diff --git a/packages/cordis/tool-cordis/tests/mount.spec.ts b/packages/cordis/tool-cordis/tests/mount.spec.ts
index 09011c77de..2ddee8dbca 100644
--- a/packages/cordis/tool-cordis/tests/mount.spec.ts
+++ b/packages/cordis/tool-cordis/tests/mount.spec.ts
@@ -175,9 +175,13 @@ describe('cordis_mount', () => {
     // The registered schema is canonical JSON Schema derived from the DSL:
     // the required array survived, integer became number, extra is optional.
     const schema = ctx.tools.schemas().find(s => s.name === 'json_schema_tool')!
-    const parameters = schema.parameters as { properties: Record<string, { type: string; enum?: string[] }>; required?: string[] }
+    const parameters = schema.parameters as {
+      properties: Record<string, { type: string; enum?: string[]; default?: unknown }>
+      required?: string[]
+    }
     expect(parameters.required).toEqual(['text'])
     expect(parameters.properties.count!.type).toBe('number')
+    expect(parameters.properties.count!.default).toBe(1)
     expect(parameters.properties.mode!.enum).toEqual(['fast', 'slow'])
     // Arg validation enforces the normalized spec: text required, extra not.
     expect((await call(ctx, 'json_schema_tool', { count: 2 })).isError).toBe(true)
diff --git a/packages/core/tools/README.md b/packages/core/tools/README.md
index cfe2fdf2fa..bfd75fdcb6 100644
--- a/packages/core/tools/README.md
+++ b/packages/core/tools/README.md
@@ -154,7 +154,7 @@ The available tools:
 
 - **Concurrency policy is not an event seam** — `executionMode()` reads the resolved tool definition directly; plugins can only declare a classifier on definitions they own.
 - **`tools/pre-execute` deliberately cannot rewrite `exec.arguments`** — logged and rendered args would desync from what ran; the rewrite design is [a proposed RFC](../../../docs/rfc/proposed/feature/2026-06-30-pre-tool-input-rewrite.md).
-- **`defineTool`'s schema DSL is a deliberate subset** — string/number/boolean/object/array with string-only `enum`; `validateArgs` tolerates extra keys and never applies `default` (`XXX(unused-default)` flags removing that field); raw-registered JSON-Schema tools validate their own input.
+- **`defineTool`'s schema DSL is a deliberate subset** — string/number/boolean/object/array with string-only `enum`; `validateArgs` tolerates extra keys and preserves `default` as a model-visible JSON Schema annotation without applying it during validation; dynamic Cordis mounts may supply defaults even though first-party definitions do not, while raw-registered JSON-Schema tools validate their own input.
 - **`timeoutMs` on a definition is declarative only** — the registry never enforces deadlines; enforcement requires the `@deepseek-ai/dsh-timeout-policy` wrapper.
 - **Code Mode is TypeScript-only and the presentation mode is service-wide** — `mode: code`/`both` rejects prompt assembly unless `ctx.codeRuntime.language === 'typescript'`; scoped restrictions/shadows still choose each agent's visible bindings, but one tool cannot be native-only while another is code-only.
 - **Code Mode bindings return text only** — non-text content blocks in a sub-call result collapse to `[<type> content]` placeholders.
diff --git a/packages/core/tools/src/schema.ts b/packages/core/tools/src/schema.ts
index c61c819618..f2b62669b1 100644
--- a/packages/core/tools/src/schema.ts
+++ b/packages/core/tools/src/schema.ts
@@ -21,12 +21,8 @@ export interface SchemaProp {
   /** Enum of allowed values (strings only). */
   enum?: string[]
   /**
-   * Default value, emitted into the JSON Schema only (validation never applies
-   * it — see the validator note below).
-   *
-   * XXX(unused-default): no tool definition in the repo sets `default`; it rides
-   * into the wire schema for a model that no tool surfaces it to. Drop the field
-   * and its converter line unless a real tool needs a model-visible default.
+   * Model-visible JSON Schema default annotation. Validation does not apply it;
+   * dynamic tool mounts may supply it even though first-party definitions do not.
    */
   default?: unknown
   /** Nested properties for type: 'object'. */