diff --git a/package-lock.json b/package-lock.json
index 64e4a6d4e0..430c5a09ea 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -19,7 +19,6 @@
         "es6-error": "4.1.1",
         "firefox-profile": "4.7.1",
         "fx-runner": "1.7.0",
-        "https-proxy-agent": "^7.0.0",
         "jose": "5.9.6",
         "jszip": "3.10.2",
         "multimatch": "6.0.0",
@@ -31,6 +30,7 @@
         "strip-bom": "5.0.0",
         "strip-json-comments": "5.0.3",
         "tmp": "0.2.7",
+        "undici": "^6.29.0",
         "update-notifier": "7.3.1",
         "watchpack": "2.5.2",
         "yargs": "17.7.3",
@@ -2815,15 +2815,6 @@
         "node": ">=14.0"
       }
     },
-    "node_modules/agent-base": {
-      "version": "7.1.4",
-      "resolved": "https://registry.npmjs.org/agent-base/-/agent-base-7.1.4.tgz",
-      "integrity": "sha512-MnA+YT8fwfJPgBx3m60MNqakm30XOkyIoH1y6huTQvC0PwZG7ki8NacLBcrPbNoo8vEZy7Jpuk7+jMO+CUovTQ==",
-      "license": "MIT",
-      "engines": {
-        "node": ">= 14"
-      }
-    },
     "node_modules/aggregate-error": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/aggregate-error/-/aggregate-error-3.1.0.tgz",
@@ -3564,6 +3555,15 @@
         "url": "https://github.com/sponsors/fb55"
       }
     },
+    "node_modules/cheerio/node_modules/undici": {
+      "version": "7.30.0",
+      "resolved": "https://registry.npmjs.org/undici/-/undici-7.30.0.tgz",
+      "integrity": "sha512-dkrQXeHSaoamnItlYbmzG0wFYrM0ZwDxCIg0A7aKjTyyhh9svRzCNFEzV+Vm05/yehjCzjDZ31KXfGEjYSztDQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.18.1"
+      }
+    },
     "node_modules/chokidar": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-5.0.0.tgz",
@@ -5738,19 +5738,6 @@
         "url": "https://github.com/fb55/entities?sponsor=1"
       }
     },
-    "node_modules/https-proxy-agent": {
-      "version": "7.0.6",
-      "resolved": "https://registry.npmjs.org/https-proxy-agent/-/https-proxy-agent-7.0.6.tgz",
-      "integrity": "sha512-vK9P5/iUfdl95AI+JVyUuIcVtd4ofvtrOr3HNtM2yxC9bnMbEdp3x01OhQNnjb8IJYi38VlTE3mBXwcfvywuSw==",
-      "license": "MIT",
-      "dependencies": {
-        "agent-base": "^7.1.2",
-        "debug": "4"
-      },
-      "engines": {
-        "node": ">= 14"
-      }
-    },
     "node_modules/iconv-lite": {
       "version": "0.6.3",
       "resolved": "https://registry.npmjs.org/iconv-lite/-/iconv-lite-0.6.3.tgz",
@@ -10214,12 +10201,12 @@
       }
     },
     "node_modules/undici": {
-      "version": "7.30.0",
-      "resolved": "https://registry.npmjs.org/undici/-/undici-7.30.0.tgz",
-      "integrity": "sha512-dkrQXeHSaoamnItlYbmzG0wFYrM0ZwDxCIg0A7aKjTyyhh9svRzCNFEzV+Vm05/yehjCzjDZ31KXfGEjYSztDQ==",
+      "version": "6.29.0",
+      "resolved": "https://registry.npmjs.org/undici/-/undici-6.29.0.tgz",
+      "integrity": "sha512-R+RODBqp6i2pPflGdq+xIOUkl+RNfGgHwoinecKu/JCuf2uO06cOKoDbI2P7Dn6KcswdKwrczbU6IYJ6K8X+wg==",
       "license": "MIT",
       "engines": {
-        "node": ">=20.18.1"
+        "node": ">=18.17"
       }
     },
     "node_modules/undici-types": {
diff --git a/package.json b/package.json
index cd5225f093..14bec77c7c 100644
--- a/package.json
+++ b/package.json
@@ -66,7 +66,6 @@
     "es6-error": "4.1.1",
     "firefox-profile": "4.7.1",
     "fx-runner": "1.7.0",
-    "https-proxy-agent": "^7.0.0",
     "jose": "5.9.6",
     "jszip": "3.10.2",
     "multimatch": "6.0.0",
@@ -78,6 +77,7 @@
     "strip-bom": "5.0.0",
     "strip-json-comments": "5.0.3",
     "tmp": "0.2.7",
+    "undici": "^6.29.0",
     "update-notifier": "7.3.1",
     "watchpack": "2.5.2",
     "yargs": "17.7.3",
diff --git a/src/util/submit-addon.js b/src/util/submit-addon.js
index 32b01f98d2..e8e0a523d5 100644
--- a/src/util/submit-addon.js
+++ b/src/util/submit-addon.js
@@ -5,7 +5,7 @@ import { promises as streamPromises } from 'stream';
 
 import { SignJWT } from 'jose';
 import JSZip from 'jszip';
-import { HttpsProxyAgent } from 'https-proxy-agent';
+import { ProxyAgent } from 'undici';
 
 import { isErrorWithCode } from '../errors.js';
 import { createLogger } from './../util/logger.js';
@@ -110,8 +110,18 @@ export default class Client {
     return new File([fileData], basename(filePath));
   }
 
-  nodeFetch(url, { method, headers, body, agent }) {
-    return fetch(url, { method, headers, body, agent });
+  async nodeFetch(url, { method, headers, body, dispatcher }) {
+    try {
+      return await fetch(url, { method, headers, body, dispatcher });
+    } finally {
+      // Close gracefully after the response body has been consumed. Awaiting
+      // close here would prevent the caller from receiving and reading it.
+      if (dispatcher) {
+        dispatcher.close().catch((error) => {
+          log.debug(`Closing proxy dispatcher failed: ${error}`);
+        });
+      }
+    }
   }
 
   async doUploadSubmit(xpiPath, channel) {
@@ -339,11 +349,11 @@ export default class Client {
         'Content-Type': 'application/json',
       };
     }
-    let agent;
+    let dispatcher;
     if (this.apiProxy) {
-      agent = new HttpsProxyAgent(this.apiProxy);
+      dispatcher = new ProxyAgent(this.apiProxy);
     }
-    return this.nodeFetch(url, { method, body, headers, agent });
+    return this.nodeFetch(url, { method, body, headers, dispatcher });
   }
 
   returnResult(addonId, downloadedFiles) {
diff --git a/tests/unit/test-util/test.submit-addon-proxy.js b/tests/unit/test-util/test.submit-addon-proxy.js
new file mode 100644
index 0000000000..72f0f0c88a
--- /dev/null
+++ b/tests/unit/test-util/test.submit-addon-proxy.js
@@ -0,0 +1,177 @@
+import { once } from 'node:events';
+import { createServer } from 'node:http';
+import { connect } from 'node:net';
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
+
+import { assert } from 'chai';
+import { afterEach, beforeEach, describe, it } from 'mocha';
+
+import Client from '../../../src/util/submit-addon.js';
+import { withTempDir } from '../../../src/util/temp-dir.js';
+
+describe('util.submit-addon proxy transport', () => {
+  let origin;
+  let proxy;
+  let baseUrl;
+  let apiProxy;
+  let requests;
+  let tunnels;
+  let sockets;
+  let rejectProxy;
+
+  beforeEach(async () => {
+    requests = [];
+    tunnels = [];
+    sockets = new Set();
+    rejectProxy = false;
+    origin = createServer(async (request, response) => {
+      const chunks = [];
+      for await (const chunk of request) {
+        chunks.push(chunk);
+      }
+      requests.push({
+        method: request.method,
+        url: request.url,
+        headers: request.headers,
+        body: Buffer.concat(chunks).toString(),
+      });
+      if (request.url === '/signed.xpi') {
+        response.write(Buffer.from([0x50, 0x4b]));
+        setTimeout(() => response.end(Buffer.from([0, 0xff])), 20);
+      } else {
+        response.setHeader('Content-Type', 'application/json');
+        response.end(JSON.stringify({ received: true }));
+      }
+    });
+    origin.listen(0, '127.0.0.1');
+    await once(origin, 'listening');
+    baseUrl = new URL(`http://127.0.0.1:${origin.address().port}/`);
+
+    proxy = createServer();
+    proxy.on('connect', (request, socket, head) => {
+      tunnels.push({ url: request.url, headers: request.headers });
+      sockets.add(socket);
+      if (rejectProxy) {
+        socket.end('HTTP/1.1 502 Bad Gateway\r\n\r\n');
+        return;
+      }
+      const upstream = connect(origin.address().port, '127.0.0.1');
+      sockets.add(upstream);
+      upstream.on('error', () => socket.destroy());
+      socket.on('error', () => upstream.destroy());
+      upstream.on('connect', () => {
+        socket.write('HTTP/1.1 200 Connection Established\r\n\r\n');
+        upstream.write(head);
+        socket.pipe(upstream).pipe(socket);
+      });
+    });
+    proxy.listen(0, '127.0.0.1');
+    await once(proxy, 'listening');
+    apiProxy = `http://127.0.0.1:${proxy.address().port}`;
+  });
+
+  afterEach(async () => {
+    for (const socket of sockets) {
+      socket.destroy();
+    }
+    origin.closeAllConnections();
+    proxy.closeAllConnections();
+    await Promise.all([
+      new Promise((resolve) => origin.close(resolve)),
+      new Promise((resolve) => proxy.close(resolve)),
+    ]);
+  });
+
+  const makeClient = (options = {}) =>
+    new Client({
+      baseUrl,
+      apiProxy,
+      apiAuth: { getAuthHeader: async () => 'JWT test-token' },
+      userAgentString: 'web-ext-proxy-test',
+      ...options,
+    });
+
+  const checkProxy = () => {
+    assert.lengthOf(tunnels, 1);
+    assert.equal(tunnels[0].url, baseUrl.host);
+    assert.lengthOf(requests, 1);
+    assert.equal(requests[0].headers.authorization, 'JWT test-token');
+    assert.equal(requests[0].headers['user-agent'], 'web-ext-proxy-test');
+  };
+
+  it('routes JSON polling requests through the configured proxy', async () => {
+    assert.deepEqual(await makeClient().fetchJson(baseUrl), { received: true });
+    checkProxy();
+    assert.equal(requests[0].method, 'GET');
+  });
+
+  it('preserves JSON version submissions through the proxy', async () => {
+    const body = JSON.stringify({ version: { upload: 'test-upload' } });
+    await makeClient().fetchJson(baseUrl, 'PUT', body);
+    checkProxy();
+    assert.equal(requests[0].method, 'PUT');
+    assert.equal(requests[0].body, body);
+    assert.equal(requests[0].headers['content-type'], 'application/json');
+  });
+
+  for (const method of ['POST', 'PATCH']) {
+    it(`preserves multipart ${method} uploads through the proxy`, async () => {
+      const body = new FormData();
+      body.set('channel', 'unlisted');
+      body.set('upload', new Blob(['extension-bytes']), 'extension.xpi');
+      await makeClient().fetchJson(baseUrl, method, body);
+      checkProxy();
+      assert.equal(requests[0].method, method);
+      assert.match(
+        requests[0].headers['content-type'],
+        /^multipart\/form-data; boundary=/,
+      );
+      assert.include(requests[0].body, 'filename="extension.xpi"');
+      assert.include(requests[0].body, 'extension-bytes');
+      assert.include(requests[0].body, 'unlisted');
+    });
+  }
+
+  it('streams signed file downloads through the proxy', () =>
+    withTempDir(async (tmpDir) => {
+      const result = await makeClient({
+        downloadDir: tmpDir.path(),
+      }).downloadSignedFile(new URL('signed.xpi', baseUrl), 'test-addon');
+      checkProxy();
+      assert.deepEqual(result.downloadedFiles, ['signed.xpi']);
+      assert.deepEqual(
+        readFileSync(path.join(tmpDir.path(), 'signed.xpi')),
+        Buffer.from([0x50, 0x4b, 0, 0xff]),
+      );
+    }));
+
+  it('supports URL-encoded proxy credentials', async () => {
+    const proxyUrl = new URL(apiProxy);
+    proxyUrl.username = 'user';
+    proxyUrl.password = 'p%40ss';
+    await makeClient({ apiProxy: proxyUrl.href }).fetchJson(baseUrl);
+    checkProxy();
+    assert.equal(
+      tunnels[0].headers['proxy-authorization'],
+      `Basic ${Buffer.from('user:p@ss').toString('base64')}`,
+    );
+    assert.notProperty(requests[0].headers, 'proxy-authorization');
+  });
+
+  it('does not bypass a proxy that rejects the connection', async () => {
+    rejectProxy = true;
+    await assert.isRejected(makeClient().fetchJson(baseUrl), 'fetch failed');
+    assert.lengthOf(tunnels, 1);
+    assert.lengthOf(requests, 0);
+  });
+
+  it('keeps direct requests working when no proxy is configured', async () => {
+    assert.deepEqual(
+      await makeClient({ apiProxy: undefined }).fetchJson(baseUrl),
+      { received: true },
+    );
+    assert.lengthOf(tunnels, 0);
+    assert.lengthOf(requests, 1);
+  });
+});
diff --git a/tests/unit/test-util/test.submit-addon.js b/tests/unit/test-util/test.submit-addon.js
index 0d4e5d5fea..9f8d2af789 100644
--- a/tests/unit/test-util/test.submit-addon.js
+++ b/tests/unit/test-util/test.submit-addon.js
@@ -11,6 +11,7 @@ import { File } from 'node:buffer';
 import CRC32 from 'crc-32';
 import { assert, expect } from 'chai';
 import JSZip from 'jszip';
+import { ProxyAgent } from 'undici';
 import { afterEach, before, beforeEach, describe, it } from 'mocha';
 import * as sinon from 'sinon';
 
@@ -1364,10 +1365,9 @@ describe('util.submit-addon', () => {
 
         await client.fetch(baseUrl, 'POST');
 
-        assert.equal(
-          nodeFetchStub.firstCall.args[1].agent.proxy.host,
-          apiProxyHost,
-        );
+        const { dispatcher } = nodeFetchStub.firstCall.args[1];
+        assert.instanceOf(dispatcher, ProxyAgent);
+        await dispatcher.close();
         sinon.assert.calledOnce(nodeFetchStub);
       });
     });