StackMaster is a mature Ruby CLI gem for managing AWS CloudFormation stacks. It provides a rich command set (apply, diff, drift, delete, events, outputs, resources, status, validate, lint, nag, compile, tidy, init) and supports multiple template formats (JSON, YAML, ERB, SparkleFormation DSL, CfnDsl) through a pluggable compiler registry. A standout feature is the parameter resolution pipeline: 13+ pluggable resolvers (SSM Parameter Store, ACM certificates, AMI finders, stack outputs, 1Password, EJSON, environment variables, security groups, SSO group IDs) that fetch values at apply/compile time, making cross-account and cross-region stack composition practical. The architecture is cleanly modular — commands are standalone classes sharing a base module, template compilers and parameter resolvers are registered via simple registry interfaces, and the stack event streamer provides live sync feedback during updates. The codebase demonstrates solid engineering craftsmanship: a custom Erubis subclass generates CloudFormation Fn::Join arrays from ERB templates, a role assumer caches STS credentials per (account, role) and temporarily swaps Aws.config to support cross-account operations, and a generic PagedResponseAccumulator cleanly handles AWS SDK pagination. Test coverage is extensive — RSpec unit specs plus Cucumber/Aruba feature tests exercise every command and resolver against stubbed AWS responses, and a dedicated in-memory test driver (TestDriver::CloudFormation/S3) enables isolated testing without network calls. The changelog is meticulously maintained with per-release notes, and the gem supports a wide Ruby range (2.4–4.0). However, there are signs of accumulated technical debt: a large .rubocop_todo.yml exclusions list indicates code quality lapses, the codebase relies on autoload (deprecated in Ruby 3.2+), and heavy stubbing in tests may mask real AWS API integration regressions. Dependency pins on SparkleFormation (~> 3) and CfnDsl (~> 1) could become brittler as those upstream libraries evolve. This is a maintainer-mode tool — stable, well-documented, and battle-tested, but not rapidly evolving.