diff --git a/private/aws-client-api-test/src/client-interface-tests/endpoint-requiredness-variations.spec.ts b/private/aws-client-api-test/src/client-interface-tests/endpoint-requiredness-variations.spec.ts
new file mode 100644
index 000000000000..06c0f4d2255e
--- /dev/null
+++ b/private/aws-client-api-test/src/client-interface-tests/endpoint-requiredness-variations.spec.ts
@@ -0,0 +1,108 @@
+import { EC2ProtocolClient } from "@aws-sdk/aws-protocoltests-ec2";
+import { S3Client } from "@aws-sdk/client-s3";
+import { WeatherClient } from "@aws-sdk/weather";
+import { describe, expect, test as it } from "vitest";
+
+describe("variations in the config endpoint property", () => {
+  describe("for an official AWS SDK client", () => {
+    it("endpoint is not required", async () => {
+      const client = new S3Client({});
+      expect(client.config.endpoint).toBeUndefined();
+      expect(await client.config.endpoint?.()).toBeUndefined();
+    });
+
+    it("endpoint can be set, but must be queried with an undef-check", async () => {
+      const client = new S3Client({
+        endpoint: "https://localhost",
+        forcePathStyle: true,
+      });
+      expect(await client.config.endpoint?.()).toEqual({
+        hostname: "localhost",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+  });
+
+  describe("for a Smithy client SDK outside the context of AWS", () => {
+    it("requires an endpoint if using the default endpointRuleSet", async () => {
+      const client = new WeatherClient({
+        endpoint: "https://localhost",
+      });
+    });
+
+    it("config.endpoint may be queried without undef-check since it was required on input", async () => {
+      const client = new WeatherClient({
+        endpoint: "https://localhost",
+      });
+      expect(await client.config.endpoint()).toEqual({
+        hostname: "localhost",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+  });
+
+  describe("for an internal generated client that uses default AWS regional endpoints", () => {
+    // EC2ProtocolClient is a protocol test client that has the aws.api#Service trait,
+    // and here it represents generated clients outside the AWS SDK but still an "AWS Service" as far as the
+    // code generator can tell.
+
+    it("endpoint is not required", async () => {
+      const client = new EC2ProtocolClient({});
+      expect(client.config.endpoint).toBeInstanceOf(Function);
+    });
+
+    it("endpoint may be set", async () => {
+      const client = new EC2ProtocolClient({
+        endpoint: "https://localhost",
+      });
+      expect(await client.config.endpoint()).toEqual({
+        hostname: "localhost",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+
+    it("config.endpoint may be queried without undef-check", async () => {
+      const client = new EC2ProtocolClient({});
+      expect(await client.config.endpoint()).toEqual({
+        hostname: "awsec2.us-west-2.amazonaws.com",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+
+    it("fips", async () => {
+      const client = new EC2ProtocolClient({
+        useFipsEndpoint: true,
+      });
+      expect(await client.config.endpoint()).toEqual({
+        hostname: "awsec2-fips.us-west-2.amazonaws.com",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+
+    it("dualstack", async () => {
+      const client = new EC2ProtocolClient({ useDualstackEndpoint: true });
+      expect(await client.config.endpoint()).toEqual({
+        hostname: "awsec2.us-west-2.api.aws",
+        path: "/",
+        port: undefined,
+        protocol: "https:",
+        query: undefined,
+      });
+    });
+  });
+});