/**
 * Protocol-side contract for inserting a user message into the user's
 * most-recently-active chat session. The backend implementation finds
 * the session and persists the message via the existing chat-message
 * insertion path.
 */
export interface ChatMessageWriter {
    /**
     * Insert a user message into the user's most-recent chat session.
     *
     * @param userId - The owning user
     * @param content - The flattened user-message text
     * @returns The sessionId written to, or `null` if the user has no chat session
     */
    addUserMessage(userId: string, content: string): Promise<{
        sessionId: string;
    } | null>;
}