sst-cdk is a lightweight fork of AWS CDK. It's used internally in SST.
It allows SST to call specific CDK CLI methods programmatically. Instead of using the CLI.
Versions are kept in sync with AWS CDK releases. With additional updates released as release candidates. For example, 1.61.0-rc.4 is the 4th internal release after merging with AWS CDK 1.61.0.