Host-prefixed requests fail when using a custom endpoint

[joe4dev]

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

AWS SDKs add the host prefix defined in the Smithy model (e.g., sync- for sfn.StartSyncExecution) for custom endpoints (e.g., AWS_ENDPOINT_URL=http://127.0.0.1:4566), even if technically incompatible, for example sync-127.0.0.1.
This behavior is technically wrong, incompatible with the AWS standard for custom endpoints (https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html), and breaks emulator compatibility (e.g., LocalStack) of all host-prefixed operations (counting >562) for many AWS tools, including the AWS CLI, the AWS Toolkit, etc.

Regression Issue

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

SDK version number

@aws-sdk/**@3.830.0 (Example: @aws-sdk/client-sfn@3.830.0)

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v22.14.0

Reproduction Steps

1. npm install @aws-sdk/client-sfn@3.830.0

2. Run node minimal_example.js using the following script:

   import { SFNClient, StartSyncExecutionCommand } from "@aws-sdk/client-sfn";
   
   const client = new SFNClient({
       endpoint: "http://127.0.0.1:4566",
       region: "us-east-1",
       credentials: {
           accessKeyId: "test",
           secretAccessKey: "test",
       },
       maxAttempts: 1,
   });
   
   client.send(new StartSyncExecutionCommand({
       stateMachineArn: "arn:aws:states:us-east-1:000000000000:stateMachine:express_statemachine_83b68bea",
       input: "{}",
   }));

Link reproducer test suite (including botocore comparison): https://github.com/joe4dev/host-prefix-testing

Observed Behavior

The AWS SDKs add the host prefix for incompatible endpoint URLs, causing connection errors such as:

Error: AWS SDK error wrapper for Error: getaddrinfo ENOTFOUND sync-127.0.0.1

Expected Behavior

The AWS SDKs should omit the host prefix for incompatible endpoint URLs such as 127.0.0.1 and localhost. Maybe, the host prefix can be dropped altogether when using a custom endpoint.

Possible Solution

The implementation could be similar to the IPv4 detection in aws_is_virtual_hostable_s3_bucket applied through the endpoint rule set via aws.isVirtualHostableS3Bucket.

Additional Information/Context

The issue appears to be related to Smithy code generation, given that all AWS SDKs are affected.

Disclaimer: I work for LocalStack