diff --git a/package.json b/package.json
index 9bbfd91..ea731c2 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@osskit/fetch-enhancers",
-  "version": "4.6.0",
+  "version": "4.7.0",
   "repository": {
     "url": "https://github.com/osskit/fetch-enhancers"
   },
@@ -49,4 +49,4 @@
     "*.{ts,js,json}": "prettier --write"
   },
   "packageManager": "yarn@1.22.22+sha512.a6b2f7906b721bba3d67d4aff083df04dad64c399707841b7acf00f6b133b7ac24255f2652fa22ae3534329dc6180534e98d17432037ff6fd140556e2bb3137e"
-}
+}
\ No newline at end of file
diff --git a/src/enhancers/withHeaders.ts b/src/enhancers/withHeaders.ts
index eb59e94..b388d36 100644
--- a/src/enhancers/withHeaders.ts
+++ b/src/enhancers/withHeaders.ts
@@ -1,12 +1,28 @@
 import type { Fetch } from '../types.js';
 
+const headersToObject = (headers?: HeadersInit): Record<string, string> | undefined => {
+  if (!headers) {
+    return undefined;
+  }
+  if (headers instanceof Headers) {
+    return Object.fromEntries(headers.entries());
+  }
+  if (Array.isArray(headers)) {
+    return Object.fromEntries(headers);
+  }
+  if (typeof headers === 'object') {
+    return { ...headers };
+  }
+  return undefined;
+};
+
 export const withHeaders =
-  (fetch: Fetch, getHeaders: (headers?: HeadersInit) => HeadersInit): Fetch =>
-  (url, init) =>
+  (fetch: Fetch, getHeaders: (headers?: HeadersInit) => HeadersInit | Promise<HeadersInit>): Fetch =>
+  async (url, init) =>
     fetch(url, {
       ...init,
       headers: {
-        ...init?.headers,
-        ...getHeaders(init?.headers),
+        ...headersToObject(init?.headers),
+        ...headersToObject(await getHeaders(init?.headers)),
       },
     });
diff --git a/src/enhancers/withToken.ts b/src/enhancers/withToken.ts
index f35413b..80f490c 100644
--- a/src/enhancers/withToken.ts
+++ b/src/enhancers/withToken.ts
@@ -1,17 +1,8 @@
 import type { Fetch } from '../types.js';
+import { withHeaders } from './withHeaders.js';
 
-export const withToken =
-  (getToken: () => Promise<string> | string, fetch: Fetch): Fetch =>
-  async (url, innerInit) => {
-    const innerFetch = fetch;
-    const token = await getToken();
-
-    return innerFetch(url, {
-      ...innerInit,
-      headers: {
-        ...innerInit?.headers,
-        // eslint-disable-next-line @typescript-eslint/naming-convention
-        Authorization: `Bearer ${token}`,
-      },
-    });
-  };
+export const withToken = (getToken: () => Promise<string> | string, fetch: Fetch): Fetch =>
+  withHeaders(fetch, async () => ({
+    // eslint-disable-next-line @typescript-eslint/naming-convention
+    Authorization: `Bearer ${await getToken()}`,
+  }));
diff --git a/tests/specs/withHeaders.spec.ts b/tests/specs/withHeaders.spec.ts
index 97d2b46..3f01450 100644
--- a/tests/specs/withHeaders.spec.ts
+++ b/tests/specs/withHeaders.spec.ts
@@ -37,7 +37,70 @@ describe('withHeaders', () => {
 
     const { port } = server.address() as AddressInfo;
     const res = await fetchWithExtraHeaders(`http://127.0.0.1:${port}`, { headers: { id: 'id', bar: 'bar' } });
-    await expect(res.text()).resolves.toMatchInlineSnapshot(`"{"id":"override","bar":"bar","foo":"foo"}"`);
+    await expect(res.json()).resolves.toStrictEqual({ id: 'override', bar: 'bar', foo: 'foo' });
+    server.close();
+  });
+
+  it('add headers when getHeaders returns a promise', async () => {
+    const fetchWithAsyncHeaders = withHeaders(fetch, async () => ({ foo: 'foo', id: 'override' }));
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({ id: req.headers.id, bar: req.headers.bar, foo: req.headers.foo });
+      res.writeHead(200);
+      res.end(response);
+    });
+
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const res = await fetchWithAsyncHeaders(`http://127.0.0.1:${port}`, { headers: { id: 'id', bar: 'bar' } });
+    await expect(res.json()).resolves.toStrictEqual({ id: 'override', bar: 'bar', foo: 'foo' });
+    server.close();
+  });
+
+  it('merges headers when init.headers is a Headers instance', async () => {
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({ foo: req.headers.foo, bar: req.headers.bar });
+      res.writeHead(200);
+      res.end(response);
+    });
+
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const headers = new Headers({ bar: 'baz' });
+    const fetchWithHeadersInstance = withHeaders(fetch, () => ({ foo: 'bar' }));
+    const res = await fetchWithHeadersInstance(`http://127.0.0.1:${port}`, { headers });
+    await expect(res.json()).resolves.toStrictEqual({ foo: 'bar', bar: 'baz' });
+    server.close();
+  });
+
+  it('merges headers when init.headers is an object and getHeaders returns a Headers instance', async () => {
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({ foo: req.headers.foo, bar: req.headers.bar });
+      res.writeHead(200);
+      res.end(response);
+    });
+
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const headers = new Headers({ bar: 'baz' });
+    const fetchWithHeadersInstance = withHeaders(fetch, () => new Headers({ foo: 'bar' }));
+    const res = await fetchWithHeadersInstance(`http://127.0.0.1:${port}`, { headers });
+    await expect(res.json()).resolves.toStrictEqual({ foo: 'bar', bar: 'baz' });
+    server.close();
+  });
+
+  it('merges headers when init.headers is a Headers instance and getHeaders returns a Headers instance', async () => {
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({ foo: req.headers.foo, bar: req.headers.bar });
+      res.writeHead(200);
+      res.end(response);
+    });
+
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const headers = new Headers({ bar: 'baz' });
+    const fetchWithHeadersInstance = withHeaders(fetch, () => new Headers({ foo: 'bar' }));
+    const res = await fetchWithHeadersInstance(`http://127.0.0.1:${port}`, { headers });
+    await expect(res.json()).resolves.toStrictEqual({ foo: 'bar', bar: 'baz' });
     server.close();
   });
 });
diff --git a/tests/specs/withToken.spec.ts b/tests/specs/withToken.spec.ts
new file mode 100644
index 0000000..1f4f9a8
--- /dev/null
+++ b/tests/specs/withToken.spec.ts
@@ -0,0 +1,41 @@
+import { createServer } from 'node:http';
+import type { AddressInfo } from 'node:net';
+import { describe, it, expect } from 'vitest';
+import { withToken } from '../../src/index.js';
+import { waitForServer } from '../services/waitForServer.js';
+
+describe('withToken', () => {
+  it('adds token to headers', async () => {
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({
+        authorization: req.headers.authorization,
+      });
+      res.writeHead(200);
+      res.end(response);
+    });
+
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const fetchWithToken = withToken(() => 'mytoken', fetch);
+    const res = await fetchWithToken(`http://127.0.0.1:${port}`);
+    await expect(res.text()).resolves.toBe('{"authorization":"Bearer mytoken"}');
+  });
+
+  it('merges headers when init.headers is a Headers instance', async () => {
+    const server = createServer((req, res) => {
+      const response = JSON.stringify({
+        authorization: req.headers.authorization,
+        foo: req.headers.foo,
+      });
+      res.writeHead(200);
+      res.end(response);
+    });
+    await waitForServer(server);
+    const { port } = server.address() as AddressInfo;
+    const headers = new Headers({ foo: 'bar' });
+    const fetchWithToken = withToken(() => 'mytoken', fetch);
+    const res = await fetchWithToken(`http://127.0.0.1:${port}`, { headers });
+    await expect(res.text()).resolves.toBe('{"authorization":"Bearer mytoken","foo":"bar"}');
+    server.close();
+  });
+});