diff --git a/packages/core/tools/README.md b/packages/core/tools/README.md
index c856ca1474..600cc1766a 100644
--- a/packages/core/tools/README.md
+++ b/packages/core/tools/README.md
@@ -89,7 +89,7 @@ ctx.tools.register(defineTool({
 
 The unified schema DSL uses `ParameterSchemaSpec` for the implicit open parameter object and `ValueSchemaSpec` for any JSON-value root. It supports `string`, `number`, `integer`, `boolean`, `null`, `array`, `object`, author-only `json`, and exact-one `oneOf`; scalar `enum`/`const` values are type-correct. Every explicit DSL object declares `additionalProperties: true | false`, while the implicit parameter root and raw JSON Schema keep the standard open default.
 
-A `defineTool` definition validates model arguments before execution and turns missing required values, wrong primitives, invalid enum members, and nested violations into `ToolArgsError` (`INVALID_ARGS`) for the normal error-result path. It also infers the body return and pure output projectors from `output.schema`; the registry snapshots and validates the returned lossless JSON before presentation. Extra parameter keys are allowed, defaults are not applied, and object or array fields without `properties` or `items` receive only a type check. Raw-registered tools own input validation but still declare and receive registry-enforced output.
+A `defineTool` definition validates model arguments before execution and turns missing required values, wrong primitives, invalid enum members, and nested violations into `ToolArgsError` (`INVALID_ARGS`) for the normal error-result path. It also infers the body return and pure output projectors from `output.schema`; the registry snapshots and validates the returned lossless JSON before presentation. The implicit parameter root is open; an explicit object accepts extra keys only with `additionalProperties: true`, and a closed object with no declared properties accepts only `{}`. Raw JSON Schema objects remain open unless they explicitly set `additionalProperties: false`. Defaults are not applied; open objects without `properties` and arrays without `items` receive only a container type check. Raw-registered tools own input validation but still declare and receive registry-enforced output.
 
 See `defineTool`, `validateArgs`, `ToolArgsError`, `ValueSchemaSpec`, `ParameterSchemaSpec`, `InferValue`, `InferArgs`, `valueSchemaSpecToJsonSchema`, and `parameterSchemaSpecToJsonSchema` in the public API for details.