diff --git a/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.spec.ts b/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.spec.ts
new file mode 100644
index 000000000000..d518134cee93
--- /dev/null
+++ b/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.spec.ts
@@ -0,0 +1,10 @@
+import { describe, expect, test as it } from "vitest";
+
+import { resolveAccountIdEndpointModeConfig } from "./AccountIdEndpointModeConfigResolver";
+
+describe(resolveAccountIdEndpointModeConfig.name, () => {
+  it("maintains object custody", () => {
+    const input = {};
+    expect(resolveAccountIdEndpointModeConfig(input)).toBe(input);
+  });
+});
diff --git a/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.ts b/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.ts
index 36ae35492651..b9166c184c39 100644
--- a/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.ts
+++ b/packages/core/src/submodules/account-id-endpoint/AccountIdEndpointModeConfigResolver.ts
@@ -35,11 +35,9 @@ export interface AccountIdEndpointModeResolvedConfig {
 export const resolveAccountIdEndpointModeConfig = <T>(
   input: T & AccountIdEndpointModeInputConfig & PreviouslyResolved
 ): T & AccountIdEndpointModeResolvedConfig => {
-  const accountIdEndpointModeProvider = normalizeProvider(
-    input.accountIdEndpointMode ?? DEFAULT_ACCOUNT_ID_ENDPOINT_MODE
-  );
-  return {
-    ...input,
+  const { accountIdEndpointMode } = input;
+  const accountIdEndpointModeProvider = normalizeProvider(accountIdEndpointMode ?? DEFAULT_ACCOUNT_ID_ENDPOINT_MODE);
+  return Object.assign(input, {
     accountIdEndpointMode: async () => {
       const accIdMode = await accountIdEndpointModeProvider();
       if (!validateAccountIdEndpointMode(accIdMode)) {
@@ -49,5 +47,5 @@ export const resolveAccountIdEndpointModeConfig = <T>(
       }
       return accIdMode;
     },
-  };
+  });
 };
diff --git a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4AConfig.spec.ts b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4AConfig.spec.ts
index 7a258e16bb68..c51290d15856 100644
--- a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4AConfig.spec.ts
+++ b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4AConfig.spec.ts
@@ -9,4 +9,9 @@ describe(resolveAwsSdkSigV4AConfig.name, () => {
     expect(typeof config.sigv4aSigningRegionSet).toEqual("function");
     expect(await config.sigv4aSigningRegionSet()).toEqual(undefined);
   });
+
+  it("maintains object custody", () => {
+    const input = {};
+    expect(resolveAwsSdkSigV4AConfig(input)).toBe(input);
+  });
 });
diff --git a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.spec.ts b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.spec.ts
index 3f3b1a5fabb8..eb3c2455bf4b 100644
--- a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.spec.ts
+++ b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.spec.ts
@@ -4,6 +4,17 @@ import { describe, expect, test as it, vi } from "vitest";
 import { resolveAwsSdkSigV4Config } from "./resolveAwsSdkSigV4Config";
 
 describe(resolveAwsSdkSigV4Config.name, () => {
+  it("maintains object custody", () => {
+    const input = {
+      region: "",
+      sha256: vi.fn(),
+      serviceId: "",
+      useFipsEndpoint: async () => false,
+      useDualstackEndpoint: async () => false,
+    };
+    expect(resolveAwsSdkSigV4Config(input)).toBe(input);
+  });
+
   it("should allow one argument to be passed to the resolved credentials function", async () => {
     const fn = vi.fn();
 
diff --git a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.ts b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.ts
index 9079d9def144..b71b04c607f7 100644
--- a/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.ts
+++ b/packages/core/src/submodules/httpAuthSchemes/aws_sdk/resolveAwsSdkSigV4Config.ts
@@ -221,8 +221,7 @@ export const resolveAwsSdkSigV4Config = <T>(
     };
   }
 
-  return {
-    ...config,
+  return Object.assign(config, {
     systemClockOffset,
     signingEscapePath,
     credentials: isUserSupplied
@@ -232,7 +231,7 @@ export const resolveAwsSdkSigV4Config = <T>(
           )
       : boundCredentialsProvider!,
     signer,
-  };
+  });
 };
 
 /**
diff --git a/packages/middleware-api-key/src/apiKeyConfiguration.spec.ts b/packages/middleware-api-key/src/apiKeyConfiguration.spec.ts
index d67bae14a948..25a508d41493 100644
--- a/packages/middleware-api-key/src/apiKeyConfiguration.spec.ts
+++ b/packages/middleware-api-key/src/apiKeyConfiguration.spec.ts
@@ -3,6 +3,12 @@ import { describe, expect, test as it } from "vitest";
 import { resolveApiKeyConfig } from "./index";
 
 describe("ApiKeyConfig", () => {
+  it("maintains object custody", () => {
+    const config = {
+      apiKey: () => Promise.resolve("example-api-key"),
+    };
+    expect(resolveApiKeyConfig(config)).toBe(config);
+  });
   it("should return the input unchanged", () => {
     const config = {
       apiKey: () => Promise.resolve("example-api-key"),
diff --git a/packages/middleware-api-key/src/apiKeyConfiguration.ts b/packages/middleware-api-key/src/apiKeyConfiguration.ts
index a3050a27fee5..dc4360cfd4c3 100644
--- a/packages/middleware-api-key/src/apiKeyConfiguration.ts
+++ b/packages/middleware-api-key/src/apiKeyConfiguration.ts
@@ -31,8 +31,8 @@ export interface ApiKeyResolvedConfig {
 export const resolveApiKeyConfig = <T>(
   input: T & ApiKeyPreviouslyResolved & ApiKeyInputConfig
 ): T & ApiKeyResolvedConfig => {
-  return {
-    ...input,
-    apiKey: input.apiKey ? normalizeProvider(input.apiKey) : undefined,
-  };
+  const { apiKey } = input;
+  return Object.assign(input, {
+    apiKey: apiKey ? normalizeProvider(apiKey) : undefined,
+  });
 };
diff --git a/packages/middleware-bucket-endpoint/src/configurations.spec.ts b/packages/middleware-bucket-endpoint/src/configurations.spec.ts
new file mode 100644
index 000000000000..b413f7514af7
--- /dev/null
+++ b/packages/middleware-bucket-endpoint/src/configurations.spec.ts
@@ -0,0 +1,15 @@
+import { describe, expect, test as it, vi } from "vitest";
+
+import { resolveBucketEndpointConfig } from "./configurations";
+
+describe(resolveBucketEndpointConfig.name, () => {
+  it("maintains object custody", () => {
+    const input = {
+      region: async () => "",
+      regionInfoProvider: vi.fn(),
+      useFipsEndpoint: async () => false,
+      useDualstackEndpoint: async () => false,
+    };
+    expect(resolveBucketEndpointConfig(input)).toBe(input);
+  });
+});
diff --git a/packages/middleware-bucket-endpoint/src/configurations.ts b/packages/middleware-bucket-endpoint/src/configurations.ts
index f555ad0484f8..831466103fa0 100644
--- a/packages/middleware-bucket-endpoint/src/configurations.ts
+++ b/packages/middleware-bucket-endpoint/src/configurations.ts
@@ -93,8 +93,7 @@ export function resolveBucketEndpointConfig<T>(
     useArnRegion = false,
     disableMultiregionAccessPoints = false,
   } = input;
-  return {
-    ...input,
+  return Object.assign(input, {
     bucketEndpoint,
     forcePathStyle,
     useAccelerateEndpoint,
@@ -103,5 +102,5 @@ export function resolveBucketEndpointConfig<T>(
       typeof disableMultiregionAccessPoints === "function"
         ? disableMultiregionAccessPoints
         : () => Promise.resolve(disableMultiregionAccessPoints),
-  };
+  });
 }
diff --git a/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.spec.ts b/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.spec.ts
index f435d191eef3..5462144461e4 100644
--- a/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.spec.ts
+++ b/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.spec.ts
@@ -7,18 +7,26 @@ vi.mock("@aws-sdk/endpoint-cache");
 
 describe(resolveEndpointDiscoveryConfig.name, () => {
   const endpointDiscoveryCommandCtor = vi.fn();
-  const mockInput = {
+  const mockInput = () => ({
     isCustomEndpoint: false,
     credentials: vi.fn(),
     endpointDiscoveryEnabledProvider: vi.fn(),
-  };
+  });
 
   afterEach(() => {
     vi.clearAllMocks();
   });
 
+  it("maintains object custody", () => {
+    const input = {
+      credentials: vi.fn(),
+      endpointDiscoveryEnabledProvider: async () => false,
+    };
+    expect(resolveEndpointDiscoveryConfig(input, { endpointDiscoveryCommandCtor })).toBe(input);
+  });
+
   it("assigns endpointDiscoveryCommandCtor in resolvedConfig", () => {
-    const resolvedConfig = resolveEndpointDiscoveryConfig(mockInput, { endpointDiscoveryCommandCtor });
+    const resolvedConfig = resolveEndpointDiscoveryConfig(mockInput(), { endpointDiscoveryCommandCtor });
     expect(resolvedConfig.endpointDiscoveryCommandCtor).toStrictEqual(endpointDiscoveryCommandCtor);
   });
 
@@ -27,7 +35,7 @@ describe(resolveEndpointDiscoveryConfig.name, () => {
       const endpointCacheSize = 100;
       resolveEndpointDiscoveryConfig(
         {
-          ...mockInput,
+          ...mockInput(),
           endpointCacheSize,
         },
         { endpointDiscoveryCommandCtor }
@@ -36,28 +44,30 @@ describe(resolveEndpointDiscoveryConfig.name, () => {
     });
 
     it("creates cache of size 1000 if endpointCacheSize not passed", () => {
-      resolveEndpointDiscoveryConfig(mockInput, { endpointDiscoveryCommandCtor });
+      resolveEndpointDiscoveryConfig(mockInput(), { endpointDiscoveryCommandCtor });
       expect(EndpointCache).toBeCalledWith(1000);
     });
   });
 
   describe("endpointDiscoveryEnabled", () => {
     it.each<boolean>([false, true])(`sets to value passed in the config: %s`, async (endpointDiscoveryEnabled) => {
+      const input = mockInput();
       const resolvedConfig = resolveEndpointDiscoveryConfig(
         {
-          ...mockInput,
+          ...input,
           endpointDiscoveryEnabled,
         },
         { endpointDiscoveryCommandCtor }
       );
       await expect(resolvedConfig.endpointDiscoveryEnabled()).resolves.toBe(endpointDiscoveryEnabled);
-      expect(mockInput.endpointDiscoveryEnabledProvider).not.toHaveBeenCalled();
+      expect(input.endpointDiscoveryEnabledProvider).not.toHaveBeenCalled();
       expect(resolvedConfig.isClientEndpointDiscoveryEnabled).toStrictEqual(true);
     });
 
     it(`sets to endpointDiscoveryEnabledProvider if value is not passed`, () => {
-      const resolvedConfig = resolveEndpointDiscoveryConfig(mockInput, { endpointDiscoveryCommandCtor });
-      expect(resolvedConfig.endpointDiscoveryEnabled).toBe(mockInput.endpointDiscoveryEnabledProvider);
+      const input = mockInput();
+      const resolvedConfig = resolveEndpointDiscoveryConfig(input, { endpointDiscoveryCommandCtor });
+      expect(resolvedConfig.endpointDiscoveryEnabled).toBe(input.endpointDiscoveryEnabledProvider);
       expect(resolvedConfig.isClientEndpointDiscoveryEnabled).toStrictEqual(false);
     });
   });
diff --git a/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.ts b/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.ts
index 732b373ad7b8..2dfc63d1f757 100644
--- a/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.ts
+++ b/packages/middleware-endpoint-discovery/src/resolveEndpointDiscoveryConfig.ts
@@ -68,13 +68,16 @@ export interface EndpointDiscoveryConfigOptions {
 export const resolveEndpointDiscoveryConfig = <T>(
   input: T & PreviouslyResolved & EndpointDiscoveryInputConfig,
   { endpointDiscoveryCommandCtor }: EndpointDiscoveryConfigOptions
-): T & EndpointDiscoveryResolvedConfig => ({
-  ...input,
-  endpointDiscoveryCommandCtor,
-  endpointCache: new EndpointCache(input.endpointCacheSize ?? 1000),
-  endpointDiscoveryEnabled:
-    input.endpointDiscoveryEnabled !== undefined
-      ? () => Promise.resolve(input.endpointDiscoveryEnabled)
-      : input.endpointDiscoveryEnabledProvider,
-  isClientEndpointDiscoveryEnabled: input.endpointDiscoveryEnabled !== undefined,
-});
+): T & EndpointDiscoveryResolvedConfig => {
+  const { endpointCacheSize, endpointDiscoveryEnabled, endpointDiscoveryEnabledProvider } = input;
+
+  return Object.assign(input, {
+    endpointDiscoveryCommandCtor,
+    endpointCache: new EndpointCache(endpointCacheSize ?? 1000),
+    endpointDiscoveryEnabled:
+      endpointDiscoveryEnabled !== undefined
+        ? () => Promise.resolve(endpointDiscoveryEnabled)
+        : endpointDiscoveryEnabledProvider,
+    isClientEndpointDiscoveryEnabled: endpointDiscoveryEnabled !== undefined,
+  });
+};
diff --git a/packages/middleware-eventstream/src/eventStreamConfiguration.ts b/packages/middleware-eventstream/src/eventStreamConfiguration.ts
index b82fa0ffa2f7..47796b798329 100644
--- a/packages/middleware-eventstream/src/eventStreamConfiguration.ts
+++ b/packages/middleware-eventstream/src/eventStreamConfiguration.ts
@@ -36,13 +36,12 @@ export function resolveEventStreamConfig<T>(
 ): T & EventStreamResolvedConfig {
   const eventSigner = input.signer;
   const messageSigner = input.signer;
-  const eventStreamPayloadHandler = input.eventStreamPayloadHandlerProvider({
-    ...input,
+  const newInput = Object.assign(input, {
+    eventSigner,
     messageSigner,
   });
-  return {
-    ...input,
-    eventSigner,
+  const eventStreamPayloadHandler = newInput.eventStreamPayloadHandlerProvider(newInput);
+  return Object.assign(newInput, {
     eventStreamPayloadHandler,
-  };
+  });
 }
diff --git a/packages/middleware-eventstream/src/middleware-eventstream.integ.spec.ts b/packages/middleware-eventstream/src/middleware-eventstream.integ.spec.ts
index b8cf25b96cbc..4f5aedd1267b 100644
--- a/packages/middleware-eventstream/src/middleware-eventstream.integ.spec.ts
+++ b/packages/middleware-eventstream/src/middleware-eventstream.integ.spec.ts
@@ -1,9 +1,11 @@
 import { LexRuntimeV2 } from "@aws-sdk/client-lex-runtime-v2";
 import { RekognitionStreaming } from "@aws-sdk/client-rekognitionstreaming";
 import { TranscribeStreaming } from "@aws-sdk/client-transcribe-streaming";
-import { describe, expect, test as it } from "vitest";
+import { Decoder, Encoder, EventStreamPayloadHandlerProvider } from "@smithy/types";
+import { describe, expect, test as it, vi } from "vitest";
 
 import { requireRequestsFrom } from "../../../private/aws-util-test/src";
+import { resolveEventStreamConfig } from "./eventStreamConfiguration";
 
 describe("middleware-eventstream", () => {
   const logger = {
@@ -14,6 +16,18 @@ describe("middleware-eventstream", () => {
     error() {},
   };
 
+  describe("config resolver", () => {
+    it("maintains object custody", () => {
+      const input = {
+        utf8Encoder: vi.fn(),
+        utf8Decoder: vi.fn(),
+        signer: vi.fn(),
+        eventStreamPayloadHandlerProvider: vi.fn(),
+      };
+      expect(resolveEventStreamConfig(input)).toBe(input);
+    });
+  });
+
   // TODO: http2 in CI
   describe.skip(LexRuntimeV2.name, () => {
     it("should set streaming headers", async () => {
diff --git a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
index 9f340834a480..7d8de834ee0b 100644
--- a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
+++ b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.spec.ts
@@ -20,6 +20,11 @@ describe(resolveFlexibleChecksumsConfig.name, () => {
     vi.clearAllMocks();
   });
 
+  it("maintains object custody", () => {
+    const input = {};
+    expect(resolveFlexibleChecksumsConfig(input)).toBe(input);
+  });
+
   it("returns default client checksums configuration, if not provided", () => {
     const resolvedConfig = resolveFlexibleChecksumsConfig({});
     expect(resolvedConfig).toEqual({
diff --git a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.ts b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.ts
index 235b7625f01e..df38da5429ca 100644
--- a/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.ts
+++ b/packages/middleware-flexible-checksums/src/resolveFlexibleChecksumsConfig.ts
@@ -54,13 +54,11 @@ export interface FlexibleChecksumsResolvedConfig {
  */
 export const resolveFlexibleChecksumsConfig = <T>(
   input: T & FlexibleChecksumsInputConfig
-): T & FlexibleChecksumsResolvedConfig => ({
-  ...input,
-  requestChecksumCalculation: normalizeProvider(
-    input.requestChecksumCalculation ?? DEFAULT_REQUEST_CHECKSUM_CALCULATION
-  ),
-  responseChecksumValidation: normalizeProvider(
-    input.responseChecksumValidation ?? DEFAULT_RESPONSE_CHECKSUM_VALIDATION
-  ),
-  requestStreamBufferSize: Number(input.requestStreamBufferSize ?? 0),
-});
+): T & FlexibleChecksumsResolvedConfig => {
+  const { requestChecksumCalculation, responseChecksumValidation, requestStreamBufferSize } = input;
+  return Object.assign(input, {
+    requestChecksumCalculation: normalizeProvider(requestChecksumCalculation ?? DEFAULT_REQUEST_CHECKSUM_CALCULATION),
+    responseChecksumValidation: normalizeProvider(responseChecksumValidation ?? DEFAULT_RESPONSE_CHECKSUM_VALIDATION),
+    requestStreamBufferSize: Number(requestStreamBufferSize ?? 0),
+  });
+};
diff --git a/packages/middleware-host-header/src/index.spec.ts b/packages/middleware-host-header/src/index.spec.ts
index 52e945c28429..81f13865a4ea 100644
--- a/packages/middleware-host-header/src/index.spec.ts
+++ b/packages/middleware-host-header/src/index.spec.ts
@@ -1,7 +1,7 @@
 import { HttpRequest } from "@smithy/protocol-http";
 import { beforeEach, describe, expect, test as it, vi } from "vitest";
 
-import { hostHeaderMiddleware } from "./index";
+import { hostHeaderMiddleware, resolveHostHeaderConfig } from "./index";
 describe("hostHeaderMiddleware", () => {
   const mockNextHandler = vi.fn();
 
@@ -9,6 +9,13 @@ describe("hostHeaderMiddleware", () => {
     vi.clearAllMocks();
   });
 
+  it("maintains object custody", () => {
+    const input = {
+      requestHandler: vi.fn() as any,
+    };
+    expect(resolveHostHeaderConfig(input)).toBe(input);
+  });
+
   it("should set host header if not already set", async () => {
     expect.assertions(2);
     const middleware = hostHeaderMiddleware({ requestHandler: {} as any });
diff --git a/packages/middleware-location-constraint/src/configuration.ts b/packages/middleware-location-constraint/src/configuration.ts
index 88fe114eb04e..bd1b496593c9 100644
--- a/packages/middleware-location-constraint/src/configuration.ts
+++ b/packages/middleware-location-constraint/src/configuration.ts
@@ -18,5 +18,5 @@ export interface LocationConstraintResolvedConfig {
 export function resolveLocationConstraintConfig<T>(
   input: T & LocationConstraintInputConfig & PreviouslyResolved
 ): T & LocationConstraintResolvedConfig {
-  return { ...input };
+  return input;
 }
diff --git a/packages/middleware-location-constraint/src/index.spec.ts b/packages/middleware-location-constraint/src/index.spec.ts
index 320c4be1cd18..ac02e39e3929 100644
--- a/packages/middleware-location-constraint/src/index.spec.ts
+++ b/packages/middleware-location-constraint/src/index.spec.ts
@@ -1,6 +1,7 @@
 import { beforeEach, describe, expect, test as it, vi } from "vitest";
 
 import { locationConstraintMiddleware } from "./";
+import { resolveLocationConstraintConfig } from "./configuration";
 
 describe("locationConstrainMiddleware", () => {
   const next = vi.fn();
@@ -12,6 +13,13 @@ describe("locationConstrainMiddleware", () => {
     vi.clearAllMocks();
   });
 
+  describe("config resolver", () => {
+    it("maintains object custody", () => {
+      const input = {} as any;
+      expect(resolveLocationConstraintConfig(input)).toBe(input);
+    });
+  });
+
   describe("for region us-east-1", () => {
     const handler = locationConstraintMiddleware({
       region: () => Promise.resolve("us-east-1"),
diff --git a/packages/middleware-sdk-s3-control/src/configurations.spec.ts b/packages/middleware-sdk-s3-control/src/configurations.spec.ts
new file mode 100644
index 000000000000..411383e6bbf7
--- /dev/null
+++ b/packages/middleware-sdk-s3-control/src/configurations.spec.ts
@@ -0,0 +1,10 @@
+import { describe, expect, test as it } from "vitest";
+
+import { resolveS3ControlConfig } from "./configurations";
+
+describe(resolveS3ControlConfig.name, () => {
+  it("maintains object custody", () => {
+    const input = {} as any;
+    expect(resolveS3ControlConfig(input)).toBe(input);
+  });
+});
diff --git a/packages/middleware-sdk-s3-control/src/configurations.ts b/packages/middleware-sdk-s3-control/src/configurations.ts
index a7d34107d222..55ff29a12117 100644
--- a/packages/middleware-sdk-s3-control/src/configurations.ts
+++ b/packages/middleware-sdk-s3-control/src/configurations.ts
@@ -52,8 +52,7 @@ export function resolveS3ControlConfig<T>(
   input: T & PreviouslyResolved & S3ControlInputConfig
 ): T & S3ControlResolvedConfig {
   const { useArnRegion = false } = input;
-  return {
-    ...input,
+  return Object.assign(input, {
     useArnRegion: typeof useArnRegion === "function" ? useArnRegion : () => Promise.resolve(useArnRegion),
-  };
+  });
 }
diff --git a/packages/middleware-sdk-s3/src/s3Configuration.spec.ts b/packages/middleware-sdk-s3/src/s3Configuration.spec.ts
new file mode 100644
index 000000000000..4465a63fd962
--- /dev/null
+++ b/packages/middleware-sdk-s3/src/s3Configuration.spec.ts
@@ -0,0 +1,14 @@
+import { describe, expect, test as it, vi } from "vitest";
+
+import { resolveS3Config } from "./s3Configuration";
+
+describe(resolveS3Config.name, () => {
+  it("maintains object custody", () => {
+    const input = {};
+    expect(
+      resolveS3Config(input, {
+        session: [() => null, vi.fn()],
+      })
+    ).toBe(input);
+  });
+});
diff --git a/packages/middleware-sdk-s3/src/s3Configuration.ts b/packages/middleware-sdk-s3/src/s3Configuration.ts
index 6dfd9d3544ec..3b3ed7f3b48c 100644
--- a/packages/middleware-sdk-s3/src/s3Configuration.ts
+++ b/packages/middleware-sdk-s3/src/s3Configuration.ts
@@ -3,9 +3,8 @@ import type { Client, Command } from "@smithy/types";
 import { S3ExpressIdentityProvider, S3ExpressIdentityProviderImpl } from "./s3-express";
 
 /**
- * @public
- *
  * All endpoint parameters with built-in bindings of AWS::S3::*
+ * @public
  */
 export interface S3InputConfig {
   /**
@@ -28,7 +27,7 @@ export interface S3InputConfig {
    * This feature should only be used as a last resort if you do not know the region of your bucket(s) ahead of time.
    */
   followRegionRedirects?: boolean;
-  /*
+  /**
    * Identity provider for an S3 feature.
    */
   s3ExpressIdentityProvider?: S3ExpressIdentityProvider;
@@ -39,16 +38,16 @@ export interface S3InputConfig {
 }
 
 /**
- * @internal
  * This is a placeholder for the actual
  * S3Client type from \@aws-sdk/client-s3. It is not explicitly
  * imported to avoid a circular dependency.
+ * @internal
  */
 type PlaceholderS3Client = Client<any, any, any> & any;
 
 /**
- * @internal
  * Placeholder for the constructor for CreateSessionCommand.
+ * @internal
  */
 type PlaceholderCreateSessionCommandCtor = { new (args: any): Command<any, any, any, any, any> };
 
@@ -70,14 +69,22 @@ export const resolveS3Config = <T>(
   }
 ): T & S3ResolvedConfig => {
   const [s3ClientProvider, CreateSessionCommandCtor] = session;
-  return {
-    ...input,
-    forcePathStyle: input.forcePathStyle ?? false,
-    useAccelerateEndpoint: input.useAccelerateEndpoint ?? false,
-    disableMultiregionAccessPoints: input.disableMultiregionAccessPoints ?? false,
-    followRegionRedirects: input.followRegionRedirects ?? false,
+  const {
+    forcePathStyle,
+    useAccelerateEndpoint,
+    disableMultiregionAccessPoints,
+    followRegionRedirects,
+    s3ExpressIdentityProvider,
+    bucketEndpoint,
+  } = input;
+
+  return Object.assign(input, {
+    forcePathStyle: forcePathStyle ?? false,
+    useAccelerateEndpoint: useAccelerateEndpoint ?? false,
+    disableMultiregionAccessPoints: disableMultiregionAccessPoints ?? false,
+    followRegionRedirects: followRegionRedirects ?? false,
     s3ExpressIdentityProvider:
-      input.s3ExpressIdentityProvider ??
+      s3ExpressIdentityProvider ??
       new S3ExpressIdentityProviderImpl(async (key: string) =>
         s3ClientProvider().send(
           new CreateSessionCommandCtor({
@@ -85,6 +92,6 @@ export const resolveS3Config = <T>(
           })
         )
       ),
-    bucketEndpoint: input.bucketEndpoint ?? false,
-  };
+    bucketEndpoint: bucketEndpoint ?? false,
+  });
 };
diff --git a/packages/middleware-sdk-sqs/src/queue-url.spec.ts b/packages/middleware-sdk-sqs/src/queue-url.spec.ts
index 7750ac3fa109..889fbe984c13 100644
--- a/packages/middleware-sdk-sqs/src/queue-url.spec.ts
+++ b/packages/middleware-sdk-sqs/src/queue-url.spec.ts
@@ -2,7 +2,7 @@ import { HttpRequest } from "@aws-sdk/protocol-http";
 import { FinalizeHandlerArguments, HandlerExecutionContext } from "@aws-sdk/types";
 import { afterEach, beforeEach, describe, expect, test as it, vi } from "vitest";
 
-import { queueUrlMiddleware } from "./queue-url";
+import { queueUrlMiddleware, resolveQueueUrlConfig } from "./queue-url";
 
 describe("queueUrlMiddleware", () => {
   const mockNextHandler = vi.fn();
@@ -27,6 +27,13 @@ describe("queueUrlMiddleware", () => {
     vi.resetAllMocks();
   });
 
+  describe("config resolver", () => {
+    it("maintains object custody", () => {
+      const input = {};
+      expect(resolveQueueUrlConfig(input)).toBe(input);
+    });
+  });
+
   it("should use the QueueUrl hostname as the endpoint if useQueueUrlAsEndpoint is true", async () => {
     const middleware = queueUrlMiddleware({ useQueueUrlAsEndpoint: true });
     const input = { QueueUrl: "https://xyz.com/123/MyQueue" };
diff --git a/packages/middleware-sdk-sqs/src/queue-url.ts b/packages/middleware-sdk-sqs/src/queue-url.ts
index 659029b6a280..901960f751ff 100644
--- a/packages/middleware-sdk-sqs/src/queue-url.ts
+++ b/packages/middleware-sdk-sqs/src/queue-url.ts
@@ -36,10 +36,9 @@ export interface PreviouslyResolved {
 export const resolveQueueUrlConfig = <T>(
   config: T & PreviouslyResolved & QueueUrlInputConfig
 ): T & QueueUrlResolvedConfig => {
-  return {
-    ...config,
+  return Object.assign(config, {
     useQueueUrlAsEndpoint: config.useQueueUrlAsEndpoint ?? true,
-  };
+  });
 };
 
 /**
diff --git a/packages/middleware-sdk-sts/src/index.ts b/packages/middleware-sdk-sts/src/index.ts
index b2c43704182c..436d83603525 100644
--- a/packages/middleware-sdk-sts/src/index.ts
+++ b/packages/middleware-sdk-sts/src/index.ts
@@ -47,8 +47,10 @@ export interface StsAuthConfigOptions {
 export const resolveStsAuthConfig = <T>(
   input: T & PreviouslyResolved & StsAuthInputConfig,
   { stsClientCtor }: StsAuthConfigOptions
-): T & StsAuthResolvedConfig =>
-  resolveAwsAuthConfig({
-    ...input,
-    stsClientCtor,
-  });
+): T & StsAuthResolvedConfig => {
+  return resolveAwsAuthConfig(
+    Object.assign(input, {
+      stsClientCtor,
+    })
+  );
+};
diff --git a/packages/middleware-sdk-sts/src/middleware-sdk-sts.integ.spec.ts b/packages/middleware-sdk-sts/src/middleware-sdk-sts.integ.spec.ts
index bf0618073200..61a1932f5258 100644
--- a/packages/middleware-sdk-sts/src/middleware-sdk-sts.integ.spec.ts
+++ b/packages/middleware-sdk-sts/src/middleware-sdk-sts.integ.spec.ts
@@ -1,6 +1,8 @@
 import { STS, STSClient } from "@aws-sdk/client-sts";
 import { describe, expect, test as it } from "vitest";
 
+import { resolveStsAuthConfig } from "./index";
+
 describe("middleware-sdk-sts", () => {
   describe(STS.name, () => {
     it("sets the sts constructor in config", async () => {
@@ -10,5 +12,13 @@ describe("middleware-sdk-sts", () => {
 
       expect(client.config.stsClientCtor).toBe(STSClient);
     });
+
+    it("maintains object custody", () => {
+      const client = new STS({
+        region: "us-west-2",
+      });
+      const input = client.config;
+      expect(resolveStsAuthConfig(input, { stsClientCtor: STSClient })).toBe(input);
+    });
   });
 });
diff --git a/packages/middleware-signing/src/awsAuthConfiguration.spec.ts b/packages/middleware-signing/src/awsAuthConfiguration.spec.ts
index 1423d1b3de85..aa10fe8c3969 100644
--- a/packages/middleware-signing/src/awsAuthConfiguration.spec.ts
+++ b/packages/middleware-signing/src/awsAuthConfiguration.spec.ts
@@ -12,7 +12,7 @@ describe("AuthConfig", () => {
   };
 
   describe("resolveAwsAuthConfig", () => {
-    const inputParams = {
+    const inputParams = () => ({
       credentialDefaultProvider: () => () => Promise.resolve({ accessKeyId: "key", secretAccessKey: "secret" }),
       region: vi.fn().mockImplementation(() => Promise.resolve("us-foo-1")),
       regionInfoProvider: () => Promise.resolve({ hostname: "foo.com", partition: "aws" }),
@@ -24,27 +24,29 @@ describe("AuthConfig", () => {
       credentials: vi.fn().mockResolvedValue({ accessKeyId: "key", secretAccessKey: "secret" }),
       useFipsEndpoint: () => Promise.resolve(false),
       useDualstackEndpoint: () => Promise.resolve(false),
-    };
+    });
 
     beforeEach(() => {
       vi.clearAllMocks();
     });
 
     it("should memoize custom credential provider", async () => {
-      const { signer: signerProvider } = resolveAwsAuthConfig(inputParams);
+      const input = inputParams();
+      const spy = input.credentials;
+      const { signer: signerProvider } = resolveAwsAuthConfig(input);
       const signer = await signerProvider(authScheme);
       const request = new HttpRequest({});
       const repeats = 10;
       for (let i = 0; i < repeats; i++) {
         await signer.sign(request);
       }
-      expect(inputParams.credentials).toBeCalledTimes(1);
+      expect(spy).toBeCalledTimes(1);
     });
 
     it("should refresh custom credential provider if expired", async () => {
       const FOUR_MINUTES_AND_59_SEC = 299 * 1000;
       const input = {
-        ...inputParams,
+        ...inputParams(),
         credentials: vi
           .fn()
           .mockResolvedValueOnce({
@@ -54,6 +56,7 @@ describe("AuthConfig", () => {
           })
           .mockResolvedValue({ accessKeyId: "key", secretAccessKey: "secret" }),
       };
+      const spy = input.credentials;
       const { signer: signerProvider } = resolveAwsAuthConfig(input);
       const signer = await signerProvider(authScheme);
       const request = new HttpRequest({});
@@ -61,12 +64,12 @@ describe("AuthConfig", () => {
       for (let i = 0; i < repeats; i++) {
         await signer.sign(request);
       }
-      expect(input.credentials).toBeCalledTimes(2);
+      expect(spy).toBeCalledTimes(2);
     });
   });
 
   describe("resolveSigV4AuthConfig", () => {
-    const inputParams = {
+    const inputParams = () => ({
       credentialDefaultProvider: () => () => Promise.resolve({ accessKeyId: "key", secretAccessKey: "secret" }),
       region: vi.fn().mockImplementation(() => Promise.resolve("us-foo-1")),
       signingName: "foo",
@@ -75,27 +78,29 @@ describe("AuthConfig", () => {
         digest: vi.fn().mockReturnValue("SHA256 hash"),
       }),
       credentials: vi.fn().mockResolvedValue({ accessKeyId: "key", secretAccessKey: "secret" }),
-    };
+    });
 
     beforeEach(() => {
       vi.clearAllMocks();
     });
 
     it("should memoize custom credential provider", async () => {
-      const { signer: signerProvider } = resolveSigV4AuthConfig(inputParams);
+      const input = inputParams();
+      const spy = input.credentials;
+      const { signer: signerProvider } = resolveSigV4AuthConfig(input);
       const signer = await signerProvider(authScheme);
       const request = new HttpRequest({});
       const repeats = 10;
       for (let i = 0; i < repeats; i++) {
         await signer.sign(request);
       }
-      expect(inputParams.credentials).toBeCalledTimes(1);
+      expect(spy).toBeCalledTimes(1);
     });
 
     it("should refresh custom credential provider if expired", async () => {
       const FOUR_MINUTES_AND_59_SEC = 299 * 1000;
       const input = {
-        ...inputParams,
+        ...inputParams(),
         credentials: vi
           .fn()
           .mockResolvedValueOnce({
@@ -105,14 +110,16 @@ describe("AuthConfig", () => {
           })
           .mockResolvedValue({ accessKeyId: "key", secretAccessKey: "secret" }),
       };
-      const { signer: signerProvider } = resolveSigV4AuthConfig(input);
+      const spy = input.credentials;
+      const { signer: signerProvider, credentials } = resolveSigV4AuthConfig(input);
       const signer = await signerProvider(authScheme);
       const request = new HttpRequest({});
       const repeats = 10;
       for (let i = 0; i < repeats; i++) {
         await signer.sign(request);
       }
-      expect(input.credentials).toBeCalledTimes(2);
+      console.log("what is credentials", credentials);
+      expect(spy).toBeCalledTimes(2);
     });
   });
 });
diff --git a/packages/middleware-signing/src/awsAuthConfiguration.ts b/packages/middleware-signing/src/awsAuthConfiguration.ts
index 010d55a5c1c4..1e8321804c48 100644
--- a/packages/middleware-signing/src/awsAuthConfiguration.ts
+++ b/packages/middleware-signing/src/awsAuthConfiguration.ts
@@ -246,13 +246,12 @@ export const resolveAwsAuthConfig = <T>(
     };
   }
 
-  return {
-    ...input,
+  return Object.assign(input, {
     systemClockOffset,
     signingEscapePath,
     credentials: normalizedCreds,
     signer,
-  };
+  });
 };
 
 /**
@@ -279,13 +278,12 @@ export const resolveSigV4AuthConfig = <T>(
       })
     );
   }
-  return {
-    ...input,
+  return Object.assign(input, {
     systemClockOffset,
     signingEscapePath,
     credentials: normalizedCreds,
     signer,
-  };
+  });
 };
 
 /**
diff --git a/packages/middleware-token/src/resolveTokenConfig.spec.ts b/packages/middleware-token/src/resolveTokenConfig.spec.ts
index 18ab6f5b5886..f10284e1b83d 100644
--- a/packages/middleware-token/src/resolveTokenConfig.spec.ts
+++ b/packages/middleware-token/src/resolveTokenConfig.spec.ts
@@ -21,6 +21,11 @@ describe(resolveTokenConfig.name, () => {
     vi.clearAllMocks();
   });
 
+  it("maintains object custody", () => {
+    const input = {};
+    expect(resolveTokenConfig(input)).toBe(input);
+  });
+
   describe("sets token from normalizeTokenProvider if token is provided", () => {
     beforeEach(() => {
       vi.mocked(normalizeTokenProvider).mockReturnValue(mockOutputToken);
diff --git a/packages/middleware-token/src/resolveTokenConfig.ts b/packages/middleware-token/src/resolveTokenConfig.ts
index d783821f5b0f..cc45ba3baafe 100644
--- a/packages/middleware-token/src/resolveTokenConfig.ts
+++ b/packages/middleware-token/src/resolveTokenConfig.ts
@@ -5,7 +5,9 @@ import { tokenDefaultProvider } from "./tokenDefaultProvider";
 /**
  * @internal
  */
-export const resolveTokenConfig = <T>(input: T & TokenInputConfig): T & TokenResolvedConfig => ({
-  ...input,
-  token: input.token ? normalizeTokenProvider(input.token) : tokenDefaultProvider(input as any),
-});
+export const resolveTokenConfig = <T>(input: T & TokenInputConfig): T & TokenResolvedConfig => {
+  const { token } = input;
+  return Object.assign(input, {
+    token: token ? normalizeTokenProvider(token) : tokenDefaultProvider(input as any),
+  });
+};
diff --git a/packages/middleware-user-agent/src/configurations.spec.ts b/packages/middleware-user-agent/src/configurations.spec.ts
new file mode 100644
index 000000000000..7f433b5c1772
--- /dev/null
+++ b/packages/middleware-user-agent/src/configurations.spec.ts
@@ -0,0 +1,13 @@
+import { describe, expect, test as it } from "vitest";
+
+import { resolveUserAgentConfig } from "./configurations";
+
+describe(resolveUserAgentConfig.name, () => {
+  it("maintains object custody", () => {
+    const input = {
+      defaultUserAgentProvider: async () => [["", ""] as [string, string?]],
+      runtime: "node",
+    };
+    expect(resolveUserAgentConfig(input)).toBe(input);
+  });
+});
diff --git a/packages/middleware-user-agent/src/configurations.ts b/packages/middleware-user-agent/src/configurations.ts
index af973ba931d5..654c8e781b24 100644
--- a/packages/middleware-user-agent/src/configurations.ts
+++ b/packages/middleware-user-agent/src/configurations.ts
@@ -57,9 +57,11 @@ export function resolveUserAgentConfig<T>(
   input: T & PreviouslyResolved & UserAgentInputConfig
 ): T & UserAgentResolvedConfig {
   const normalizedAppIdProvider = normalizeProvider(input.userAgentAppId ?? DEFAULT_UA_APP_ID);
-  return {
-    ...input,
-    customUserAgent: typeof input.customUserAgent === "string" ? [[input.customUserAgent]] : input.customUserAgent,
+  const { customUserAgent } = input;
+
+  return Object.assign(input, {
+    customUserAgent:
+      typeof customUserAgent === "string" ? ([[customUserAgent]] as [string, string?][]) : customUserAgent,
     userAgentAppId: async () => {
       const appId = await normalizedAppIdProvider();
       if (!isValidUserAgentAppId(appId)) {
@@ -72,5 +74,5 @@ export function resolveUserAgentConfig<T>(
       }
       return appId;
     },
-  };
+  });
 }
diff --git a/packages/middleware-websocket/src/websocket-configuration.spec.ts b/packages/middleware-websocket/src/websocket-configuration.spec.ts
new file mode 100644
index 000000000000..a5c75d07eb71
--- /dev/null
+++ b/packages/middleware-websocket/src/websocket-configuration.spec.ts
@@ -0,0 +1,13 @@
+import { describe, expect, test as it, vi } from "vitest";
+
+import { resolveWebSocketConfig } from "./websocket-configuration";
+
+describe(resolveWebSocketConfig.name, () => {
+  it("maintains object custody", () => {
+    const input = {
+      signer: vi.fn(),
+      requestHandler: vi.mocked(vi.fn()) as any,
+    };
+    expect(resolveWebSocketConfig(input)).toBe(input);
+  });
+});
diff --git a/packages/middleware-websocket/src/websocket-configuration.ts b/packages/middleware-websocket/src/websocket-configuration.ts
index 189483cd4784..ca1976338123 100644
--- a/packages/middleware-websocket/src/websocket-configuration.ts
+++ b/packages/middleware-websocket/src/websocket-configuration.ts
@@ -26,15 +26,18 @@ export interface WebSocketResolvedConfig {
 
 export const resolveWebSocketConfig = <T>(
   input: T & WebSocketInputConfig & PreviouslyResolved
-): T & WebSocketResolvedConfig => ({
-  ...input,
-  signer: async (authScheme: AuthScheme) => {
-    const signerObj = await input.signer(authScheme);
-    if (validateSigner(signerObj)) {
-      return new WebsocketSignatureV4({ signer: signerObj });
-    }
-    throw new Error("Expected WebsocketSignatureV4 signer, please check the client constructor.");
-  },
-});
+): T & WebSocketResolvedConfig => {
+  const { signer } = input;
+
+  return Object.assign(input, {
+    signer: async (authScheme: AuthScheme) => {
+      const signerObj = await signer(authScheme);
+      if (validateSigner(signerObj)) {
+        return new WebsocketSignatureV4({ signer: signerObj });
+      }
+      throw new Error("Expected WebsocketSignatureV4 signer, please check the client constructor.");
+    },
+  });
+};
 
 const validateSigner = (signer: any): signer is BaseSignatureV4 => !!signer;
diff --git a/packages/region-config-resolver/src/extensions/index.ts b/packages/region-config-resolver/src/extensions/index.ts
index 27c40a409f60..c37b5fd1e235 100644
--- a/packages/region-config-resolver/src/extensions/index.ts
+++ b/packages/region-config-resolver/src/extensions/index.ts
@@ -7,23 +7,12 @@ export type RegionExtensionRuntimeConfigType = Partial<{ region: string | Provid
  * @internal
  */
 export const getAwsRegionExtensionConfiguration = (runtimeConfig: RegionExtensionRuntimeConfigType) => {
-  let runtimeConfigRegion: Provider<string> = async () => {
-    if (runtimeConfig.region === undefined) {
-      throw new Error("Region is missing from runtimeConfig");
-    }
-    const region = runtimeConfig.region!;
-    if (typeof region === "string") {
-      return region;
-    }
-    return region();
-  };
-
   return {
     setRegion(region: Provider<string>): void {
-      runtimeConfigRegion = region;
+      runtimeConfig.region = region;
     },
     region(): Provider<string> {
-      return runtimeConfigRegion;
+      return runtimeConfig.region as Provider<string>;
     },
   };
 };
diff --git a/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.spec.ts b/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.spec.ts
index b3aef695b8ff..599819ef750f 100644
--- a/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.spec.ts
+++ b/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.spec.ts
@@ -22,6 +22,13 @@ describe("RegionConfig", () => {
     vi.clearAllMocks();
   });
 
+  it("maintains object custody", () => {
+    const input = {
+      region: "mockRegion",
+    };
+    expect(resolveRegionConfig(input)).toBe(input);
+  });
+
   describe("region", () => {
     it("return normalized value with real region if passed as a string", async () => {
       const resolvedRegionConfig = resolveRegionConfig({ region: mockRegion, useFipsEndpoint: mockUseFipsEndpoint });
diff --git a/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.ts b/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.ts
index d1ee3c8561b1..a062736b8967 100644
--- a/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.ts
+++ b/packages/region-config-resolver/src/regionConfig/resolveRegionConfig.ts
@@ -48,8 +48,7 @@ export const resolveRegionConfig = <T>(input: T & RegionInputConfig & Previously
     throw new Error("Region is missing");
   }
 
-  return {
-    ...input,
+  return Object.assign(input, {
     region: async () => {
       if (typeof region === "string") {
         return getRealRegion(region);
@@ -64,5 +63,5 @@ export const resolveRegionConfig = <T>(input: T & RegionInputConfig & Previously
       }
       return typeof useFipsEndpoint !== "function" ? Promise.resolve(!!useFipsEndpoint) : useFipsEndpoint();
     },
-  };
+  });
 };