/mix-compression

name: 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.
effort: low

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](https://github.com/rtk-ai/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** — `/phx:work` or `/phx:full` hitting context limits from mix output
• **Debugging loops** — `/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

which rtk && rtk --version

Read `${CLAUDE_SKILL_DIR}/references/install.md` if rtk is missing — covers homebrew install + shell hook setup.

Step 2: Seed `.rtk/filters.toml`

Reference filters live at `${CLAUDE_SKILL_DIR}/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 "${CLAUDE_SKILL_DIR}/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 `${CLAUDE_SKILL_DIR}/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

• `${CLAUDE_SKILL_DIR}/references/rtk-filters.toml` — bundled filter set
• `${CLAUDE_SKILL_DIR}/references/install.md` — rtk install + shell hook setup
• [rtk on GitHub](https://github.com/rtk-ai/rtk)