diff --git a/bin/cli/commands/oauth.mjs b/bin/cli/commands/oauth.mjs
index 1cdaeb8267..c3a070228a 100644
--- a/bin/cli/commands/oauth.mjs
+++ b/bin/cli/commands/oauth.mjs
@@ -54,6 +54,34 @@ async function openBrowser(url) {
   }
 }
 
+// Mirrors src/lib/oauth/providers.ts::isLoopbackHostname — used here to detect
+// when the redirect_uri the server resolved (and the authorize URL now
+// advertises) points at a loopback address the CLI never binds a listener on
+// (issue #12413). Returns false on an unparseable URI rather than throwing.
+function isLoopbackHost(uri) {
+  try {
+    return /^(localhost|127\.0\.0\.1|\[::1\]|::1)$/i.test(new URL(uri).hostname);
+  } catch {
+    return false;
+  }
+}
+
+function printLoopbackRedirectWarning(providerId, redirectUri) {
+  process.stdout.write(
+    `Note: the authorize URL below advertises ${redirectUri}, but this CLI does not\n` +
+      "listen on that port. Right after you approve, the browser is expected to\n" +
+      "show a connection error (e.g. \"This site can't be reached\" / \n" +
+      "ERR_CONNECTION_REFUSED) — that is normal, not a failure. Copy the full URL\n" +
+      "from the address bar anyway and paste it below.\n"
+  );
+  if (providerId === "antigravity") {
+    process.stdout.write(
+      "Tip: `omniroute login antigravity` captures the code automatically and\n" +
+        "avoids that error page entirely.\n"
+    );
+  }
+}
+
 function targetApiOptions(opts = {}) {
   return {
     baseUrl: opts.baseUrl,
@@ -110,6 +138,10 @@ async function runBrowserFlow(def, opts) {
   const { codeVerifier, state, redirectUri: returnedRedirectUri } = start;
   const finalRedirectUri = returnedRedirectUri || redirectUri;
 
+  if (finalRedirectUri && isLoopbackHost(finalRedirectUri)) {
+    printLoopbackRedirectWarning(def.id, finalRedirectUri);
+  }
+
   process.stdout.write(`\nOpen this URL to authorize:\n  ${url}\n\n`);
   if (opts.browser !== false) await openBrowser(url);
   process.stdout.write(
diff --git a/changelog.d/fixes/12413-antigravity-oauth-redirect-hint.md b/changelog.d/fixes/12413-antigravity-oauth-redirect-hint.md
new file mode 100644
index 0000000000..96b6358d47
--- /dev/null
+++ b/changelog.d/fixes/12413-antigravity-oauth-redirect-hint.md
@@ -0,0 +1 @@
+- fix(oauth): warn before the dead localhost:8080 redirect in antigravity/gemini `oauth start` (#12413)
diff --git a/tests/unit/probe-12413-antigravity-oauth-redirect-hint.test.ts b/tests/unit/probe-12413-antigravity-oauth-redirect-hint.test.ts
new file mode 100644
index 0000000000..55d07f7e76
--- /dev/null
+++ b/tests/unit/probe-12413-antigravity-oauth-redirect-hint.test.ts
@@ -0,0 +1,161 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { Readable } from "node:stream";
+
+// Repro probe for issue #12413: `omniroute oauth start --provider antigravity`
+// prints the Google authorize URL (which advertises
+// redirect_uri=http://localhost:8080/callback) and then silently waits for a
+// pasted callback URL/code — nothing in the CLI output warns the operator
+// that their browser is about to land on a dead port (ERR_CONNECTION_REFUSED)
+// and that seeing that error is expected, not a failure.
+//
+// This asserts the CLI's browser-flow instructions proactively mention the
+// expected browser error (e.g. "can't be reached" / "connection refused" /
+// "ERR_CONNECTION_REFUSED") BEFORE the user is sent to authorize.
+
+function makeResp(data: unknown, status = 200) {
+  return {
+    ok: status < 400,
+    status,
+    json: () => Promise.resolve(data),
+    text: () => Promise.resolve(JSON.stringify(data)),
+    headers: new Headers(),
+  };
+}
+
+async function captureStdout(fn: () => Promise<void>) {
+  const chunks: string[] = [];
+  const orig = process.stdout.write.bind(process.stdout);
+  (process.stdout.write as unknown) = (c: string | Uint8Array) => {
+    chunks.push(typeof c === "string" ? c : Buffer.from(c).toString("utf8"));
+    return true;
+  };
+  try {
+    await fn();
+  } finally {
+    process.stdout.write = orig;
+  }
+  return chunks.join("");
+}
+
+function makeCmd() {
+  return { optsWithGlobals: () => ({ output: "json", quiet: true }) };
+}
+
+// readline's `close` fallback (bin/cli/io.mjs) only fires once per stdin EOF —
+// reusing the real (already-ended) process.stdin across tests in the same file
+// means the second readline.createInterface() never sees another `end`/`close`
+// event and hangs forever. Swap in a fresh, already-ended Readable per test
+// instead of calling `process.stdin.push(null)` on the shared real stream.
+async function withEndedStdin<T>(fn: () => Promise<T>): Promise<T> {
+  const origStdin = process.stdin;
+  const fakeStdin = new Readable({ read() {} });
+  fakeStdin.push(null);
+  Object.defineProperty(process, "stdin", { value: fakeStdin, configurable: true });
+  try {
+    return await fn();
+  } finally {
+    Object.defineProperty(process, "stdin", { value: origStdin, configurable: true });
+  }
+}
+
+test("runOAuthStart browser flow warns antigravity users before the dead localhost:8080 redirect (#12413)", async () => {
+  const origFetch = globalThis.fetch;
+  const origExit = process.exit;
+  let exitErr: Error | null = null;
+
+  (globalThis.fetch as unknown) = (url: string) => {
+    if (url.includes("/api/oauth/antigravity/authorize")) {
+      return Promise.resolve(
+        makeResp({
+          authUrl:
+            "https://accounts.google.com/o/oauth2/v2/auth?redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fcallback",
+          codeVerifier: "verifier",
+          state: "state123",
+          redirectUri: "http://localhost:8080/callback",
+        })
+      );
+    }
+    return Promise.reject(new Error(`Unexpected fetch: ${url}`));
+  };
+
+  (process.exit as unknown) = (code?: number) => {
+    exitErr = new Error(`exit ${code}`);
+    throw exitErr;
+  };
+
+  let out = "";
+  try {
+    const { runOAuthStart } = await import("../../bin/cli/commands/oauth.mjs");
+    out = await withEndedStdin(() =>
+      captureStdout(async () => {
+        try {
+          await runOAuthStart({ provider: "antigravity", browser: false }, makeCmd());
+        } catch (e) {
+          if (e !== exitErr) throw e;
+        }
+      })
+    );
+  } finally {
+    globalThis.fetch = origFetch;
+    process.exit = origExit;
+  }
+
+  assert.ok(out.includes("8080"), "should print the advertised (dead) redirect port");
+
+  const mentionsExpectedBrowserError =
+    /can't be reached|connection refused|err_connection_refused|won't load|expected/i.test(out);
+  assert.ok(
+    mentionsExpectedBrowserError,
+    `expected the CLI to warn about the dead-redirect browser error BEFORE the user hits it, got:\n${out}`
+  );
+});
+
+test("runOAuthStart browser flow does NOT warn for a non-loopback redirect (claude-code)", async () => {
+  const origFetch = globalThis.fetch;
+  const origExit = process.exit;
+  let exitErr: Error | null = null;
+
+  (globalThis.fetch as unknown) = (url: string) => {
+    if (url.includes("/api/oauth/claude/authorize")) {
+      return Promise.resolve(
+        makeResp({
+          authUrl: "https://platform.claude.com/oauth/authorize?redirect_uri=fixed",
+          codeVerifier: "verifier",
+          state: "state123",
+          redirectUri: "https://platform.claude.com/oauth/code/callback",
+        })
+      );
+    }
+    return Promise.reject(new Error(`Unexpected fetch: ${url}`));
+  };
+
+  (process.exit as unknown) = (code?: number) => {
+    exitErr = new Error(`exit ${code}`);
+    throw exitErr;
+  };
+
+  let out = "";
+  try {
+    const { runOAuthStart } = await import("../../bin/cli/commands/oauth.mjs");
+    out = await withEndedStdin(() =>
+      captureStdout(async () => {
+        try {
+          await runOAuthStart({ provider: "claude-code", browser: false }, makeCmd());
+        } catch (e) {
+          if (e !== exitErr) throw e;
+        }
+      })
+    );
+  } finally {
+    globalThis.fetch = origFetch;
+    process.exit = origExit;
+  }
+
+  const mentionsExpectedBrowserError =
+    /can't be reached|connection refused|err_connection_refused|won't load/i.test(out);
+  assert.ok(
+    !mentionsExpectedBrowserError,
+    `did not expect a dead-redirect warning for a non-loopback provider, got:\n${out}`
+  );
+});