Bug: persistentKeys are not persisted across children

[iamgerg]

Expected Behavior

When creating a child logger I expect persistentKeys from the parent to be merged into the child's.

Current Behavior

When calling logger.createChild with the persistentKeys object, existing persistent keys on the parent are completely cleared out instead of merged together.

Code snippet

const logger = new Logger({
  persistentKeys: {
    foo: 'hello',
    overridable: 1,
  },
})

logger.appendKeys({
  resettableKey: 'some-id',
})

logger.appendPersistentKeys({
  dynamic: 'stays',
})

const child = logger.createChild({
  persistentKeys: {
    bar: 'world',
    overridable: 2,
  },
})

child.info('Log Message')
child.resetKeys()
child.info('Log Message 2')

Expected Output:

{
  "level": "INFO",
  "message": "Log Message",
  "timestamp": "2025-11-06T18:54:49.202Z",
  "service": "service_undefined",
  "sampling_rate": 0,
  "foo": "hello",
  "overridable": 2,
  "dynamic": "stays",
  "bar": "world",
  "resettableKey": "some-id"
}
{
  "level": "INFO",
  "message": "Log Message 2",
  "timestamp": "2025-11-06T18:54:49.204Z",
  "service": "service_undefined",
  "sampling_rate": 0,
  "foo": "hello",
  "overridable": 2,
  "dynamic": "stays",
  "bar": "world"
}

Current Output:

{
  "level": "INFO",
  "message": "Log Message",
  "timestamp": "2025-11-06T18:57:06.315Z",
  "service": "service_undefined",
  "sampling_rate": 0,
  "bar": "world",
  "overridable": 2,
  "resettableKey": "some-id"
}
{
  "level": "INFO",
  "message": "Log Message 2",
  "timestamp": "2025-11-06T18:57:06.315Z",
  "service": "service_undefined",
  "sampling_rate": 0,
  "bar": "world",
  "overridable": 2
}

Steps to Reproduce

1. Create file with code snippet from above
2. Run the code and see the console output

Possible Solution

I believe this is due to this line where creating a child relies on setting the deprecated persistentLogAttributes instead of the newer persistentKeys attribute. Which is ignored during instantiation if both are used seen here. This usage of the deprecated api also contributes to why when creating a child and setting persistentKeys on it, the

Both persistentLogAttributes and persistentKeys options were provided.

warning is displayed regardless of callers actually using that deprecated attribute or not. Related issue

I can help contribute this if it is a desired fix

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

22.x

Packaging format used

npm

Execution logs

[boring-cyborg]

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[phipag]

Thanks for opening this issue @iamgerg. I just tested this and can confirm this behavior.

When looking at the PR from the issue you referenced (#4627) it looks like this only suppresses the warning when creating a child logger where the parent logger has no persistent keys. So this needs some re-work regarding the warning.

Now, regarding the behavior you described there seemed to be confusion if this is actually the expected behavior (see comment1, comment2).

According to the documentation and the code sample provided here persistent keys should be inherited by child loggers. I would also expect this based on the code snippet referenced calling the merge function.

Since @svozza and @sdangol commented on the other issue, can you please provide your opinion here?

(1) We need to re-work the suppression of the warning message from PR #4627
(2) We need to decide if this is a bug or expected behavior. I think that this is a bug, please confirm.

[svozza]

Imo if you create a child logger I would expect it to inherit the attributes of the parent so I think this is a bug. @iamgerg if you are happy to work on this then I can assign the issue to you.

[powertools-for-aws-oss-automation]

Note

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