diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index cb40dada2..f082646b4 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -19,6 +19,7 @@
 - Cloud Code Assist Gemini 3.6/3.7 Flash requests at `minimal` now send `thinkingLevel: LOW` on the aliased `-low` SKU instead of `MINIMAL`, which the API rejects with HTTP 400.
 - Answer Cursor `interaction_query` permission gates (hosted web search, Exa, unnamed field-9 WebFetch) so the Run RPC continues instead of sitting silent until the 300s idle watchdog.
 - Fixed provider tool calls arriving with flattened array argument paths (e.g. Gemini's `questions[0].id`) being stripped and rejected by argument validation; well-formed flattened paths are now rebuilt into the nested arrays the tool schema expects ([#8886](https://github.com/can1357/oh-my-pi/issues/8886)).
+- Fixed OAuth login (Codex `localhost:1455`, and any `localhost` callback flow) failing on hosts with IPv6 disabled at the kernel (`ipv6.disable=1`). The `::1` companion listener added in #8081 fails there with Bun's generic "Is port X in use?" message (oven-sh/bun#7187), which the in-use check misread as a real collision — tearing down the healthy IPv4 listener and surfacing a bogus "port 1455 is in use" error. The dual-bind path now detects the missing IPv6 loopback up front and serves IPv4 alone ([#8814](https://github.com/can1357/oh-my-pi/issues/8814)).
 
 ## [17.3.7] - 2026-08-17
 
diff --git a/packages/ai/src/registry/oauth/callback-server.ts b/packages/ai/src/registry/oauth/callback-server.ts
index cbb45e563..d3e5a399a 100644
--- a/packages/ai/src/registry/oauth/callback-server.ts
+++ b/packages/ai/src/registry/oauth/callback-server.ts
@@ -10,6 +10,7 @@
  * - generateAuthUrl(): Build provider-specific authorization URL
  * - exchangeToken(): Exchange authorization code for tokens
  */
+import * as os from "node:os";
 import * as AIError from "../../error";
 import templateHtml from "./oauth.html" with { type: "text" };
 import type { OAuthController, OAuthCredentials } from "./types";
@@ -58,6 +59,27 @@ function isAddressInUse(error: unknown): boolean {
 	return error instanceof Error && /EADDRINUSE|in use/i.test(error.message);
 }
 
+/**
+ * Whether this host exposes an IPv6 loopback (`::1`) the companion listener can
+ * bind. A kernel with IPv6 disabled (`ipv6.disable=1`) lists no internal IPv6
+ * address, and the `::1` companion bind there fails with a generic Bun error
+ * {@link isAddressInUse} cannot distinguish from a real collision — Bun reuses
+ * its "Is port X in use?" message for every listen failure (oven-sh/bun#7187) —
+ * so the dual-bind path must be skipped up front rather than misread as a
+ * conflict (issue #8814).
+ */
+function ipv6LoopbackAvailable(): boolean {
+	const interfaces = os.networkInterfaces();
+	for (const name in interfaces) {
+		const addresses = interfaces[name];
+		if (!addresses) continue;
+		for (const address of addresses) {
+			if (address.internal && address.family === "IPv6") return true;
+		}
+	}
+	return false;
+}
+
 export interface OAuthCallbackFlowOptions {
 	preferredPort: number;
 	callbackPath?: string;
@@ -335,12 +357,21 @@ export abstract class OAuthCallbackFlow {
 		if (this.callbackHostname !== DEFAULT_HOSTNAME) {
 			return this.#serve(this.callbackHostname, port, expectedState);
 		}
+		// A host with IPv6 disabled at the kernel exposes no `::1`, so the
+		// companion bind cannot succeed there. Bun reports that failure with the
+		// same generic "Is port X in use?" message it uses for a real collision
+		// (oven-sh/bun#7187), which the catch below would misread — tearing down
+		// the healthy IPv4 listener and, for a pinned port, throwing a bogus
+		// "port in use" ConfigurationError. Detecting the missing stack up front
+		// lets the IPv4 listener serve alone (issue #8814).
+		const dualStack = ipv6LoopbackAvailable();
 		for (let attempt = 0; ; attempt++) {
 			const primary = this.#serve(IPV4_LOOPBACK, port, expectedState);
 			const boundPort = primary.port;
 			// A non-TCP endpoint has no port for the companion to target;
 			// #resolveServerPort reports that case precisely.
 			if (typeof boundPort !== "number") return primary;
+			if (!dualStack) return primary;
 			let companion: Bun.Server<unknown>;
 			try {
 				companion = this.#serve(IPV6_LOOPBACK, boundPort, expectedState);
diff --git a/packages/ai/test/callback-server-dual-stack.test.ts b/packages/ai/test/callback-server-dual-stack.test.ts
index ae0084c38..55c5a4d89 100644
--- a/packages/ai/test/callback-server-dual-stack.test.ts
+++ b/packages/ai/test/callback-server-dual-stack.test.ts
@@ -1,4 +1,5 @@
 import { afterEach, describe, expect, it, vi } from "bun:test";
+import * as os from "node:os";
 import { OAuthCallbackFlow } from "@oh-my-pi/pi-ai/registry/oauth/callback-server";
 import type { OAuthCredentials } from "@oh-my-pi/pi-ai/registry/oauth/types";
 
@@ -117,4 +118,54 @@ describe("OAuthCallbackFlow loopback address families", () => {
 		expect(flow.lastRedirectUri).toBe(`http://localhost:${port}/callback`);
 		expect(progress.some(msg => msg.includes("unavailable"))).toBe(false);
 	});
+
+	it("keeps a pinned port when IPv6 is disabled and the companion bind reports a misleading in-use error", async () => {
+		// Reproduce a host with IPv6 disabled at the kernel (ipv6.disable=1): the
+		// loopback interface exposes only 127.0.0.1, no ::1 (issue #8814).
+		vi.spyOn(os, "networkInterfaces").mockReturnValue({
+			lo: [
+				{
+					address: "127.0.0.1",
+					netmask: "255.0.0.0",
+					family: "IPv4",
+					mac: "00:00:00:00:00:00",
+					internal: true,
+					cidr: "127.0.0.1/8",
+				},
+			],
+		});
+		// Bun surfaces the IPv6-unavailable companion failure with the same
+		// generic "Is port X in use?" message it uses for a real collision
+		// (oven-sh/bun#7187), so the in-use message regex cannot tell them apart.
+		const realServe = Bun.serve.bind(Bun) as typeof Bun.serve;
+		vi.spyOn(Bun, "serve").mockImplementation(((options: { hostname?: string; port?: number }) => {
+			if (options.hostname === "::1") {
+				throw Object.assign(new Error(`Failed to start server. Is port ${options.port} in use?`), {
+					code: "EADDRINUSE",
+				});
+			}
+			return realServe(options as Parameters<typeof Bun.serve>[0]);
+		}) as typeof Bun.serve);
+
+		const port = freeLoopbackPort();
+		const progress: string[] = [];
+		const cancel = new AbortController();
+		const flow = new TestCallbackFlow(
+			{
+				onAuth: () => cancel.abort("advertised"),
+				onProgress: msg => progress.push(msg),
+				signal: cancel.signal,
+			},
+			// Pinned redirect URI reproduces the Codex flow: a misclassified
+			// companion failure would abort with a bogus port-in-use error here.
+			{ preferredPort: port, redirectUri: `http://localhost:${port}/callback` },
+		);
+
+		await expect(flow.login()).rejects.toThrow();
+		// The IPv4 listener is the only reachable endpoint, so the flow must serve
+		// it and advertise the pinned URI rather than misread the companion
+		// failure as a collision and throw a ConfigurationError before onAuth.
+		expect(flow.lastRedirectUri).toBe(`http://localhost:${port}/callback`);
+		expect(progress.some(msg => msg.includes("unavailable"))).toBe(false);
+	});
 });