(flags): cdk flags --set --unconfigured --recommended writes false when exactly one flag is unconfigured #2048

Describe the bug

When exactly one boolean feature flag is unconfigured, cdk flags --set --unconfigured --recommended shows the recommended value in its preview, but writes false to cdk.json.

This is the bug that #1183 fixed for --all. The fix added !params.all to the single-flag check in modifyValues, so --unconfigured still takes the single-flag path.

Regression Issue

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

Last Known Working CDK Version

No response

Expected Behavior

The flag is written with its recommended value, as the preview shows.

Current Behavior

Flag changes:
  @aws-cdk/aws-cloudfront:defaultViewerProtocolPolicyRedirectToHttps: <unset> → true
Setting flag '@aws-cdk/aws-cloudfront:defaultViewerProtocolPolicyRedirectToHttps' to: false
Flag value(s) updated successfully.

cdk.json then contains "@aws-cdk/aws-cloudfront:defaultViewerProtocolPolicyRedirectToHttps": false.

Reproduction Steps

1. Create an app with aws-cdk-lib 2.273.0 whose cdk.json sets every boolean feature flag except @aws-cdk/aws-cloudfront:defaultViewerProtocolPolicyRedirectToHttps (recommended value true).
2. Run cdk flags --unstable=flags --set --unconfigured --recommended --yes.
3. The flag is written as false.

With two or more unconfigured flags, the recommended values are written correctly.

Possible Solution

In FlagOperations.modifyValues (packages/aws-cdk/lib/commands/flags/operations.ts), the single-flag branch is meant for cdk flags --set FLAGNAME --value X, but it does not check that --value was given:

if (flagNames.length === 1 && !params.safe && !params.all) {
  const boolValue = params.value === 'true';   // undefined → false

buildUpdateObject uses flagNames.length === 1 && params.value !== undefined. Using the same condition in modifyValues would cover --unconfigured and any other multi-flag mode, not only --all.

Additional Information/Context

Related: #1183 (same bug, fixed for --all only).

CDK CLI Version

2.1145.0 (build 13877dc)

Framework Version

aws-cdk-lib 2.273.0

Node.js Version

v24.20.0

OS

Linux (Amazon Linux 2023)

Language

TypeScript

Language Version

No response

Other information

No response