Fix baseURL double-prefix when request path starts with /

src/instance.ts

@@ -23,7 +23,8 @@ export function createInstance(defaults?: Partial<Config>): Instance {
       ...merged,
       url: fullUrl,
     });
-    return request(instanceConfig.url!, instanceConfig, instanceChain);
+    const { baseURL: _baseURL, url: _url, ...resolvedInstanceConfig } = instanceConfig;
+    return request(instanceConfig.url!, resolvedInstanceConfig, instanceChain);
   };
 
   const methodFn = (method: HttpMethod): RequestFn => {

test/axios/compatibility.test.ts

@@ -15,6 +15,10 @@ const server = setupServer(
     await new Promise((resolve) => setTimeout(resolve, 1000));
     return HttpResponse.json([{ id: 1 }]);
   }),
+  http.post('https://api.example.com/api/auth/login', async ({ request: req }) => {
+    const body = await req.json();
+    return HttpResponse.json({ token: 'abc', ...body });
+  }),
 );
 
 beforeAll(() => server.listen());
@@ -74,4 +78,10 @@ describe('axios compatibility', () => {
     ]);
     expect(results).toHaveLength(2);
   });
+
+  it('axios.create() with baseURL containing a path prepends full baseURL when path starts with /', async () => {
+    const api = axios.create({ baseURL: 'https://api.example.com/api' });
+    const res = await api.post('/auth/login', { email: 'test@test.com', password: 'test' });
+    expect(res.data).toMatchObject({ token: 'abc', email: 'test@test.com' });
+  });
 });

test/unit/instance-baseurl.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { createInstance } from '../../src/instance';
+
+describe('baseURL + leading slash path (relative baseURL)', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('does not double-prefix baseURL when path starts with /', async () => {
+    const capturedUrls: string[] = [];
+
+    // Spy on global fetch to capture the URL actually used
+    vi.stubGlobal('fetch', vi.fn(async (url: string) => {
+      capturedUrls.push(url);
+      return new Response(JSON.stringify({ ok: true }), {
+        status: 200,
+        headers: { 'Content-Type': 'application/json' },
+      });
+    }));
+
+    const instance = createInstance({ baseURL: '/api' });
+    await instance.post('/auth/login', { body: { email: 'test@test.com' } });
+
+    expect(capturedUrls).toHaveLength(1);
+    // Should be /api/auth/login, NOT /api/api/auth/login
+    expect(capturedUrls[0]).toBe('/api/auth/login');
+  });
+
+  it('does not double-prefix baseURL when path does not start with /', async () => {
+    const capturedUrls: string[] = [];
+
+    vi.stubGlobal('fetch', vi.fn(async (url: string) => {
+      capturedUrls.push(url);
+      return new Response(JSON.stringify({ ok: true }), {
+        status: 200,
+        headers: { 'Content-Type': 'application/json' },
+      });
+    }));
+
+    const instance = createInstance({ baseURL: '/api' });
+    await instance.get('users');
+
+    expect(capturedUrls).toHaveLength(1);
+    expect(capturedUrls[0]).toBe('/api/users');
+  });
+});