Joser

Claude Code Plugin for Elixir: Custom Skills and Hooks for Better Code

Quality

I’ve been experimenting with Claude Code for Elixir development and built a
custom plugin to improve code quality. To test whether it actually makes a
difference, I built the same Phoenix LiveView application twice once with
minimal configuration and once with the full plugin.

The Experiment

I created an image gallery application with Phoenix LiveView featuring:

Image uploads with progress tracking
Folder organization
Tab-based navigation
Responsive TailwindCSS UI
Local file storage with PostgreSQL metadata

Build 1 (Baseline): Minimal configuration, just project guidelines and
specs
Build 2 (Plugin): Custom skills, hooks, agent docs, and comprehensive
project documentation

What’s in the Plugin?

4 Custom Skills:

Elixir patterns (pattern matching, pipe operators, with statements)
Phoenix LiveView (lifecycle, uploads, streams)
Ecto database (schemas, changesets, queries)
Error handling (tagged tuples, error patterns)

5 Validation Hooks:

Warn on nested if/else (prefer pattern matching)
Block hardcoded config values (require Application.get_env/2)
Warn on Enum.map |> Enum.filter chains
Block missing @impl true on callbacks
Warn on string concatenation in loops

Agent Documentation:

Project structure and context boundaries
LiveView implementation checklists
Ecto conventions and best practices
Testing guides

The Results

Metric	Build 1 (Baseline)	Build 2 (Plugin)
Lines of Code	~550	~2,000+ (with tests)
Automated Tests	0	46 tests
Test Coverage	Manual only	28/28 context tests passing
Credo Score	Not run	Excellent (7 minor issues)
Convention Adherence	Good	Excellent

Where the Plugin Excelled

1. Comprehensive Testing
Build 2 included 46 automated tests covering all context operations, edge
cases, and error handling from the start.

2. Better Code Quality
Consistent pattern matching, proper pipe operator usage, tagged tuples for
error handling, and @impl true on all callbacks.

3. Idiomatic Elixir
The plugin encouraged idiomatic patterns throughout:

# Pattern matching over if/else
def process({:ok, result}), do: transform(result)
def process({:error, reason}), do: handle_error(reason)

# with statements for multi-step operations
def create_image_with_folder(attrs, folder_id) do
  with {:ok, folder} <- get_folder(folder_id),
       {:ok, image} <- create_image(attrs) do
    {:ok, %{image: image, folder: folder}}
  end
end

Important Limitation

Both builds required manual testing to catch functional issues. The plugin
helps write better code with proper patterns and conventions, but it cannot
replace human validation of feature functionality.

The Verdict

The plugin produces higher-quality, more maintainable code with better test
coverage. If you're building Elixir applications professionally, the
investment is worth it.

Try It Yourself

The plugin configuration is available on GitHub:

https://github.com/j-morgan6/elixir-claude-optimization

The package includes:
- Skills for Elixir patterns, LiveView, Ecto, and error handling
- Hooks that enforce conventions and warn about anti-patterns
- Agent documentation for project structure and testing
- Sample CLAUDE.md for project setup

This is a one time investment that's reusable across all your Elixir projects.
 I'd love to hear your thoughts or experiences with AI-assisted Elixir
development!

5 comments

#best-practices #testing #ai #code-quality #dev-tools

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2026-03-26 18:56:53 UTC

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Joser

GitHub - j-morgan6/elixir-phoenix-guide · GitHub Link to the repo!

Post #1

Joser

Released v1.1.0 of my Elixir Claude optimization plugin

The problem: Claude Code had the right skills but wasn’t invoking them consistently. Skills existed but sat unused.

What changed:

• New skill-discovery system for systematic skill selection

• Replaced passive “Use when” with mandatory “INVOKE BEFORE” directives

• File pattern detection for automatic skill recommendations

• 50%+ increase in skill utilization

The impact:

Claude Code now consistently identifies applicable skills BEFORE writing code. The plugin finally does what it should: ensure Elixir/Phoenix best practices are followed from the start, not fixed after the fact.

More updates coming soon. If you’re using Claude Code for Elixir/Phoenix work, this one’s worth checking out.

GitHub: https://lnkd.in/eU-Ghh6x

Post #2

kjwvanijk

Looks very interesting, I have been using Claude code for a bit and ran into the same issues. Yesterday I tried out gtp-5.3 codex and it performed a lot better in writing clean idiomatic elixir.

What I wonder though, how does this plugin interact with usage_rules if at all?

Post #3

Joser

v1.3.0 adds one thing: a testing-essentials skill.

The plugin now has 5 skills. The new one activates before any _test.exs file and enforces 8 testing rules covering DataCase/ConnCase setup, fixture patterns, LiveView and
context test skeletons, and a light TDD workflow.

All 4 existing skills now route to testing-essentials when you’re working on test files. The testing-guide.md agent doc was refactored into a deep reference so there’s no
duplication between it and the skill.

Plugin at a glance:

5 skills: Elixir, LiveView, Ecto, uploads, testing
10 hooks: real-time blocking/warnings for @impl, hardcoded paths, nested if/else, etc.
4 agent docs: detailed reference for complex tasks

Install / update:

/plugin marketplace add j-morgan6/elixir-phoenix-guide
/plugin

Already installed? Open /plugin and update from the menu.

GitHub: GitHub - j-morgan6/elixir-phoenix-guide · GitHub

Post #4

Joser

v2.3.0 adds five new skills and six security hooks.

The plugin now covers 19 skills across security, deployment, channels, telemetry, and JSON APIs alongside the existing LiveView, Ecto, and OTP coverage. The new security hooks run as you write code, blocking SQL injection, atom exhaustion, and open redirects, and warning on raw/1 usage, sensitive data in logs, and timing-unsafe comparisons.

New skills:

security-essentials - 7 rules: atom exhaustion, SQL injection, XSS, open redirects, sensitive logging, timing attacks, dependency auditing
deployment-gotchas - 7 rules: runtime.exs, release migrations, PHX_HOST/PHX_SERVER, asset deployment, secrets, health endpoints, log levels
phoenix-channels-essentials - 6 rules: socket auth, topic authorization, handle_in/push/broadcast, Presence, testing
telemetry-essentials - 6 rules: structured logging, handler attachment, Ecto telemetry events, LiveDashboard, metadata tagging
phoenix-json-api - 7 rules: :api pipeline, FallbackController, pagination, URL versioning, Bearer token auth, json/2

New security hooks:

atom-from-user-input (Blocking) - String.to_atom/1 on user input
unparameterized-sql-fragment (Blocking) - String interpolation in Ecto fragment
unsafe-redirect (Blocking) - Redirect to user-controlled URLs
raw-html-warning (Warning) - raw/1 usage
sensitive-logging (Warning) - password/token/secret in Logger calls
timing-unsafe-compare (Warning) - == with tokens/secrets

Plugin at a glance:

19 skills: Elixir, LiveView, Ecto, OTP, uploads, testing, auth, nested associations, changesets, PubSub, code quality, security, deployment, channels, telemetry, JSON API, authorization, auth customization, LiveView auth
27 hooks: real-time blocking and warnings
4 agent docs: detailed reference for complex tasks

GitHub: GitHub - j-morgan6/elixir-phoenix-guide · GitHub

Post #5

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