---
id: oliver-kriska/claude-elixir-phoenix/mix-compression
version: "df175d5d"
license: MIT
install: manual
updated: 2026-07-27
---

# mix-compression — Mix Output Compression installs rtk filters that intercept mix test, credo, dialyzer, and compile output before it enters your transcript. The filters collapse successful runs to single lines while keeping all failures, errors, and stack traces intact, typically saving 5-15% of tokens per session.

Publisher: oliver-kriska · Stars: 507 · Updated: 2026-07-27

Install (manual): `git clone https://github.com/oliver-kriska/claude-elixir-phoenix`

## SKILL.md

# 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

```bash
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:

```bash
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

```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 `${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)


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