/**
 * JSONL append-only store for observability events.
 *
 * Slice A of v2.11.1 (slice topology observability). Pure file I/O —
 * no schema knowledge lives here. Schema/validation lives in
 * `observability-service.ts`.
 *
 * Layout: `.peaks/_runtime/<sessionId>/metrics/slices.jsonl` — one file
 * per session, append-only. Mtime-based cross-session prune keeps at
 * most `MAX_METRICS_FILES` files in the repo (mirrors the
 * `peaks session checkpoint` contract).
 *
 * Write path is fire-and-forget: errors (disk full, permission
 * denied, ENOENT mid-write) are swallowed silently per PRD Q4
 * (full-auto must never fail-loud on observability).
 */
export declare const METRICS_DIR = "metrics";
export declare const METRICS_FILENAME = "slices.jsonl";
export declare const MAX_METRICS_FILES = 10;
/** Absolute path to a session's metrics JSONL file. */
export declare function metricsFilePath(projectRoot: string, sessionId: string): string;
/** Absolute path to a session's metrics directory. */
export declare function metricsDirPath(projectRoot: string, sessionId: string): string;
/**
 * Absolute path to a session's metrics JSONL file, or `null` when the
 * session id names no session directory.
 *
 * Total sibling of `metricsFilePath`, built on the axis's own total
 * entry (`tryGetSessionDir`) so the two agree on the predicate. It
 * exists because this module has frames whose contract is never-throws
 * (`readMetricLines`, and through it `readObservabilityEvents`) and
 * `metricsFilePath` was the one call in them that could still throw.
 * `null` here means exactly one thing — the id could not be resolved —
 * and every caller below handles it in the same statement that reads it.
 */
export declare function tryMetricsFilePath(projectRoot: string, sessionId: string): string | null;
/**
 * Append one line to the session's metrics JSONL file. Creates the
 * directory tree on demand. Returns true on success, false on any
 * error (the caller MUST treat false as "drop silently" per Q4).
 */
export declare function appendMetricLine(projectRoot: string, sessionId: string, line: string): boolean;
/**
 * Read all non-empty lines from a session's metrics JSONL file.
 * Returns [] when the file does not exist. Does NOT parse — callers
 * decide what counts as valid (see `observability-service.ts`
 * `readObservabilityEvents` for the schema-aware reader).
 *
 * Returns [] for an unresolvable session id too: this is the read half
 * of a fire-and-forget store, and `readObservabilityEvents` documents
 * that a session with no readable metrics file has no events. Throwing
 * here used to escape as a raw guard error out of a documented
 * never-throws reader (repair R6).
 */
export declare function readMetricLines(projectRoot: string, sessionId: string): string[];
export type SessionMetricsEntry = {
    sessionId: string;
    mtimeMs: number;
    path: string;
};
/**
 * List every session that currently has a metrics JSONL on disk,
 * paired with its mtime. Ordered by sessionId for determinism; the
 * caller decides mtime ordering (see `pruneMetricsFiles`).
 */
export declare function listSessionDirsWithMetrics(projectRoot: string): SessionMetricsEntry[];
/**
 * Prune the metrics files across all sessions, retaining at most
 * `MAX_METRICS_FILES` by mtime (newest kept). Returns the list of
 * removed absolute paths. Best-effort: a failed removal is silently
 * skipped (matches Q4 fire-and-forget contract).
 */
export declare function pruneMetricsFiles(projectRoot: string): string[];
export declare const JSONL_STORE_CONSTANTS: {
    readonly METRICS_DIR: "metrics";
    readonly METRICS_FILENAME: "slices.jsonl";
    readonly MAX_METRICS_FILES: 10;
};