/**
 * In-context auto-compaction (agent-loop-capability-port sprint 7).
 *
 * A pure, testable helper that summarizes the older ("head") messages of a
 * conversation into a single replacement message via ONE extra `client.chat`
 * call, so `runAgenticLoop` can keep long runs inside the context window
 * without the coarse sprint-boundary reset (`summarizeOlderSprints` /
 * `contextReset` — a different, unrelated layer; see `context-handoff.ts`).
 *
 * Fails open: any error from the summarization call is caught, logged, and
 * surfaced as `undefined` so the caller can skip compaction for that turn
 * and continue uncompacted (sc-7-4). Never throws.
 */
import type { LLMClient, Message } from "../providers/types.js";
export interface CompactionParams {
    /** Provider-agnostic LLM client — the SAME client the run is using. */
    client: LLMClient;
    /** Model ID — the SAME model the run is using. */
    model: string;
    /** The HEAD messages being summarized away (older, completed turns). */
    head: Message[];
    /** Optional caller steering appended to the base summarization prompt. */
    instructions?: string;
    /** Bounded output cap for the summary call. Default 4096. */
    maxTokens?: number;
}
export interface CompactionOutcome {
    /** The single replacement message: { role:"user", content:"[Conversation summary] ..." }. */
    summaryMessage: Message;
    /** Token usage of the summarization call itself (to be charged by the caller). */
    usage: {
        inputTokens: number;
        outputTokens: number;
    };
    /** USD cost of the summarization call, when the provider reports one. */
    costUsd?: number;
}
/**
 * Summarize `head` into a single replacement message via ONE `client.chat`
 * call. Passes NO `tools` and a bounded `maxTokens` — mirrors the existing
 * one-shot `coerceJsonOutput` precedent (`agentic-loop.ts`).
 *
 * @returns `undefined` on any failure (fail-open, sc-7-4) — the caller must
 *   skip compaction for that turn and continue uncompacted, never throw.
 */
export declare function summarizeMessages(params: CompactionParams): Promise<CompactionOutcome | undefined>;
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