Bug: POWERTOOLS_DEV as true does not seem to have any effect

[ColinMorris83]

Expected Behavior

From reading the docs: https://docs.aws.amazon.com/powertools/typescript/latest/features/metrics/#testing-your-code setting POWERTOOLS_DEV env var as true should suppress any console logs

Current Behavior

However still the unit tests output this:

No application metrics to publish. The cold-start metric may be published if enabled. If application metrics should never be empty, consider using `throwOnEmptyMetrics

Code snippet

Using latest version 4 of Vitest, but also tried on v3 with same issue.

This example just sets the env var in the specific test, but also tried it in a beforeAll block and even in a setup tests file configured as part of the vitest config.

it('should not log about no application metrics to publish', async () => {
    process.env.POWERTOOLS_DEV = 'true';
    const metrics = new Metrics();
    const mainHandler: ConnectContactFlowHandler = async (event: ConnectContactFlowEvent) => {
      return await Promise.resolve({
        eventName: event.Name,
        result: 'hello',
      });
    };

    const mockContext: Context = {
      awsRequestId: '',
      callbackWaitsForEmptyEventLoop: true,
      done: () => 1,
      fail: () => 1,
      functionName: 'handler',
      functionVersion: '1',
      getRemainingTimeInMillis: () => 1,
      invokedFunctionArn: '',
      logGroupName: '',
      logStreamName: '',
      memoryLimitInMB: '512',
      succeed: () => 1,
    };

    const mockConnectContactFlowEvent: ConnectContactFlowEvent = {
      Details: {
        ContactData: {
          Attributes: {
            attr: '1234',
          },
          Channel: 'CHAT',
          ContactId: '123',
          CustomerEndpoint: null,
          InitialContactId: '',
          InitiationMethod: 'INBOUND',
          InstanceARN: '',
          MediaStreams: {
            Customer: {
              Audio: null,
            },
          },
          PreviousContactId: '',
          Queue: null,
          SystemEndpoint: null,
        },
        Parameters: {},
      },
      Name: 'ContactFlowEvent',
    };
    await middy(mainHandler).use(logMetrics(metrics))(mockConnectContactFlowEvent, mockContext);
    expect(metrics.hasStoredMetrics()).toBe(false);
  });

Steps to Reproduce

Node22, Vitest 3 or 4
Run a unit test on a lambda wrapped with middy and logMetrics middleware
Console logs when no metrics were published, even with the environment variable set to true

Possible Solution

No response

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

22.x

Packaging format used

npm

Execution logs

[boring-cyborg]

Thanks for opening your first issue here! We'll come back to you as soon as we can.
In the meantime, check out the #typescript channel on our Powertools for AWS Lambda Discord: Invite link

[sdangol]

Hi @ColinMorris83, the log suppression mentioned in the documentation refers the suppressing the EMF Metrics logs that would be emitted by the utility, not the warning/error logs. If there were any metrics logged in your handler you would normally see logs like these in your tests as well.

{"_aws":{"Timestamp":1761740525840,"CloudWatchMetrics":[{"Namespace":"default_namespace","Dimensions":[["service"]],"Metrics":[{"Name":"test","Unit":"Count"}]}]},"service":"service_undefined","test":1}

Setting POWERTOOLS_DEV to true would suppress these logs.

[sdangol]

If you want to suppress the warning logs, you can mock the console methods to prevent their output during the tests by doing

vi.spyOn(console, 'warn').mockReturnValue();

[ColinMorris83]

Thanks for the clarification @sdangol
So basically there isn't currently any way to stop these from appearing, even in production logs

No application metrics to publish. The cold-start metric may be published......

As we are only publishing a metric when certain rare errors occur, it means every single lambda invocation is now logging out the above. It would be nice to have a way to prevent that

[sdangol]

You can pass a custom logger to the Metrics utility so that you can control whether or not warning logs are emitted.

const logger = new Logger({ logLevel: LogLevel.ERROR });
const metrics = new Metrics({
  logger,
});

Here, the logger that is passed would only log errors and not warnings.

[ColinMorris83]

Thanks, that works perfectly. Will close this issue down.