }),
+    tenantPrisma.ticket.count({ where }),
+  ]);
+
+  return { tickets, total, page, limit };
+}
+
+// ---------------------------------------------------------------------------
+// transitionTicketStatus
+// ---------------------------------------------------------------------------
+
+/**
+ * Transition a ticket to a new status, enforcing the guard map.
+ *
+ * Valid transitions:
+ *   OPEN -> ASSIGNED | CLOSED
+ *   ASSIGNED -> OPEN | RESOLVED
+ *   RESOLVED -> CLOSED | OPEN
+ *   CLOSED -> (none — terminal)
+ *
+ * Sets resolvedAt on transition to RESOLVED.
+ * Sets closedAt on transition to CLOSED.
+ *
+ * @throws Error if ticket not found, or transition is invalid
+ */
+export async function transitionTicketStatus(
+  tenantPrisma: TenantPrismaClient,
+  ticketId: string,
+  newStatus: TicketStatus
+) {
+  const ticket = await tenantPrisma.ticket.findFirst({
+    where: { id: ticketId },
+    select: { id: true, status: true, ticketNumber: true },
+  });
+
+  if (!ticket) {
+    throw new Error(`Ticket not found: ${ticketId}`);
+  }
+
+  const currentStatus = ticket.status as TicketStatus;
+  const allowedTransitions = VALID_TICKET_TRANSITIONS[currentStatus];
+
+  if (!allowedTransitions.includes(newStatus)) {
+    throw new Error(
+      `Invalid status transition: ${currentStatus} -> ${newStatus}. ` +
+      `Allowed: ${allowedTransitions.length > 0 ? allowedTransitions.join(", ") : "none (terminal state)"}`
+    );
+  }
+
+  const data: Record<string, unknown> = { status: newStatus };
+  if (newStatus === TicketStatus.RESOLVED) {
+    data.resolvedAt = new Date();
+  }
+  if (newStatus === TicketStatus.CLOSED) {
+    data.closedAt = new Date();
+  }
+
+  return tenantPrisma.ticket.update({
+    where: { id: ticketId },
+    data,
+    include: {
+      category: true,
+      subscriber: true,
+      createdBy: {
+        select: { id: true, firstName: true, lastName: true, email: true },
+      },
+    },
+  });
+}
+
+// ---------------------------------------------------------------------------
+// resolveTicket (idempotent)
+// ---------------------------------------------------------------------------
+
+/**
+ * Mark a ticket as RESOLVED. Idempotent: if already RESOLVED, returns silently.
+ *
+ * This prevents race conditions when multiple job orders complete simultaneously
+ * and each triggers auto-resolve — only the first transition fires, the rest no-op.
+ *
+ * @throws Error if ticket not found, or transition from current status to RESOLVED is invalid
+ *         (except when already RESOLVED — that is silently ignored)
+ */
+export async function resolveTicket(
+  tenantPrisma: TenantPrismaClient,
+  ticketId: string
+): Promise<void> {
+  const ticket = await tenantPrisma.ticket.findFirst({
+    where: { id: ticketId },
+    select: { id: true, status: true },
+  });
+
+  if (!ticket) {
+    throw new Error(`Ticket not found: ${ticketId}`);
+  }
+
+  // Idempotent: already resolved, nothing to do
+  if (ticket.status === TicketStatus.RESOLVED) {
+    return;
+  }
+
+  await transitionTicketStatus(tenantPrisma, ticketId, TicketStatus.RESOLVED);
+}