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Agent skill

Phx Mix Compression

by oliver-kriska in oliver-kriska/claude-elixir-phoenix

Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude.

MITAuto-check passed

Install Phx Mix Compression

skills CLI
$ npx skills add oliver-kriska/claude-elixir-phoenix --skill phx-mix-compression -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install oliver-kriska/claude-elixir-phoenix phx-mix-compression --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/oliver-kriska/claude-elixir-phoenix.git skills-src && mkdir -p .claude/skills && cp -r skills-src/targets/pi/skills/phx-mix-compression .claude/skills/phx-mix-compression && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
phx-mix-compression
GitHub stars
565
Token cost
~999 tokens
SKILL.md length
445 words
Files
3 (incl. references)
Skills in repo
109
Repo updated
First seen
Licence
MIT

At a glance

Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude.

  • Works in 4 steps: Detect rtk → Seed .rtk/filters.toml → Verify filters work → …
  • Long mix output floods context
  • SKILL.md covers When to use, Iron Laws, Workflow and Customization, plus 2 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Phx Mix Compression is an agent skill from oliver-kriska/claude-elixir-phoenix. Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude. Use when long mix output floods context.

Its SKILL.md is about 1000 tokens, which your agent loads only when the skill is triggered. The skill folder holds 3 other files, including reference files (for example `references/install.md`).

The repository describes itself as: Claude Code plugin for Elixir/Phoenix/LiveView — 26 specialist agents, Iron Laws enforcement, and Tidewave MCP integration. Plan features with parallel research agents, execute… The licence is MIT.

When your agent uses it

  • Long mix output floods context

Example prompts

  • “/phx-mix-compression”

Workflow steps

4 steps, taken from the step headings in SKILL.md.

  1. Detect rtk
  2. Seed .rtk/filters.toml
  3. Verify filters work
  4. Confirm shell hook

What it can do on your machine

Read from SKILL.md and the folder at commit 9767a82. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md (its code samples are bash).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • github.com

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Phx Mix Compression loads about 999 tokens when it runs, and up to ~5.8k if it reads all its reference files. Until then it costs about 53 tokens; SKILL.md has 445 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~53
When it runs · the whole SKILL.md, loaded when a task matches
~999
With references · SKILL.md plus every file in references/, read only if the agent opens them
~5.8k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from oliver-kriska/claude-elixir-phoenix at commit 9767a82, republished under its MIT licence (© oliver-kriska). 445 words, ~999 tokens.

Download SKILL.mdSave it as .claude/skills/phx-mix-compression/SKILL.md (or your agent's skills folder). This skill also uses 2 other files; get the full folder from GitHub.
name
phx-mix-compression
description
Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude. Use when long mix output floods context.

Mix Output Compression

Mix commands (mix test, mix credo, mix dialyzer, mix compile) emit verbose, repetitive output that consumes context fast. This skill installs rtk — a CLI proxy that filters tool output before it lands in the transcript.

The filters short-circuit happy paths to a single line (mix test: all pass) while preserving full failure blocks, compile errors, and stack traces. Net win: 5-15% per-session token reduction on mix-heavy workflows.

When to use

  • Long sessions — /skill:phx-work or /skill:phx-full hitting context limits from mix output
  • Debugging loops — /skill:phx-investigate retrying mix compile/mix test repeatedly
  • Dialyzer-heavy projects — mix dialyzer output dominates the transcript

Iron Laws

  1. NEVER strip critical signals — compile errors (** (CompileError), == Compilation error in), test failures (FAILURES, 0 failures — preserved even on short-circuit), dialyzer warnings, and stack traces with file:line MUST pass through unchanged
  2. Verify after install — run rtk verify (or rtk verify --filter mix-test) to confirm the bundled test fixtures pass before declaring success
  3. Never overwrite existing .rtk/filters.toml — diff and merge instead

Workflow

Step 1: Detect rtk
bash
which rtk && rtk --version

Read references/install.md if rtk is missing — covers homebrew install + shell hook setup.

Step 2: Seed .rtk/filters.toml

Reference filters live at references/rtk-filters.toml. Six production-tested filters covering:

  • mix-test — short-circuits all-pass, preserves failure blocks + compile errors
  • mix-credo — collapses clean runs, preserves violation blocks
  • mix-dialyzer — drops PLT progress, keeps warnings + summary
  • mix-deps-get — collapses unchanged package lists
  • mix-ecto-migrate — strips compile prefix, short-circuits "already up"
  • mix-compile — handles parallel worker prefixes (N>) and MIX_ENV

Run this if the project has no .rtk/filters.toml yet:

bash
mkdir -p .rtk
cp "references/rtk-filters.toml" .rtk/filters.toml

Read both files if one already exists. Present a diff to the user. Merge only the filters they don't already have.

Show full SKILL.md (179 more words)Show less
Step 3: Verify filters work
bash
rtk verify                       # runs all embedded [[tests.*]] fixtures
rtk verify --filter mix-test     # one filter only

Check that all report "passed". Flag and stop if any fail — usually means the user has a custom rtk version with regex differences.

Step 4: Confirm shell hook

Run rtk init zsh (or rtk init bash) to install the transparent rewrite hook that turns mix X into rtk mix X. Re-running is safe (idempotent). Skip this step and mix calls run unfiltered.

Customization

Add custom regex patterns to strip_lines_matching for project-specific noise sources (e.g., third-party hex deps spamming stack traces). See the inline example in references/rtk-filters.toml lines 57-59.

What this is NOT

  • Not a hook — Claude Code's PostToolUse hooks fire after the tool result is in the transcript and cannot shrink it. rtk works at the subprocess layer (the only layer where transcript-shortening is possible).
  • Not project-analysis — the bundled filter set is universal across Phoenix projects. No mix.exs inspection needed.
  • Not telemetry — rtk has telemetry off by default (enabled = false in config.toml). Filters run locally, no data leaves the machine.

References

  • references/rtk-filters.toml — bundled filter set
  • references/install.md — rtk install + shell hook setup
  • rtk on GitHub

© oliver-kriska, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 2 other files (references) in targets/pi/skills/phx-mix-compression of oliver-kriska/claude-elixir-phoenix.

  • SKILL.md
  • references/install.md
  • references/rtk-filters.toml

Open the folder on GitHubat commit 9767a82

Compare with similar skills

Phx Mix Compression next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Phx Mix Compression compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Phx Mix Compression this skilloliver-kriska/claude-elixir-phoenix565—~999Automated safety check: PassMIT
Compressionthedaviddias/Front-End-Checklist74k—~421Automated safety check: PassMIT
OmniRoute Prompt Compressiondiegosouzapw/OmniRoute75k—~1.6kAutomated safety check: PassMIT
Reduced Motionthedaviddias/Front-End-Checklist74k—~534Automated safety check: PassMIT
Image Compressionthedaviddias/Front-End-Checklist74k—~405Automated safety check: PassMIT
Mixed Precisionaiming-lab/AutoResearchClaw15k—~275Automated safety check: PassMIT

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Questions about Phx Mix Compression

What does Phx Mix Compression do?

Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude. Phx Mix Compression is an agent skill from oliver-kriska/claude-elixir-phoenix. Reduce mix output noise (5-15% token savings) by installing rtk filters that compress mix test/credo/dialyzer/compile output before it reaches Claude.

When should I use Phx Mix Compression?

Phx Mix Compression fits situations like: long mix output floods context.

How do I install Phx Mix Compression in Claude Code?

Run `npx skills add oliver-kriska/claude-elixir-phoenix --skill phx-mix-compression -a claude-code`. Or copy the skill folder (targets/pi/skills/phx-mix-compression in oliver-kriska/claude-elixir-phoenix) into .claude/skills/phx-mix-compression in your project. Claude Code loads it when a task matches its description.

How do I install Phx Mix Compression in Codex?

Run `npx skills add oliver-kriska/claude-elixir-phoenix --skill phx-mix-compression -a codex`. Or copy the skill folder (targets/pi/skills/phx-mix-compression in oliver-kriska/claude-elixir-phoenix) into .agents/skills/phx-mix-compression in your project. Codex loads it when a task matches its description.

Can I use Phx Mix Compression in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add oliver-kriska/claude-elixir-phoenix --skill phx-mix-compression -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/phx-mix-compression, .gemini/skills/phx-mix-compression, .github/skills/phx-mix-compression and .opencode/skills/phx-mix-compression in your project.

What does Phx Mix Compression need to run?

SKILL.md names no scripts, command-line tools or credentials: Phx Mix Compression is instructions for the agent only.

Does Phx Mix Compression access the network?

SKILL.md names 1 domain. As links in the text: github.com. This is read from the text; nothing was executed.

Is Phx Mix Compression safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Phx Mix Compression use?

Phx Mix Compression is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Phx Mix Compression use?

About 999 tokens (SKILL.md is roughly 4k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 4.8k tokens, read only when the agent opens those files.

What are the alternatives to Phx Mix Compression?

Skills that share tags, products or a category with Phx Mix Compression: Compression (thedaviddias/Front-End-Checklist, 74k stars), OmniRoute Prompt Compression (diegosouzapw/OmniRoute, 75k stars), Reduced Motion (thedaviddias/Front-End-Checklist, 74k stars) and Image Compression (thedaviddias/Front-End-Checklist, 74k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Phx Mix Compression?

oliver-kriska (a GitHub user) maintains it in oliver-kriska/claude-elixir-phoenix, which has 565 GitHub stars. The repository holds 109 skills in this directory. The repository was last updated on October 5, 2026.

Source: oliver-kriska/claude-elixir-phoenix on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.