docs: document package KV cache effects

packages/workflow/workflow-workerthread/README.md

@@ -89,12 +89,16 @@ The host keeps a ledger of forwarded child starts. A graceful worker supplies th
 
 **Token effect**: Potentially many independent child contexts are paid, bounded by `maxConcurrentAgents`, `maxTotalAgents`, and `maxItemsPerCall`; they never join the parent history directly.
 
+**KV Cache effect**: Independent of the parent request cache and of sibling children. Each child can reuse only a byte-identical prefix under its own provider, model, prompt, and schema; its later history grows append-only.
+
 ### Parent tool result, indirectly
 
 **What the model sees**: Through [`dsh-tool-workflow`](../tool-workflow/README.md), success exposes only the materialized final JSON value and child count in that consumer's wrapper. This engine supplies stable errors including `workflow script does not parse: <error>`, `invalid meta: <violations>`, `agent() requires a non-empty prompt string`, `agent() could not start a child: <error>`, `child agent run failed: <error>`, and its exact `parallel()`, `pipeline()`, `phase()`, option, schema, and JSON-boundary validation messages. Intermediate child outputs are available to the script but not the parent model.
 
 **Token effect**: Zero direct parent tokens from this engine. Final result size is capped by the tool consumer and retained until compaction.
 
+**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
 
 - **The worker/vm is not a security boundary** — model-written code can escape `node:vm` and reach the worker's process authority; a hostile-code deployment needs a separate-process or container engine.