/**
 * Where a span id waits for a resume.
 *
 * A span's id is random, and a resumed run rebuilds every open span in a
 * new process. A site whose span can still be open when a checkpoint is
 * taken saves the id in a slot: a key under `SPAN_SLOT_PREFIX` in a record
 * the checkpoint serializes, such as a frame's locals. `reopenSpan` on the
 * statelog client reads the slot and opens the span with the saved id, so
 * the resumed events land in the span the paused run started.
 *
 * The slot's lifetime is the id's lifetime. A frame local dies when its
 * frame is popped, which happens on completion and on a throw but not on a
 * pause. A site whose frame outlives the span by a long way (the tool loop,
 * which can run hundreds of tools in one frame) calls `forgetSpan` once the
 * span has closed for good, so checkpoints do not carry every old id.
 *
 * A saved id is reused only inside the trace that minted it. A rewind or a
 * CLI resume that starts a new trace gets fresh spans: the trace check
 * fails and a new id is written over the old one.
 */
export declare const SPAN_SLOT_PREFIX = "__span_";
/** A slot: a key under the prefix in `locals`. Sites name the slot without
 *  the prefix, as `"llmCall"` or a tool invocation's key. */
export type SpanSlot = {
    locals: Record<string, unknown>;
    key: string;
};
export declare function spanSlot(locals: Record<string, unknown>, name: string): SpanSlot;
/** What a slot holds: the span id and the trace it belongs to. */
export type SavedSpan = {
    traceId: string;
    spanId: string;
};
/** The span id saved in `slot`, if it is well formed and belongs to
 *  `traceId`. The checkpoint is the program, so the shape is checked before
 *  the id goes into every event's `span_id`. */
export declare function savedSpanIdFor(slot: SpanSlot, traceId: string): string | undefined;
export declare function saveSpan(slot: SpanSlot, saved: SavedSpan): void;
/** Drop a saved id once its span has closed for good. */
export declare function forgetSpan(locals: Record<string, unknown>, name: string): void;