Show HN: Lowfat – pluggable CLI filter that saved 91.8% of my LLM tokens

lowfat is a lightweight CLI tool that reduces AI token costs by filtering CLI output and file content before it reaches your agent.

Core focus

• Lightweight — Small single binary, small core; but extensible.

• Local-first — No telemetry; you own your data.

• Composable — UNIX-style pipes, mix built-ins and your own filters; not magic.

• User-owned — lowfat history shows what you run most; allow you to customize for your usecase.

Before / after

git status at the full level — same signal, less noise.

Before — raw git status:

On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   crates/lowfat-cli/src/commands/plugin.rs
	modified:   crates/lowfat-cli/src/main.rs

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	plugins/git/git-compact/samples/

no changes added to commit (use "git add" and/or "git commit -a")

After — lowfat git status:

On branch main
Changes not staged for commit:
	modified:   crates/lowfat-cli/src/commands/plugin.rs
	modified:   crates/lowfat-cli/src/main.rs
Untracked files:
	plugins/git/git-compact/samples/

Reduction of raw command output, measured on the bundled samples (crates/lowfat-plugin/embedded/*/samples/). Reproduce with cat <sample> | lowfat filter <plugin>/filter.lf --sub=<sub> --level=<level>:

command	lite	full	ultra
git diff	-16%	-38%	-96%
git log	-53%	-80%	-91%
git status	-62%	-62%	-74%
docker ps	-38%	-38%	-85%
docker images	-48%	-58%	-86%
ls -la	-2%	-75%	-87%

These percentages are the reduction of a single command's output, not your end-to-end agent token usage. savings depend on how much of your context is command output and how lossy a level you pick — higher levels drop more, so verify your agent still has what it needs. treat the table as a ceiling on the output slice, not a promise on the total.

Install

cargo install lowfat
# or
brew install zdk/tools/lowfat

Pre-built binaries on GitHub Releases.

Setup

Pick one of:

Claude Code hook — add to .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [{ "type": "command", "command": "lowfat hook" }]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Read",
        "hooks": [{ "type": "command", "command": "lowfat post-read" }]
      }
    ]
  }
}

PreToolUse rewrites Bash commands through lowfat filters. PostToolUse compresses file content after Read — strips comments, collapses function bodies, summarizes lock files.

The filtering pattern Anthropic recommends, but via lowfat.

Shell integration — auto-activates inside agent environments (CLAUDECODE=1, CODEX_ENV), or set LOWFAT_ENABLE=1 to force it on any shell:

echo 'eval "$(lowfat shell-init zsh)"' >> ~/.zshrc   # or ~/.bashrc

OpenCode plugin — one command, no config editing:

lowfat opencode install   # writes ~/.config/opencode/plugins/lowfat.ts

Restart OpenCode; commands are rewritten transparently before they run. Uninstall with lowfat opencode uninstall.

Direct usage — prefix any command:

lowfat git status
lowfat docker ps
lowfat ls -la

Pi agent — in ~/.pi/agent/settings.json:

{ "shellCommandPrefix": "eval \"$(lowfat shell-init zsh)\"; " }

Usage highlights

# See what's configured and how loud each filter is being
lowfat info                       # status badge + active filters
lowfat info git                   # pipeline for `git`
lowfat info --config              # full resolved config

# See what lowfat has saved you
lowfat stats                      # lifetime token savings
lowfat stats --audit              # recent plugin executions
lowfat history                    # rank commands by potential savings

# Dial the aggressiveness
lowfat level ultra                # max compression
LOWFAT_LEVEL=lite lowfat git log  # one-off override

# Write a plugin
lowfat plugin new terraform       # scaffold ~/.lowfat/plugins/terraform/
lowfat plugin doctor              # check plugins (and pre-install any Python deps)

# Test a plugin against a sample without installing it
cat samples/git-diff-full.txt | lowfat filter --explain ./filter.lf --sub=diff --level=ultra

File content compression (post-read)

When Claude reads files, lowfat post-read compresses the content before it enters the context:

Content type	What it does
Source code (Rust, Python, Go, Elixir, JS/TS, Java, Ruby, C/C++, Shell)	Strip comments, normalize blanks; at ultra: collapse function bodies to signatures
Markdown	Strip badges, HTML comments; truncate code blocks and tables
HTML / Vue / Svelte	Strip <style>, <script>, class attributes; at ultra: text extraction only
JSON / JSONC	Truncate large arrays, collapse deep nesting
Lock files (Cargo.lock, package-lock.json, yarn.lock, ...)	Replace with summary: package count + top deps
Unknown	Head + tail with line count

Compression level follows LOWFAT_LEVEL (lite/full/ultra). Files with <10% savings pass through unchanged.

Learn more

• docs/ARCHITECTURE.md — high-level diagram: CLI, Runner, Plugins, Builtins

• docs/CONFIG.md — .lowfat file, env vars, pipeline DSL, built-in processors, the history ranking

• docs/PLUGINS.md — lf-filter (the .lf plugin DSL), shell escape hatches, PEP 723 + uv, AI agent prompt

Alternatives

• rtk

• context-mode

• lean-ctx

• tokf

• tamp

• ecotokens

• token-enhancer

vs rtk

rtk is the closest tool but differ in philosophy: rtk is batteries-included; lowfat is a minimal core you extend yourself.

	lowfat	rtk
Built-in commands	6 curated (git, docker, grep, find, ls, tree)	100+ across many ecosystems
Custom filters	.lf DSL + shell + Python (PEP 723/uv)	TOML DSL
Levels	lite / full / ultra	-l aggressive, --ultra-compact
File-content filtering	post-read hook (code, markdown, HTML, data, lock files)	rtk read / smart (signatures, summaries)
Agent integrations	Claude Code, OpenCode, shell, Pi	14 tools (Claude Code, Copilot, Gemini, Codex, …)
Telemetry	None — local-only	Opt-in, off by default (anonymous aggregate)
Savings analytics	lowfat stats / history (local)	rtk gain / discover (local)

Token savings, head-to-head

Same commands, same repo, same cwd, run through both tools. Output tokens counted with tiktoken (cl100k_base); savings are vs the raw command output:

command	raw tokens	lowfat full	lowfat ultra	rtk
git status	81	-91%	-91%	-79%
git diff	1241	-15%	-97%	-9%
git log	3350	-93%	-97%	-56%
ls -la	153	-77%	-89%	-86%
find	535	-0%	-58%	-66%

Honest read: lowfat compresses git harder; rtk edges out find; ls is close. find only engages at lowfat's ultra level. rtk's --ultra-compact gave near-identical numbers to its default here, so the default is shown. This is a single small run on one repo — directional, not a benchmark; measure on your own workload before trusting any of it.

License

Apache-2.0

AI notice

Multiple AI tools were used for this project

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