Phx Mix Compression Skill for Claude

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

Iron Laws

• 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

• Verify after install — run rtk verify (or rtk verify --filter mix-test) to

confirm the bundled test fixtures pass before declaring success

• Never overwrite existing .rtk/filters.toml — diff and merge instead

Workflow

Step 1: Detect rtk

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:

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.

Step 3: Verify filters work

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