/**
 * Migration shield for threads checkpointed before the system prompt moved out of the
 * message history.
 *
 * Turns used to be started with `[SystemMessage, HumanMessage]`, so every such turn left
 * a system message inside the thread's persisted state. Those messages are still in old
 * checkpoints and would now arrive mid-history — both an invalid shape for providers that
 * take the system prompt out-of-band, and a stale duplicate of a prompt the agent already
 * supplies. So they are dropped, not re-hoisted: the authoritative prompt is
 * `createAgent({ systemPrompt })` plus DynamicSystemPromptMiddleware.
 *
 * Nothing writes system messages into state any more, so for new threads this is a no-op.
 * It stays as a cheap guard (and can be deleted once no pre-refactor checkpoints remain).
 */
export declare function createLegacySystemMessagesMiddleware(): import("langchain").AgentMiddleware<undefined, undefined, unknown, readonly (import("@langchain/core/tools").ClientTool | import("@langchain/core/tools").ServerTool)[]>;