fix(fetch-adapter): set User-Agent header to match http adapter (#10772)

The fetch adapter was not setting a User-Agent header, causing requests
to use Node.js's default 'node' user agent instead of 'axios/<version>'.
This mirrors the existing behavior in the HTTP adapter.

Fixes #7494

Co-authored-by: Ofek Danny <ofek@kaps.co.il>
Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/adapters/fetch.js

@@ -12,6 +12,7 @@ import {
 import resolveConfig from '../helpers/resolveConfig.js';
 import settle from '../core/settle.js';
 import estimateDataURLDecodedBytes from '../helpers/estimateDataURLDecodedBytes.js';
+import { VERSION } from '../env/data.js';
 
 const DEFAULT_CHUNK_SIZE = 64 * 1024;
 
@@ -277,6 +278,9 @@ const factory = (env) => {
         }
       }
 
+      // Set User-Agent header if not already set (fetch defaults to 'node' in Node.js)
+      headers.set('User-Agent', 'axios/' + VERSION, false);
+
       const resolvedOptions = {
         ...fetchOptions,
         signal: composedSignal,

tests/unit/adapters/fetch.test.js

@@ -14,6 +14,7 @@ import stream from 'stream';
 import { AbortController } from 'abortcontroller-polyfill/dist/cjs-ponyfill.js';
 import util from 'util';
 import NodeFormData from 'form-data';
+import { VERSION } from '../../../lib/env/data.js';
 
 const SERVER_PORT = 8010;
 const LOCAL_SERVER_URL = `http://localhost:${SERVER_PORT}`;
@@ -619,6 +620,52 @@ describe.runIf(typeof fetch === 'function')('supports fetch with nodejs', () =>
     });
   });
 
+  describe('fetch adapter - User-Agent header', () => {
+    it('should set User-Agent header to axios/<version> by default', async () => {
+      const server = await startHTTPServer(
+        (req, res) => {
+          res.setHeader('Content-Type', 'application/json');
+          res.end(JSON.stringify({ userAgent: req.headers['user-agent'] }));
+        },
+        { port: SERVER_PORT }
+      );
+
+      try {
+        const { data } = await fetchAxios.post(`http://localhost:${server.address().port}/`, {
+          payload: 'test',
+        });
+
+        assert.strictEqual(data.userAgent, `axios/${VERSION}`);
+      } finally {
+        await stopHTTPServer(server);
+      }
+    });
+
+    it('should not override a user-provided User-Agent header', async () => {
+      const customUA = 'my-custom-agent/1.0';
+
+      const server = await startHTTPServer(
+        (req, res) => {
+          res.setHeader('Content-Type', 'application/json');
+          res.end(JSON.stringify({ userAgent: req.headers['user-agent'] }));
+        },
+        { port: SERVER_PORT }
+      );
+
+      try {
+        const { data } = await fetchAxios.post(
+          `http://localhost:${server.address().port}/`,
+          { payload: 'test' },
+          { headers: { 'User-Agent': customUA } }
+        );
+
+        assert.strictEqual(data.userAgent, customUA);
+      } finally {
+        await stopHTTPServer(server);
+      }
+    });
+  });
+
   describe('env config', () => {
     it('should respect env fetch API configuration', async () => {
       const { data, headers } = await fetchAxios.get('/', {