[FEATURE]: Set prompt_cache_ttl for OpenRouter provider #16848
Description
[FEATURE]: Set prompt_cache_ttl for OpenRouter provider
Feature hasn't been suggested before.
- I have verified this feature I'm about to request hasn't been suggested before.
(Note: #5416 and #5422 cover broader caching config. This is about one missing parameter on an existing code path.)
Describe the enhancement you want to request
We already set prompt_cache_key for OpenRouter in ProviderTransform.options() (packages/opencode/src/provider/transform.ts, line 790), but we don't set a TTL. OpenRouter defaults to 5 minutes.
Without a longer TTL, any idle gap over 5 minutes between prompts triggers a full cache miss and rewrite on the next turn. For Anthropic models this is common in real usage — reviewing output, editing files, thinking, etc. The 5-minute default ends up costing more overall because the cache is constantly being rewritten.
Caution
The fix originally sketched below is broken. The top-level prompt_cache_ttl parameter is silently ignored by OpenRouter — no error, no cache write, nothing. See the conclusive empirical research in #16850, including a 3-way verification script that tested with a 6-minute delay past the default TTL.
The fix is one line next to the existing prompt_cache_key assignment:
// BROKEN — silently ignored by OpenRouter
if (input.model.providerID === "openrouter") {
result["prompt_cache_key"] = input.sessionID
result["prompt_cache_ttl"] = 3600 // does nothing
}The correct documented mechanism is cache_control: { type: "ephemeral", ttl: "1h" } on message content (OpenRouter docs). This is a one-line change in the existing applyCaching function. The fix is implemented and tested in #16850.