diff --git a/packages/api/src/__tests__/assistant-tools-client-delete-errors.test.ts b/packages/api/src/__tests__/assistant-tools-client-delete-errors.test.ts
new file mode 100644
index 0000000..6e8d684
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-client-delete-errors.test.ts
@@ -0,0 +1,114 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { SystemRole } from "@capakraken/shared";
+
+vi.mock("@capakraken/application", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/application")>();
+  return {
+    ...actual,
+    countPlanningEntries: vi.fn().mockResolvedValue({ countsByRoleId: new Map() }),
+    getDashboardBudgetForecast: vi.fn().mockResolvedValue([]),
+    getDashboardPeakTimes: vi.fn().mockResolvedValue([]),
+    listAssignmentBookings: vi.fn().mockResolvedValue([]),
+  };
+});
+
+import { countPlanningEntries } from "@capakraken/application";
+import { executeTool } from "../router/assistant-tools.js";
+import { createToolContext } from "./assistant-tools-master-data-mutation-test-helpers.js";
+
+describe("assistant client delete tool - errors", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.mocked(countPlanningEntries).mockResolvedValue({ countsByRoleId: new Map() });
+  });
+
+  it("returns a stable error when deleting a missing client", async () => {
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn().mockResolvedValue(null),
+        },
+      },
+      { userRole: SystemRole.ADMIN },
+    );
+
+    const result = await executeTool(
+      "delete_client",
+      JSON.stringify({ id: "client_missing" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error: "Client not found with the given criteria.",
+      }),
+    );
+  });
+
+  it("returns a stable error when deleting a client that still has projects", async () => {
+    const clientRecord = {
+      id: "client_1",
+      name: "Acme Mobility",
+      code: "ACM",
+      parentId: null,
+      isActive: true,
+      sortOrder: 3,
+      tags: ["auto", "priority"],
+      _count: { projects: 2, children: 0 },
+    };
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn().mockResolvedValueOnce(clientRecord).mockResolvedValueOnce(clientRecord),
+        },
+      },
+      { userRole: SystemRole.ADMIN },
+    );
+
+    const result = await executeTool(
+      "delete_client",
+      JSON.stringify({ id: "client_1" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error: "Client cannot be deleted while it still has projects. Deactivate it instead.",
+      }),
+    );
+  });
+
+  it("returns a stable error when deleting a client that still has child clients", async () => {
+    const clientRecord = {
+      id: "client_1",
+      name: "Acme Mobility",
+      code: "ACM",
+      parentId: null,
+      isActive: true,
+      sortOrder: 3,
+      tags: ["auto", "priority"],
+      _count: { projects: 0, children: 2 },
+    };
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn().mockResolvedValueOnce(clientRecord).mockResolvedValueOnce(clientRecord),
+        },
+      },
+      { userRole: SystemRole.ADMIN },
+    );
+
+    const result = await executeTool(
+      "delete_client",
+      JSON.stringify({ id: "client_1" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error:
+          "Client cannot be deleted while it still has child clients. Remove or reassign them first.",
+      }),
+    );
+  });
+});
diff --git a/packages/api/src/__tests__/assistant-tools-client-delete-success.test.ts b/packages/api/src/__tests__/assistant-tools-client-delete-success.test.ts
new file mode 100644
index 0000000..290200a
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-client-delete-success.test.ts
@@ -0,0 +1,75 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { SystemRole } from "@capakraken/shared";
+
+vi.mock("@capakraken/application", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/application")>();
+  return {
+    ...actual,
+    countPlanningEntries: vi.fn().mockResolvedValue({ countsByRoleId: new Map() }),
+    getDashboardBudgetForecast: vi.fn().mockResolvedValue([]),
+    getDashboardPeakTimes: vi.fn().mockResolvedValue([]),
+    listAssignmentBookings: vi.fn().mockResolvedValue([]),
+  };
+});
+
+import { countPlanningEntries } from "@capakraken/application";
+import { executeTool } from "../router/assistant-tools.js";
+import { createToolContext } from "./assistant-tools-master-data-mutation-test-helpers.js";
+
+describe("assistant client delete tool - success", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.mocked(countPlanningEntries).mockResolvedValue({ countsByRoleId: new Map() });
+  });
+
+  it("returns the expected assistant payload for deleting a client", async () => {
+    const db = {
+      client: {
+        findUnique: vi.fn(async ({ where }: { where: { id?: string; code?: string } }) => {
+          if (where.id === "client_1") {
+            return {
+              id: "client_1",
+              name: "Acme Mobility",
+              code: "ACM",
+              parentId: null,
+              sortOrder: 3,
+              tags: ["auto", "priority"],
+              isActive: true,
+              _count: { projects: 0, children: 0 },
+            };
+          }
+          return null;
+        }),
+        delete: vi.fn().mockResolvedValue({
+          id: "client_1",
+          name: "Acme Mobility",
+          code: "ACM",
+          parentId: null,
+          sortOrder: 3,
+          tags: ["auto", "priority"],
+          isActive: true,
+          _count: { projects: 0, children: 0 },
+        }),
+      },
+      auditLog: {
+        create: vi.fn().mockResolvedValue({ id: "audit_1" }),
+      },
+    };
+    const ctx = createToolContext(db, { userRole: SystemRole.ADMIN });
+
+    const deleteClientResult = await executeTool(
+      "delete_client",
+      JSON.stringify({ id: "client_1" }),
+      ctx,
+    );
+
+    expect(JSON.parse(deleteClientResult.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        message: "Deleted client: Acme Mobility",
+      }),
+    );
+    expect(db.client.delete).toHaveBeenCalledWith({ where: { id: "client_1" } });
+    expect(db.auditLog.create).toHaveBeenCalled();
+  });
+});
diff --git a/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-errors.test.ts b/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-errors.test.ts
new file mode 100644
index 0000000..7cc79fa
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-errors.test.ts
@@ -0,0 +1,102 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { SystemRole } from "@capakraken/shared";
+
+vi.mock("@capakraken/application", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/application")>();
+  return {
+    ...actual,
+    countPlanningEntries: vi.fn().mockResolvedValue({ countsByRoleId: new Map() }),
+    getDashboardBudgetForecast: vi.fn().mockResolvedValue([]),
+    getDashboardPeakTimes: vi.fn().mockResolvedValue([]),
+    listAssignmentBookings: vi.fn().mockResolvedValue([]),
+  };
+});
+
+import { countPlanningEntries } from "@capakraken/application";
+import { executeTool } from "../router/assistant-tools.js";
+import { createToolContext } from "./assistant-tools-master-data-mutation-test-helpers.js";
+
+describe("assistant client create and update tools - errors", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.mocked(countPlanningEntries).mockResolvedValue({ countsByRoleId: new Map() });
+  });
+
+  it("returns a stable error when creating a client with a duplicate code", async () => {
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn(async ({
+            where,
+          }: {
+            where: { id?: string; code?: string };
+          }) => {
+            if (where.code === "ACM") {
+              return { id: "client_existing", code: "ACM", name: "Existing Client" };
+            }
+            return null;
+          }),
+        },
+      },
+      { userRole: SystemRole.MANAGER },
+    );
+
+    const result = await executeTool(
+      "create_client",
+      JSON.stringify({ name: "Acme Mobility", code: "ACM" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error: "A client with this code already exists.",
+      }),
+    );
+  });
+
+  it("returns a stable error when creating a client with a missing parent", async () => {
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn().mockResolvedValue(null),
+        },
+      },
+      { userRole: SystemRole.MANAGER },
+    );
+
+    const result = await executeTool(
+      "create_client",
+      JSON.stringify({ name: "Acme Mobility", parentId: "client_missing" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error: "Parent client not found with the given criteria.",
+      }),
+    );
+  });
+
+  it("returns a stable error when updating a missing client", async () => {
+    const ctx = createToolContext(
+      {
+        client: {
+          findUnique: vi.fn().mockResolvedValue(null),
+        },
+      },
+      { userRole: SystemRole.MANAGER },
+    );
+
+    const result = await executeTool(
+      "update_client",
+      JSON.stringify({ id: "client_missing", name: "Acme Mobility" }),
+      ctx,
+    );
+
+    expect(JSON.parse(result.content)).toEqual(
+      expect.objectContaining({
+        error: "Client not found with the given criteria.",
+      }),
+    );
+  });
+});
diff --git a/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-success.test.ts b/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-success.test.ts
new file mode 100644
index 0000000..7411dcd
--- /dev/null
+++ b/packages/api/src/__tests__/assistant-tools-client-mutations-create-update-success.test.ts
@@ -0,0 +1,181 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { SystemRole } from "@capakraken/shared";
+
+vi.mock("@capakraken/application", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/application")>();
+  return {
+    ...actual,
+    countPlanningEntries: vi.fn().mockResolvedValue({ countsByRoleId: new Map() }),
+    getDashboardBudgetForecast: vi.fn().mockResolvedValue([]),
+    getDashboardPeakTimes: vi.fn().mockResolvedValue([]),
+    listAssignmentBookings: vi.fn().mockResolvedValue([]),
+  };
+});
+
+import { countPlanningEntries } from "@capakraken/application";
+import { executeTool } from "../router/assistant-tools.js";
+import { createToolContext } from "./assistant-tools-master-data-mutation-test-helpers.js";
+
+describe("assistant client create and update tools - success", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.mocked(countPlanningEntries).mockResolvedValue({ countsByRoleId: new Map() });
+  });
+
+  it("routes create and update client mutations through their backing router", async () => {
+    const db = {
+      client: {
+        findUnique: vi.fn().mockImplementation(async (args?: { where?: { id?: string; code?: string } }) => {
+          if (args?.where?.id === "client_1") {
+            return {
+              id: "client_1",
+              name: "Acme",
+              code: "ACME",
+              parentId: null,
+              isActive: true,
+              sortOrder: 0,
+              tags: [],
+            };
+          }
+          if (args?.where?.code === "ACME-NEW") {
+            return null;
+          }
+          return null;
+        }),
+        create: vi.fn().mockResolvedValue({
+          id: "client_1",
+          name: "Acme",
+          code: "ACME",
+          parentId: null,
+          isActive: true,
+          sortOrder: 2,
+          tags: ["key"],
+        }),
+        update: vi.fn().mockResolvedValue({
+          id: "client_1",
+          name: "Acme Updated",
+          code: "ACME-NEW",
+          parentId: null,
+          isActive: false,
+          sortOrder: 3,
+          tags: ["vip"],
+        }),
+      },
+      auditLog: {
+        create: vi.fn().mockResolvedValue({ id: "audit_1" }),
+      },
+    };
+    const ctx = createToolContext(db, { userRole: SystemRole.ADMIN });
+
+    const createClientResult = await executeTool(
+      "create_client",
+      JSON.stringify({ name: "Acme", code: "ACME", sortOrder: 2, tags: ["key"] }),
+      ctx,
+    );
+    const updateClientResult = await executeTool(
+      "update_client",
+      JSON.stringify({
+        id: "client_1",
+        name: "Acme Updated",
+        code: "ACME-NEW",
+        sortOrder: 3,
+        isActive: false,
+        tags: ["vip"],
+      }),
+      ctx,
+    );
+
+    expect(JSON.parse(createClientResult.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        clientId: "client_1",
+        message: "Created client: Acme",
+      }),
+    );
+    expect(JSON.parse(updateClientResult.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        clientId: "client_1",
+        message: "Updated client: Acme Updated",
+      }),
+    );
+
+    expect(db.client.create).toHaveBeenCalled();
+    expect(db.client.update).toHaveBeenCalled();
+    expect(db.auditLog.create).toHaveBeenCalled();
+  });
+
+  it("returns the expected assistant payloads for create and update client mutations", async () => {
+    const db = {
+      client: {
+        findUnique: vi.fn(async ({ where }: { where: { id?: string; code?: string } }) => {
+          if (where.id === "client_1") {
+            return {
+              id: "client_1",
+              name: "Acme",
+              code: "AC",
+              sortOrder: 0,
+              isActive: true,
+              parentId: null,
+              tags: [],
+              _count: { projects: 0, children: 0 },
+            };
+          }
+
+          return null;
+        }),
+        create: vi.fn().mockResolvedValue({
+          id: "client_1",
+          name: "Acme",
+          code: "AC",
+          parentId: null,
+          sortOrder: 2,
+          tags: ["auto"],
+        }),
+        update: vi.fn().mockResolvedValue({
+          id: "client_1",
+          name: "Acme Mobility",
+          code: "ACM",
+          parentId: null,
+          sortOrder: 3,
+          tags: ["auto", "priority"],
+          isActive: true,
+        }),
+      },
+      auditLog: {
+        create: vi.fn().mockResolvedValue({ id: "audit_1" }),
+      },
+    };
+    const ctx = createToolContext(db, { userRole: SystemRole.ADMIN });
+
+    const createClientResult = await executeTool(
+      "create_client",
+      JSON.stringify({ name: "Acme", code: "AC", sortOrder: 2, tags: ["auto"] }),
+      ctx,
+    );
+    const updateClientResult = await executeTool(
+      "update_client",
+      JSON.stringify({
+        id: "client_1",
+        name: "Acme Mobility",
+        code: "ACM",
+        sortOrder: 3,
+        tags: ["auto", "priority"],
+      }),
+      ctx,
+    );
+
+    expect(JSON.parse(createClientResult.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        message: "Created client: Acme",
+      }),
+    );
+    expect(JSON.parse(updateClientResult.content)).toEqual(
+      expect.objectContaining({
+        success: true,
+        message: "Updated client: Acme Mobility",
+      }),
+    );
+  });
+});