import type { TextBlock } from '../types/blocks.js';
/**
 * Derive a stable, provider-agnostic cache-partition key from a frozen
 * system-prompt epoch. Requests that share the same stable prefix produce the
 * same key, so provider backends route them to the same automatic-cache
 * partition — this is what OpenAI's `prompt_cache_key` (and Gemini implicit
 * routing) needs to actually hit the cache on load-balanced deployments.
 *
 * Keyed off the volatile-free `ctx.systemPrompt` epoch array (the per-turn
 * ledger/next-steps blocks are appended AFTER this array, so they never enter
 * the key). Cached by array identity in a WeakMap — the sha-256 runs once per
 * epoch (a new array = a new epoch, e.g. on mode switch), not per request.
 *
 * Anthropic ignores this field (it uses `ttl` + `cache_control` markers), so
 * setting it is harmless there; only the wires that read `req.cache.key` act on
 * it, gated by their own capability flags.
 */
export declare function deriveCachePrefixKey(systemPrompt: readonly TextBlock[]): string;
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