You’re very close — the error you’re seeing (CredentialsProviderError: Could not load credentials from any providers) isn’t really about your endpoint, but rather about AWS SDK v3 trying to sign the request even though it’s hitting your local serverless-offline WebSocket server.

Let’s walk through what’s happening and how to fix it.

🧩 What’s Happening

When you do:

const apiGatewayClient = new ApiGatewayManagementApiClient({ endpoint });
await apiGatewayClient.send(new PostToConnectionCommand(payload));

The AWS SDK v3 automatically assumes it’s talking to real AWS API Gateway, so it:

• Attempts to sign the request with AWS credentials.

• Fails because serverless-offline doesn’t need or support signed requests.

• Hence: Could not load credentials from any providers.

So even though your endpoint (http://localhost:3001) is correct, the client is still trying to sign requests as if it were AWS.

✅ Fix: Disable Signature / Credentials for Local Use

When using serverless-offline for WebSocket testing, you need to give the ApiGatewayManagementApiClient dummy credentials and a local region.

Here’s a working local setup:

const {
  ApiGatewayManagementApiClient,
  PostToConnectionCommand,
} = require("@aws-sdk/client-apigatewaymanagementapi");

exports.message = async (event, context) => {
  // Use the same port that serverless-offline shows for websocket
  const endpoint = "http://localhost:3001";

  const connectionId = event.requestContext.connectionId;
  const payload = {
    ConnectionId: connectionId,
    Data: "pong",
  };

  const apiGatewayClient = new ApiGatewayManagementApiClient({
    endpoint,
    region: "us-east-1",
    // Dummy credentials to satisfy SDK signer
    credentials: {
      accessKeyId: "dummy",
      secretAccessKey: "dummy",
    },
  });

  try {
    await apiGatewayClient.send(new PostToConnectionCommand(payload));
  } catch (err) {
    console.error("PostToConnection error:", err);
  }

  return { statusCode: 200, body: "pong sent" };
};

💡 Why Dummy Credentials Work

The AWS SDK v3 doesn’t let you completely disable signing, but it’s happy if you provide any credentials.
Since serverless-offline ignores them, “dummy” values are perfectly fine locally.

🧪 Verify It Works

1. Start serverless offline:

   npx serverless offline
   
   

2. Connect via WebSocket client (e.g., wscat):

   npx wscat -c ws://localhost:3001
   
   

3. Type a message — you should see "pong" echoed back.

⚙️ Summary

ProblemFixSDK tries to sign local requestsProvide dummy credentialsWrong endpointUse http://localhost:3001 (as printed by serverless-offline)Missing regionAdd region: "us-east-1"

If you want, I can also show you how to make this conditional (so it automatically switches between local and AWS endpoints depending on IS_OFFLINE), which makes deployments smoother. Would you like that?