Why does cetas no longer ship rtk?

cetas once integrated rtk. An independent benchmark explains why it is not shipped, and how to wire it back if you want it.

rtk (Rust Token Killer) is a command-line proxy: it rewrites commands like pnpm test or go test -v ./... into rtk test-style forms, compressing output before it enters the context. Its README promises 60–90% token savings — an attractive promise for anyone living in test output and build logs.

cetas integrated it once. The repository’s posoco-ext-rtk was written for exactly that: a tool-call hook handing every agent-initiated shell command to rtk rewrite, with the rewritten command then executed.

We stopped shipping it. The reason is not the integration — the hook still works and the tests still pass — but rtk’s premise itself.

Compression has a price

Compressing output means rtk makes editing judgments on the model’s behalf: which lines matter, which can be dropped. It frequently drops the wrong ones.

JetBrains’ independent benchmark put numbers on it: Claude Code with rtk ran SkillsBench’s 86 tasks in a paired A/B over 425 billed trials. The advertised 60–90% did not appear; at low reasoning effort each task was 7.6% more expensive (p=0.004) with 13.8% more turns. Quality was unchanged — the model, fed truncated information, re-ran commands and re-read files, burning the savings back. At high reasoning effort the gap closed to zero, but rtk saved nothing at either level.

The most telling detail is rtk’s own scoreboard: across the whole run, rtk gain reported 96.2 million tokens saved while the actual bill for the same trials went up. The scoreboard measures against “raw output before compression”, but most of that output would have been truncated by the host or discounted by caching anyway — the model never received it. The original article’s conclusion is worth quoting:

a tool’s self-reported savings are a claim about its counterfactual, not about your bill.

So we do not ship it. A tool that silently truncates information and then makes the model spend more turns recovering it saves display numbers and costs real money.

Still want it? Two steps

# Install rtk itself (brew, or per its repository)
rtk --help

# Wire up the cetas hook extension

The hook’s wiring is fully demonstrated in the posoco-ext-rtk source and tests. Once connected, every shell command the agent runs passes through rtk rewrite first: rewritten when possible, executed verbatim otherwise. Whether it is worth it, borrow JetBrains’ methodology: run it paired, measure the real bill, and ignore the tool’s own scoreboard.