diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 20ef357ef..ab2f19c99 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- `OAuthCallbackFlow` now serves a `GET /launch` route on its loopback callback server that 302-redirects to the pending authorization URL, and exposes that short URL as `OAuthAuthInfo.launchUrl`. UIs can advertise it as a truncation-safe copy target (~30 chars) instead of the full authorize URL, so terminals narrower than the composed row cannot silently drop OAuth query parameters like `code_challenge_method=S256` ([#4418](https://github.com/can1357/oh-my-pi/issues/4418)).
+
 ## [16.3.4] - 2026-07-03
 
 ### Added
diff --git a/packages/ai/src/auth-storage.ts b/packages/ai/src/auth-storage.ts
index 5a835f2ba..fc71c9f70 100644
--- a/packages/ai/src/auth-storage.ts
+++ b/packages/ai/src/auth-storage.ts
@@ -16,7 +16,13 @@ import * as AIError from "./error";
 import { isUsageLimitOutcome } from "./error/rate-limit";
 import { getProviderDefinition, PASTE_CODE_LOGIN_PROVIDERS } from "./registry";
 import { getOAuthApiKey, getOAuthProvider, refreshOAuthToken } from "./registry/oauth";
-import type { OAuthController, OAuthCredentials, OAuthProvider, OAuthProviderId } from "./registry/oauth/types";
+import type {
+	OAuthAuthInfo,
+	OAuthController,
+	OAuthCredentials,
+	OAuthProvider,
+	OAuthProviderId,
+} from "./registry/oauth/types";
 import { getEnvApiKey, getEnvApiKeyName } from "./stream";
 import type { Provider } from "./types";
 import type {
@@ -1863,7 +1869,7 @@ export class AuthStorage {
 		provider: OAuthProviderId,
 		ctrl: OAuthController & {
 			/** onAuth is required by auth-storage but optional in OAuthController */
-			onAuth: (info: { url: string; instructions?: string }) => void;
+			onAuth: (info: OAuthAuthInfo) => void;
 			/** onPrompt is required for some providers (github-copilot, openai-codex) */
 			onPrompt: (prompt: { message: string; placeholder?: string }) => Promise<string>;
 		},
diff --git a/packages/ai/src/registry/oauth/callback-server.ts b/packages/ai/src/registry/oauth/callback-server.ts
index b0f084d7d..3f7cf6884 100644
--- a/packages/ai/src/registry/oauth/callback-server.ts
+++ b/packages/ai/src/registry/oauth/callback-server.ts
@@ -17,6 +17,14 @@ import type { OAuthController, OAuthCredentials } from "./types";
 const DEFAULT_TIMEOUT = 300_000;
 const DEFAULT_HOSTNAME = "localhost";
 const CALLBACK_PATH = "/callback";
+/**
+ * Path served by {@link OAuthCallbackFlow} that 302-redirects to the pending
+ * authorization URL. Kept out of {@link OAuthCallbackFlowOptions} because it
+ * lives on the loopback callback server alongside {@link CALLBACK_PATH} and
+ * must never clash with a provider-registered redirect URI (all known
+ * providers register `/callback`-shaped paths).
+ */
+const LAUNCH_PATH = "/launch";
 
 export type CallbackResult = { code: string; state: string };
 
@@ -54,6 +62,14 @@ export abstract class OAuthCallbackFlow {
 	allowPortFallback: boolean;
 	#callbackResolve?: (result: CallbackResult) => void;
 	#callbackReject?: (error: string) => void;
+	/**
+	 * Authorization URL the `/launch` route currently redirects to. Set by
+	 * {@link login} after {@link generateAuthUrl} and before {@link OAuthController.onAuth}
+	 * fires, cleared when the server stops. `undefined` before the flow reaches
+	 * that point and after it finishes, so `/launch` returns 503 rather than
+	 * a stale URL.
+	 */
+	#pendingAuthUrl?: string;
 
 	constructor(
 		ctrl: OAuthController,
@@ -120,7 +136,7 @@ export abstract class OAuthCallbackFlow {
 		this.#throwIfCancelled();
 
 		// Start callback server first to get actual redirect URI
-		const { server, redirectUri } = await this.#startCallbackServer(state);
+		const { server, redirectUri, launchUrl } = await this.#startCallbackServer(state);
 
 		try {
 			this.#throwIfCancelled();
@@ -128,8 +144,14 @@ export abstract class OAuthCallbackFlow {
 			const { url: authUrl, instructions } = await this.generateAuthUrl(state, redirectUri);
 			this.#throwIfCancelled();
 
+			// Publish the auth URL to the `/launch` route BEFORE handing it to
+			// callers. `onAuth` immediately renders a UI that advertises the
+			// launch URL as a copy target, so `/launch` must already resolve if
+			// the user clicks/pastes it during the same render pass.
+			this.#pendingAuthUrl = authUrl;
+
 			// Notify controller that auth is ready
-			this.ctrl.onAuth?.({ url: authUrl, instructions });
+			this.ctrl.onAuth?.({ url: authUrl, launchUrl, instructions });
 			this.ctrl.onProgress?.("Waiting for browser authentication...");
 
 			// Wait for callback or manual input
@@ -140,6 +162,7 @@ export abstract class OAuthCallbackFlow {
 
 			return await this.exchangeToken(code, state, redirectUri);
 		} finally {
+			this.#pendingAuthUrl = undefined;
 			server.stop();
 		}
 	}
@@ -147,14 +170,21 @@ export abstract class OAuthCallbackFlow {
 	/**
 	 * Start callback server, trying preferred port first, falling back to random.
 	 */
-	async #startCallbackServer(expectedState: string): Promise<{ server: Bun.Server<unknown>; redirectUri: string }> {
+	async #startCallbackServer(
+		expectedState: string,
+	): Promise<{ server: Bun.Server<unknown>; redirectUri: string; launchUrl: string }> {
 		try {
 			const server = this.#createServer(this.preferredPort, expectedState);
+			// `preferredPort: 0` opts into a random port — read the actual bound
+			// port from the server so both the redirect URI and launch URL point at
+			// a reachable socket, not the sentinel.
+			const actualPort = this.#resolveServerPort(server);
+			const launchUrl = this.#launchUrlFor(actualPort);
 			if (this.redirectUri) {
-				return { server, redirectUri: this.redirectUri };
+				return { server, redirectUri: this.redirectUri, launchUrl };
 			}
-			const redirectUri = `http://${this.callbackHostname}:${this.preferredPort}${this.callbackPath}`;
-			return { server, redirectUri };
+			const redirectUri = `http://${this.callbackHostname}:${actualPort}${this.callbackPath}`;
+			return { server, redirectUri, launchUrl };
 		} catch (cause) {
 			if (this.redirectUri) {
 				throw new AIError.ConfigurationError(
@@ -169,13 +199,39 @@ export abstract class OAuthCallbackFlow {
 				);
 			}
 			const server = this.#createServer(0, expectedState);
-			const actualPort = server.port;
+			const actualPort = this.#resolveServerPort(server);
 			const redirectUri = `http://${this.callbackHostname}:${actualPort}${this.callbackPath}`;
+			const launchUrl = this.#launchUrlFor(actualPort);
 			this.ctrl.onProgress?.(`Preferred port ${this.preferredPort} unavailable, using port ${actualPort}`);
-			return { server, redirectUri };
+			return { server, redirectUri, launchUrl };
 		}
 	}
 
+	/**
+	 * Read the numeric port a callback server bound to. `Bun.Server.port` is
+	 * declared `number | undefined` because Unix-socket servers have no port,
+	 * but every callback flow uses TCP; a missing port here indicates a
+	 * configuration error rather than a fallback case.
+	 */
+	#resolveServerPort(server: Bun.Server<unknown>): number {
+		const port = server.port;
+		if (typeof port !== "number") {
+			throw new AIError.ConfigurationError(
+				"OAuth callback server bound to a non-TCP endpoint; expected a numeric port. Check `oauth.callbackPort`/`oauth.redirectUri`.",
+			);
+		}
+		return port;
+	}
+
+	/**
+	 * Build the `/launch` URL served by the callback server bound to `port`.
+	 * Kept short (~30 chars) so UIs can advertise it as a viewport-truncation-safe
+	 * copy target for the full authorization URL.
+	 */
+	#launchUrlFor(port: number): string {
+		return `http://${this.callbackHostname}:${port}${LAUNCH_PATH}`;
+	}
+
 	/**
 	 * Create HTTP server for OAuth callback.
 	 */
@@ -190,11 +246,22 @@ export abstract class OAuthCallbackFlow {
 	}
 
 	/**
-	 * Handle OAuth callback HTTP request.
+	 * Handle OAuth callback HTTP request. Two routes on the same loopback server:
+	 * - `callbackPath` (default `/callback`) — the provider redirect target.
+	 * - `LAUNCH_PATH` (`/launch`) — 302 to the pending authorization URL so
+	 *   viewport-safe copy targets can survive TUI truncation.
 	 */
 	#handleCallback(req: Request, expectedState: string): Response {
 		const url = new URL(req.url);
 
+		if (url.pathname === LAUNCH_PATH) {
+			const pending = this.#pendingAuthUrl;
+			if (!pending) {
+				return new Response("OAuth launch URL is no longer active", { status: 503 });
+			}
+			return Response.redirect(pending, 302);
+		}
+
 		if (url.pathname !== this.callbackPath) {
 			return new Response("Not Found", { status: 404 });
 		}
diff --git a/packages/ai/src/registry/oauth/types.ts b/packages/ai/src/registry/oauth/types.ts
index 20bc43219..a11b4e5fd 100644
--- a/packages/ai/src/registry/oauth/types.ts
+++ b/packages/ai/src/registry/oauth/types.ts
@@ -23,7 +23,21 @@ export type OAuthPrompt = {
 };
 
 export type OAuthAuthInfo = {
+	/**
+	 * Full authorization URL. Suitable for direct browser launch, OSC 8
+	 * hyperlinks, and clipboard when the target UI can guarantee the full
+	 * string reaches the user unmodified.
+	 */
 	url: string;
+	/**
+	 * Short loopback URL that 302-redirects to {@link url}. Provided by flows
+	 * that host the redirect on the same callback server they already run
+	 * ({@link OAuthCallbackFlow}). UIs SHOULD prefer this as the copy target
+	 * so viewport truncation cannot corrupt OAuth query parameters. Undefined
+	 * for flows without a loopback callback server (device code, paste-code
+	 * providers with fixed non-loopback redirects, etc.).
+	 */
+	launchUrl?: string;
 	instructions?: string;
 };
 
diff --git a/packages/ai/test/callback-server-launch-route.test.ts b/packages/ai/test/callback-server-launch-route.test.ts
new file mode 100644
index 000000000..9cc040a4d
--- /dev/null
+++ b/packages/ai/test/callback-server-launch-route.test.ts
@@ -0,0 +1,116 @@
+import { afterEach, describe, expect, it, vi } from "bun:test";
+import { OAuthCallbackFlow } from "@oh-my-pi/pi-ai/registry/oauth/callback-server";
+import type { OAuthAuthInfo, OAuthCredentials } from "@oh-my-pi/pi-ai/registry/oauth/types";
+
+/**
+ * Regression harness for #4418 — the `/launch` route the callback server hosts
+ * so UIs can advertise a short (~30-char) copy target that survives TUI viewport
+ * truncation. Without it, the full authorize URL (~260+ chars on Linear/GitHub/…)
+ * gets silently truncated mid-parameter and downgrades the flow to plain PKCE.
+ */
+class LaunchProbeFlow extends OAuthCallbackFlow {
+	authUrls: string[] = [];
+	// Long enough that a 270-col TUI would clip `code_challenge_method=S256`.
+	static readonly PADDING = "x".repeat(200);
+
+	async generateAuthUrl(state: string, redirectUri: string): Promise<{ url: string }> {
+		const url =
+			"https://mcp.example.com/authorize?" +
+			new URLSearchParams({
+				response_type: "code",
+				client_id: "test-client",
+				redirect_uri: redirectUri,
+				state,
+				scope: LaunchProbeFlow.PADDING,
+				code_challenge: "test-challenge",
+				code_challenge_method: "S256",
+			}).toString();
+		this.authUrls.push(url);
+		return { url };
+	}
+
+	async exchangeToken(): Promise<OAuthCredentials> {
+		return { access: "unused", refresh: "unused", expires: Date.now() + 60_000 };
+	}
+}
+
+/**
+ * Start a flow and resolve once `onAuth` fires — that's the exact instant
+ * `/launch` becomes live, so tests can hit it without a wall-clock sleep.
+ * Returns the captured auth info, the abort controller (so tests can shut the
+ * flow down), and the pending `login` promise (so tests can await teardown).
+ */
+async function startFlowAndWaitForAuth(): Promise<{
+	info: OAuthAuthInfo;
+	abort: AbortController;
+	login: Promise<void>;
+}> {
+	const abort = new AbortController();
+	const authFired = Promise.withResolvers<OAuthAuthInfo>();
+	const flow = new LaunchProbeFlow(
+		{
+			onAuth: info => {
+				authFired.resolve(info);
+			},
+			signal: abort.signal,
+		},
+		{ preferredPort: 0, allowPortFallback: true },
+	);
+	// Kick off login in the background; tests own its lifetime via `abort`.
+	const login = flow.login().catch(() => undefined) as Promise<void>;
+	const info = await authFired.promise;
+	return { info, abort, login };
+}
+
+afterEach(() => {
+	vi.restoreAllMocks();
+});
+
+describe("OAuthCallbackFlow /launch route", () => {
+	it("advertises a short launch URL and 302s it to the pending authorization URL", async () => {
+		const { info, abort, login } = await startFlowAndWaitForAuth();
+
+		// Contract 1 — launch URL is short and shaped like a loopback URL. A
+		// terminal that truncates below ~40 columns is degenerate; anything above
+		// that keeps the launch URL intact regardless of the full URL length.
+		expect(info.launchUrl).toBeDefined();
+		expect(info.launchUrl!.length).toBeLessThan(40);
+		expect(info.launchUrl).toMatch(/^http:\/\/localhost:\d+\/launch$/);
+
+		// Contract 2 — GET /launch returns 302 pointing at the pending authorize URL,
+		// byte-for-byte (the whole point: no truncation surface between UI and provider).
+		const response = await fetch(info.launchUrl!, { redirect: "manual" });
+		expect(response.status).toBe(302);
+		expect(response.headers.get("location")).toBe(info.url);
+
+		abort.abort("test done");
+		await login;
+	});
+
+	it("stops answering /launch once the flow completes so no stale URL is redirected", async () => {
+		const { info, abort, login } = await startFlowAndWaitForAuth();
+		expect(info.launchUrl).toBeDefined();
+
+		abort.abort("test done");
+		await login;
+
+		// Server has stopped and `#pendingAuthUrl` was cleared — the launch URL
+		// no longer connects. The correct end-state is that the redirect NEVER
+		// points at a stale URL; the loopback socket is gone so `fetch` rejects.
+		await expect(fetch(info.launchUrl!)).rejects.toThrow();
+	});
+
+	it("routes `/callback` and `/launch` on the same server without interfering", async () => {
+		const { info, abort, login } = await startFlowAndWaitForAuth();
+		expect(info.launchUrl).toBeDefined();
+
+		// A GET at an unrelated path still 404s — `/launch` is additive, not a
+		// blanket catch-all.
+		const origin = new URL(info.launchUrl!).origin;
+		const stray = await fetch(`${origin}/nope`);
+		expect(stray.status).toBe(404);
+
+		abort.abort("test done");
+		await login;
+	});
+});
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 860266c53..39812cf13 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed `/mcp reauth` against S256-only OAuth providers (Linear, and OAuth 2.1 flows generally) failing on Windows and other environments where the browser cannot auto-open. Two independent defects converged on Linear's `The plain PKCE method is not allowed. Use S256 instead.` error page: `openPath` invoked bare `rundll32` and silently swallowed the resulting `Executable not found in $PATH` when the Windows machine PATH no longer referenced `System32`, and `MCPAuthorizationLinkPrompt` composed a `Copy URL:` line wider than the viewport that `TUI#prepareLine` then silently truncated — trimming the trailing `code_challenge_method=S256` (RFC 7636 §4.3 treats a request with `code_challenge` and no method as `plain`). The MCP OAuth fallback, `/login`, setup wizard, auth-broker CLI, and login-dialog now advertise the short `OAuthAuthInfo.launchUrl` (loopback `/launch` route hosted by `OAuthCallbackFlow`) as the copy target; the OSC 8 hyperlink still carries the full URL for click-through; the MCP flow additionally stages the copy target on the clipboard via OSC 52; and the Windows opener now resolves `rundll32.exe` through `%SystemRoot%\System32\`, logging spawn failures and non-zero exits instead of dropping them ([#4418](https://github.com/can1357/oh-my-pi/issues/4418)).
+
 ## [16.3.4] - 2026-07-03
 
 ### Fixed
diff --git a/packages/coding-agent/src/cli/auth-broker-cli.ts b/packages/coding-agent/src/cli/auth-broker-cli.ts
index 2d04ee61f..0f504f6f1 100644
--- a/packages/coding-agent/src/cli/auth-broker-cli.ts
+++ b/packages/coding-agent/src/cli/auth-broker-cli.ts
@@ -222,8 +222,17 @@ async function runLocalLogin(provider: OAuthProvider): Promise<void> {
 		// for non-paste-code providers, so this is defense-in-depth on the same gate.
 		const usesManualInput = PASTE_CODE_LOGIN_PROVIDERS.has(provider);
 		await storage.login(provider, {
-			onAuth({ url, instructions }) {
-				process.stdout.write(`\nOpen this URL in your browser:\n${url}\n`);
+			onAuth({ url, launchUrl, instructions }) {
+				process.stdout.write("\nOpen this URL in your browser:\n");
+				// Advertise the short launch URL first when the flow exposes one — it
+				// survives narrow terminals that would truncate the trailing
+				// `code_challenge_method=S256` (or worse, drop `state`/`code_challenge`
+				// entirely) from the full authorize URL. The full URL still prints
+				// beneath it so headless callers can capture it programmatically.
+				if (launchUrl && launchUrl !== url) {
+					process.stdout.write(`${launchUrl}\n(redirects to)\n`);
+				}
+				process.stdout.write(`${url}\n`);
 				if (instructions) process.stdout.write(`${instructions}\n`);
 				process.stdout.write("\n");
 			},
diff --git a/packages/coding-agent/src/modes/components/login-dialog.ts b/packages/coding-agent/src/modes/components/login-dialog.ts
index 628cb8691..27c4ed6a5 100644
--- a/packages/coding-agent/src/modes/components/login-dialog.ts
+++ b/packages/coding-agent/src/modes/components/login-dialog.ts
@@ -68,12 +68,20 @@ export class LoginDialogComponent extends Container {
 	}
 
 	/**
-	 * Called by onAuth callback - show URL and optional instructions
+	 * Called by the OAuth `onAuth` callback. Renders the copy target on its own
+	 * row and attaches an OSC 8 hyperlink carrying the full URL so terminals
+	 * that support it can click-through even when the copy target is truncated.
+	 *
+	 * `launchUrl` (when present) is a short loopback URL that 302s to `url`.
+	 * Preferred as the copy target because viewport truncation on a long
+	 * authorize URL silently drops trailing OAuth query parameters — e.g.
+	 * `code_challenge_method=S256`.
 	 */
-	showAuth(url: string, instructions?: string): void {
+	showAuth(url: string, instructions?: string, launchUrl?: string): void {
+		const copyTarget = launchUrl ?? url;
 		this.#contentContainer.clear();
 		this.#contentContainer.addChild(new Spacer(1));
-		this.#contentContainer.addChild(new Text(theme.fg("accent", url), 1, 0));
+		this.#contentContainer.addChild(new Text(theme.fg("accent", copyTarget), 1, 0));
 
 		const clickHint = process.platform === "darwin" ? "Cmd+click to open" : "Ctrl+click to open";
 		const hyperlink = `\x1b]8;;${url}\x07${clickHint}\x1b]8;;\x07`;
diff --git a/packages/coding-agent/src/modes/controllers/mcp-command-controller.ts b/packages/coding-agent/src/modes/controllers/mcp-command-controller.ts
index 8e0e0aaec..a72507ba6 100644
--- a/packages/coding-agent/src/modes/controllers/mcp-command-controller.ts
+++ b/packages/coding-agent/src/modes/controllers/mcp-command-controller.ts
@@ -43,6 +43,7 @@ import {
 import type { MCPAuthConfig, MCPServerConfig, MCPServerConnection } from "../../mcp/types";
 import { shortenPath } from "../../tools/render-utils";
 import { urlHyperlinkAlways } from "../../tui";
+import { copyToClipboard } from "../../utils/clipboard";
 import { openPath } from "../../utils/open";
 import { ChatBlock } from "../components/chat-block";
 import { MCPAddWizard } from "../components/mcp-add-wizard";
@@ -73,22 +74,33 @@ function raceAbortSignal<T>(promise: Promise<T>, signal: AbortSignal, createErro
 	});
 }
 
-/** Renders the MCP OAuth fallback URL without hard-wrapping the copy target. */
+/**
+ * Renders the MCP OAuth fallback URL without hard-wrapping the copy target.
+ *
+ * When the flow's callback server hosts a `/launch` short URL (`launchUrl`),
+ * that is advertised as the copy target instead of the full authorization
+ * URL: a Linear-shaped authorize URL routinely exceeds 260 columns, and the
+ * TUI silently truncates any composed row wider than the viewport — dropping
+ * trailing OAuth parameters like `code_challenge_method=S256`. The OSC 8
+ * hyperlink still carries the full URL for terminals that support it.
+ */
 export class MCPAuthorizationLinkPrompt implements Component {
-	readonly #url: string;
+	readonly #fullUrl: string;
+	readonly #copyTarget: string;
 
-	constructor(url: string) {
-		this.#url = url;
+	constructor(url: string, launchUrl?: string) {
+		this.#fullUrl = url;
+		this.#copyTarget = launchUrl ?? url;
 	}
 
 	invalidate(): void {}
 
 	render(_width: number): readonly string[] {
-		const link = urlHyperlinkAlways(this.#url, "Click here to authorize");
+		const link = urlHyperlinkAlways(this.#fullUrl, "Click here to authorize");
 		return [
 			` ${theme.fg("success", "Open authorization URL:")}`,
 			` ${theme.fg("accent", link)}`,
-			` ${theme.fg("muted", `Copy URL: ${replaceTabs(this.#url)}`)}`,
+			` ${theme.fg("muted", `Copy URL: ${replaceTabs(this.#copyTarget)}`)}`,
 		];
 	}
 }
@@ -689,7 +701,7 @@ export class MCPCommandController {
 					stripSameOriginResource: opts?.stripSameOriginResource,
 				},
 				{
-					onAuth: (info: { url: string; instructions?: string }) => {
+					onAuth: (info: { url: string; launchUrl?: string; instructions?: string }) => {
 						// Show auth URL prominently in chat as one block
 						const block = new TranscriptBlock();
 						this.ctx.present(block);
@@ -707,24 +719,23 @@ export class MCPCommandController {
 						block.addChild(new Text(theme.fg("muted", MCP_MANUAL_LOGIN_TIP), 1, 0));
 						block.addChild(new Spacer(1));
 						block.addChild(new Text(theme.fg("accent", "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"), 1, 0));
-						// Try to open browser automatically
-						try {
-							openPath(info.url);
-
-							// Show confirmation that browser should open
-							block.addChild(new Spacer(1));
-							block.addChild(new Text(theme.fg("success", "→ Opening browser automatically..."), 1, 0));
-							block.addChild(new Spacer(1));
-							block.addChild(new Text(theme.fg("muted", "Alternative if browser did not open:"), 1, 0));
-							block.addChild(new MCPAuthorizationLinkPrompt(info.url));
-							this.ctx.ui.requestRender();
-						} catch (_error) {
-							// Show error if browser doesn't open
-							block.addChild(new Spacer(1));
-							block.addChild(new Text(theme.fg("warning", "→ Could not open browser automatically"), 1, 0));
-							block.addChild(new MCPAuthorizationLinkPrompt(info.url));
-							this.ctx.ui.requestRender();
-						}
+						// `openPath` is best-effort — it logs spawn failures but never
+						// throws, so we always render the copy-URL fallback beneath the
+						// "attempting to open browser" line and no earlier try/catch is
+						// worth keeping.
+						openPath(info.url);
+						const copyTarget = info.launchUrl ?? info.url;
+						// Stage the copy target on the clipboard via OSC 52 (same
+						// pattern the setup wizard uses). Best-effort: falls back to
+						// the visible "Copy URL:" line whether or not the terminal
+						// honors OSC 52.
+						void copyToClipboard(copyTarget).catch(() => {});
+						block.addChild(new Spacer(1));
+						block.addChild(new Text(theme.fg("success", "→ Attempting to open browser..."), 1, 0));
+						block.addChild(new Spacer(1));
+						block.addChild(new Text(theme.fg("muted", "Alternative if browser did not open:"), 1, 0));
+						block.addChild(new MCPAuthorizationLinkPrompt(info.url, info.launchUrl));
+						this.ctx.ui.requestRender();
 					},
 					onProgress: (message: string) => {
 						this.ctx.present([new Spacer(1), new Text(theme.fg("muted", message), 1, 0)]);
diff --git a/packages/coding-agent/src/modes/controllers/selector-controller.ts b/packages/coding-agent/src/modes/controllers/selector-controller.ts
index ec061edae..dac8f48bd 100644
--- a/packages/coding-agent/src/modes/controllers/selector-controller.ts
+++ b/packages/coding-agent/src/modes/controllers/selector-controller.ts
@@ -1117,9 +1117,10 @@ export class SelectorController {
 		const useManualInput = PASTE_CODE_LOGIN_PROVIDERS.has(providerId);
 		try {
 			await this.ctx.session.modelRegistry.authStorage.login(providerId as OAuthProvider, {
-				onAuth: (info: { url: string; instructions?: string }) => {
+				onAuth: (info: { url: string; launchUrl?: string; instructions?: string }) => {
+					const copyTarget = info.launchUrl ?? info.url;
 					const block = new TranscriptBlock();
-					block.addChild(new Text(theme.fg("dim", info.url), 1, 0));
+					block.addChild(new Text(theme.fg("dim", copyTarget), 1, 0));
 					const hyperlink = `\x1b]8;;${info.url}\x07Click here to login\x1b]8;;\x07`;
 					block.addChild(new Text(theme.fg("accent", hyperlink), 1, 0));
 					if (info.instructions) {
diff --git a/packages/coding-agent/src/modes/rpc/rpc-mode.ts b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
index 5c4cabd89..ac2a59ac1 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-mode.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
@@ -1213,6 +1213,7 @@ export async function runRpcMode(
 								id: Snowflake.next() as string,
 								method: "open_url",
 								url: info.url,
+								launchUrl: info.launchUrl,
 								instructions: info.instructions,
 							} as RpcExtensionUIRequest);
 						},
diff --git a/packages/coding-agent/src/modes/rpc/rpc-types.ts b/packages/coding-agent/src/modes/rpc/rpc-types.ts
index 10863bc79..a877079cd 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-types.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-types.ts
@@ -370,7 +370,19 @@ export type RpcExtensionUIRequest =
 	  }
 	| { type: "extension_ui_request"; id: string; method: "setTitle"; title: string }
 	| { type: "extension_ui_request"; id: string; method: "set_editor_text"; text: string }
-	| { type: "extension_ui_request"; id: string; method: "open_url"; url: string; instructions?: string };
+	| {
+			type: "extension_ui_request";
+			id: string;
+			method: "open_url";
+			url: string;
+			/**
+			 * Short loopback URL that 302-redirects to {@link url}. When present,
+			 * hosts SHOULD surface it as the copy target so terminal viewport
+			 * truncation cannot corrupt OAuth query parameters on the full URL.
+			 */
+			launchUrl?: string;
+			instructions?: string;
+	  };
 
 // ============================================================================
 // Host Tool Frames (bidirectional)
diff --git a/packages/coding-agent/src/modes/setup-wizard/scenes/sign-in.ts b/packages/coding-agent/src/modes/setup-wizard/scenes/sign-in.ts
index c0896c4da..7735e39ce 100644
--- a/packages/coding-agent/src/modes/setup-wizard/scenes/sign-in.ts
+++ b/packages/coding-agent/src/modes/setup-wizard/scenes/sign-in.ts
@@ -189,7 +189,11 @@ export class SignInTab implements SetupTab {
 			await this.#authStorage.login(providerId as OAuthProvider, {
 				signal: this.#loginAbort.signal,
 				onAuth: info => {
-					this.#authUrl = info.url;
+					// Store the short launch URL when available; the setup wizard
+					// renders `#authUrl` as a plain text row (no OSC 8 hyperlink), so
+					// the copy/display target must survive viewport truncation. The
+					// browser is still launched against the full URL.
+					this.#authUrl = info.launchUrl ?? info.url;
 					this.#statusLines = [];
 					if (info.instructions) {
 						this.#statusLines.push(theme.fg("warning", info.instructions));
diff --git a/packages/coding-agent/src/utils/open.ts b/packages/coding-agent/src/utils/open.ts
index db4090e5d..ca3390438 100644
--- a/packages/coding-agent/src/utils/open.ts
+++ b/packages/coding-agent/src/utils/open.ts
@@ -1,7 +1,7 @@
 import * as fs from "node:fs";
 import * as path from "node:path";
 import * as url from "node:url";
-import * as piUtils from "@oh-my-pi/pi-utils";
+import { $which, logger } from "@oh-my-pi/pi-utils";
 
 const URL_SCHEME_PATTERN = /^[a-zA-Z][a-zA-Z\d+.-]*:/;
 
@@ -9,7 +9,7 @@ function getExistingWslLocalPath(urlOrPath: string): string | undefined {
 	if (
 		process.platform !== "linux" ||
 		!(process.env.WSL_DISTRO_NAME || process.env.WSL_INTEROP) ||
-		!piUtils.$which("wslview")
+		!$which("wslview")
 	) {
 		return undefined;
 	}
@@ -31,6 +31,24 @@ function getExistingWslLocalPath(urlOrPath: string): string | undefined {
 	}
 }
 
+/**
+ * Resolve the Windows `rundll32.exe` command used to hand a URL/path to the
+ * user's registered protocol handler. Anchoring to `%SystemRoot%\System32`
+ * (rather than relying on `rundll32` being on `PATH`) survives environments
+ * where the machine `PATH` no longer references `System32` — a common
+ * real-world misconfiguration where `System32\Wbem` / `WindowsPowerShell` /
+ * `OpenSSH` survive but `System32` itself is dropped. Bare `rundll32` on
+ * such boxes throws `Executable not found in $PATH: "rundll32"` from
+ * `Bun.spawn` before ShellExecute ever sees the URL.
+ */
+function windowsOpenerCommand(target: string): string[] {
+	const systemRoot = process.env.SystemRoot?.trim() || process.env.SYSTEMROOT?.trim() || "C:\\Windows";
+	// `path.win32` (not the platform-adaptive `path.join`) keeps Windows path
+	// separators when tests run under a POSIX host and matches Windows call
+	// conventions on the real target.
+	const rundll32 = path.win32.join(systemRoot, "System32", "rundll32.exe");
+	return [rundll32, "url.dll,FileProtocolHandler", target];
+}
 /** Open a URL or file path in the default browser/application. Best-effort, never throws. */
 export function openPath(urlOrPath: string): void {
 	let cmd: string[];
@@ -39,7 +57,7 @@ export function openPath(urlOrPath: string): void {
 			cmd = ["open", urlOrPath];
 			break;
 		case "win32":
-			cmd = ["rundll32", "url.dll,FileProtocolHandler", urlOrPath];
+			cmd = windowsOpenerCommand(urlOrPath);
 			break;
 		default: {
 			const wslPath = getExistingWslLocalPath(urlOrPath);
@@ -47,9 +65,36 @@ export function openPath(urlOrPath: string): void {
 			break;
 		}
 	}
+	let child: Bun.Subprocess | undefined;
 	try {
-		Bun.spawn(cmd, { stdin: "ignore", stdout: "ignore", stderr: "ignore" });
-	} catch {
-		// Best-effort: browser opening is non-critical
+		child = Bun.spawn(cmd, { stdin: "ignore", stdout: "ignore", stderr: "ignore" });
+	} catch (error) {
+		// Spawn threw synchronously (missing binary, denied exec, sandbox
+		// restriction, …). Best-effort: log so the failure isn't invisible while
+		// still letting the caller advertise a copy-URL fallback.
+		logger.warn("Failed to open external URL/path", {
+			command: cmd[0],
+			target: urlOrPath,
+			error: error instanceof Error ? error.message : String(error),
+		});
+		return;
 	}
+	// Detect delayed failures (exec succeeded but the opener exited non-zero)
+	// without blocking the caller. Recording them makes silent misconfigurations
+	// (e.g. `xdg-open` present but no MIME handler for `https`) diagnosable from
+	// `~/.omp/logs/omp.*.log`.
+	child.exited.then(
+		exitCode => {
+			if (typeof exitCode === "number" && exitCode !== 0) {
+				logger.warn("External opener exited with non-zero status", {
+					command: cmd[0],
+					target: urlOrPath,
+					exitCode,
+				});
+			}
+		},
+		() => {
+			// Ignore — awaiting the subprocess is best-effort telemetry.
+		},
+	);
 }
diff --git a/packages/coding-agent/test/modes/controllers/mcp-authorization-link.test.ts b/packages/coding-agent/test/modes/controllers/mcp-authorization-link.test.ts
index 666ce0b47..6e77e95de 100644
--- a/packages/coding-agent/test/modes/controllers/mcp-authorization-link.test.ts
+++ b/packages/coding-agent/test/modes/controllers/mcp-authorization-link.test.ts
@@ -37,4 +37,22 @@ describe("MCPAuthorizationLinkPrompt", () => {
 		expect(plainLines[1]).toContain("Click here to authorize");
 		expect(plainLines[2]).toBe(` Copy URL: ${LONG_AUTH_URL}`);
 	});
+
+	it("advertises the launch URL as the copy target while keeping OSC 8 pointing at the full URL", () => {
+		const launchUrl = "http://localhost:14570/launch";
+		const lines = new MCPAuthorizationLinkPrompt(LONG_AUTH_URL, launchUrl).render(80);
+		const plainLines = lines.map(line => stripVTControlCharacters(line));
+
+		expect(lines).toHaveLength(3);
+		// OSC 8 hyperlink still carries the full URL — click-through targets
+		// the provider directly on terminals that support the escape.
+		expect(extractLinkUri(lines[1])).toBe(LONG_AUTH_URL);
+		expect(plainLines[1]).toContain("Click here to authorize");
+		// Copy target is the short loopback URL. Terminals that don't render
+		// OSC 8, and every copy-paste operation, hit this line — and it must
+		// survive viewport truncation without dropping OAuth parameters like
+		// `code_challenge_method=S256`.
+		expect(plainLines[2]).toBe(` Copy URL: ${launchUrl}`);
+		expect(plainLines[2].length).toBeLessThan(50);
+	});
 });
diff --git a/packages/coding-agent/test/utils/open.test.ts b/packages/coding-agent/test/utils/open.test.ts
index 6495da1dc..467c28f4e 100644
--- a/packages/coding-agent/test/utils/open.test.ts
+++ b/packages/coding-agent/test/utils/open.test.ts
@@ -132,4 +132,51 @@ describe("openPath", () => {
 		expect(spawnSyncSpy).not.toHaveBeenCalled();
 		expect(spawnCalls.map(call => call.cmd)).toEqual([["xdg-open", existingLinuxPath]]);
 	});
+
+	it("resolves rundll32 through %SystemRoot% so a broken machine PATH cannot silence the opener", () => {
+		setPlatform("win32");
+		const originalSystemRoot = process.env.SystemRoot;
+		process.env.SystemRoot = "D:\\CustomWindows";
+		try {
+			const spawnCalls: SpawnCall[] = [];
+			spySpawn(spawnCalls);
+
+			openPath("https://mcp.linear.app/authorize?state=xyz&code_challenge_method=S256");
+
+			expect(spawnCalls).toHaveLength(1);
+			const [call] = spawnCalls;
+			// Absolute rundll32 path — bare `rundll32` was the whole bug on Windows
+			// boxes where the machine PATH no longer references System32.
+			expect(call?.cmd[0]).toBe("D:\\CustomWindows\\System32\\rundll32.exe");
+			// Handler + URL forwarded verbatim as a single argv slot so `&` in the
+			// query string cannot be interpreted as a shell separator.
+			expect(call?.cmd.slice(1)).toEqual([
+				"url.dll,FileProtocolHandler",
+				"https://mcp.linear.app/authorize?state=xyz&code_challenge_method=S256",
+			]);
+		} finally {
+			if (originalSystemRoot === undefined) delete process.env.SystemRoot;
+			else process.env.SystemRoot = originalSystemRoot;
+		}
+	});
+
+	it("falls back to C:\\Windows for rundll32 when SystemRoot is unset", () => {
+		setPlatform("win32");
+		const originalSystemRoot = process.env.SystemRoot;
+		const originalSystemRootLower = process.env.SYSTEMROOT;
+		delete process.env.SystemRoot;
+		delete process.env.SYSTEMROOT;
+		try {
+			const spawnCalls: SpawnCall[] = [];
+			spySpawn(spawnCalls);
+
+			openPath("https://example.com");
+
+			expect(spawnCalls).toHaveLength(1);
+			expect(spawnCalls[0]?.cmd[0]).toBe("C:\\Windows\\System32\\rundll32.exe");
+		} finally {
+			if (originalSystemRoot !== undefined) process.env.SystemRoot = originalSystemRoot;
+			if (originalSystemRootLower !== undefined) process.env.SYSTEMROOT = originalSystemRootLower;
+		}
+	});
 });