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, /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: