diff --git a/chartlets.js/packages/lib/src/api/fetchCallback.test.ts b/chartlets.js/packages/lib/src/api/fetchCallback.test.ts
new file mode 100644
index 00000000..b5516a1f
--- /dev/null
+++ b/chartlets.js/packages/lib/src/api/fetchCallback.test.ts
@@ -0,0 +1,88 @@
+import {
+  describe,
+  it,
+  vi,
+  expect,
+  beforeEach,
+  afterEach,
+  type Mock,
+} from "vitest";
+import { fetchCallback } from "./fetchCallback";
+
+describe("fetchCallback", () => {
+  beforeEach(() => {
+    // Mock the global fetch function
+    globalThis.fetch = vi.fn();
+  });
+
+  afterEach(() => {
+    // Restore the original fetch implementation
+    vi.restoreAllMocks();
+  });
+
+  it("should return expected data on success", async () => {
+    // Define a mock response
+    const expectedResult = {
+      contribPoint: "panels",
+      contribIndex: 0,
+      stateChanges: [
+        {
+          id: "checkbox0",
+          property: "value",
+          value: true,
+        },
+      ],
+    };
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({
+        result: expectedResult,
+      }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchCallback([], {
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "ok",
+      data: expectedResult,
+    });
+  });
+
+  it("should return error for bad API responses", async () => {
+    // Define a mock response
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({ message: "Hello, world!" }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchCallback([], {
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "failed",
+      error: {
+        message: "unexpected response from https://chartlets-test/api/callback",
+      },
+    });
+  });
+});
diff --git a/chartlets.js/packages/lib/src/api/fetchContributions.test.ts b/chartlets.js/packages/lib/src/api/fetchContributions.test.ts
new file mode 100644
index 00000000..ea95b046
--- /dev/null
+++ b/chartlets.js/packages/lib/src/api/fetchContributions.test.ts
@@ -0,0 +1,96 @@
+import {
+  describe,
+  it,
+  vi,
+  expect,
+  beforeEach,
+  afterEach,
+  type Mock,
+} from "vitest";
+import { fetchContributions } from "./fetchContributions";
+
+describe("fetchContributions", () => {
+  beforeEach(() => {
+    // Mock the global fetch function
+    globalThis.fetch = vi.fn();
+  });
+
+  afterEach(() => {
+    // Restore the original fetch implementation
+    vi.restoreAllMocks();
+  });
+
+  it("should return expected data on success", async () => {
+    // Define a mock response
+    const expectedResult = {
+      extensions: [{ name: "e0", version: "1", contributes: ["panels"] }],
+      contributions: {
+        panels: [{ name: "p0", extension: "e0", layout: {} }],
+      },
+    };
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({
+        result: expectedResult,
+      }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchContributions({
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "ok",
+      data: {
+        extensions: [{ name: "e0", version: "1", contributes: ["panels"] }],
+        contributions: {
+          panels: [
+            {
+              name: "p0",
+              extension: "e0",
+              layout: { inputs: [], outputs: [] },
+              callbacks: [],
+            },
+          ],
+        },
+      },
+    });
+  });
+
+  it("should return error for bad responses", async () => {
+    // Define a mock response
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({ message: "Hello, world!" }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchContributions({
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "failed",
+      error: {
+        message:
+          "unexpected response from https://chartlets-test/api/contributions",
+      },
+    });
+  });
+});
diff --git a/chartlets.js/packages/lib/src/api/fetchLayout.test.ts b/chartlets.js/packages/lib/src/api/fetchLayout.test.ts
new file mode 100644
index 00000000..f8a26531
--- /dev/null
+++ b/chartlets.js/packages/lib/src/api/fetchLayout.test.ts
@@ -0,0 +1,82 @@
+import {
+  describe,
+  it,
+  vi,
+  expect,
+  beforeEach,
+  afterEach,
+  type Mock,
+} from "vitest";
+import { fetchLayout } from "./fetchLayout";
+
+describe("fetchLayout", () => {
+  beforeEach(() => {
+    // Mock the global fetch function
+    globalThis.fetch = vi.fn();
+  });
+
+  afterEach(() => {
+    // Restore the original fetch implementation
+    vi.restoreAllMocks();
+  });
+
+  it("should return expected data on success", async () => {
+    // Define a mock response
+    const expectedResult = {
+      type: "Button",
+      text: "Click me!",
+    };
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({
+        result: expectedResult,
+      }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchLayout("panels", 0, [], {
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "ok",
+      data: expectedResult,
+    });
+  });
+
+  it("should return error for bad responses", async () => {
+    // Define a mock response
+    const mockResponse = {
+      ok: true,
+      status: 200,
+      statusText: "ok",
+      json: vi.fn().mockResolvedValue({ message: "Hello, world!" }),
+    };
+
+    // Mock fetch to resolve with the mock response
+    (globalThis.fetch as Mock).mockResolvedValue(mockResponse);
+
+    // Call fetch
+    const response = await fetchLayout("panels", 0, [], {
+      serverUrl: "https://chartlets-test",
+      endpointName: "api",
+    });
+    // Assertions
+    expect(fetch).toHaveBeenCalledOnce();
+    expect(response).toEqual({
+      status: "failed",
+      error: {
+        message:
+          "unexpected response from https://chartlets-test/api/layout/panels/0",
+      },
+    });
+  });
+});