Chat Compression Cache Sharing

Context

Chat compression currently sends a cold side query with a dedicated system instruction, no main-session tool declarations, and a media-slimmed copy of the conversation. Providers whose prompt-cache key starts with tools and the system instruction cannot reuse the main session's cached prefix.

Design

Compression first attempts a specialized single-turn request when all of the following are true:

• the compression model is the current main model;
• the active provider is Anthropic or DashScope and cache control is enabled;
• the chat has a provider-reported prompt token count to anchor the estimate;
• the effective prompt token count plus the bounded compression output reserve fits the model's context window.

The request uses the current turn's effective generation config, including per-request tool overrides used by subagents, and the complete curated history, including media. The normal model-modality filtering is applied when the request is sent, so supported media remains unchanged and unsupported media uses the same placeholders as other model requests. The existing compression instruction is appended as the final user message. Nothing consumes or executes function calls from this request. A response containing a function call, an empty response, a malformed state snapshot, or a request error is discarded and retried once through the existing cold side query. Its media-slimmed input is built lazily only when that fallback is needed. Cancellation does not trigger the fallback.

Using the current GeminiChat keeps the request scoped to the live session. The process-global fork cache is intentionally not used because it retains only a short history tail and can belong to another concurrent session.

Sessions using a distinct compaction model stay on the existing path because their cache identity differs from the main session. Media-bearing histories use the shared path first so the unchanged provider-facing prefix can reuse the main session's cache.

Verification

Unit tests assert exact system, tools, full-history, and trailing-directive construction; provider/model gates; media preservation on the shared path; window preflight; media slimming after fallback; tool-call and malformed-response fallback; and cancellation behavior. Provider testing should compare the serialized request prefix and cached-token usage for the main turn and compression request.