import type { OAuth2Config } from "../core/types.js";
/** Default Google OAuth2 token endpoint. */
export declare const GOOGLE_TOKEN_URL = "https://oauth2.googleapis.com/token";
/** Microsoft OAuth2 token endpoint (common tenant). */
export declare const MICROSOFT_TOKEN_URL = "https://login.microsoftonline.com/common/oauth2/v2.0/token";
/** OAuth2 token endpoint response shape. */
export interface TokenResponse {
    /** Bearer access token for API or SMTP XOAUTH2. */
    access_token: string;
    /** Token lifetime in seconds. */
    expires_in: number;
    /** Token type (typically `"Bearer"`). */
    token_type: string;
}
/**
 * OAuth2 client with in-memory token cache and automatic refresh.
 */
export declare class OAuth2Client {
    /** OAuth2 configuration supplied at construction. */
    private readonly config;
    /** Cached access token, or null before the first fetch. */
    private cachedToken;
    /** Expiry timestamp (ms) for the cached access token. */
    private expiresAt;
    /** In-flight refresh promise used to deduplicate concurrent refreshes. */
    private refreshPromise;
    /** Creates an OAuth2 client from configuration. */
    constructor(config: OAuth2Config);
    /**
     * Get a valid access token (cached when still valid, refreshed otherwise).
     */
    getAccessToken(): Promise<string>;
    /**
     * Force-refresh the access token regardless of expiry.
     */
    refreshAccessToken(): Promise<string>;
    /**
     * Build the XOAUTH2 SASL string for SMTP AUTH (base64-encoded).
     */
    buildXOAUTH2(): Promise<string>;
}