From 3307240195ece4ef772b45e5323c683f4181f1e5 Mon Sep 17 00:00:00 2001
From: jif-oai <jif@openai.com>
Date: Thu, 28 May 2026 12:55:08 +0200
Subject: [PATCH] Use stable Guardian prompt cache keys (#24803)

## Why

Guardian review sessions are reusable across forks when their
`GuardianReviewSessionReuseKey` is unchanged, but the underlying
Responses request was still using the child thread ID as
`prompt_cache_key`. That meant forked Guardian reviews that should share
cache context produced different cache keys, reducing prompt cache reuse
and weakening the reuse invariant.

## What Changed

- Adds a `ModelClient` prompt cache key override and uses it for
`ResponsesApiRequest.prompt_cache_key`.
- Computes Guardian review cache keys as
`guardian:<sha1(parent_thread_id:reuse_key)>`, scoped to the parent
thread plus the reuse-sensitive Guardian config.
- Wires session construction to apply that override only for Guardian
sub-agent sessions.

## Testing

- Added coverage that Guardian cache keys are stable for the same
parent/reuse key, change when either the parent thread or reuse key
changes, fit within the Responses API length limit, and are absent for
non-Guardian sessions.
- Extended the parallel review test to assert forked Guardian reviews
send the same `prompt_cache_key`.