diff --git a/packages/durabletask-js/src/client/client.ts b/packages/durabletask-js/src/client/client.ts
index 62a7b346..af90fc7c 100644
--- a/packages/durabletask-js/src/client/client.ts
+++ b/packages/durabletask-js/src/client/client.ts
@@ -998,6 +998,14 @@ export class TaskHubGrpcClient {
     input?: unknown,
     options?: SignalEntityOptions,
   ): Promise<void> {
+    if (!id) {
+      throw new Error("signalEntity: 'id' is required.");
+    }
+
+    if (!operationName) {
+      throw new Error("signalEntity: 'operationName' is required and cannot be empty.");
+    }
+
     const req = new pb.SignalEntityRequest();
     req.setInstanceid(id.toString());
     req.setRequestid(randomUUID());
@@ -1051,6 +1059,10 @@ export class TaskHubGrpcClient {
     id: EntityInstanceId,
     includeState: boolean = true,
   ): Promise<EntityMetadata<T> | undefined> {
+    if (!id) {
+      throw new Error("getEntity: 'id' is required.");
+    }
+
     const req = new pb.GetEntityRequest();
     req.setInstanceid(id.toString());
     req.setIncludestate(includeState);
diff --git a/packages/durabletask-js/test/entity-client.spec.ts b/packages/durabletask-js/test/entity-client.spec.ts
index 910fec60..def880a4 100644
--- a/packages/durabletask-js/test/entity-client.spec.ts
+++ b/packages/durabletask-js/test/entity-client.spec.ts
@@ -396,6 +396,65 @@ describe("EntityInstanceId.fromString", () => {
   });
 });
 
+describe("Entity Client Input Validation", () => {
+  // Validation guards throw before any gRPC call, so no connection is needed.
+  const { TaskHubGrpcClient } = require("../src");
+  let client: InstanceType<typeof TaskHubGrpcClient>;
+
+  beforeEach(() => {
+    client = new TaskHubGrpcClient({ hostAddress: "localhost:4001" });
+  });
+
+  describe("signalEntity", () => {
+    it("should throw when id is null", async () => {
+      await expect(client.signalEntity(null as any, "op")).rejects.toThrow(
+        "signalEntity: 'id' is required.",
+      );
+    });
+
+    it("should throw when id is undefined", async () => {
+      await expect(client.signalEntity(undefined as any, "op")).rejects.toThrow(
+        "signalEntity: 'id' is required.",
+      );
+    });
+
+    it("should throw when operationName is empty string", async () => {
+      const entityId = new EntityInstanceId("counter", "my-counter");
+      await expect(client.signalEntity(entityId, "")).rejects.toThrow(
+        "signalEntity: 'operationName' is required and cannot be empty.",
+      );
+    });
+
+    it("should throw when operationName is null", async () => {
+      const entityId = new EntityInstanceId("counter", "my-counter");
+      await expect(client.signalEntity(entityId, null as any)).rejects.toThrow(
+        "signalEntity: 'operationName' is required and cannot be empty.",
+      );
+    });
+
+    it("should throw when operationName is undefined", async () => {
+      const entityId = new EntityInstanceId("counter", "my-counter");
+      await expect(client.signalEntity(entityId, undefined as any)).rejects.toThrow(
+        "signalEntity: 'operationName' is required and cannot be empty.",
+      );
+    });
+  });
+
+  describe("getEntity", () => {
+    it("should throw when id is null", async () => {
+      await expect(client.getEntity(null as any)).rejects.toThrow(
+        "getEntity: 'id' is required.",
+      );
+    });
+
+    it("should throw when id is undefined", async () => {
+      await expect(client.getEntity(undefined as any)).rejects.toThrow(
+        "getEntity: 'id' is required.",
+      );
+    });
+  });
+});
+
 describe("getEntities query normalization", () => {
   const { TaskHubGrpcClient } = require("../src");