diff --git a/public/scripts/main.js b/public/scripts/main.js
index 215d6e6..80fe340 100644
--- a/public/scripts/main.js
+++ b/public/scripts/main.js
@@ -1,23 +1,16 @@
 /* global bootstrap */
-import { parseRepositoryURL, generateWorkflow } from "./workflow.js";
+import {
+  parseRepositoryURL,
+  detectDefaultBranch,
+  generateWorkflow,
+} from "./workflow.js";
 
 function navigateTo(url) {
   window.open(url, "_blank");
 }
 window.navigateTo = navigateTo;
 
-function generateWorkflowWithFormInputs() {
-  return generateWorkflow({
-    useBatchToken: document.getElementById("use-batch-token").checked,
-    useVirtualDisplay: document.getElementById("use-virtual-display").checked,
-    buildAcrossPlatforms: document.getElementById("build-across-platforms")
-      .checked,
-    siteUrl:
-      window.location.origin + window.location.pathname.replace(/\/[^/]*$/, ""),
-  });
-}
-
-function handleFormSubmit(e) {
+async function handleFormSubmit(e) {
   e.preventDefault();
 
   const repoField = document.getElementById("repo");
@@ -28,7 +21,18 @@ function handleFormSubmit(e) {
   }
   repoField.classList.remove("is-invalid");
 
-  const workflow = generateWorkflowWithFormInputs();
+  let branch = await detectDefaultBranch(repoInfo);
+  if (!branch) branch = "main";
+
+  const workflow = generateWorkflow({
+    useBatchToken: document.getElementById("use-batch-token").checked,
+    useVirtualDisplay: document.getElementById("use-virtual-display").checked,
+    buildAcrossPlatforms: document.getElementById("build-across-platforms")
+      .checked,
+    siteUrl:
+      window.location.origin + window.location.pathname.replace(/\/[^/]*$/, ""),
+    branch,
+  });
 
   const encoded = encodeURIComponent(workflow);
   const filePath = ".github/workflows/matlab.yml";
@@ -37,7 +41,7 @@ function handleFormSubmit(e) {
   if (repoInfo.enterprise) {
     url += `/enterprises/${repoInfo.enterprise}`;
   }
-  url += `/${repoInfo.owner}/${repoInfo.repo}/new/main?filename=${filePath}&value=${encoded}`;
+  url += `/${repoInfo.owner}/${repoInfo.repo}/new/${branch}?filename=${filePath}&value=${encoded}`;
 
   window.navigateTo(url);
 
@@ -50,10 +54,24 @@ function showDownloadAlert() {
   alert.focus();
 }
 
-function handleDownloadClick(e) {
+async function handleDownloadClick(e) {
   e.preventDefault();
 
-  const workflow = generateWorkflowWithFormInputs();
+  const repoField = document.getElementById("repo");
+  const repoInfo = parseRepositoryURL(repoField.value.trim());
+
+  let branch = await detectDefaultBranch(repoInfo);
+  if (!branch) branch = "main";
+
+  const workflow = generateWorkflow({
+    useBatchToken: document.getElementById("use-batch-token").checked,
+    useVirtualDisplay: document.getElementById("use-virtual-display").checked,
+    buildAcrossPlatforms: document.getElementById("build-across-platforms")
+      .checked,
+    siteUrl:
+      window.location.origin + window.location.pathname.replace(/\/[^/]*$/, ""),
+    branch,
+  });
 
   const blob = new Blob([workflow], { type: "text/yaml" });
   const url = URL.createObjectURL(blob);
diff --git a/public/scripts/workflow.js b/public/scripts/workflow.js
index b4560d2..97cd73c 100644
--- a/public/scripts/workflow.js
+++ b/public/scripts/workflow.js
@@ -33,6 +33,7 @@ function parseRepositoryURL(repoURL) {
       // Enterprise: http(s)://github.com/enterprises/enterprise/owner/repo
       return {
         origin: url.origin,
+        hostname: url.hostname,
         enterprise: parts[1],
         owner: parts[2],
         repo: parts[3],
@@ -41,6 +42,7 @@ function parseRepositoryURL(repoURL) {
       // Standard: http(s)://host/owner/repo
       return {
         origin: url.origin,
+        hostname: url.hostname,
         owner: parts[0],
         repo: parts[1],
       };
@@ -51,11 +53,88 @@ function parseRepositoryURL(repoURL) {
   return null;
 }
 
+async function detectDefaultBranch(repoInfo) {
+  if (!repoInfo) return null;
+
+  async function fetchWithTimeout(url, options = {}) {
+    const controller = new AbortController();
+    const timeout = setTimeout(() => controller.abort(), 3000); // 3 seconds timeout
+    try {
+      const resp = await fetch(url, {
+        ...options,
+        signal: controller.signal,
+      });
+      return resp;
+    } finally {
+      clearTimeout(timeout);
+    }
+  }
+
+  // Try to detect default branch using the GitHub API
+  async function tryApi() {
+    let apiUrl;
+    if (repoInfo.hostname.replace(/^www\./, "") === "github.com") {
+      apiUrl = `https://api.github.com/repos/${repoInfo.owner}/${repoInfo.repo}`;
+    } else {
+      apiUrl = `${repoInfo.origin}/api/v3/repos/${repoInfo.owner}/${repoInfo.repo}`;
+    }
+
+    try {
+      const resp = await fetchWithTimeout(apiUrl, {
+        headers: { Accept: "application/vnd.github+json" },
+      });
+      if (!resp.ok) return null;
+
+      const data = await resp.json();
+      return data.default_branch || null;
+    } catch {
+      // Network or abort error, fallback
+      return null;
+    }
+  }
+
+  // Fallback: Probe common branch names by checking for a README.md
+  async function tryBlobFallback() {
+    let repoBaseUrl;
+    if (repoInfo.hostname.replace(/^www\./, "") === "github.com") {
+      repoBaseUrl = "https://github.com";
+    } else {
+      repoBaseUrl = `${repoInfo.origin}`;
+    }
+    if (repoInfo.enterprise) {
+      repoBaseUrl += `/enterprises/${repoInfo.enterprise}`;
+    }
+    repoBaseUrl += `/${repoInfo.owner}/${repoInfo.repo}`;
+
+    const branches = ["main", "master", "develop"];
+    const checks = branches.map(async (branch) => {
+      const url = `${repoBaseUrl}/blob/${branch}/README.md`;
+      try {
+        const resp = await fetchWithTimeout(url, {
+          method: "HEAD",
+          redirect: "manual",
+        });
+        return resp.ok ? branch : null;
+      } catch {
+        return null;
+      }
+    });
+    const results = await Promise.all(checks);
+    return results.find((branch) => branch !== null) || null;
+  }
+
+  // Main logic: Try API first, then fallback if needed
+  const apiBranch = await tryApi();
+  if (apiBranch) return apiBranch;
+  return await tryBlobFallback();
+}
+
 function generateWorkflow({
   useBatchToken = false,
   useVirtualDisplay = false,
   buildAcrossPlatforms = false,
   siteUrl = "http://localhost/",
+  branch = "main",
 }) {
   return dedent(`
   # This workflow was generated using the GitHub Actions Workflow Generator for MATLAB.
@@ -65,9 +144,9 @@ function generateWorkflow({
 
   on:
     push:
-      branches: [main]
+      branches: [${branch}]
     pull_request:
-      branches: [main]
+      branches: [${branch}]
     workflow_dispatch: {}
   ${
     useBatchToken
@@ -145,4 +224,4 @@ function dedent(str) {
   return match ? str.replace(new RegExp("^" + match[0], "gm"), "") : str;
 }
 
-export { parseRepositoryURL, generateWorkflow };
+export { parseRepositoryURL, detectDefaultBranch, generateWorkflow };
diff --git a/tests/main.test.js b/tests/main.test.js
index b7a838f..00ac194 100644
--- a/tests/main.test.js
+++ b/tests/main.test.js
@@ -20,38 +20,49 @@ beforeEach(async () => {
     <div id="download-alert" class="d-none"></div>
   `;
 
+  const realWorkflow = await import("../public/scripts/workflow.js");
+  jest.unstable_mockModule("../public/scripts/workflow.js", () => ({
+    ...realWorkflow,
+    detectDefaultBranch: jest.fn().mockResolvedValue("main"),
+  }));
+
   await import("../public/scripts/main.js");
   window.navigateTo = jest.fn();
 });
 
-test("form submit with invalid repo shows error", () => {
+const flushPromises = () => new Promise((r) => setTimeout(r, 0));
+
+test("form submit with invalid repo shows error", async () => {
   const repoInput = document.getElementById("repo");
   expect(repoInput.classList.contains("is-invalid")).toBe(false);
   repoInput.value = "invalidrepo";
   document
     .getElementById("generate-form")
     .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
   expect(repoInput.classList.contains("is-invalid")).toBe(true);
   expect(window.navigateTo).not.toHaveBeenCalled();
 });
 
-test("form submit with valid slug works", () => {
+test("form submit with valid slug works", async () => {
   const repoInput = document.getElementById("repo");
   repoInput.value = "owner/repo";
   document
     .getElementById("generate-form")
     .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
   expect(window.navigateTo).toHaveBeenCalledWith(
     expect.stringContaining("https://github.com/owner/repo/new/main?filename="),
   );
 });
 
-test("form submit with valid URL works", () => {
+test("form submit with valid URL works", async () => {
   const repoInput = document.getElementById("repo");
   repoInput.value = "https://github.com/octocat/Hello-World";
   document
     .getElementById("generate-form")
     .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
   expect(window.navigateTo).toHaveBeenCalledWith(
     expect.stringContaining(
       "https://github.com/octocat/Hello-World/new/main?filename=",
@@ -59,12 +70,13 @@ test("form submit with valid URL works", () => {
   );
 });
 
-test("form submit with valid cloud-hosted enterprise URL works", () => {
+test("form submit with valid cloud-hosted enterprise URL works", async () => {
   const repoInput = document.getElementById("repo");
   repoInput.value = "https://github.com/enterprises/gh/octocat/Hello-World";
   document
     .getElementById("generate-form")
     .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
   expect(window.navigateTo).toHaveBeenCalledWith(
     expect.stringContaining(
       "https://github.com/enterprises/gh/octocat/Hello-World/new/main?filename=",
@@ -72,12 +84,57 @@ test("form submit with valid cloud-hosted enterprise URL works", () => {
   );
 });
 
+test("form submit uses detected default branch", async () => {
+  jest.resetModules();
+  jest.unstable_mockModule("../public/scripts/workflow.js", () => ({
+    parseRepositoryURL: (v) => ({
+      origin: "https://github.com",
+      owner: "o",
+      repo: "r",
+    }),
+    detectDefaultBranch: jest.fn().mockResolvedValue("master"),
+    generateWorkflow: () => "yaml-content",
+  }));
+  await import("../public/scripts/main.js");
+  window.navigateTo = jest.fn();
+  document
+    .getElementById("generate-form")
+    .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
+  expect(window.navigateTo).toHaveBeenCalledWith(
+    expect.stringContaining("https://github.com/o/r/new/master?filename="),
+  );
+});
+
+test("form submit defaults to main branch if detection fails", async () => {
+  jest.resetModules();
+  jest.unstable_mockModule("../public/scripts/workflow.js", () => ({
+    parseRepositoryURL: (v) => ({
+      origin: "https://github.com",
+      owner: "o",
+      repo: "r",
+    }),
+    detectDefaultBranch: jest.fn().mockResolvedValue(null),
+    generateWorkflow: () => "yaml-content",
+  }));
+  await import("../public/scripts/main.js");
+  window.navigateTo = jest.fn();
+  document
+    .getElementById("generate-form")
+    .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
+  expect(window.navigateTo).toHaveBeenCalledWith(
+    expect.stringContaining("https://github.com/o/r/new/main?filename="),
+  );
+});
+
 test("advanced options are passed to generateWorkflow", async () => {
   // Re-import main.js with a spy on generateWorkflow
   jest.resetModules();
   const workflowSpy = jest.fn(() => "yaml-content");
   jest.unstable_mockModule("../public/scripts/workflow.js", () => ({
     parseRepositoryURL: (v) => ({ owner: "o", repo: "r" }),
+    detectDefaultBranch: jest.fn().mockResolvedValue("main"),
     generateWorkflow: workflowSpy,
   }));
   document.getElementById("repo").value = "o/r";
@@ -89,15 +146,17 @@ test("advanced options are passed to generateWorkflow", async () => {
   document
     .getElementById("generate-form")
     .dispatchEvent(new Event("submit", { bubbles: true, cancelable: true }));
+  await flushPromises();
   expect(workflowSpy).toHaveBeenCalledWith({
     useBatchToken: true,
     useVirtualDisplay: false,
     buildAcrossPlatforms: true,
     siteUrl: "http://localhost",
+    branch: "main",
   });
 });
 
-test("download link triggers file download", () => {
+test("download link triggers file download", async () => {
   const repoInput = document.getElementById("repo");
   repoInput.value = "owner/repo";
 
@@ -116,6 +175,8 @@ test("download link triggers file download", () => {
 
   document.getElementById("download-alert-link").click();
 
+  await flushPromises();
+
   expect(mockCreateObjectURL).toHaveBeenCalled();
   expect(clickSpy).toHaveBeenCalled();
   expect(mockRevokeObjectURL).toHaveBeenCalled();
diff --git a/tests/workflow.test.js b/tests/workflow.test.js
index a0b4087..ed31ebb 100644
--- a/tests/workflow.test.js
+++ b/tests/workflow.test.js
@@ -1,3 +1,4 @@
+import { jest } from "@jest/globals";
 import {
   parseRepositoryURL,
   generateWorkflow,
@@ -9,6 +10,7 @@ describe("parseRepositoryURL", () => {
     test("shorthand owner/repo", () => {
       expect(parseRepositoryURL("owner/repo")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "owner",
         repo: "repo",
       });
@@ -18,6 +20,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -25,6 +28,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("http://github.com/octocat/hello-world"),
       ).toEqual({
         origin: "http://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -36,6 +40,7 @@ describe("parseRepositoryURL", () => {
         ),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         enterprise: "mycompany",
         owner: "octocat",
         repo: "hello-world",
@@ -44,6 +49,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://mycompany.github.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://mycompany.github.com",
+        hostname: "mycompany.github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -51,6 +57,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.mycompany.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://github.mycompany.com",
+        hostname: "github.mycompany.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -58,6 +65,7 @@ describe("parseRepositoryURL", () => {
     test("URLs without protocol", () => {
       expect(parseRepositoryURL("github.com/octocat/hello-world")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -65,6 +73,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("mycompany.github.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://mycompany.github.com",
+        hostname: "mycompany.github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -72,6 +81,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("github.mycompany.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://github.mycompany.com",
+        hostname: "github.mycompany.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -81,6 +91,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.com/octocat/hello-world/"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -88,6 +99,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.com/octocat/hello-world/README.md"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -95,6 +107,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.com/octocat/hello-world.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -107,6 +120,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git@github.com:octocat/hello-world.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -114,6 +128,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("ssh://git@github.com/octocat/hello-world.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -123,6 +138,7 @@ describe("parseRepositoryURL", () => {
         ),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         enterprise: "mycompany",
         owner: "octocat",
         repo: "hello-world",
@@ -133,6 +149,7 @@ describe("parseRepositoryURL", () => {
         ),
       ).toEqual({
         origin: "https://mycompany.github.com",
+        hostname: "mycompany.github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -140,6 +157,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git@github.mycompany.com:octocat/hello-world.git"),
       ).toEqual({
         origin: "https://github.mycompany.com",
+        hostname: "github.mycompany.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -149,6 +167,7 @@ describe("parseRepositoryURL", () => {
         ),
       ).toEqual({
         origin: "https://github.mycompany.com",
+        hostname: "github.mycompany.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -156,6 +175,7 @@ describe("parseRepositoryURL", () => {
     test("SSH URLs without .git", () => {
       expect(parseRepositoryURL("git@github.com:octocat/hello-world")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -165,6 +185,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git://github.com/octocat/hello-world.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -172,6 +193,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git://github.com/octocat/hello-world"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -182,11 +204,13 @@ describe("parseRepositoryURL", () => {
     test("protocol/host case insensitivity, preserve owner/repo case", () => {
       expect(parseRepositoryURL("OWNER/REPO")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OWNER",
         repo: "REPO",
       });
       expect(parseRepositoryURL("GitHub.com/OctoCat/Hello-World")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OctoCat",
         repo: "Hello-World",
       });
@@ -194,6 +218,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("HTTPS://GITHUB.COM/OctoCat/Hello-World"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OctoCat",
         repo: "Hello-World",
       });
@@ -201,6 +226,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git@github.com:OctoCat/Hello-World.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OctoCat",
         repo: "Hello-World",
       });
@@ -208,6 +234,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("ssh://git@github.com/OctoCat/Hello-World.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OctoCat",
         repo: "Hello-World",
       });
@@ -215,6 +242,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("git://github.com/OctoCat/Hello-World.git"),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "OctoCat",
         repo: "Hello-World",
       });
@@ -240,6 +268,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("https://github.com:8080/octocat/hello-world"),
       ).toEqual({
         origin: "https://github.com:8080",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -249,6 +278,7 @@ describe("parseRepositoryURL", () => {
         ),
       ).toEqual({
         origin: "https://github.mycompany.com:8080",
+        hostname: "github.mycompany.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -256,6 +286,7 @@ describe("parseRepositoryURL", () => {
     test("inputs with whitespace", () => {
       expect(parseRepositoryURL("  owner/repo  ")).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "owner",
         repo: "repo",
       });
@@ -263,6 +294,7 @@ describe("parseRepositoryURL", () => {
         parseRepositoryURL("  https://github.com/octocat/hello-world  "),
       ).toEqual({
         origin: "https://github.com",
+        hostname: "github.com",
         owner: "octocat",
         repo: "hello-world",
       });
@@ -270,6 +302,163 @@ describe("parseRepositoryURL", () => {
   });
 });
 
+describe("detectDefaultBranch", () => {
+  // Mock fetch globally for all tests in this suite
+  const originalFetch = global.fetch;
+  afterEach(() => {
+    global.fetch = originalFetch;
+    jest.clearAllMocks();
+  });
+
+  test("returns null for null input", async () => {
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const branch = await detectDefaultBranch(null);
+    expect(branch).toBeNull();
+  });
+
+  test("detects default branch via GitHub API", async () => {
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true,
+      json: () => Promise.resolve({ default_branch: "dev" }),
+    });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBe("dev");
+    expect(global.fetch).toHaveBeenCalledWith(
+      "https://api.github.com/repos/o/r",
+      expect.any(Object),
+    );
+  });
+
+  test("detects default branch via GitHub Enterprise API", async () => {
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true,
+      json: () => Promise.resolve({ default_branch: "dev" }),
+    });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://mycompany.github.com",
+      hostname: "mycompany.github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBe("dev");
+    expect(global.fetch).toHaveBeenCalledWith(
+      "https://mycompany.github.com/api/v3/repos/o/r",
+      expect.any(Object),
+    );
+  });
+
+  test("returns null if GitHub API response is not ok", async () => {
+    global.fetch = jest.fn().mockResolvedValue({ ok: false });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBeNull();
+  });
+
+  test("detects 'main' branch on fallback", async () => {
+    global.fetch = jest.fn().mockImplementation((url) => {
+      return Promise.resolve({ ok: url.includes("main") });
+    });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBe("main");
+  });
+
+  test("detects 'master' branch on fallback", async () => {
+    global.fetch = jest.fn().mockImplementation((url) => {
+      return Promise.resolve({ ok: url.includes("master") });
+    });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBe("master");
+  });
+
+  test("detects 'develop' branch on fallback", async () => {
+    global.fetch = jest.fn().mockImplementation((url) => {
+      return Promise.resolve({ ok: url.includes("develop") });
+    });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBe("develop");
+  });
+
+  test("returns null if no fallback branches found", async () => {
+    global.fetch = jest.fn().mockResolvedValue({ ok: false });
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBeNull();
+  });
+
+  test("handles fetch errors gracefully", async () => {
+    global.fetch = jest.fn().mockRejectedValue(new Error("Network error"));
+    const { detectDefaultBranch } = await import(
+      "../public/scripts/workflow.js"
+    );
+    const repoInfo = {
+      origin: "https://github.com",
+      hostname: "github.com",
+      owner: "o",
+      repo: "r",
+    };
+    const branch = await detectDefaultBranch(repoInfo);
+    expect(branch).toBeNull();
+  });
+});
+
 describe("generateWorkflow", () => {
   test("default workflow", () => {
     const yaml = generateWorkflow({});