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I do see this: https://github.com/pulumi/pulumi-cdk/issues/54

You mean AWS CDK v2.x? CDK bootstrap has its own versioning, which is at v8, v9 or something like that nowadays.

Yep, I meant the template that gets used with CDK v2, but I didn’t realize the templates themselves had independent versioning. Do CDK users normally interact with that version number, or do they rely on their CDK version to manage it? My understanding from the AWS docs is that there are basically two versions, “legacy” (CDK v1.x) and “modern” (v2.x), and that

cdk bootstrap

handles everything based the CLI version.

DIfferent versions of the CDK, and not just 1.x or 2.x, require different minimal version of the bootstrap. The version of the bootstrap setup is stored in a parameter store value, and Cloudformation templates generated by AWS CDK has a rule that checks if this value is high enough. In the generated Cloudformation for a stack you will find a section like this one (usually at the end):

Parameters:
  BootstrapVersion:
    Type: AWS::SSM::Parameter::Value<String>
    Default: /cdk-bootstrap/hnb659fds/version
    Description: Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]
Rules:
  CheckBootstrapVersion:
    Assertions:
      - Assert:
          Fn::Not:
            - Fn::Contains:
                - - "1"
                  - "2"
                  - "3"
                  - "4"
                  - "5"
                - Ref: BootstrapVersion
        AssertDescription: CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI.