diff --git a/src/openai/client.js b/src/openai/client.js
index f207005..cbc0ab0 100644
--- a/src/openai/client.js
+++ b/src/openai/client.js
@@ -1 +1,18 @@
 // OpenAI API client wrapper.
+
+import OpenAI from "openai";
+
+/**
+ * Create an OpenAI SDK client from the loaded config.
+ * @param {object} config - Config object from loadConfig()
+ * @param {string} config.openaiApiKey - Required. The OpenAI API key.
+ * @returns {OpenAI} The configured OpenAI SDK client instance.
+ */
+export function createOpenAIClient(config) {
+  if (!config?.openaiApiKey) {
+    throw new Error("Missing config.openaiApiKey.");
+  }
+  return new OpenAI({
+    apiKey: config.openaiApiKey,
+  });
+}
diff --git a/test/openai/client.test.js b/test/openai/client.test.js
new file mode 100644
index 0000000..cf03bcc
--- /dev/null
+++ b/test/openai/client.test.js
@@ -0,0 +1,39 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const MockClient = vi.fn(function (opts) {
+  this.apiKey = opts?.apiKey;
+});
+
+// Mock the OpenAI SDK — Vitest hoists vi.mock automatically.
+vi.mock("openai", () => ({ default: MockClient }));
+
+const { createOpenAIClient } = await import("../../src/openai/client.js");
+
+describe("createOpenAIClient", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("creates OpenAI client with supplied apiKey", () => {
+    const config = { openaiApiKey: "sk-test-key" };
+    const client = createOpenAIClient(config);
+
+    expect(MockClient).toHaveBeenCalledTimes(1);
+    expect(MockClient).toHaveBeenCalledWith({ apiKey: "sk-test-key" });
+    expect(client.apiKey).toBe("sk-test-key");
+  });
+
+  it("throws when openaiApiKey is missing", () => {
+    const config = {};
+
+    expect(() => createOpenAIClient(config)).toThrow("Missing config.openaiApiKey.");
+    expect(MockClient).not.toHaveBeenCalled();
+  });
+
+  it("throws when openaiApiKey is empty string", () => {
+    const config = { openaiApiKey: "" };
+
+    expect(() => createOpenAIClient(config)).toThrow("Missing config.openaiApiKey.");
+    expect(MockClient).not.toHaveBeenCalled();
+  });
+});