diff --git a/lib/octokit.js b/lib/octokit.js
index 264eaa2b..8e0823e3 100644
--- a/lib/octokit.js
+++ b/lib/octokit.js
@@ -14,7 +14,7 @@ import { throttling } from "@octokit/plugin-throttling";
 import urljoin from "url-join";
 import { HttpProxyAgent } from "http-proxy-agent";
 import { HttpsProxyAgent } from "https-proxy-agent";
-import { ProxyAgent, fetch as undiciFetch } from "undici";
+import { Agent, ProxyAgent, fetch as undiciFetch } from "undici";
 
 import { RETRY_CONF } from "./definitions/retry.js";
 import { THROTTLE_CONF } from "./definitions/throttle.js";
@@ -75,7 +75,7 @@ export function toOctokitOptions(options) {
       ...opts,
       dispatcher: options.proxy
         ? new ProxyAgent({ uri: options.proxy })
-        : undefined,
+        : new Agent(),
     });
 
   return {
diff --git a/lib/publish.js b/lib/publish.js
index 9a5b623c..2ee3b908 100644
--- a/lib/publish.js
+++ b/lib/publish.js
@@ -138,7 +138,6 @@ export default async function publish(pluginConfig, context, { Octokit }) {
         name: fileName,
         headers: {
           "content-type": mime.getType(extname(fileName)) || "text/plain",
-          "content-length": file.size,
         },
       };
 
diff --git a/test/publish.test.js b/test/publish.test.js
index 86bb6b7c..35942043 100644
--- a/test/publish.test.js
+++ b/test/publish.test.js
@@ -494,6 +494,101 @@ test("Publish a release with one asset", async (t) => {
   t.true(fetch.done());
 });
 
+test("Publish upload request does not set content-length manually", async (t) => {
+  const owner = "test_user";
+  const repo = "test_repo";
+  const env = { GITHUB_TOKEN: "github_token" };
+  const pluginConfig = {
+    assets: [".dotfile"],
+  };
+  const nextRelease = {
+    gitTag: "v1.0.0",
+    name: "v1.0.0",
+    notes: "Test release note body",
+  };
+  const options = { repositoryUrl: `https://github.com/${owner}/${repo}.git` };
+  const untaggedReleaseUrl = `https://github.com/${owner}/${repo}/releases/untagged-123`;
+  const releaseUrl = `https://github.com/${owner}/${repo}/releases/${nextRelease.version}`;
+  const assetUrl = `https://github.com/${owner}/${repo}/releases/download/${nextRelease.version}/.dotfile`;
+  const releaseId = 1;
+  const uploadOrigin = "https://uploads.github.local";
+  const uploadUri = `/api/uploads/repos/${owner}/${repo}/releases/${releaseId}/assets`;
+  const uploadUrl = `${uploadOrigin}${uploadUri}{?name,label}`;
+  const branch = "test_branch";
+  const assetUploadUrl = `${uploadOrigin}${uploadUri}?name=${encodeURIComponent(".dotfile")}&`;
+
+  /** @type {Record<string, string>} */
+  let uploadHeaders;
+
+  const fetch = async (url, request = {}) => {
+    if (
+      url === `https://api.github.local/repos/${owner}/${repo}/releases` &&
+      request.method === "POST"
+    ) {
+      return new Response(
+        JSON.stringify({
+          upload_url: uploadUrl,
+          html_url: untaggedReleaseUrl,
+          id: releaseId,
+        }),
+        { headers: { "content-type": "application/json" } },
+      );
+    }
+
+    if (
+      url ===
+        `https://api.github.local/repos/${owner}/${repo}/releases/${releaseId}` &&
+      request.method === "PATCH"
+    ) {
+      return new Response(JSON.stringify({ html_url: releaseUrl }), {
+        headers: { "content-type": "application/json" },
+      });
+    }
+
+    if (url === assetUploadUrl && request.method === "POST") {
+      uploadHeaders = request.headers;
+      return new Response(JSON.stringify({ browser_download_url: assetUrl }), {
+        headers: { "content-type": "application/json" },
+      });
+    }
+
+    throw new Error(`Unexpected request ${request.method} ${url}`);
+  };
+
+  const result = await publish(
+    pluginConfig,
+    {
+      cwd,
+      env,
+      options,
+      branch: { name: branch, type: "release", main: true },
+      nextRelease,
+      logger: t.context.logger,
+    },
+    {
+      Octokit: TestOctokit.defaults((options) => ({
+        ...options,
+        request: { ...options.request, fetch },
+      })),
+    },
+  );
+
+  t.is(result.url, releaseUrl);
+  t.true(t.context.log.calledWith("Published GitHub release: %s", releaseUrl));
+  t.true(t.context.log.calledWith("Published file %s", assetUrl));
+  t.truthy(uploadHeaders);
+
+  const normalizedHeaders = Object.fromEntries(
+    Object.entries(uploadHeaders).map(([name, value]) => [
+      name.toLowerCase(),
+      String(value),
+    ]),
+  );
+
+  t.is(normalizedHeaders["content-type"], "text/plain");
+  t.falsy(normalizedHeaders["content-length"]);
+});
+
 test("Publish a release with one asset and custom github url", async (t) => {
   const owner = "test_user";
   const repo = "test_repo";
diff --git a/test/to-octokit-options.test.js b/test/to-octokit-options.test.js
index 57b1dc07..f84c68b3 100644
--- a/test/to-octokit-options.test.js
+++ b/test/to-octokit-options.test.js
@@ -1,9 +1,8 @@
-import { createServer as _createServer } from "node:https";
+import { createServer } from "node:http";
 
 import test from "ava";
 import { HttpProxyAgent } from "http-proxy-agent";
 import { HttpsProxyAgent } from "https-proxy-agent";
-import { ProxyAgent } from "undici";
 
 import { toOctokitOptions } from "../lib/octokit.js";
 
@@ -157,3 +156,93 @@ test("only fetch is provided when no proxy is set", async (t) => {
     auth: "github_token",
   });
 });
+
+test("fetch without proxy stays on direct undici dispatcher", async (t) => {
+  const server = createServer((req, res) => {
+    req.resume();
+    req.on("end", () => {
+      res.writeHead(200, { "Content-Type": "text/plain" });
+      res.end("ok");
+    });
+  });
+
+  await new Promise((resolve) => {
+    server.listen(0, "127.0.0.1", resolve);
+  });
+
+  const { port } = server.address();
+  const url = `http://127.0.0.1:${port}/upload`;
+
+  try {
+    // Initialize Node's built-in fetch first to mimic real runtime usage.
+    await fetch(url);
+
+    const options = toOctokitOptions({
+      githubToken: "github_token",
+      githubApiUrl: `http://127.0.0.1:${port}`,
+    });
+
+    // throws because the content-length does not match the body length
+    const error = await t.throwsAsync(
+      options.request.fetch(url, {
+        method: "POST",
+        headers: { "content-length": "10" },
+        body: "abc",
+      }),
+    );
+
+    const causeStack = error?.cause?.stack || "";
+    t.false(causeStack.includes("node:internal/deps/undici/undici"));
+    t.true(causeStack.includes("/node_modules/undici/"));
+  } finally {
+    await new Promise((resolve) => {
+      server.close(resolve);
+    });
+  }
+});
+
+test("fetch function sets content-length for Buffer body", async (t) => {
+  let requestContentLength;
+  let receivedBodyLength = 0;
+  const body = Buffer.from("upload-body");
+
+  const server = createServer((req, res) => {
+    requestContentLength = req.headers["content-length"];
+    req.on("data", (chunk) => {
+      receivedBodyLength += chunk.length;
+    });
+    req.on("end", () => {
+      res.writeHead(200, { "Content-Type": "application/json" });
+      res.end(JSON.stringify({ ok: true }));
+    });
+  });
+
+  await new Promise((resolve) => {
+    server.listen(0, "127.0.0.1", resolve);
+  });
+
+  const { port } = server.address();
+  const url = `http://127.0.0.1:${port}/upload`;
+
+  try {
+    const options = toOctokitOptions({
+      githubToken: "github_token",
+      githubApiUrl: `http://127.0.0.1:${port}`,
+    });
+
+    const response = await options.request.fetch(url, {
+      method: "POST",
+      headers: { "content-type": "application/octet-stream" },
+      body,
+    });
+
+    t.is(response.status, 200);
+    t.deepEqual(await response.json(), { ok: true });
+    t.is(requestContentLength, `${body.byteLength}`);
+    t.is(receivedBodyLength, body.byteLength);
+  } finally {
+    await new Promise((resolve) => {
+      server.close(resolve);
+    });
+  }
+});