diff --git a/src/cliproxy/auth-handler.ts b/src/cliproxy/auth-handler.ts
index b854312f..23b92fd2 100644
--- a/src/cliproxy/auth-handler.ts
+++ b/src/cliproxy/auth-handler.ts
@@ -27,6 +27,7 @@ import {
   registerAccount,
   touchAccount,
 } from './account-manager';
+import { preflightOAuthCheck } from '../management/oauth-port-diagnostics';
 
 /**
  * OAuth callback ports used by CLIProxyAPI (hardcoded in binary)
@@ -90,26 +91,50 @@ function killProcessOnPort(port: number, verbose: boolean): boolean {
 
 /**
  * Detect if running in a headless environment (no browser available)
+ *
+ * IMPROVED: Avoids false positives on Windows desktop environments
+ * where isTTY may be undefined due to terminal wrapper behavior.
+ *
+ * Case study: Vietnamese Windows users reported "command hangs" because
+ * their terminal (PowerShell via npm) didn't set isTTY correctly.
  */
 function isHeadlessEnvironment(): boolean {
-  // SSH session
+  // SSH session - always headless
   if (process.env.SSH_TTY || process.env.SSH_CLIENT || process.env.SSH_CONNECTION) {
     return true;
   }
 
-  // No display (Linux/X11)
+  // No display on Linux (X11/Wayland) - headless
   if (process.platform === 'linux' && !process.env.DISPLAY && !process.env.WAYLAND_DISPLAY) {
     return true;
   }
 
-  // Non-interactive (piped stdin) - skip on Windows
-  // Windows npm wrappers don't set isTTY correctly (returns undefined, not true)
+  // Windows desktop - NEVER headless unless SSH (already checked above)
+  // This fixes false positive where Windows npm wrappers don't set isTTY correctly
   // Windows desktop environments always have browser capability
-  if (process.platform !== 'win32' && !process.stdin.isTTY) {
-    return true;
+  if (process.platform === 'win32') {
+    return false;
   }
 
-  return false;
+  // macOS - check for proper terminal
+  if (process.platform === 'darwin') {
+    // Non-interactive stdin on macOS means likely piped/scripted
+    if (!process.stdin.isTTY) {
+      return true;
+    }
+    return false;
+  }
+
+  // Linux with display - check TTY
+  if (process.platform === 'linux') {
+    if (!process.stdin.isTTY) {
+      return true;
+    }
+    return false;
+  }
+
+  // Default fallback for unknown platforms
+  return !process.stdin.isTTY;
 }
 
 /**
@@ -422,6 +447,19 @@ export async function triggerOAuth(
     }
   };
 
+  // Pre-flight check: verify OAuth callback port is available
+  const preflight = await preflightOAuthCheck(provider);
+  if (!preflight.ready) {
+    console.log('');
+    console.log('[!] OAuth pre-flight check failed:');
+    for (const issue of preflight.issues) {
+      console.log(`    ${issue}`);
+    }
+    console.log('');
+    console.log('[i] Resolve the port conflict and try again.');
+    return null;
+  }
+
   // Ensure binary exists
   let binaryPath: string;
   try {
diff --git a/src/management/doctor.ts b/src/management/doctor.ts
index 6f4058a3..cfb101f9 100644
--- a/src/management/doctor.ts
+++ b/src/management/doctor.ts
@@ -19,6 +19,8 @@ import {
   CLIPROXY_DEFAULT_PORT,
 } from '../cliproxy';
 import { getPortProcess, isCLIProxyProcess } from '../utils/port-utils';
+import { getEnvironmentDiagnostics } from './environment-diagnostics';
+import { checkAuthCodePorts } from './oauth-port-diagnostics';
 
 // Make ora optional (might not be available during npm install postinstall)
 // ora v9+ is an ES module, need to use .default for CommonJS
@@ -146,31 +148,41 @@ class Doctor {
     this.checkCcsDirectory();
     console.log('');
 
-    // Group 2: Configuration
+    // Group 2: Environment (NEW - OAuth readiness diagnostics)
+    console.log(header('ENVIRONMENT'));
+    this.checkEnvironment();
+    console.log('');
+
+    // Group 3: Configuration
     console.log(header('CONFIGURATION'));
     this.checkConfigFiles();
     this.checkClaudeSettings();
     console.log('');
 
-    // Group 3: Profiles & Delegation
+    // Group 4: Profiles & Delegation
     console.log(header('PROFILES & DELEGATION'));
     this.checkProfiles();
     this.checkInstances();
     this.checkDelegation();
     console.log('');
 
-    // Group 4: System Health
+    // Group 5: System Health
     console.log(header('SYSTEM HEALTH'));
     this.checkPermissions();
     this.checkCcsSymlinks();
     this.checkSettingsSymlinks();
     console.log('');
 
-    // Group 5: CLIProxy (OAuth profiles)
+    // Group 6: CLIProxy (OAuth profiles)
     console.log(header('CLIPROXY (OAUTH PROFILES)'));
     await this.checkCLIProxy();
     console.log('');
 
+    // Group 7: OAuth Readiness (NEW - port availability)
+    console.log(header('OAUTH READINESS'));
+    await this.checkOAuthPorts();
+    console.log('');
+
     this.showReport();
     return this.results;
   }
@@ -766,6 +778,117 @@ class Doctor {
     }
   }
 
+  /**
+   * Check 10.1: Environment detection (OAuth readiness)
+   * Helps diagnose Windows headless false positives
+   */
+  private checkEnvironment(): void {
+    const spinner = ora('Checking environment').start();
+    const diag = getEnvironmentDiagnostics();
+
+    // Determine overall environment health
+    let envStatus: 'OK' | 'WARN' = 'OK';
+    let envMessage = 'Browser available';
+
+    // Check for potential issues
+    if (diag.detectedHeadless) {
+      if (diag.platform === 'win32' && diag.ttyStatus === 'undefined') {
+        // Windows false positive - this is actually a warning
+        envStatus = 'WARN';
+        envMessage = 'Headless detected (may be false positive on Windows)';
+      } else if (diag.sshSession) {
+        envMessage = 'SSH session (headless mode)';
+      } else {
+        envMessage = 'Headless environment';
+      }
+    }
+
+    if (envStatus === 'WARN') {
+      spinner.warn();
+      console.log(`  ${warn('Environment'.padEnd(22))}  ${envMessage}`);
+    } else {
+      spinner.succeed();
+      console.log(`  ${ok('Environment'.padEnd(22))}  ${envMessage}`);
+    }
+
+    // Show key environment details
+    console.log(`  ${''.padEnd(24)}  Platform: ${diag.platformName}`);
+    if (diag.sshSession) {
+      console.log(`  ${''.padEnd(24)}  SSH: Yes (${diag.sshReason})`);
+    }
+    if (diag.ttyStatus === 'undefined') {
+      console.log(`  ${''.padEnd(24)}  TTY: undefined [!]`);
+    }
+    console.log(`  ${''.padEnd(24)}  Browser: ${diag.browserReason}`);
+
+    this.results.addCheck(
+      'Environment',
+      envStatus === 'OK' ? 'success' : 'warning',
+      envMessage,
+      envStatus === 'WARN' ? 'If browser opens correctly, this warning can be ignored' : undefined,
+      {
+        status: envStatus,
+        info: envMessage,
+      }
+    );
+  }
+
+  /**
+   * Check 10.2: OAuth callback ports availability
+   * Pre-flight check for OAuth authentication
+   */
+  private async checkOAuthPorts(): Promise<void> {
+    const spinner = ora('Checking OAuth callback ports').start();
+    const portDiagnostics = await checkAuthCodePorts();
+
+    // Count issues
+    const conflicts = portDiagnostics.filter((d) => d.status === 'occupied');
+
+    if (conflicts.length === 0) {
+      spinner.succeed();
+      console.log(`  ${ok('OAuth Ports'.padEnd(22))}  All callback ports available`);
+      this.results.addCheck('OAuth Ports', 'success', undefined, undefined, {
+        status: 'OK',
+        info: 'All callback ports available',
+      });
+    } else {
+      spinner.warn();
+      console.log(`  ${warn('OAuth Ports'.padEnd(22))}  ${conflicts.length} port conflict(s)`);
+      this.results.addCheck(
+        'OAuth Ports',
+        'warning',
+        `${conflicts.length} port conflict(s)`,
+        'Close conflicting applications before OAuth',
+        { status: 'WARN', info: `${conflicts.length} conflict(s)` }
+      );
+    }
+
+    // Show individual port status
+    for (const diag of portDiagnostics) {
+      const providerName = diag.provider.charAt(0).toUpperCase() + diag.provider.slice(1);
+      const portStr = diag.port !== null ? `(${diag.port})` : '';
+
+      let statusIcon: string;
+      switch (diag.status) {
+        case 'free':
+        case 'cliproxy':
+          statusIcon = ok(`${providerName} ${portStr}`.padEnd(20));
+          break;
+        case 'occupied':
+          statusIcon = warn(`${providerName} ${portStr}`.padEnd(20));
+          break;
+        default:
+          statusIcon = info(`${providerName} ${portStr}`.padEnd(20));
+      }
+
+      console.log(`  ${statusIcon}  ${diag.message}`);
+
+      if (diag.recommendation && diag.status === 'occupied') {
+        console.log(`  ${''.padEnd(24)}  Fix: ${diag.recommendation}`);
+      }
+    }
+  }
+
   /**
    * Check 11: CLIProxy health (OAuth profiles: gemini, codex, agy, qwen)
    */
diff --git a/src/management/environment-diagnostics.ts b/src/management/environment-diagnostics.ts
new file mode 100644
index 00000000..7083256a
--- /dev/null
+++ b/src/management/environment-diagnostics.ts
@@ -0,0 +1,310 @@
+/**
+ * Environment Diagnostics Module
+ *
+ * Provides detailed environment detection diagnostics for troubleshooting
+ * OAuth authentication issues, particularly for Windows users experiencing
+ * false headless detection.
+ *
+ * Case study: Vietnamese Windows users reported "command hangs" because
+ * Windows terminal wrappers don't set isTTY correctly, triggering
+ * headless mode when browser should be available.
+ */
+
+/**
+ * Detailed environment diagnostic information
+ */
+export interface EnvironmentDiagnostics {
+  /** Operating system platform */
+  platform: NodeJS.Platform;
+  /** Platform display name */
+  platformName: string;
+  /** Current shell (from SHELL env or detected) */
+  shell: string;
+  /** Terminal program (TERM_PROGRAM env) */
+  termProgram: string | null;
+  /** Whether running in SSH session */
+  sshSession: boolean;
+  /** SSH detection reason */
+  sshReason: string | null;
+  /** stdin TTY status */
+  ttyStatus: boolean | 'undefined';
+  /** stdout TTY status */
+  stdoutTty: boolean | 'undefined';
+  /** Display environment (Linux X11/Wayland) */
+  display: string | null;
+  /** Windows Terminal detected (WT_SESSION env) */
+  windowsTerminal: boolean;
+  /** VS Code integrated terminal detected */
+  vsCodeTerminal: boolean;
+  /** CI environment detected */
+  ciEnvironment: boolean;
+  /** Final headless detection result */
+  detectedHeadless: boolean;
+  /** Reasons why headless was detected (or not) */
+  headlessReasons: string[];
+  /** Browser capability assessment */
+  browserCapability: 'available' | 'unlikely' | 'unknown';
+  /** Browser capability reason */
+  browserReason: string;
+}
+
+/**
+ * Detect current shell from environment
+ */
+function detectShell(): string {
+  if (process.env.SHELL) {
+    return process.env.SHELL;
+  }
+  if (process.platform === 'win32') {
+    if (process.env.PSModulePath) {
+      return 'PowerShell';
+    }
+    if (process.env.COMSPEC) {
+      return process.env.COMSPEC;
+    }
+    return 'cmd.exe';
+  }
+  return 'unknown';
+}
+
+/**
+ * Check if running in SSH session
+ */
+function checkSshSession(): { isSsh: boolean; reason: string | null } {
+  if (process.env.SSH_TTY) {
+    return { isSsh: true, reason: 'SSH_TTY set' };
+  }
+  if (process.env.SSH_CLIENT) {
+    return { isSsh: true, reason: 'SSH_CLIENT set' };
+  }
+  if (process.env.SSH_CONNECTION) {
+    return { isSsh: true, reason: 'SSH_CONNECTION set' };
+  }
+  return { isSsh: false, reason: null };
+}
+
+/**
+ * Check browser capability based on environment
+ */
+function assessBrowserCapability(
+  platform: NodeJS.Platform,
+  sshSession: boolean,
+  display: string | null,
+  windowsTerminal: boolean,
+  vsCodeTerminal: boolean
+): { capability: 'available' | 'unlikely' | 'unknown'; reason: string } {
+  // SSH session - no browser
+  if (sshSession) {
+    return { capability: 'unlikely', reason: 'SSH session detected' };
+  }
+
+  // Windows desktop - should always have browser
+  if (platform === 'win32') {
+    if (windowsTerminal || vsCodeTerminal) {
+      return { capability: 'available', reason: 'Windows desktop terminal' };
+    }
+    // Even plain cmd.exe on Windows desktop should have browser access
+    return { capability: 'available', reason: 'Windows desktop environment' };
+  }
+
+  // macOS - always has browser unless SSH
+  if (platform === 'darwin') {
+    return { capability: 'available', reason: 'macOS desktop environment' };
+  }
+
+  // Linux - depends on DISPLAY/WAYLAND
+  if (platform === 'linux') {
+    if (display) {
+      return { capability: 'available', reason: `Display available: ${display}` };
+    }
+    return { capability: 'unlikely', reason: 'No DISPLAY or WAYLAND_DISPLAY' };
+  }
+
+  return { capability: 'unknown', reason: 'Unknown platform' };
+}
+
+/**
+ * Determine if environment is headless (improved detection)
+ * Returns both result and reasons for transparency
+ */
+function detectHeadless(): { isHeadless: boolean; reasons: string[] } {
+  const reasons: string[] = [];
+
+  // SSH session
+  if (process.env.SSH_TTY || process.env.SSH_CLIENT || process.env.SSH_CONNECTION) {
+    reasons.push('SSH session detected');
+    return { isHeadless: true, reasons };
+  }
+
+  // Linux without display
+  if (process.platform === 'linux' && !process.env.DISPLAY && !process.env.WAYLAND_DISPLAY) {
+    reasons.push('Linux without DISPLAY/WAYLAND_DISPLAY');
+    return { isHeadless: true, reasons };
+  }
+
+  // Windows desktop - NEVER headless unless SSH (already checked above)
+  // This fixes false positive on Windows where isTTY is undefined
+  if (process.platform === 'win32') {
+    reasons.push('Windows desktop - browser available');
+    return { isHeadless: false, reasons };
+  }
+
+  // macOS - check for proper terminal
+  if (process.platform === 'darwin') {
+    if (!process.stdin.isTTY) {
+      reasons.push('Non-interactive stdin on macOS');
+      return { isHeadless: true, reasons };
+    }
+    reasons.push('macOS with TTY - browser available');
+    return { isHeadless: false, reasons };
+  }
+
+  // Linux with display - check TTY
+  if (process.platform === 'linux') {
+    if (!process.stdin.isTTY) {
+      reasons.push('Non-interactive stdin on Linux');
+      return { isHeadless: true, reasons };
+    }
+    reasons.push('Linux with display and TTY');
+    return { isHeadless: false, reasons };
+  }
+
+  // Default fallback
+  reasons.push('Default: assuming headless');
+  return { isHeadless: true, reasons };
+}
+
+/**
+ * Get comprehensive environment diagnostics
+ */
+export function getEnvironmentDiagnostics(): EnvironmentDiagnostics {
+  const platform = process.platform;
+  const ssh = checkSshSession();
+  const display =
+    process.platform === 'linux'
+      ? process.env.DISPLAY || process.env.WAYLAND_DISPLAY || null
+      : null;
+  const windowsTerminal = !!process.env.WT_SESSION;
+  const vsCodeTerminal = !!(
+    process.env.VSCODE_GIT_IPC_HANDLE ||
+    process.env.VSCODE_IPC_HOOK_CLI ||
+    process.env.TERM_PROGRAM === 'vscode'
+  );
+  const ciEnvironment = !!(
+    process.env.CI ||
+    process.env.GITHUB_ACTIONS ||
+    process.env.GITLAB_CI ||
+    process.env.JENKINS_URL
+  );
+
+  const browser = assessBrowserCapability(
+    platform,
+    ssh.isSsh,
+    display,
+    windowsTerminal,
+    vsCodeTerminal
+  );
+  const headless = detectHeadless();
+
+  // Platform display name
+  const platformNames: Record<string, string> = {
+    win32: 'Windows',
+    darwin: 'macOS',
+    linux: 'Linux',
+    freebsd: 'FreeBSD',
+    openbsd: 'OpenBSD',
+  };
+
+  return {
+    platform,
+    platformName: platformNames[platform] || platform,
+    shell: detectShell(),
+    termProgram: process.env.TERM_PROGRAM || null,
+    sshSession: ssh.isSsh,
+    sshReason: ssh.reason,
+    ttyStatus: process.stdin.isTTY === undefined ? 'undefined' : process.stdin.isTTY,
+    stdoutTty: process.stdout.isTTY === undefined ? 'undefined' : process.stdout.isTTY,
+    display,
+    windowsTerminal,
+    vsCodeTerminal,
+    ciEnvironment,
+    detectedHeadless: headless.isHeadless,
+    headlessReasons: headless.reasons,
+    browserCapability: browser.capability,
+    browserReason: browser.reason,
+  };
+}
+
+/**
+ * Check if environment should use headless OAuth flow
+ * Uses improved detection that avoids Windows false positives
+ */
+export function shouldUseHeadlessAuth(): boolean {
+  return detectHeadless().isHeadless;
+}
+
+/**
+ * Format diagnostics for display
+ */
+export function formatEnvironmentDiagnostics(diag: EnvironmentDiagnostics): string[] {
+  const lines: string[] = [];
+
+  // Platform
+  lines.push(`Platform                ${diag.platformName} (${diag.platform})`);
+
+  // Shell
+  lines.push(`Shell                   ${diag.shell}`);
+
+  // Terminal
+  if (diag.termProgram) {
+    lines.push(`Terminal Program        ${diag.termProgram}`);
+  }
+  if (diag.windowsTerminal) {
+    lines.push(`Windows Terminal        Yes`);
+  }
+  if (diag.vsCodeTerminal) {
+    lines.push(`VS Code Terminal        Yes`);
+  }
+
+  // SSH
+  lines.push(`SSH Session             ${diag.sshSession ? `Yes (${diag.sshReason})` : 'No'}`);
+
+  // TTY
+  const ttyDisplay =
+    diag.ttyStatus === 'undefined' ? 'undefined [!]' : diag.ttyStatus ? 'Yes' : 'No';
+  lines.push(`stdin TTY               ${ttyDisplay}`);
+
+  // Display (Linux)
+  if (diag.platform === 'linux') {
+    lines.push(`Display                 ${diag.display || 'Not set'}`);
+  }
+
+  // CI
+  if (diag.ciEnvironment) {
+    lines.push(`CI Environment          Yes`);
+  }
+
+  // Browser capability
+  const browserIcon =
+    diag.browserCapability === 'available'
+      ? '[OK]'
+      : diag.browserCapability === 'unlikely'
+        ? '[!]'
+        : '[?]';
+  lines.push(`Browser Capability      ${browserIcon} ${diag.browserReason}`);
+
+  // Headless detection result
+  const headlessIcon = diag.detectedHeadless ? '[!]' : '[OK]';
+  lines.push(`Headless Mode           ${headlessIcon} ${diag.detectedHeadless ? 'Yes' : 'No'}`);
+  for (const reason of diag.headlessReasons) {
+    lines.push(`                        → ${reason}`);
+  }
+
+  return lines;
+}
+
+export default {
+  getEnvironmentDiagnostics,
+  shouldUseHeadlessAuth,
+  formatEnvironmentDiagnostics,
+};
diff --git a/src/management/oauth-port-diagnostics.ts b/src/management/oauth-port-diagnostics.ts
new file mode 100644
index 00000000..ccc255a0
--- /dev/null
+++ b/src/management/oauth-port-diagnostics.ts
@@ -0,0 +1,236 @@
+/**
+ * OAuth Port Diagnostics Module
+ *
+ * Pre-flight checks for OAuth callback ports to detect conflicts
+ * before users attempt authentication.
+ *
+ * OAuth flows require specific localhost ports for callbacks:
+ * - Gemini: 8085
+ * - Codex: 1455
+ * - Agy: 51121
+ * - Qwen: Device Code Flow (no port needed)
+ */
+
+import { getPortProcess, PortProcess, isCLIProxyProcess } from '../utils/port-utils';
+import { CLIProxyProvider } from '../cliproxy/types';
+
+/**
+ * OAuth callback ports for each provider
+ * Extracted from CLIProxyAPI source
+ */
+export const OAUTH_CALLBACK_PORTS: Record<CLIProxyProvider, number | null> = {
+  gemini: 8085,
+  codex: 1455,
+  agy: 51121,
+  qwen: null, // Device Code Flow - no callback port
+  iflow: null, // Device Code Flow - no callback port
+};
+
+/**
+ * OAuth flow types
+ */
+export type OAuthFlowType = 'authorization_code' | 'device_code';
+
+/**
+ * OAuth flow type per provider
+ */
+export const OAUTH_FLOW_TYPES: Record<CLIProxyProvider, OAuthFlowType> = {
+  gemini: 'authorization_code',
+  codex: 'authorization_code',
+  agy: 'authorization_code',
+  qwen: 'device_code',
+  iflow: 'device_code',
+};
+
+/**
+ * Port diagnostic result
+ */
+export interface OAuthPortDiagnostic {
+  /** Provider name */
+  provider: CLIProxyProvider;
+  /** OAuth flow type */
+  flowType: OAuthFlowType;
+  /** Callback port (null for device code flow) */
+  port: number | null;
+  /** Port status */
+  status: 'free' | 'occupied' | 'cliproxy' | 'not_applicable';
+  /** Process occupying the port (if any) */
+  process: PortProcess | null;
+  /** Human-readable status message */
+  message: string;
+  /** Recommendation for fixing (if issue detected) */
+  recommendation: string | null;
+}
+
+/**
+ * Check OAuth port availability for a single provider
+ */
+export async function checkOAuthPort(provider: CLIProxyProvider): Promise<OAuthPortDiagnostic> {
+  const port = OAUTH_CALLBACK_PORTS[provider];
+  const flowType = OAUTH_FLOW_TYPES[provider];
+
+  // Device code flow doesn't need callback port
+  if (port === null) {
+    return {
+      provider,
+      flowType,
+      port: null,
+      status: 'not_applicable',
+      process: null,
+      message: 'Uses Device Code Flow (no callback port needed)',
+      recommendation: null,
+    };
+  }
+
+  // Check if port is in use
+  const portProcess = await getPortProcess(port);
+
+  if (!portProcess) {
+    return {
+      provider,
+      flowType,
+      port,
+      status: 'free',
+      process: null,
+      message: `Port ${port} is available`,
+      recommendation: null,
+    };
+  }
+
+  // Check if it's CLIProxy (expected if proxy is running)
+  if (isCLIProxyProcess(portProcess)) {
+    return {
+      provider,
+      flowType,
+      port,
+      status: 'cliproxy',
+      process: portProcess,
+      message: `Port ${port} in use by CLIProxy (expected)`,
+      recommendation: null,
+    };
+  }
+
+  // Port is occupied by another process
+  return {
+    provider,
+    flowType,
+    port,
+    status: 'occupied',
+    process: portProcess,
+    message: `Port ${port} occupied by ${portProcess.processName}`,
+    recommendation: `Kill process: kill ${portProcess.pid} (or close ${portProcess.processName})`,
+  };
+}
+
+/**
+ * Check OAuth ports for all providers
+ */
+export async function checkAllOAuthPorts(): Promise<OAuthPortDiagnostic[]> {
+  const providers: CLIProxyProvider[] = ['gemini', 'codex', 'agy', 'qwen', 'iflow'];
+  const results: OAuthPortDiagnostic[] = [];
+
+  for (const provider of providers) {
+    const diagnostic = await checkOAuthPort(provider);
+    results.push(diagnostic);
+  }
+
+  return results;
+}
+
+/**
+ * Check OAuth ports for providers that use Authorization Code flow only
+ */
+export async function checkAuthCodePorts(): Promise<OAuthPortDiagnostic[]> {
+  const providers: CLIProxyProvider[] = ['gemini', 'codex', 'agy'];
+  const results: OAuthPortDiagnostic[] = [];
+
+  for (const provider of providers) {
+    const diagnostic = await checkOAuthPort(provider);
+    results.push(diagnostic);
+  }
+
+  return results;
+}
+
+/**
+ * Get providers with port conflicts
+ */
+export async function getPortConflicts(): Promise<OAuthPortDiagnostic[]> {
+  const allPorts = await checkAllOAuthPorts();
+  return allPorts.filter((d) => d.status === 'occupied');
+}
+
+/**
+ * Format OAuth port diagnostics for display
+ */
+export function formatOAuthPortDiagnostics(diagnostics: OAuthPortDiagnostic[]): string[] {
+  const lines: string[] = [];
+
+  for (const diag of diagnostics) {
+    const providerName = diag.provider.charAt(0).toUpperCase() + diag.provider.slice(1);
+    const portStr = diag.port !== null ? `(${diag.port})` : '';
+
+    let statusIcon: string;
+    switch (diag.status) {
+      case 'free':
+        statusIcon = '[OK]';
+        break;
+      case 'cliproxy':
+        statusIcon = '[OK]';
+        break;
+      case 'occupied':
+        statusIcon = '[!]';
+        break;
+      case 'not_applicable':
+        statusIcon = '[i]';
+        break;
+      default:
+        statusIcon = '[?]';
+    }
+
+    const label = `${providerName} ${portStr}`.padEnd(20);
+    lines.push(`${statusIcon} ${label} ${diag.message}`);
+
+    if (diag.recommendation) {
+      lines.push(`                        → ${diag.recommendation}`);
+    }
+  }
+
+  return lines;
+}
+
+/**
+ * Pre-flight check before OAuth - returns issues or empty array if OK
+ */
+export async function preflightOAuthCheck(provider: CLIProxyProvider): Promise<{
+  ready: boolean;
+  issues: string[];
+}> {
+  const diagnostic = await checkOAuthPort(provider);
+  const issues: string[] = [];
+
+  if (diagnostic.status === 'occupied' && diagnostic.process) {
+    issues.push(
+      `OAuth callback port ${diagnostic.port} is blocked by ${diagnostic.process.processName} (PID ${diagnostic.process.pid})`
+    );
+    if (diagnostic.recommendation) {
+      issues.push(`Fix: ${diagnostic.recommendation}`);
+    }
+  }
+
+  return {
+    ready: issues.length === 0,
+    issues,
+  };
+}
+
+export default {
+  OAUTH_CALLBACK_PORTS,
+  OAUTH_FLOW_TYPES,
+  checkOAuthPort,
+  checkAllOAuthPorts,
+  checkAuthCodePorts,
+  getPortConflicts,
+  formatOAuthPortDiagnostics,
+  preflightOAuthCheck,
+};