S3Client throws "CredentialsProviderError: Not found in ENV: AWS_DEFAULTS_MODE_ENV" even when defaultsMode: 'legacy' is explicitly set

[GOPIKRISHNA11596]

Checkboxes for prior research

• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow.
• I've searched for previous similar issues and didn't find any solution.

Describe the bug

• When using the latest AWS SDK for JavaScript v3 (Smithy-based), creating an S3Client for non-AWS S3-compatible endpoints (e.g., Google Cloud Storage) fails with:
  CredentialsProviderError: Not found in ENV: AWS_DEFAULTS_MODE_ENV
• This happens even when defaultsMode: 'legacy' is explicitly set in the client configuration.
• The error originates from the internal @smithy/node-config-provider → @smithy/util-defaults-mode-node provider chain, which still tries to read from process.env.AWS_DEFAULTS_MODE_ENV.
• This behavior breaks all S3-compatible integrations that don’t use AWS credentials or environment setup.

Environment Details

• SDK Package: @aws-sdk/client-s3
• SDK Version: 3.608.0 (also reproducible on 3.610.x)
• Node.js Version: v18.20.3
• Use Case: GCS S3-compatible API
• Region: Not applicable (using custom endpoint)

Regression Issue

• Select this option if this issue appears to be a regression.

SDK version number

@aws-sdk/package-name@version, ...

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

Node.js Version: v18.20.3

Reproduction Steps

import { S3Client } from "@aws-sdk/client-s3";

const s3 = new S3Client({
  endpoint: "https://storage.googleapis.com/",
  region: "us-east-1",
  credentials: {
    accessKeyId: "GOOGXXXXXX",
    secretAccessKey: "YYYYYYYYYY"
  },
  defaultsMode: "legacy", // explicitly set
});

Expected Behavior:
Client should initialize successfully and use legacy defaults mode without requiring any AWS environment variables.

Actual Behavior:
Throws immediately on client creation:
CredentialsProviderError: Not found in ENV: AWS_DEFAULTS_MODE_ENV at @smithy/node-config-provider/dist-cjs/index.js:28:15

Workarounds Tested

✅ Adding environment variable works:

export AWS_DEFAULTS_MODE=legacy

✅ Adding before import also works:

process.env.AWS_DEFAULTS_MODE = 'legacy';
const s3 = new S3Client(...);

❌ Setting defaultsMode: 'legacy' in constructor → still fails
❌ Passing via credentials object → still fails

Observed Behavior

Throws immediately on client creation:
CredentialsProviderError: Not found in ENV: AWS_DEFAULTS_MODE_ENV at @smithy/node-config-provider/dist-cjs/index.js:28:15

Expected Behavior

Expected Behavior:
Client should initialize successfully and use legacy defaults mode without requiring any AWS environment variables.

Possible Solution

No response

Additional Information/Context

No response

[aBurmeseDev]

Hi @GOPIKRISHNA11596 - thanks for reaching out.

We couldn't reproduce the issue with the code you provided. Can you share your stack trace or anything that might help to investigate?

import { S3Client } from "@aws-sdk/client-s3";

const s3 = new S3Client({
  endpoint: "https://storage.googleapis.com/", // access denied 
  region: "us-east-1",
  defaultsMode: "legacy", 
});

Fo reference, this's the source code of where the error comes from and technically the credential provider should be able to retrieve the value from config.