diff --git a/extensions/openai/base-url.test.ts b/extensions/openai/base-url.test.ts
new file mode 100644
index 00000000000..e430a47b63a
--- /dev/null
+++ b/extensions/openai/base-url.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+import { isOpenAIApiBaseUrl, isOpenAICodexBaseUrl } from "./base-url.js";
+
+describe("openai base URL helpers", () => {
+  it("recognizes direct OpenAI API routes", () => {
+    expect(isOpenAIApiBaseUrl("https://api.openai.com")).toBe(true);
+    expect(isOpenAIApiBaseUrl("https://api.openai.com/v1")).toBe(true);
+    expect(isOpenAIApiBaseUrl("https://api.openai.com/v1/")).toBe(true);
+  });
+
+  it("rejects proxy or unrelated API routes", () => {
+    expect(isOpenAIApiBaseUrl("https://proxy.example.com/v1")).toBe(false);
+    expect(isOpenAIApiBaseUrl("https://chatgpt.com/backend-api")).toBe(false);
+    expect(isOpenAIApiBaseUrl(undefined)).toBe(false);
+  });
+
+  it("recognizes Codex ChatGPT backend routes", () => {
+    expect(isOpenAICodexBaseUrl("https://chatgpt.com/backend-api")).toBe(true);
+    expect(isOpenAICodexBaseUrl("https://chatgpt.com/backend-api/")).toBe(true);
+  });
+
+  it("rejects non-Codex backend routes", () => {
+    expect(isOpenAICodexBaseUrl("https://api.openai.com/v1")).toBe(false);
+    expect(isOpenAICodexBaseUrl("https://chatgpt.com")).toBe(false);
+    expect(isOpenAICodexBaseUrl(undefined)).toBe(false);
+  });
+});