diff --git a/known-issues.md b/known-issues.md
index 5958fbe..e6c4a87 100644
--- a/known-issues.md
+++ b/known-issues.md
@@ -158,22 +158,24 @@ convention in CLAUDE.md.
 ## KI-006 — `anyOf` schemas cause `INVALID_ARGUMENTS` on valid inputs
 
 **File:** `src/tools/plugins.ts`, `src/tools/workspaces.ts`
-**Status:** Resolved (PR: `fix/kind-ki006-anyof` #5)
+**Status:** Resolved
+**Resolved in:** PR `fix/kind-ki006-anyof` (#5, plugins.ts) and PR #10 (workspaces.ts)
 **Severity:** Medium
 
 ### Resolution
-The root cause was `z.string().optional().nullable()` (zod chain order) in the
-`update_workspace` tool's `parent_id` schema. `zod-to-json-schema` with
-`strictUnions: true` produces `anyOf` for the `optional().nullable()` chain, but
-`nullable().optional()` produces a clean `type: ["string","null"]` with no `anyOf`.
+The root cause is `z.string().optional().nullable()` (zod chain order).
+`zod-to-json-schema` with `strictUnions: true` produces `anyOf` for the
+`optional().nullable()` chain, but `nullable().optional()` produces a clean
+`type: ["string","null"]` with no `anyOf`.
 
-Fix: changed `z.string().nullable().optional()` → `z.string().optional().nullable()`
-in `src/tools/workspaces.ts:122`. Semantically equivalent (string | null | undefined),
-no runtime behaviour change.
+- **plugins.ts:** already used safe `nullable().optional()` order (PR #5).
+- **workspaces.ts:** `parent_id: z.string().optional().nullable()` → `z.string().nullable().optional()`.
+  Semantically equivalent (string | null | undefined); no runtime behaviour change.
 
-Regression guard added in `tests/__tests__/plugins-schema.test.ts`: mirrors all 6
-plugin tool schemas and asserts no `anyOf` in JSON Schema output. Includes a control
-test documenting the known `optional().nullable()` zod-to-json-schema quirk.
+Regression guard in `tests/__tests__/plugins-schema.test.ts`: mirrors all 6
+plugin tool schemas and the `update_workspace` workspace schema; asserts no
+`anyOf` in JSON Schema output. Includes a control test documenting the
+`optional().nullable()` → `anyOf` quirk.
 
 ---
 
diff --git a/src/tools/workspaces.ts b/src/tools/workspaces.ts
index ee65309..b57f7f6 100644
--- a/src/tools/workspaces.ts
+++ b/src/tools/workspaces.ts
@@ -119,7 +119,7 @@ export function registerWorkspaceTools(srv: McpServer) {
       name: z.string().optional(),
       role: z.string().optional(),
       tier: z.number().optional(),
-      parent_id: z.string().optional().nullable().describe("Set parent for nesting, null to un-nest"),
+      parent_id: z.string().nullable().optional().describe("Set parent for nesting, null to un-nest"),
     },
     handleUpdateWorkspace
   );
diff --git a/tests/__tests__/plugins-schema.test.ts b/tests/__tests__/plugins-schema.test.ts
index edc453d..50ffa47 100644
--- a/tests/__tests__/plugins-schema.test.ts
+++ b/tests/__tests__/plugins-schema.test.ts
@@ -54,6 +54,24 @@ describe("KI-006: plugin tool schemas are anyOf-free", () => {
     }),
   } as const;
 
+  // -------------------------------------------------------------------------
+  // Schema fixtures — mirrors src/tools/workspaces.ts
+  // -------------------------------------------------------------------------
+
+  const workspaceSchemas = {
+    update_workspace: z.object({
+      workspace_id: z.string(),
+      name: z.string().optional(),
+      role: z.string().optional(),
+      tier: z.number().optional(),
+      // NOTE: nullable must come BEFORE optional in the Zod chain.
+      // z.string().optional().nullable() → zod-to-json-schema produces `anyOf`,
+      // which causes INVALID_ARGUMENTS on valid null/undefined inputs in some
+      // MCP hosts. The safe order is z.string().nullable().optional().
+      parent_id: z.string().nullable().optional().describe("Set parent for nesting, null to un-nest"),
+    }),
+  } as const;
+
   // -------------------------------------------------------------------------
   // Tests
   // -------------------------------------------------------------------------
@@ -67,6 +85,15 @@ describe("KI-006: plugin tool schemas are anyOf-free", () => {
     });
   }
 
+  for (const [tool, schema] of Object.entries(workspaceSchemas)) {
+    describe(tool, () => {
+      const json = zodToJsonSchema(schema, { strictUnions: true });
+      it("has no anyOf", () => {
+        expect(hasAnyOf(json)).toBe(false);
+      });
+    });
+  }
+
   // -------------------------------------------------------------------------
   // Control: document the optional().nullable() zod-to-json-schema quirk
   // -------------------------------------------------------------------------