Interface or docu wrong

[holonsoftFounder]

Option A: make it tri-state

Change signature to:

Task<bool?> IsFeatureEnabledAsync(...)

Implementation:

public async Task<bool?> IsFeatureEnabledAsync(
    string featureKey,
    string distinctId,
    FeatureFlagOptions? options,
    CancellationToken cancellationToken)
{
    var result = await GetFeatureFlagAsync(featureKey, distinctId, options, cancellationToken);

    if (result is null) return null;          // undefined / not existing
    return result.IsEnabled;                  // true/false if known
}

Updated doc (clear semantics):

true = enabled
false = explicitly disabled
null = undefined / not found

This matches your “not existing is silently hidden” point and preserves information.

Option B (non-breaking): fix the doc to match behavior

Keep Task and update to something like:

true if enabled
false if disabled or undefined / not found

Example:

/// <returns>
/// <c>true</c> if the feature is enabled for the user.
/// <c>false</c> if the feature is disabled, undefined, or does not exist.
/// </returns>

This is consistent with the current implementation.

Option C (non-breaking, add) PREFERRED

If you want both without breaking callers

Add a new API and keep the old one as a convenience wrapper:

Task<bool?> TryIsFeatureEnabledAsync(...);

Task IsFeatureEnabledAsync(...) // legacy
=> (await TryIsFeatureEnabledAsync(...)) == true;

That way callers who care about “undefined” can opt in, and existing callers keep their boolean.