docs: document package KV cache effects

packages/web/web-search-deepseek/README.md

@@ -47,12 +47,16 @@ Provider failures become `WEB_PROVIDER_ERROR`; caller cancellation becomes `WEB_
 
 **Token effect**: Separate provider input and output tokens are incurred for each search; `maxTokens` caps generated output and `maxUses` caps native search uses.
 
+**KV Cache effect**: Independent of the conversation request cache. The auxiliary instruction and native tool definition can form a stable prefix, but each changed query or model route prevents reuse from its first difference.
+
 ### Conversation tool result, indirectly
 
 **What the model sees**: Through [`dsh-tool-web`](../tool-web/README.md), the conversation model sees deduplicated URLs, titles, dates, and citation snippets from structured search blocks; provider prose is not trusted as an answer. This provider's exact failures are `DeepSeek search aborted`, `DeepSeek search request failed: <error>`, `DeepSeek returned no web_search_tool_result blocks; the request may not have triggered native web search`, and `DeepSeek returned an unprocessable response body: <error>`; HTTP failures preserve the provider message. The consumer owns the error wrapper.
 
 **Token effect**: Zero direct conversation tokens from registration. Result tokens scale with returned sources and snippets, then the seam enforces the requested source bound.
 
+**KV Cache effect**: Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.
+
 ## Known Limitations and Deferred Work
 
 - **One search costs a full Messages model turn** — latency plus generated tokens, with up to `maxUses` server-side searches; DeepSeek exposes no dedicated retrieval endpoint.