diff --git a/Makefile b/Makefile
index c538995c4861..7dd135969ef1 100644
--- a/Makefile
+++ b/Makefile
@@ -18,6 +18,10 @@ test-unit: build-s3-browser-bundle
 	yarn g:vitest run -c vitest.config.clients.unit.ts
 	npx jest -c jest.config.js
 
+# typecheck for test code.
+test-types:
+	npx tsc -p tsconfig.test.json
+
 test-protocols: build-s3-browser-bundle
 	yarn g:vitest run -c vitest.config.protocols.integ.ts
 
@@ -26,6 +30,7 @@ test-integration: build-s3-browser-bundle
 	yarn g:vitest run -c vitest.config.integ.ts
 	npx jest -c jest.config.integ.js
 	make test-protocols;
+	make test-types;
 
 test-e2e: build-s3-browser-bundle
 	yarn g:vitest run -c vitest.config.e2e.ts --retry=4
diff --git a/clients/client-s3/test/e2e/S3.e2e.spec.ts b/clients/client-s3/test/e2e/S3.e2e.spec.ts
index e1307c8170b6..91f7024c62fa 100644
--- a/clients/client-s3/test/e2e/S3.e2e.spec.ts
+++ b/clients/client-s3/test/e2e/S3.e2e.spec.ts
@@ -77,7 +77,7 @@ describe("@aws-sdk/client-s3", () => {
       const body = createBuffer("1MB");
       let bodyChecksum = "";
 
-      const bodyChecksumReader = (next) => async (args) => {
+      const bodyChecksumReader = (next: any) => async (args: any) => {
         const checksumValue = args.request.headers["x-amz-checksum-crc32"];
         if (checksumValue) {
           bodyChecksum = checksumValue;
diff --git a/clients/client-s3/test/unit/flexibleChecksums.spec.ts b/clients/client-s3/test/unit/flexibleChecksums.spec.ts
index 985fe9547b85..710b513c1b30 100644
--- a/clients/client-s3/test/unit/flexibleChecksums.spec.ts
+++ b/clients/client-s3/test/unit/flexibleChecksums.spec.ts
@@ -84,7 +84,7 @@ describe("Flexible Checksums", () => {
             expect(headers["transfer-encoding"]).to.equal("chunked");
             expect(headers["x-amz-content-sha256"]).to.equal("STREAMING-UNSIGNED-PAYLOAD-TRAILER");
             expect(headers["x-amz-trailer"]).to.equal(checksumHeader);
-            body.on("data", (data) => {
+            body.on("data", (data: any) => {
               const stringValue = data.toString();
               if (stringValue.startsWith(checksumHeader)) {
                 const receivedChecksum = stringValue.replace("\r\n", "").split(":")[1];
diff --git a/packages/credential-provider-sso/src/resolveSSOCredentials.spec.ts b/packages/credential-provider-sso/src/resolveSSOCredentials.spec.ts
index 6f1628b6a10a..ffd505cfe472 100644
--- a/packages/credential-provider-sso/src/resolveSSOCredentials.spec.ts
+++ b/packages/credential-provider-sso/src/resolveSSOCredentials.spec.ts
@@ -160,7 +160,7 @@ describe(resolveSSOCredentials.name, () => {
 
     it("creates SSO client with provided region, if client is not passed", async () => {
       const mockCustomSsoSend = vi.fn().mockResolvedValue({ roleCredentials: mockCreds });
-      vi.mocked(SSOClient).mockReturnValue({ send: mockCustomSsoSend });
+      vi.mocked(SSOClient as any).mockReturnValue({ send: mockCustomSsoSend });
 
       await resolveSSOCredentials({ ...mockOptions, ssoClient: undefined });
       expect(mockCustomSsoSend).toHaveBeenCalledTimes(1);
@@ -176,7 +176,7 @@ describe(resolveSSOCredentials.name, () => {
 
     it("creates SSO client with provided region, if client is not passed, and includes accountId", async () => {
       const mockCustomSsoSend = vi.fn().mockResolvedValue({ roleCredentials: mockCreds });
-      vi.mocked(SSOClient).mockReturnValue({ send: mockCustomSsoSend });
+      vi.mocked(SSOClient as any).mockReturnValue({ send: mockCustomSsoSend });
 
       const result = await resolveSSOCredentials({ ...mockOptions, ssoClient: undefined });
       expect(result).toHaveProperty("accountId", mockOptions.ssoAccountId);
diff --git a/packages/credential-providers/src/fromTemporaryCredentials.spec.ts b/packages/credential-providers/src/fromTemporaryCredentials.spec.ts
index 18baddd69b97..a3b837f9d66c 100644
--- a/packages/credential-providers/src/fromTemporaryCredentials.spec.ts
+++ b/packages/credential-providers/src/fromTemporaryCredentials.spec.ts
@@ -62,7 +62,7 @@ describe("fromTemporaryCredentials", () => {
       secretAccessKey: "SECRET_ACCESS_KEY",
       sessionToken: "SESSION_TOKEN",
     });
-    expect(vi.mocked(STSClient)).toHaveBeenCalledWith({
+    expect(vi.mocked(STSClient as any)).toHaveBeenCalledWith({
       credentials: masterCredentials,
       region,
     });
@@ -86,7 +86,7 @@ describe("fromTemporaryCredentials", () => {
       clientPlugins: [plugin],
     });
     await provider();
-    expect(vi.mocked(STSClient)).toHaveBeenCalledWith({
+    expect(vi.mocked(STSClient as any)).toHaveBeenCalledWith({
       credentials: masterCredentials,
     });
     expect(mockUsePlugin).toHaveBeenCalledTimes(1);
@@ -101,7 +101,7 @@ describe("fromTemporaryCredentials", () => {
       },
     });
     await provider();
-    expect(vi.mocked(STSClient)).toHaveBeenCalledWith({});
+    expect(vi.mocked(STSClient as any)).toHaveBeenCalledWith({});
   });
 
   it("should create a role session name if none provided", async () => {
@@ -140,22 +140,22 @@ describe("fromTemporaryCredentials", () => {
     expect(credentials.accessKeyId).toBe("access_id_from_third");
     // Creates STS Client with right master credentials and assume role with
     // expected role arn.
-    expect(vi.mocked(STSClient).mock.results.length).toBe(3);
-    const outmostClient = vi.mocked(STSClient).mock.results[0].value;
+    expect(vi.mocked(STSClient as any).mock.results.length).toBe(3);
+    const outmostClient = vi.mocked(STSClient as any).mock.results[0].value;
     expect(outmostClient.config.credentials).toEqual(expect.objectContaining({ accessKeyId: "access_id_from_second" }));
     expect((outmostClient.send as any).mock.calls.length).toBe(1);
     expect((outmostClient.send as any).mock.calls[0][0].input).toEqual(
       expect.objectContaining({ RoleArn: roleArnOf("third") })
     );
 
-    const middleClient = vi.mocked(STSClient).mock.results[1].value;
+    const middleClient = vi.mocked(STSClient as any).mock.results[1].value;
     expect(middleClient.config.credentials).toEqual(expect.objectContaining({ accessKeyId: "access_id_from_first" }));
     expect((middleClient.send as any).mock.calls.length).toBe(1);
     expect((middleClient.send as any).mock.calls[0][0].input).toEqual(
       expect.objectContaining({ RoleArn: roleArnOf("second") })
     );
 
-    const innermostClient = vi.mocked(STSClient).mock.results[2].value;
+    const innermostClient = vi.mocked(STSClient as any).mock.results[2].value;
     expect(innermostClient.config.credentials).toEqual(undefined);
     expect((innermostClient.send as any).mock.calls.length).toBe(1);
     expect((innermostClient.send as any).mock.calls[0][0].input).toEqual(
@@ -169,7 +169,7 @@ describe("fromTemporaryCredentials", () => {
 
     // Should not create extra clients if credentials is still valid
     await provider();
-    expect(vi.mocked(STSClient).mock.results.length).toBe(3);
+    expect(vi.mocked(STSClient as any).mock.results.length).toBe(3);
   });
 
   it("should support assuming a role with multi-factor authentication", async () => {
diff --git a/packages/eventstream-handler-node/src/EventStreamPayloadHandler.spec.ts b/packages/eventstream-handler-node/src/EventStreamPayloadHandler.spec.ts
index e3ffa9a2592a..330de2282f16 100644
--- a/packages/eventstream-handler-node/src/EventStreamPayloadHandler.spec.ts
+++ b/packages/eventstream-handler-node/src/EventStreamPayloadHandler.spec.ts
@@ -29,7 +29,7 @@ describe(EventStreamPayloadHandler.name, () => {
 
   beforeEach(() => {
     (EventSigningStream as unknown as any).mockImplementation(() => new PassThrough());
-    vi.mocked(EventStreamCodec).mockImplementation(() => {});
+    vi.mocked(EventStreamCodec).mockImplementation((() => {}) as any);
   });
 
   afterEach(() => {
diff --git a/packages/middleware-bucket-endpoint/src/bucketHostname.spec.ts b/packages/middleware-bucket-endpoint/src/bucketHostname.spec.ts
index 1eb2f733b48b..18e5519b1544 100644
--- a/packages/middleware-bucket-endpoint/src/bucketHostname.spec.ts
+++ b/packages/middleware-bucket-endpoint/src/bucketHostname.spec.ts
@@ -1,5 +1,5 @@
 import { parse as parseArn } from "@aws-sdk/util-arn-parser";
-import { describe, expect, expect, test as it } from "vitest";
+import { describe, expect, test as it } from "vitest";
 
 import { bucketHostname } from "./bucketHostname";
 
diff --git a/packages/middleware-endpoint-discovery/src/getEndpointDiscoveryPlugin.spec.ts b/packages/middleware-endpoint-discovery/src/getEndpointDiscoveryPlugin.spec.ts
index 882c6f9391bb..e0e6cdaabeb3 100644
--- a/packages/middleware-endpoint-discovery/src/getEndpointDiscoveryPlugin.spec.ts
+++ b/packages/middleware-endpoint-discovery/src/getEndpointDiscoveryPlugin.spec.ts
@@ -18,7 +18,7 @@ describe(getEndpointDiscoveryPlugin.name, () => {
 
   it(`applyToStack function adds endpoint discovery middleware`, () => {
     const middlewareReturn = {};
-    vi.mocked(endpointDiscoveryMiddleware).mockReturnValueOnce(middlewareReturn);
+    vi.mocked(endpointDiscoveryMiddleware).mockReturnValueOnce(middlewareReturn as any);
 
     // @ts-ignore
     const plugin = getEndpointDiscoveryPlugin(pluginConfig, middlewareConfig);
diff --git a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
index 404e3df5ec17..06b293d2cbcc 100644
--- a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
+++ b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
@@ -13,7 +13,7 @@ vi.mock("@smithy/util-middleware");
 
 describe(resolveFlexibleChecksumsConfig.name, () => {
   beforeEach(() => {
-    vi.mocked(normalizeProvider).mockImplementation((input) => input);
+    vi.mocked(normalizeProvider).mockImplementation((input) => input as any);
   });
 
   afterEach(() => {
diff --git a/packages/middleware-flexible-checksums/src/validateChecksumFromResponse.spec.ts b/packages/middleware-flexible-checksums/src/validateChecksumFromResponse.spec.ts
index 7829cdd13d2f..836512b99ba2 100644
--- a/packages/middleware-flexible-checksums/src/validateChecksumFromResponse.spec.ts
+++ b/packages/middleware-flexible-checksums/src/validateChecksumFromResponse.spec.ts
@@ -51,10 +51,12 @@ describe(validateChecksumFromResponse.name, () => {
 
   beforeEach(() => {
     vi.mocked(getChecksumLocationName).mockImplementation((algorithm) => algorithm);
-    vi.mocked(getChecksumAlgorithmListForResponse).mockImplementation((responseAlgorithms) => responseAlgorithms);
+    vi.mocked(getChecksumAlgorithmListForResponse).mockImplementation(
+      (responseAlgorithms) => responseAlgorithms as any
+    );
     vi.mocked(selectChecksumAlgorithmFunction).mockReturnValue(mockChecksumAlgorithmFn);
     vi.mocked(getChecksum).mockResolvedValue(mockChecksum);
-    vi.mocked(createChecksumStream).mockReturnValue(mockBodyStream);
+    vi.mocked(createChecksumStream).mockReturnValue(mockBodyStream as any);
   });
 
   afterEach(() => {
diff --git a/packages/middleware-host-header/src/index.spec.ts b/packages/middleware-host-header/src/index.spec.ts
index e708a1006c5a..52e945c28429 100644
--- a/packages/middleware-host-header/src/index.spec.ts
+++ b/packages/middleware-host-header/src/index.spec.ts
@@ -1,5 +1,5 @@
 import { HttpRequest } from "@smithy/protocol-http";
-import { beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { hostHeaderMiddleware } from "./index";
 describe("hostHeaderMiddleware", () => {
diff --git a/packages/middleware-location-constraint/src/middleware-location-constraint.integ.spec.ts b/packages/middleware-location-constraint/src/middleware-location-constraint.integ.spec.ts
index 3f6e84a3c02a..21722cc86a06 100644
--- a/packages/middleware-location-constraint/src/middleware-location-constraint.integ.spec.ts
+++ b/packages/middleware-location-constraint/src/middleware-location-constraint.integ.spec.ts
@@ -1,5 +1,5 @@
 import { S3 } from "@aws-sdk/client-s3";
-import { describe, expect, expect, test as it } from "vitest";
+import { describe, expect, test as it } from "vitest";
 
 import { requireRequestsFrom } from "../../../private/aws-util-test/src";
 
diff --git a/packages/middleware-sdk-rds/src/middleware-sdk-rds.integ.spec.ts b/packages/middleware-sdk-rds/src/middleware-sdk-rds.integ.spec.ts
index 8825c6671171..311fdf109ab9 100644
--- a/packages/middleware-sdk-rds/src/middleware-sdk-rds.integ.spec.ts
+++ b/packages/middleware-sdk-rds/src/middleware-sdk-rds.integ.spec.ts
@@ -1,5 +1,5 @@
 import { RDS } from "@aws-sdk/client-rds";
-import { describe, expect, expect, test as it } from "vitest";
+import { describe, expect, test as it } from "vitest";
 
 import { TestHttpHandler } from "../../../private/aws-util-test/src";
 
diff --git a/packages/middleware-sdk-route53/src/middleware-sdk-route53.integ.spec.ts b/packages/middleware-sdk-route53/src/middleware-sdk-route53.integ.spec.ts
index db1cc54faf30..f40b2d4a2233 100644
--- a/packages/middleware-sdk-route53/src/middleware-sdk-route53.integ.spec.ts
+++ b/packages/middleware-sdk-route53/src/middleware-sdk-route53.integ.spec.ts
@@ -1,6 +1,6 @@
 import { Route53 } from "@aws-sdk/client-route-53";
 import { XMLParser } from "fast-xml-parser";
-import { describe, expect, expect, test as it } from "vitest";
+import { describe, expect, test as it } from "vitest";
 
 import { requireRequestsFrom } from "../../../private/aws-util-test/src";
 
diff --git a/packages/middleware-sdk-s3-control/src/process-arnables-plugin/getProcessArnablesPlugin.spec.ts b/packages/middleware-sdk-s3-control/src/process-arnables-plugin/getProcessArnablesPlugin.spec.ts
index eaa978f28731..7d4040dc8692 100644
--- a/packages/middleware-sdk-s3-control/src/process-arnables-plugin/getProcessArnablesPlugin.spec.ts
+++ b/packages/middleware-sdk-s3-control/src/process-arnables-plugin/getProcessArnablesPlugin.spec.ts
@@ -1,7 +1,7 @@
 import { constructStack } from "@smithy/middleware-stack";
 import { HttpRequest } from "@smithy/protocol-http";
 import { Provider, RegionInfo } from "@smithy/types";
-import { beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { S3ControlResolvedConfig } from "../configurations";
 import { getProcessArnablesPlugin } from "./getProcessArnablesPlugin";
diff --git a/packages/middleware-sdk-s3/package.json b/packages/middleware-sdk-s3/package.json
index 79aca97c70c5..0be6f0a0d51c 100644
--- a/packages/middleware-sdk-s3/package.json
+++ b/packages/middleware-sdk-s3/package.json
@@ -10,7 +10,8 @@
     "build:types:downlevel": "downlevel-dts dist-types dist-types/ts3.4",
     "clean": "rimraf ./dist-* && rimraf *.tsbuildinfo",
     "test": "yarn g:vitest run",
-    "test:integration": "yarn g:vitest run -c vitest.config.integ.ts",
+    "test:types": "tsc -p tsconfig.test.json",
+    "test:integration": "yarn g:vitest run -c vitest.config.integ.ts && yarn test:types",
     "test:e2e": "yarn g:vitest run -c vitest.config.e2e.ts --mode development",
     "extract:docs": "api-extractor run --local",
     "test:watch": "yarn g:vitest watch",
diff --git a/packages/middleware-sdk-s3/src/check-content-length-header.spec.ts b/packages/middleware-sdk-s3/src/check-content-length-header.spec.ts
index 27ed5c258ad5..fccebe57f451 100644
--- a/packages/middleware-sdk-s3/src/check-content-length-header.spec.ts
+++ b/packages/middleware-sdk-s3/src/check-content-length-header.spec.ts
@@ -6,7 +6,7 @@ import { checkContentLengthHeader } from "./check-content-length-header";
 describe("checkContentLengthHeaderMiddleware", () => {
   const mockNextHandler = vi.fn();
 
-  let spy: vi.SpyInstance;
+  let spy: any;
 
   beforeEach(() => {
     spy = vi.spyOn(console, "warn");
diff --git a/packages/middleware-sdk-s3/src/middleware-sdk-s3.integ.spec.ts b/packages/middleware-sdk-s3/src/middleware-sdk-s3.integ.spec.ts
index 6e619da79e69..6ef0cadf972f 100644
--- a/packages/middleware-sdk-s3/src/middleware-sdk-s3.integ.spec.ts
+++ b/packages/middleware-sdk-s3/src/middleware-sdk-s3.integ.spec.ts
@@ -1,5 +1,5 @@
 import { S3 } from "@aws-sdk/client-s3";
-import { describe, expect, expect, test as it, vi } from "vitest";
+import { describe, expect, test as it, vi } from "vitest";
 
 import { requireRequestsFrom } from "../../../private/aws-util-test/src";
 
diff --git a/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.e2e.spec.ts b/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.e2e.spec.ts
index 971236036213..e3e1fa20fe0a 100644
--- a/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.e2e.spec.ts
+++ b/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.e2e.spec.ts
@@ -215,7 +215,7 @@ async function createClientAndRecorder() {
       }
 
       const commandName = context.commandName + s3ExpressSuffix;
-      const input = args.input;
+      const input = args.input as any;
       const commandRecorder = (recorder.calls[commandName] = recorder.calls[commandName] ?? {});
       commandRecorder[input["Bucket"] ?? "-"] |= 0;
       commandRecorder[input["Bucket"] ?? "-"]++;
diff --git a/packages/middleware-sdk-s3/src/throw-200-exceptions.spec.ts b/packages/middleware-sdk-s3/src/throw-200-exceptions.spec.ts
index ab8b0ce09ad9..0dce5f58018d 100644
--- a/packages/middleware-sdk-s3/src/throw-200-exceptions.spec.ts
+++ b/packages/middleware-sdk-s3/src/throw-200-exceptions.spec.ts
@@ -1,7 +1,7 @@
 import { HttpRequest, HttpResponse } from "@smithy/protocol-http";
 import { toUtf8 } from "@smithy/util-utf8";
 import { Readable } from "stream";
-import { beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { throw200ExceptionsMiddleware } from "./throw-200-exceptions";
 
diff --git a/packages/middleware-sdk-s3/tsconfig.test.json b/packages/middleware-sdk-s3/tsconfig.test.json
new file mode 100644
index 000000000000..22a8661f32d4
--- /dev/null
+++ b/packages/middleware-sdk-s3/tsconfig.test.json
@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "baseUrl": ".",
+    "rootDir": "src",
+    "skipLibCheck": true,
+    "noEmit": true
+  },
+  "extends": "../../tsconfig.cjs.json",
+  "include": ["src/**/s3-type-transforms.integ.spec.ts"],
+  "exclude": []
+}
diff --git a/packages/middleware-sdk-sqs/src/middleware-sdk-sqs.integ.spec.ts b/packages/middleware-sdk-sqs/src/middleware-sdk-sqs.integ.spec.ts
index 914d962d8ba0..833a7ccbdcbd 100644
--- a/packages/middleware-sdk-sqs/src/middleware-sdk-sqs.integ.spec.ts
+++ b/packages/middleware-sdk-sqs/src/middleware-sdk-sqs.integ.spec.ts
@@ -3,7 +3,7 @@ import { HttpHandler, HttpResponse } from "@smithy/protocol-http";
 import type { AwsCredentialIdentity } from "@smithy/types";
 import crypto from "crypto";
 import { Readable } from "stream";
-import { beforeEach, describe, expect, expect, test as it } from "vitest";
+import { beforeEach, describe, expect, test as it } from "vitest";
 
 import { requireRequestsFrom } from "../../../private/aws-util-test/src";
 
diff --git a/packages/middleware-signing/src/awsAuthMiddleware.spec.ts b/packages/middleware-signing/src/awsAuthMiddleware.spec.ts
index c0d5f1d2884c..84471b94f2e1 100644
--- a/packages/middleware-signing/src/awsAuthMiddleware.spec.ts
+++ b/packages/middleware-signing/src/awsAuthMiddleware.spec.ts
@@ -1,5 +1,5 @@
 import { HttpRequest } from "@smithy/protocol-http";
-import { FinalizeHandler, RequestSigner } from "@smithy/types";
+import { RequestSigner } from "@smithy/types";
 import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { AwsAuthResolvedConfig } from "./awsAuthConfiguration";
@@ -11,11 +11,11 @@ vi.mock("./utils/getUpdatedSystemClockOffset");
 vi.mock("./utils/getSkewCorrectedDate");
 
 describe(awsAuthMiddleware.name, () => {
-  let mockSignFn: vi.Mock<any, any>;
+  let mockSignFn: any;
   let mockSigner: () => Promise<RequestSigner>;
   let mockOptions: AwsAuthResolvedConfig;
 
-  const mockNext: vi.MockedFunction<FinalizeHandler<any, any>> = vi.fn();
+  const mockNext: any = vi.fn();
   const mockSystemClockOffset = 100;
   const mockUpdatedSystemClockOffset = 500;
   const mockSigningHandlerArgs = {
diff --git a/packages/middleware-token/src/getTokenPlugin.spec.ts b/packages/middleware-token/src/getTokenPlugin.spec.ts
index d91199b6c349..b1a2018400dd 100644
--- a/packages/middleware-token/src/getTokenPlugin.spec.ts
+++ b/packages/middleware-token/src/getTokenPlugin.spec.ts
@@ -18,7 +18,7 @@ describe(getTokenPlugin.name, () => {
 
   it("applyToStack adds tokenMiddleware", () => {
     const middlewareReturn = {};
-    vi.mocked(tokenMiddleware).mockReturnValueOnce(middlewareReturn);
+    vi.mocked(tokenMiddleware).mockReturnValueOnce(middlewareReturn as any);
 
     // @ts-ignore
     const plugin = getTokenPlugin(pluginConfig);
diff --git a/packages/middleware-token/src/normalizeTokenProvider.spec.ts b/packages/middleware-token/src/normalizeTokenProvider.spec.ts
index 9a4ef2033b78..8ae9412eeeb7 100644
--- a/packages/middleware-token/src/normalizeTokenProvider.spec.ts
+++ b/packages/middleware-token/src/normalizeTokenProvider.spec.ts
@@ -1,6 +1,6 @@
 import { memoize } from "@smithy/property-provider";
 import { normalizeProvider } from "@smithy/util-middleware";
-import { afterEach, beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { normalizeTokenProvider } from "./normalizeTokenProvider";
 
@@ -73,13 +73,13 @@ describe(normalizeTokenProvider.name, () => {
       });
 
       it("returns true if expiration is not defined", () => {
-        const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2];
+        const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2]!;
         const expiration = Date.now();
         expect(memoizeRefreshFn({ expiration })).toEqual(true);
       });
 
       it("returns false if expiration is not defined", () => {
-        const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2];
+        const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2]!;
         expect(memoizeRefreshFn({})).toEqual(false);
       });
     });
diff --git a/packages/middleware-user-agent/src/check-features.spec.ts b/packages/middleware-user-agent/src/check-features.spec.ts
index d52fa4040504..fb7725564da7 100644
--- a/packages/middleware-user-agent/src/check-features.spec.ts
+++ b/packages/middleware-user-agent/src/check-features.spec.ts
@@ -15,7 +15,7 @@ describe(checkFeatures.name, () => {
           identity: {
             accountId: "123456789012",
             $source: {},
-          },
+          } as any,
         },
       },
     } as AwsHandlerExecutionContext;
diff --git a/packages/middleware-user-agent/src/user-agent-middleware.spec.ts b/packages/middleware-user-agent/src/user-agent-middleware.spec.ts
index d305f77e67da..355146df8d51 100644
--- a/packages/middleware-user-agent/src/user-agent-middleware.spec.ts
+++ b/packages/middleware-user-agent/src/user-agent-middleware.spec.ts
@@ -1,7 +1,7 @@
 import { setPartitionInfo, useDefaultPartitionInfo } from "@aws-sdk/util-endpoints";
 import { HttpRequest } from "@smithy/protocol-http";
 import { UserAgentPair } from "@smithy/types";
-import { afterEach, beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { USER_AGENT, X_AMZ_USER_AGENT } from "./constants";
 import { userAgentMiddleware } from "./user-agent-middleware";
diff --git a/packages/middleware-websocket/src/EventStreamPayloadHandler.spec.ts b/packages/middleware-websocket/src/EventStreamPayloadHandler.spec.ts
index 70a93a930106..e5b04d3aa80d 100644
--- a/packages/middleware-websocket/src/EventStreamPayloadHandler.spec.ts
+++ b/packages/middleware-websocket/src/EventStreamPayloadHandler.spec.ts
@@ -1,6 +1,6 @@
 import { EventStreamCodec } from "@smithy/eventstream-codec";
 import { Decoder, Encoder, FinalizeHandler, FinalizeHandlerArguments, HttpRequest, MessageSigner } from "@smithy/types";
-import { afterEach, beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 import { ReadableStream, TransformStream } from "web-streams-polyfill";
 
 import { EventStreamPayloadHandler } from "./EventStreamPayloadHandler";
@@ -21,7 +21,7 @@ describe(EventStreamPayloadHandler.name, () => {
   beforeEach(() => {
     window.TransformStream = TransformStream;
     (getEventSigningTransformStream as unknown as any).mockImplementation(() => new TransformStream());
-    vi.mocked(EventStreamCodec).mockImplementation(() => {});
+    vi.mocked(EventStreamCodec).mockImplementation((() => {}) as any);
   });
 
   afterEach(() => {
diff --git a/packages/middleware-websocket/src/websocket-fetch-handler.spec.ts b/packages/middleware-websocket/src/websocket-fetch-handler.spec.ts
index 6194081a03de..b30c93e29dc2 100644
--- a/packages/middleware-websocket/src/websocket-fetch-handler.spec.ts
+++ b/packages/middleware-websocket/src/websocket-fetch-handler.spec.ts
@@ -2,7 +2,7 @@ import { FetchHttpHandler } from "@smithy/fetch-http-handler";
 import { HttpRequest } from "@smithy/protocol-http";
 import { WebSocket } from "mock-socket";
 import { PassThrough } from "stream";
-import { afterEach, beforeEach, describe, expect, expect, test as it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 import WS from "vitest-websocket-mock";
 
 import { WebSocketFetchHandler } from "./websocket-fetch-handler";
@@ -139,7 +139,7 @@ describe(WebSocketFetchHandler.name, () => {
         expect(err.$metadata).toBeDefined();
         expect(err.$metadata.httpStatusCode >= 500).toBe(true);
         expect(
-          ((global as any).setTimeout as any).mock.calls.filter((args) => {
+          ((global as any).setTimeout as any).mock.calls.filter((args: any) => {
             //find the 'setTimeout' call from the websocket handler
             return args[0].toString().indexOf("$metadata") >= 0;
           })[0][1]
diff --git a/packages/token-providers/src/fromSso.spec.ts b/packages/token-providers/src/fromSso.spec.ts
index 20c906ad2df1..84418060d8ca 100644
--- a/packages/token-providers/src/fromSso.spec.ts
+++ b/packages/token-providers/src/fromSso.spec.ts
@@ -74,7 +74,7 @@ describe(fromSso.name, () => {
 
   describe("throws validation error", () => {
     it("when profile is not found", async () => {
-      vi.mocked(parseKnownFiles).mockReturnValue({});
+      vi.mocked(parseKnownFiles as any).mockReturnValue({});
       const expectedError = new TokenProviderError(
         `Profile '${mockProfileName}' could not be found in shared credentials file.`,
         false
@@ -84,7 +84,7 @@ describe(fromSso.name, () => {
 
     it("when sso_session is not defined for profile", async () => {
       const { sso_session, ...mockSsoProfileWithoutSsoSession } = mockSsoProfile;
-      vi.mocked(parseKnownFiles).mockReturnValue({ [mockProfileName]: mockSsoProfileWithoutSsoSession });
+      vi.mocked(parseKnownFiles as any).mockReturnValue({ [mockProfileName]: mockSsoProfileWithoutSsoSession });
       const expectedError = new TokenProviderError(
         `Profile '${mockProfileName}' is missing required property 'sso_session'.`
       );
diff --git a/packages/token-providers/src/getNewSsoOidcToken.spec.ts b/packages/token-providers/src/getNewSsoOidcToken.spec.ts
index ea7f87aef2da..b84c51c2bf32 100644
--- a/packages/token-providers/src/getNewSsoOidcToken.spec.ts
+++ b/packages/token-providers/src/getNewSsoOidcToken.spec.ts
@@ -32,8 +32,8 @@ describe(getNewSsoOidcToken.name, () => {
 
   beforeEach(() => {
     mockSend = vi.fn().mockResolvedValueOnce(mockNewToken);
-    vi.mocked(getSsoOidcClient).mockReturnValue({ send: mockSend });
-    (CreateTokenCommand as unknown as any).mockImplementation((args) => args);
+    vi.mocked(getSsoOidcClient as any).mockReturnValue({ send: mockSend });
+    (CreateTokenCommand as unknown as any).mockImplementation((args: any) => args);
   });
 
   describe("re-throws", () => {
@@ -56,7 +56,7 @@ describe(getNewSsoOidcToken.name, () => {
 
     it("if client.send() throws", async () => {
       const mockSendWithError = vi.fn().mockRejectedValueOnce(mockError);
-      vi.mocked(getSsoOidcClient).mockReturnValueOnce({ send: mockSendWithError });
+      vi.mocked(getSsoOidcClient as any).mockReturnValueOnce({ send: mockSendWithError });
       try {
         await getNewSsoOidcToken(mockSsoToken, mockSsoRegion);
         fail(`expected ${mockError}`);
diff --git a/packages/token-providers/src/getSsoOidcClient.spec.ts b/packages/token-providers/src/getSsoOidcClient.spec.ts
index 109f897aa09c..bbaf420c3e6b 100644
--- a/packages/token-providers/src/getSsoOidcClient.spec.ts
+++ b/packages/token-providers/src/getSsoOidcClient.spec.ts
@@ -9,7 +9,7 @@ describe("getSsoOidcClient", () => {
 
   beforeEach(() => {
     vi.resetModules();
-    vi.mocked(SSOOIDCClient).mockImplementation((({ region }) => getMockClient(region)) as any);
+    vi.mocked(SSOOIDCClient as any).mockImplementation((({ region }: any) => getMockClient(region)) as any);
   });
 
   afterEach(() => {
diff --git a/packages/token-providers/src/nodeProvider.spec.ts b/packages/token-providers/src/nodeProvider.spec.ts
index d993d197ffec..dc0ae787c30b 100644
--- a/packages/token-providers/src/nodeProvider.spec.ts
+++ b/packages/token-providers/src/nodeProvider.spec.ts
@@ -29,7 +29,7 @@ describe(nodeProvider.name, () => {
       [chain, mockChainFn],
       [memoize, mockMemoizeFn],
     ].forEach(([fromFn, mockFn]) => {
-      vi.mocked(fromFn).mockReturnValue(mockFn);
+      vi.mocked(fromFn).mockReturnValue(mockFn as any);
     });
   });
 
@@ -99,13 +99,13 @@ describe(nodeProvider.name, () => {
     });
 
     it("returns true if expiration is not defined", () => {
-      const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2];
+      const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2]!;
       const expiration = Date.now();
       expect(memoizeRefreshFn({ expiration })).toEqual(true);
     });
 
     it("returns false if expiration is not defined", () => {
-      const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2];
+      const memoizeRefreshFn = vi.mocked(memoize).mock.calls[0][2]!;
       expect(memoizeRefreshFn({})).toEqual(false);
     });
   });
diff --git a/tsconfig.test.json b/tsconfig.test.json
new file mode 100644
index 000000000000..9a9c14442761
--- /dev/null
+++ b/tsconfig.test.json
@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "baseUrl": ".",
+    "noEmit": true
+  },
+  "extends": "./tsconfig.cjs.json",
+  "include": ["packages/**/*.spec.ts", "lib/**/*.spec.ts", "clients/**/*.spec.ts", "private/**/*.spec.ts"],
+  "exclude": [
+    "node_modules/",
+    "vitest.*.ts",
+    "clients/client-s3/test/e2e/S3.browser.e2e.spec.ts",
+    "private/aws-restjson-server/**/*.spec.ts",
+    "private/aws-restjson-validation-server/**/*.spec.ts"
+  ]
+}