diff --git a/js/dev/types.ts b/js/dev/types.ts
index 6a8684593..da992efea 100644
--- a/js/dev/types.ts
+++ b/js/dev/types.ts
@@ -109,7 +109,7 @@ export type SerializedParametersContainer = z.infer<
 >;
 
 export const evaluatorDefinitionSchema = z.object({
-  parameters: serializedParametersContainerSchema.optional(),
+  parameters: serializedParametersContainerSchema.nullish(),
   scores: z.array(z.object({ name: z.string() })).optional(),
   classifiers: z.array(z.object({ name: z.string() })).optional(),
 });
diff --git a/js/src/zod/zod-v3-serialization.test.ts b/js/src/zod/zod-v3-serialization.test.ts
index 826188421..c4d1097d2 100644
--- a/js/src/zod/zod-v3-serialization.test.ts
+++ b/js/src/zod/zod-v3-serialization.test.ts
@@ -19,7 +19,10 @@ import {
   serializeEvalParametersToStaticParametersSchema,
   serializeRemoteEvalParametersContainer,
 } from "../framework2";
-import { serializedParametersContainerSchema } from "../../dev/types";
+import {
+  serializedParametersContainerSchema,
+  evaluatorDefinitionSchema,
+} from "../../dev/types";
 // Detect which zod version is installed by checking for v4-specific properties
 function getInstalledZodVersion(): 3 | 4 {
   const testSchema = zodModule.z.string();
@@ -199,3 +202,33 @@ describe("serializedParametersContainerSchema parsing", () => {
     expect(result.success).toBe(true);
   });
 });
+
+describe("evaluatorDefinitionSchema parameters nullish", () => {
+  test("allows parameters: null", () => {
+    const result = evaluatorDefinitionSchema.safeParse({
+      parameters: null,
+    });
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.parameters).toBeNull();
+    }
+  });
+
+  test("allows parameters: undefined", () => {
+    const result = evaluatorDefinitionSchema.safeParse({
+      parameters: undefined,
+    });
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.parameters).toBeUndefined();
+    }
+  });
+
+  test("allows parameters omitted entirely", () => {
+    const result = evaluatorDefinitionSchema.safeParse({});
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.parameters).toBeUndefined();
+    }
+  });
+});