diff --git a/src/tools/workspaces.ts b/src/tools/workspaces.ts
index b57f7f6..ee65309 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().nullable().optional().describe("Set parent for nesting, null to un-nest"),
+      parent_id: z.string().optional().nullable().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
new file mode 100644
index 0000000..edc453d
--- /dev/null
+++ b/tests/__tests__/plugins-schema.test.ts
@@ -0,0 +1,90 @@
+/**
+ * KI-006 regression guard: verify plugin tool schemas are anyOf-free.
+ *
+ * JSON Schema `anyOf` unions are not reliably validated by all MCP client
+ * hosts.  zod-to-json-schema with `strictUnions: true` produces clean,
+ * non-anyOf schemas for simple Zod types (string, enum, number, boolean).
+ *
+ * Known zod-to-json-schema quirk: `string().optional().nullable()` produces
+ * anyOf; the safe order is `string().nullable().optional()`.
+ */
+import { z } from "zod";
+import { zodToJsonSchema } from "zod-to-json-schema";
+
+describe("KI-006: plugin tool schemas are anyOf-free", () => {
+  // -------------------------------------------------------------------------
+  // Helpers
+  // -------------------------------------------------------------------------
+
+  function hasAnyOf(schema: unknown): boolean {
+    if (typeof schema !== "object" || schema === null) return false;
+    const obj = schema as Record<string, unknown>;
+    if ("anyOf" in obj) return true;
+    for (const val of Object.values(obj)) {
+      if (typeof val === "object" && val !== null && hasAnyOf(val)) return true;
+    }
+    return false;
+  }
+
+  // -------------------------------------------------------------------------
+  // Schema fixtures — mirrors src/tools/plugins.ts
+  // -------------------------------------------------------------------------
+
+  const schemas = {
+    list_installed_plugins: z.object({
+      workspace_id: z.string().describe("Workspace ID"),
+    }),
+    install_plugin: z.object({
+      workspace_id: z.string().describe("Workspace ID"),
+      source: z.string().describe(
+        "Source URL: 'local://<name>' for platform registry, 'github://<owner>/<repo>[#<ref>]' for GitHub, or any registered scheme."
+      ),
+    }),
+    uninstall_plugin: z.object({
+      workspace_id: z.string().describe("Workspace ID"),
+      name: z.string().describe("Plugin name to remove"),
+    }),
+    list_plugin_sources: z.object({}),
+    list_available_plugins: z.object({
+      workspace_id: z.string(),
+    }),
+    check_plugin_compatibility: z.object({
+      workspace_id: z.string(),
+      runtime: z.string().describe("Target runtime"),
+    }),
+  } as const;
+
+  // -------------------------------------------------------------------------
+  // Tests
+  // -------------------------------------------------------------------------
+
+  for (const [tool, schema] of Object.entries(schemas)) {
+    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
+  // -------------------------------------------------------------------------
+
+  describe("control: optional().nullable() quirk", () => {
+    it("string().optional().nullable() → produces anyOf (known zod-to-json-schema issue)", () => {
+      const json = zodToJsonSchema(
+        z.object({ parent_id: z.string().optional().nullable() }),
+        { strictUnions: true }
+      );
+      expect(hasAnyOf(json)).toBe(true);
+    });
+    it("string().nullable().optional() → no anyOf (safe order)", () => {
+      const json = zodToJsonSchema(
+        z.object({ parent_id: z.string().nullable().optional() }),
+        { strictUnions: true }
+      );
+      expect(hasAnyOf(json)).toBe(false);
+    });
+  });
+});