🔮 Kettle::Jem

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👣 How will this project approach the September 2025 hostile takeover of RubyGems? 🚑️

I've summarized my thoughts in this blog post.

🌻 Synopsis

Kettle::Jem is an AST-aware gem templating system that keeps hundreds of Ruby gems in sync with a shared template while preserving each project's customizations. Unlike line-based copy/merge tools, Kettle::Jem understands the structure of every file it touches — Ruby via Prism, YAML via Psych, Markdown via Markly, TOML via tree-sitter, and more — so template updates land precisely where they belong, and project-specific additions are never clobbered.

Plugin authors can now use the dedicated plugin authoring guide to build kettle-jem extension gems against the supported plugin seam.

Key Features

• AST-aware merging — 10 format-specific merge engines (prism, psych, markly, toml, json, jsonc, json5, bash, dotenv, rbs, text)
• Token substitution — {KJ|TOKEN} patterns resolved from config, ENV, or auto-derived from gemspec
• Freeze blocks — protect any section from template overwrites with # kettle-jem:freeze / # kettle-jem:unfreeze
• Per-file strategies — merge, accept_template, keep_destination, or raw_copy
• Multi-phase pipeline — 11 ordered phases (service_actor-based) from config sync through duplicate checking
• SHA-pinned GitHub Actions — template uses: always wins, propagating immutable SHAs
• Convergence in one pass — a single kettle-jem install applies all changes; a second run produces zero diff
• Selftest divergence check — CI verifies that project drift stays within a configurable threshold

💡 Info you can shake a stick at

Tokens to Remember
Works with MRI Ruby 4
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
... 💖	🧊 🐙 🛖 🧪

Compatibility

Compatible with MRI Ruby 4.0.0+, and concordant releases of JRuby, and TruffleRuby. CI workflows and Appraisals are generated for MRI Ruby 4.0.0+. This test floor is configured by ruby.test_minimum in .kettle-jem.yml and may be higher than the gem's runtime compatibility floor when legacy Rubies are not practical for the current toolchain.

The amazing test matrix is powered by the kettle-dev stack.

How kettle-dev manages complexity in tests

Gem	Source	Role	Total downloads
appraisal2	GitHub	multi-dependency Appraisal matrix generation
appraisal2-rubocop	GitHub	RuboCop Appraisal generator integration
kettle-dev	GitHub	development, release, and CI workflow tooling
kettle-soup-cover	GitHub	SimpleCov coverage policy and reporting
kettle-test	GitHub	standard test runner and coverage harness
rubocop-lts	GitHub	Ruby-version-aware linting
turbo_tests2	GitHub	parallel test execution

StructuredMerge package family

This gem is part of the StructuredMerge Ruby package family. The implementation inventory, layering model, and backend notes live in the root package-family guide. Shared behavior is defined by the StructuredMerge fixtures and implemented by the Go, Ruby, Rust, and TypeScript repositories.

Merge analysis must enter parsing through tree_haver. Parser-specific gems register concrete TreeHaver backends; substrate gems register grammar mappings and keep shared format or language merge behavior in one place. Missing backends fail closed instead of falling back to direct parser-library calls.

Federated DVCS

Find this repo on federated forges (Coming soon!)

Federated DVCS Repository	Status	Issues	PRs	Wiki	CI
🧪 structuredmerge/structuredmerge-ruby on GitLab	The Truth	💚	💚	💚	🐭 Tiny Matrix
🧊 structuredmerge/structuredmerge-ruby on CodeBerg	An Ethical Mirror (Donate)	💚	💚	➖	⭕️ No Matrix
🐙 structuredmerge/structuredmerge-ruby on GitHub	Another Mirror	💚	💚	💚	💯 Full Matrix

Enterprise Support

Available as part of the Tidelift Subscription.

Need enterprise-level guarantees?

The maintainers of this and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use.

• 💡Subscribe for support guarantees covering all your FLOSS dependencies
• 💡Tidelift is part of Sonar
• 💡Tidelift pays maintainers to maintain the software you depend on!
  📊@Pointy Haired Boss: An enterprise support subscription is "never gonna let you down", and supports open source maintainers

Alternatively:

• 
• 
• 
• 

✨ Installation

Install the gem and add to the application's Gemfile by executing:

bundle add kettle-jem

If bundler is not being used to manage dependencies, install the gem by executing:

gem install kettle-jem

⚙️ Configuration

Each gem that uses Kettle::Jem has a .kettle-jem.yml file at its root. This file controls every aspect of how the template is applied.

Minimal Configuration

project_emoji: "🔮"
engines:
  - ruby
licenses:
  - MIT
tokens:
  forge:
    gh_user: "your-username"
  author:
    name: "Your Name"
    email: "you@example.com"

Full Configuration Reference

# REQUIRED — unique emoji used in badges and gemspec summary
project_emoji: "🔮"               # ENV override: KJ_PROJECT_EMOJI

# Ruby engines to include in CI matrix (remove to skip)
engines:
  - ruby
  - jruby
  - truffleruby

# The generated direct dependency-heads workflow derives its Bundler lockfile
# platform from `engines`. JRuby gets an isolated temporary lockfile with the
# `universal-java` platform; MRI and TruffleRuby use the checked-in lock
# context. This avoids making one shared lockfile satisfy incompatible engine
# platforms.

# SPDX license identifiers
licenses:
  - MIT

# Logo layout in README header: org | project | org_and_project
readme:
  top_logo_mode: org

# Bot accounts to exclude from contributor lists
machine_users:
  - dependabot

# Maximum allowed divergence (%) for selftest CI check
min_divergence_threshold: 5       # ENV override: KJ_MIN_DIVERGENCE_THRESHOLD

# Default merge behavior applied to all files
defaults:
  preference: "template"           # template | destination
  add_template_only_nodes: true    # add nodes that only exist in template
  freeze_token: "kettle-jem"       # marker for frozen sections

# Token values for {KJ|TOKEN} substitution
tokens:
  forge:
    gh_user: "github-username"    # ENV override: KJ_GH_USER
    gl_user: "gitlab-username"    # ENV override: KJ_GL_USER
    cb_user: "codeberg-username"  # ENV override: KJ_CB_USER
    sh_user: "sourcehut-user"     # ENV override: KJ_SH_USER
  author:
    name: "Full Name"             # ENV override: KJ_AUTHOR_NAME
    given_names: "Full"           # ENV override: KJ_AUTHOR_GIVEN_NAMES
    family_names: "Name"          # ENV override: KJ_AUTHOR_FAMILY_NAMES
    email: "you@example.com"      # ENV override: KJ_AUTHOR_EMAIL
    domain: "example.com"         # ENV override: KJ_AUTHOR_DOMAIN
    orcid: "0000-0000-0000-0000"  # ENV override: KJ_AUTHOR_ORCID
  funding:
    kofi: "username"              # ENV override: KJ_FUNDING_KOFI
    paypal: "username"            # ENV override: KJ_FUNDING_PAYPAL
    buymeacoffee: "username"      # ENV override: KJ_FUNDING_BUYMEACOFFEE
    liberapay: "username"         # ENV override: KJ_FUNDING_LIBERAPAY
  social:
    mastodon: "username"          # ENV override: KJ_SOCIAL_MASTODON
    bluesky: "user.bsky.social"   # ENV override: KJ_SOCIAL_BLUESKY
    linktree: "username"          # ENV override: KJ_SOCIAL_LINKTREE
    devto: "username"             # ENV override: KJ_SOCIAL_DEVTO

# Glob-based overrides (first match wins)
patterns:
  - path: "certs/**"
    strategy: raw_copy

# Per-file overrides
files:
  Rakefile:
    strategy: merge
    preference: destination        # preserve local tasks
  AGENTS.md:
    strategy: accept_template      # always use template version

Framework Matrix vs. Appraisals

workflows.preset: framework and workflows.framework_matrix are meant for a simple 2D matrix: Ruby versions × one framework gem/version axis. This is a good fit when you want kettle-jem to generate framework-version modular gemfiles, Appraisals entries, and CI matrix entries for a single framework dependency.

If you need a deeper or more complex matrix, prefer kettle-jem-appraisals, which generates Appraisals entries and is the better fit for Appraisals-style combinations.

Strategies

Strategy	Behavior
merge	Resolve tokens, then AST-merge template + destination (default)
accept_template	Resolve tokens, overwrite destination with template result
keep_destination	Skip entirely — no merge, no creation
raw_copy	Copy bytes as-is — no token resolution, no merge (for binary assets)

raw_copy exists for bootstrap files that may be needed before the full templating stack is available. Because it bypasses normal template processing, do not use it for templates that contain {KJ|...} tokens or require StructuredMerge normalization.

Token Substitution

Tokens use {KJ|TOKEN} syntax and are resolved in priority order:

1. ENV variables (highest) — e.g., KJ_AUTHOR_NAME
2. .kettle-jem.yml tokens: section — explicit values
3. Auto-derived from gemspec (lowest) — author name, email, domain

Common tokens:

Token	Source
`	GEM_NAME`	Gem name from gemspec
`	NAMESPACE`	Ruby module namespace
`	AUTHOR:NAME`	Author full name
`	AUTHOR:EMAIL`	Author email
`	GH:USER`	GitHub username
`	PROJECT_EMOJI`	Project emoji from config
`	MIN_RUBY`	Minimum Ruby version
`	FREEZE_TOKEN`	Freeze marker name

Freeze Blocks

Protect sections in any file from template overwrites:

# kettle-jem:freeze
gem "my-local-fork", path: "../custom"
# kettle-jem:unfreeze

Content between freeze/unfreeze markers is always preserved from the destination, regardless of what the template contains. Works in all supported formats (Ruby, YAML, Markdown, TOML, JSON, Bash, etc.).

Merge Engine Selection

Kettle::Jem selects the merge engine by file type:

File Pattern	Merge Engine	Key Behaviors
*.rb, Gemfile, *.gemspec, Rakefile, Appraisals	Prism::Merge	Three-phase matching, gemspec var renaming
*.yml, *.yaml	Psych::Merge	SHA-pinned uses:, per-key preferences
*.md, *.markdown	Markly::Merge	Heading/list matching, inner list merge
*.toml	Toml::Merge	Sort keys, table matching
*.json	Json::Merge	Key-based matching
*.jsonc	Json::Merge	With comment preservation
*.json5	Json::Merge	JSON5 keys, strings, comments, and trailing commas
*.sh, *.bash, .envrc	Bash::Merge	Block matching
.env*	Dotenv::Merge	KEY=value matching
*.rbs	RBS::Merge	Type signature matching
.gitignore	Text::Merge	Intentional line-based merge

No silent fallback: If a tree-sitter grammar is unavailable for a file type that requires AST merging, kettle-jem will fail (default) or skip the file — never silently degrade to text-based merging. See PARSE_ERROR_MODE below.

🔧 Basic Usage

Initial Setup

gem install kettle-jem
cd my-gem
kettle-jem

The setup CLI runs a two-phase bootstrap:

1. Bootstrap — creates .kettle-jem.yml, installs modular gemfiles, ensures dev dependencies
2. Bundled — loads the full runtime and runs kettle-jem install

Applying Template Updates

After initial setup, re-run the template process to pull in updates:

K_JEM_TEMPLATING=true bundle exec kettle-jem install

This applies all template phases, then runs the local finishing steps such as bin/setup, curated binstub generation, hooks, and lockfile normalization.

Phase	Description	Files Affected
0	Config sync	.kettle-jem.yml
1	Dev container	.devcontainer/
2	GitHub workflows	.github/workflows/, FUNDING.yml
3	Quality config	.qlty/qlty.toml
4	Modular gemfiles	gemfiles/modular/
5	Spec helper	spec/spec_helper.rb
6	Environment templates	.env.local.example
7	Remaining files	gemspec, README, LICENSE, Rakefile, …
8	Git hooks	.git-hooks/
9	License files	LICENSE*
10	Duplicate check	(validation only)

Each phase is implemented as a composable service_actor actor, enabling per-phase statistics (📄 templates, 🆕 created, 📋 pre-existing, 🟰 identical, ✏️ changed) and future slice-based workflows.

Checking Divergence

CI can verify that a project hasn't drifted too far from the template:

bundle exec rake kettle:jem:selftest

This re-applies the template in a temporary checkout and measures the diff. Output is condensed to two summary lines after the template run:

[selftest] 📄  Report - tmp/template_test/report/summary.md
[selftest] ✅  Score: 100.0% · Divergence: 0.0% · Threshold: fail when divergence reaches 5.0%

If divergence exceeds min_divergence_threshold (default 5%), the check fails.

Workflow-Specific Options

For GitHub Actions workflows, the template always wins for uses: lines (SHA-pinned action references) while destination wins for job configuration:

# Template updates this SHA automatically:
uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0

# Your matrix customizations are preserved:
matrix:
  ruby: ["3.2", "3.3", "3.4"]

Per-File Overrides

Override merge behavior for specific files in .kettle-jem.yml:

files:
  Rakefile:
    strategy: merge
    preference: destination     # keep your custom tasks
  certs/my.pem:
    strategy: raw_copy          # binary file, no merging
  generated/report.md:
    strategy: keep_destination  # never touch this file

Environment Variables & CLI Options

Kettle::Jem behavior is controlled via environment variables (which double as Rake task arguments) and CLI flags passed to kettle-jem setup.

Merge & Error Handling

Variable	CLI Flag	Default	Description
FAILURE_MODE	--failure-mode=VAL	error	How general merge failures are handled. error raises and halts; rescue logs a warning and uses unmerged content.
PARSE_ERROR_MODE	—	fail	How AST parser unavailability is handled. fail raises immediately (recommended); skip warns and preserves the destination file unchanged. There is no text-merge fallback — AST merge or nothing.

Task Control

Variable	CLI Flag	Default	Description
allowed	--allowed=VAL	true	Set to false/0/no to require manual review of env file changes before continuing.
—	--interactive	(off)	Enable interactive prompts (opt-in). Overrides the default non-interactive behavior.
KETTLE_JEM_VERBOSE	--verbose	false	Show detailed output including per-file messages and setup progress. Overrides the default quiet behavior.
only	--only=VAL	(all)	Comma-separated glob patterns — only template files matching at least one pattern are processed.
include	--include=VAL	(all)	Comma-separated glob patterns — additional files to include beyond the default set.
hook_templates	--hook_templates=VAL	(prompt)	Git hook install location: l/local, g/global, or n/none. Also via KETTLE_DEV_HOOK_TEMPLATES.
KETTLE_JEM_CHECKSUMS	--checksums=VAL	template,ignore-dest	Checksum skip mode. See "Checksum Skip Modes" below.
—	--ignore-checksums	(off)	Alias for --checksums=off; disables checksum-based skipping.

Checksum Skip Modes

Kettle::Jem writes managed template state to .structuredmerge/kettle-jem.lock. The legacy whole-template checksum inventory now lives under template_state.checksums. That inventory is useful for coarse template drift state, but it is not used to skip individual files. File skipping uses files.*.input_fingerprint, a per-destination digest that includes the selected template source SHA, recipe identity/version, resolved token digest, and kettle-jem implementation SHA. Token values are hashed into the fingerprint and are not written to the lockfile.

--checksums accepts comma-separated modes:

Mode	Behavior
template	Skip a destination file only when its input_fingerprint matches the lock. This is the default template-input check.
ignore-dest	Do not compare the destination file checksum. This is the default destination behavior.
dest	Also require the destination file checksum to match the lock before skipping. dest implies template unless ignore-template is present.
ignore-template	Do not compare input_fingerprint; useful only with dest when local destination state should be the only skip gate.
off	Disable checksum-based skipping. This cannot be combined with other modes.

Common combinations:

Value	Result
template,ignore-dest	Default. Retemplate when relevant template input changes; ignore local destination edits.
dest,template	Retemplate when relevant template input changes or the destination differs from the prior templated result.
dest,ignore-template	Skip only when the destination checksum matches; ignore template input changes.
off	Run normal templating without checksum skip shortcuts.

Config & Identity (KJ_ prefix)

These map directly to .kettle-jem.yml keys, seed freshly created configs, fill missing keys during config sync, and act as runtime overrides.

Variable	Description
KJ_PROJECT_EMOJI	Project identifying emoji (e.g. 🪙). Required in config.
KJ_MIN_DIVERGENCE_THRESHOLD	Selftest divergence threshold for min_divergence_threshold.
KJ_AUTHOR_NAME	Gem author full name
KJ_AUTHOR_EMAIL	Gem author email
KJ_AUTHOR_DOMAIN	Author website domain (derived from email if unset)
KJ_AUTHOR_GIVEN_NAMES	First/given names
KJ_AUTHOR_FAMILY_NAMES	Last/family names
KJ_AUTHOR_ORCID	ORCID identifier
FORGE_ORG	Preferred forge organization or owner used for scaffolded repository URLs when no forge remote is available
KJ_GH_ORG	GitHub-specific organization or owner fallback for scaffolded repository URLs
KJ_GH_USER	GitHub username used for maintainer profile and sponsor links
KJ_GL_USER	GitLab username
KJ_CB_USER	Codeberg username
KJ_SH_USER	SourceHut username

Workspace & Funding

Variable	Description
KETTLE_DEV_DEV	Workspace root for local sibling gems. true = ~/src/my; a path = that path; unset/false = released gems.
KETTLE_DEV_DEBUG	Set to true for verbose debug output.
FUNDING_ORG	OpenCollective organization handle for FUNDING.yml. Auto-derived from git remote if unset.
OPENCOLLECTIVE_HANDLE	Alternative to FUNDING_ORG for personal OpenCollective pages.
KJ_FUNDING_KOFI	Ko-fi handle for FUNDING.yml
KJ_FUNDING_PAYPAL	PayPal handle for FUNDING.yml
KJ_FUNDING_BUYMEACOFFEE	Buy Me a Coffee handle for funding links
KJ_FUNDING_LIBERAPAY	Liberapay handle for funding links
KJ_SOCIAL_MASTODON	Mastodon handle for social/profile links
KJ_SOCIAL_BLUESKY	Bluesky handle for social/profile links
KJ_SOCIAL_LINKTREE	Linktree handle for social/profile links
KJ_SOCIAL_DEVTO	DEV Community handle for social/profile links

Templating Examples

# Standard template update (quiet, non-interactive — the default)
K_JEM_TEMPLATING=true bundle exec kettle-jem install

# Verbose output
K_JEM_TEMPLATING=true KETTLE_JEM_VERBOSE=true bundle exec kettle-jem install

# Interactive mode (prompts before each change)
K_JEM_TEMPLATING=true bundle exec kettle-jem install --interactive

# Force re-evaluation of every selected template file
K_JEM_TEMPLATING=true bundle exec kettle-jem install --ignore-checksums

# Retemplate when either template inputs or destination checksums changed
K_JEM_TEMPLATING=true bundle exec kettle-jem install --checksums=dest,template

# Only workflow files, skip unparseable. Scoped template runs skip install
# finishing steps and are intended for surgical file updates.
K_JEM_TEMPLATING=true PARSE_ERROR_MODE=skip bundle exec kettle-jem template --only=".github/**"

# Rescue on merge failure (don't halt)
K_JEM_TEMPLATING=true FAILURE_MODE=rescue bundle exec kettle-jem install

The kettle:jem:* rake tasks are internal targets used by the executable after it prepares the templating environment; call kettle-jem directly for normal templating work.

🔐 Security

See SECURITY.md.

🤝 Contributing

If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check issues or PRs, or use the gem and think about how it could be better.

We so if you make changes, remember to update it.

See CONTRIBUTING.md for more detailed instructions.

🚀 Release Instructions

See CONTRIBUTING.md.

Code Coverage

Coverage service badges

🪇 Code of Conduct

Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .

🌈 Contributors

Made with contributors-img.

Also see GitLab Contributors: https://gitlab.com/structuredmerge/structuredmerge-ruby/-/graphs/main

⭐️ Star History

📌 Versioning

This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("kettle-jem", "~> 7.0")

📌 Is "Platform Support" part of the public API? More details inside.

Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.

To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

📄 License

The gem is available under the following licenses: AGPL-3.0-only, PolyForm-Small-Business-1.0.0. See LICENSE.md for details.

If none of the available licenses suit your use case, please contact us to discuss a custom commercial license.

© Copyright

See LICENSE.md for the official copyright notice.

Copyright holders

• Required Notice: Copyright (c) 2026 Peter H. Boling

🤑 A request for help

Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.

To join the community or get help, use the RubyForum or Discord.

To say "thanks!" ☝️ Join the community or 👇️ send money.

💌 💌

Please give the project a star ⭐ ♥.

Many parts of this project are actively managed by a kettle-jem smart template utilizing StructuredMerge.org merge contracts.

Thanks for RTFM. ☺️

Field	Value
Package	kettle-jem
Description	🔮 Kettle::Jem provides gem scaffolding, templating, and setup automation using the *-merge gem family for AST-based file merging and token-resolver for template token resolution. Includes MergerConfig presets, YAML recipes, and a complete gem template scaffold.
Homepage	https://github.com/structuredmerge/structuredmerge-ruby
Source	https://github.com/structuredmerge/structuredmerge-ruby
License	AGPL-3.0-only OR PolyForm-Small-Business-1.0.0
Funding	https://github.com/sponsors/pboling, https://ko-fi.com/pboling, https://liberapay.com/pboling/donate, https://thanks.dev/u/gh/pboling, https://tidelift.com/funding/github/rubygems/kettle-jem, https://www.buymeacoffee.com/pboling