Cache conversation state across turns

Generating the first response from a multi-turn history reprocesses every prior turn. The KV cache is the trick that skips that: save the model's internal state after the first turn, then replay it on the second turn with only the new user message.

kvCache: true on the second completion() call skips the prior turn's reprocessing. The same history prefix is required; change the prefix and the cache misses, falling back to a full reprocess.

Setting kvCache: true asks the SDK to save the model's internal state after this turn. Consider the first call with the flag on:

const r1 = completion({ modelId, history, stream: true, kvCache: true });
for await (const token of r1.tokenStream) process.stdout.write(token);
const final1 = await r1.final;

The cache key is the history prefix. To keep that prefix intact for the next hit, push cacheableAssistantContent back into history:

history.push({
  role: "assistant",
  content: final1.cacheableAssistantContent ?? final1.contentText,
});
history.push({ role: "user", content: "What about Germany?" });

Same flag, same history reference. The second turn replays the cache; comparing stats after proves it. You would call it like so:

const r2 = completion({ modelId, history, stream: true, kvCache: true });
for await (const token of r2.tokenStream) process.stdout.write(token);
const final2 = await r2.final;

console.log(`\n▸ First: ${JSON.stringify(final1.stats)}`);
console.log(`▸ Second (cached): ${JSON.stringify(final2.stats)}`);

The two stats objects show the speedup. On a long-running assistant with thousands of prior turns, the cached path runs ten to a hundred times faster than the cold path.

Note: final.cacheableAssistantContent is the exact text the cache was saved against. Fall back to final.contentText if it's undefined (some models and tool-using flows don't expose it).