diff --git a/packages/abort-controller/vitest.config.ts b/packages/abort-controller/vitest.config.mts
similarity index 100%
rename from packages/abort-controller/vitest.config.ts
rename to packages/abort-controller/vitest.config.mts
diff --git a/packages/chunked-blob-reader-native/vitest.config.ts b/packages/chunked-blob-reader-native/vitest.config.mts
similarity index 100%
rename from packages/chunked-blob-reader-native/vitest.config.ts
rename to packages/chunked-blob-reader-native/vitest.config.mts
diff --git a/packages/chunked-blob-reader/vitest.config.ts b/packages/chunked-blob-reader/vitest.config.mts
similarity index 100%
rename from packages/chunked-blob-reader/vitest.config.ts
rename to packages/chunked-blob-reader/vitest.config.mts
diff --git a/packages/config-resolver/vitest.config.ts b/packages/config-resolver/vitest.config.mts
similarity index 100%
rename from packages/config-resolver/vitest.config.ts
rename to packages/config-resolver/vitest.config.mts
diff --git a/packages/core/vitest.config.ts b/packages/core/vitest.config.mts
similarity index 100%
rename from packages/core/vitest.config.ts
rename to packages/core/vitest.config.mts
diff --git a/packages/credential-provider-imds/vitest.config.ts b/packages/credential-provider-imds/vitest.config.mts
similarity index 100%
rename from packages/credential-provider-imds/vitest.config.ts
rename to packages/credential-provider-imds/vitest.config.mts
diff --git a/packages/eventstream-codec/vitest.config.ts b/packages/eventstream-codec/vitest.config.mts
similarity index 100%
rename from packages/eventstream-codec/vitest.config.ts
rename to packages/eventstream-codec/vitest.config.mts
diff --git a/packages/eventstream-serde-config-resolver/vitest.config.ts b/packages/eventstream-serde-config-resolver/vitest.config.mts
similarity index 100%
rename from packages/eventstream-serde-config-resolver/vitest.config.ts
rename to packages/eventstream-serde-config-resolver/vitest.config.mts
diff --git a/packages/eventstream-serde-universal/vitest.config.ts b/packages/eventstream-serde-universal/vitest.config.mts
similarity index 100%
rename from packages/eventstream-serde-universal/vitest.config.ts
rename to packages/eventstream-serde-universal/vitest.config.mts
diff --git a/packages/experimental-identity-and-auth/vitest.config.ts b/packages/experimental-identity-and-auth/vitest.config.mts
similarity index 100%
rename from packages/experimental-identity-and-auth/vitest.config.ts
rename to packages/experimental-identity-and-auth/vitest.config.mts
diff --git a/packages/fetch-http-handler/vitest.config.ts b/packages/fetch-http-handler/vitest.config.mts
similarity index 100%
rename from packages/fetch-http-handler/vitest.config.ts
rename to packages/fetch-http-handler/vitest.config.mts
diff --git a/packages/hash-blob-browser/vitest.config.ts b/packages/hash-blob-browser/vitest.config.mts
similarity index 100%
rename from packages/hash-blob-browser/vitest.config.ts
rename to packages/hash-blob-browser/vitest.config.mts
diff --git a/packages/hash-node/vitest.config.ts b/packages/hash-node/vitest.config.mts
similarity index 100%
rename from packages/hash-node/vitest.config.ts
rename to packages/hash-node/vitest.config.mts
diff --git a/packages/hash-stream-node/vitest.config.ts b/packages/hash-stream-node/vitest.config.mts
similarity index 100%
rename from packages/hash-stream-node/vitest.config.ts
rename to packages/hash-stream-node/vitest.config.mts
diff --git a/packages/invalid-dependency/vitest.config.ts b/packages/invalid-dependency/vitest.config.mts
similarity index 100%
rename from packages/invalid-dependency/vitest.config.ts
rename to packages/invalid-dependency/vitest.config.mts
diff --git a/packages/is-array-buffer/vitest.config.ts b/packages/is-array-buffer/vitest.config.mts
similarity index 100%
rename from packages/is-array-buffer/vitest.config.ts
rename to packages/is-array-buffer/vitest.config.mts
diff --git a/packages/md5-js/vitest.config.ts b/packages/md5-js/vitest.config.mts
similarity index 100%
rename from packages/md5-js/vitest.config.ts
rename to packages/md5-js/vitest.config.mts
diff --git a/packages/middleware-apply-body-checksum/vitest.config.ts b/packages/middleware-apply-body-checksum/vitest.config.mts
similarity index 100%
rename from packages/middleware-apply-body-checksum/vitest.config.ts
rename to packages/middleware-apply-body-checksum/vitest.config.mts
diff --git a/packages/middleware-compression/vitest.config.ts b/packages/middleware-compression/vitest.config.mts
similarity index 100%
rename from packages/middleware-compression/vitest.config.ts
rename to packages/middleware-compression/vitest.config.mts
diff --git a/packages/middleware-endpoint/vitest.config.ts b/packages/middleware-endpoint/vitest.config.mts
similarity index 100%
rename from packages/middleware-endpoint/vitest.config.ts
rename to packages/middleware-endpoint/vitest.config.mts
diff --git a/packages/middleware-retry/vitest.config.ts b/packages/middleware-retry/vitest.config.mts
similarity index 100%
rename from packages/middleware-retry/vitest.config.ts
rename to packages/middleware-retry/vitest.config.mts
diff --git a/packages/middleware-serde/vitest.config.ts b/packages/middleware-serde/vitest.config.mts
similarity index 100%
rename from packages/middleware-serde/vitest.config.ts
rename to packages/middleware-serde/vitest.config.mts
diff --git a/packages/middleware-stack/vitest.config.ts b/packages/middleware-stack/vitest.config.mts
similarity index 100%
rename from packages/middleware-stack/vitest.config.ts
rename to packages/middleware-stack/vitest.config.mts
diff --git a/packages/node-config-provider/vitest.config.ts b/packages/node-config-provider/vitest.config.mts
similarity index 100%
rename from packages/node-config-provider/vitest.config.ts
rename to packages/node-config-provider/vitest.config.mts
diff --git a/packages/node-http-handler/vitest.config.ts b/packages/node-http-handler/vitest.config.mts
similarity index 100%
rename from packages/node-http-handler/vitest.config.ts
rename to packages/node-http-handler/vitest.config.mts
diff --git a/packages/property-provider/vitest.config.ts b/packages/property-provider/vitest.config.mts
similarity index 100%
rename from packages/property-provider/vitest.config.ts
rename to packages/property-provider/vitest.config.mts
diff --git a/packages/protocol-http/vitest.config.ts b/packages/protocol-http/vitest.config.mts
similarity index 100%
rename from packages/protocol-http/vitest.config.ts
rename to packages/protocol-http/vitest.config.mts
diff --git a/packages/querystring-parser/vitest.config.ts b/packages/querystring-parser/vitest.config.mts
similarity index 100%
rename from packages/querystring-parser/vitest.config.ts
rename to packages/querystring-parser/vitest.config.mts
diff --git a/packages/service-error-classification/vitest.config.ts b/packages/service-error-classification/vitest.config.mts
similarity index 100%
rename from packages/service-error-classification/vitest.config.ts
rename to packages/service-error-classification/vitest.config.mts
diff --git a/packages/shared-ini-file-loader/vitest.config.ts b/packages/shared-ini-file-loader/vitest.config.mts
similarity index 100%
rename from packages/shared-ini-file-loader/vitest.config.ts
rename to packages/shared-ini-file-loader/vitest.config.mts
diff --git a/packages/signature-v4/vitest.config.ts b/packages/signature-v4/vitest.config.mts
similarity index 100%
rename from packages/signature-v4/vitest.config.ts
rename to packages/signature-v4/vitest.config.mts
diff --git a/packages/signature-v4a/vitest.config.js b/packages/signature-v4a/vitest.config.js
deleted file mode 100644
index 3a143bfb2cf..00000000000
--- a/packages/signature-v4a/vitest.config.js
+++ /dev/null
@@ -1,7 +0,0 @@
-import { defineConfig } from 'vitest/config';
-
-export default defineConfig({
-  test: {
-    exclude: ['**/node_modules/**', '**/es/**'],
-  },
-});
diff --git a/packages/signature-v4a/vitest.config.mts b/packages/signature-v4a/vitest.config.mts
new file mode 100644
index 00000000000..9e5b5ec0549
--- /dev/null
+++ b/packages/signature-v4a/vitest.config.mts
@@ -0,0 +1,9 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    exclude: ["**/node_modules/**", "**/es/**"],
+    include: ["**/*.spec.ts"],
+    environment: "node",
+  },
+});
diff --git a/packages/smithy-client/vitest.config.ts b/packages/smithy-client/vitest.config.mts
similarity index 100%
rename from packages/smithy-client/vitest.config.ts
rename to packages/smithy-client/vitest.config.mts
diff --git a/packages/url-parser/vitest.config.ts b/packages/url-parser/vitest.config.mts
similarity index 100%
rename from packages/url-parser/vitest.config.ts
rename to packages/url-parser/vitest.config.mts
diff --git a/packages/util-base64/vitest.config.ts b/packages/util-base64/vitest.config.mts
similarity index 100%
rename from packages/util-base64/vitest.config.ts
rename to packages/util-base64/vitest.config.mts
diff --git a/packages/util-body-length-browser/vitest.config.ts b/packages/util-body-length-browser/vitest.config.mts
similarity index 100%
rename from packages/util-body-length-browser/vitest.config.ts
rename to packages/util-body-length-browser/vitest.config.mts
diff --git a/packages/util-body-length-node/vitest.config.ts b/packages/util-body-length-node/vitest.config.mts
similarity index 100%
rename from packages/util-body-length-node/vitest.config.ts
rename to packages/util-body-length-node/vitest.config.mts
diff --git a/packages/util-buffer-from/vitest.config.ts b/packages/util-buffer-from/vitest.config.mts
similarity index 100%
rename from packages/util-buffer-from/vitest.config.ts
rename to packages/util-buffer-from/vitest.config.mts
diff --git a/packages/util-config-provider/vitest.config.ts b/packages/util-config-provider/vitest.config.mts
similarity index 100%
rename from packages/util-config-provider/vitest.config.ts
rename to packages/util-config-provider/vitest.config.mts
diff --git a/packages/util-defaults-mode-browser/vitest.config.ts b/packages/util-defaults-mode-browser/vitest.config.mts
similarity index 100%
rename from packages/util-defaults-mode-browser/vitest.config.ts
rename to packages/util-defaults-mode-browser/vitest.config.mts
diff --git a/packages/util-defaults-mode-node/vitest.config.ts b/packages/util-defaults-mode-node/vitest.config.mts
similarity index 100%
rename from packages/util-defaults-mode-node/vitest.config.ts
rename to packages/util-defaults-mode-node/vitest.config.mts
diff --git a/packages/util-endpoints/vitest.config.ts b/packages/util-endpoints/vitest.config.mts
similarity index 100%
rename from packages/util-endpoints/vitest.config.ts
rename to packages/util-endpoints/vitest.config.mts
diff --git a/packages/util-hex-encoding/vitest.config.ts b/packages/util-hex-encoding/vitest.config.mts
similarity index 100%
rename from packages/util-hex-encoding/vitest.config.ts
rename to packages/util-hex-encoding/vitest.config.mts
diff --git a/packages/util-middleware/vitest.config.ts b/packages/util-middleware/vitest.config.mts
similarity index 100%
rename from packages/util-middleware/vitest.config.ts
rename to packages/util-middleware/vitest.config.mts
diff --git a/packages/util-retry/vitest.config.ts b/packages/util-retry/vitest.config.mts
similarity index 100%
rename from packages/util-retry/vitest.config.ts
rename to packages/util-retry/vitest.config.mts
diff --git a/packages/util-stream/vitest.config.ts b/packages/util-stream/vitest.config.mts
similarity index 100%
rename from packages/util-stream/vitest.config.ts
rename to packages/util-stream/vitest.config.mts
diff --git a/packages/util-uri-escape/vitest.config.ts b/packages/util-uri-escape/vitest.config.mts
similarity index 100%
rename from packages/util-uri-escape/vitest.config.ts
rename to packages/util-uri-escape/vitest.config.mts
diff --git a/packages/util-utf8/vitest.config.ts b/packages/util-utf8/vitest.config.mts
similarity index 100%
rename from packages/util-utf8/vitest.config.ts
rename to packages/util-utf8/vitest.config.mts
diff --git a/packages/util-waiter/vitest.config.ts b/packages/util-waiter/vitest.config.mts
similarity index 100%
rename from packages/util-waiter/vitest.config.ts
rename to packages/util-waiter/vitest.config.mts
diff --git a/private/smithy-rpcv2-cbor-schema/vite.config.js b/private/smithy-rpcv2-cbor-schema/vite.config.mjs
similarity index 100%
rename from private/smithy-rpcv2-cbor-schema/vite.config.js
rename to private/smithy-rpcv2-cbor-schema/vite.config.mjs
diff --git a/private/smithy-rpcv2-cbor/vite.config.js b/private/smithy-rpcv2-cbor/vite.config.mjs
similarity index 100%
rename from private/smithy-rpcv2-cbor/vite.config.js
rename to private/smithy-rpcv2-cbor/vite.config.mjs
diff --git a/smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/PackageJsonGenerator.java b/smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/PackageJsonGenerator.java
index 5d1360470b5..be1ae9d3f7d 100644
--- a/smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/PackageJsonGenerator.java
+++ b/smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/PackageJsonGenerator.java
@@ -32,7 +32,7 @@ final class PackageJsonGenerator {
 
     public static final String PACKAGE_JSON_FILENAME = "package.json";
     public static final String TYPEDOC_FILE_NAME = "typedoc.json";
-    public static final String VITEST_CONFIG_FILENAME = "vite.config.js";
+    public static final String VITEST_CONFIG_FILENAME = "vite.config.mjs";
 
     private PackageJsonGenerator() {}
 
@@ -67,7 +67,7 @@ static void writePackageJson(
             node = node.withMember(depEntry.getKey(), builder.build());
         }
 
-        // Add test script and vite.config.js if specs and their devDependency on vitest has been generated.
+        // Add test script and vite.config.mjs if specs and their devDependency on vitest has been generated.
         ObjectNode devDeps = node.getObjectMember("devDependencies").orElse(Node.objectNode());
         if (devDeps.containsMember(TypeScriptDependency.VITEST.packageName)) {
             ObjectNode scripts = node.getObjectMember("scripts").orElse(Node.objectNode());
diff --git a/smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/vite.config.js b/smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/vite.config.mjs
similarity index 100%
rename from smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/vite.config.js
rename to smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/vite.config.mjs
diff --git a/vitest.config.browser.ts b/vitest.config.browser.mts
similarity index 100%
rename from vitest.config.browser.ts
rename to vitest.config.browser.mts
diff --git a/vitest.config.integ.ts b/vitest.config.integ.mts
similarity index 100%
rename from vitest.config.integ.ts
rename to vitest.config.integ.mts
diff --git a/vitest.config.ts b/vitest.config.mts
similarity index 100%
rename from vitest.config.ts
rename to vitest.config.mts