/**
 * Environment variable substitution for config values.
 *
 * Supports `${VAR_NAME}` syntax in string values, substituted at config load time.
 * - Only uppercase env vars are matched: `[A-Z_][A-Z0-9_]*`
 * - Escape with `$${}` to output literal `${}`
 * - Missing env vars throw `MissingEnvVarError` with context
 *
 * @example
 * ```json5
 * {
 *   models: {
 *     providers: {
 *       "vercel-gateway": {
 *         apiKey: "${VERCEL_GATEWAY_API_KEY}"
 *       }
 *     }
 *   }
 * }
 * ```
 */
export declare class MissingEnvVarError extends Error {
    readonly varName: string;
    readonly configPath: string;
    constructor(varName: string, configPath: string);
}
export type EnvSubstitutionWarning = {
    varName: string;
    configPath: string;
};
export type SubstituteOptions = {
    /** When set, missing vars call this instead of throwing and the original placeholder is preserved. */
    onMissing?: (warning: EnvSubstitutionWarning) => void;
};
export declare function containsEnvVarReference(value: string): boolean;
/**
 * Resolves `${VAR_NAME}` environment variable references in config values.
 *
 * @param obj - The parsed config object (after JSON5 parse and $include resolution)
 * @param env - Environment variables to use for substitution (defaults to process.env)
 * @param opts - Options: `onMissing` callback to collect warnings instead of throwing.
 * @returns The config object with env vars substituted
 * @throws {MissingEnvVarError} If a referenced env var is not set or empty (unless `onMissing` is set)
 */
export declare function resolveConfigEnvVars(obj: unknown, env?: NodeJS.ProcessEnv, opts?: SubstituteOptions): unknown;