03 — RTK v0.45.0: current design

Summary

RTK is a narrow, deterministic shell-observation reducer with strong command coverage, but it cannot reduce native reads or conversation history. Cutoff: 2026-08-12. Current stable: v0.45.0, released 2026-08-07.

RTK is a deterministic command proxy. An agent hook rewrites eligible shell commands through one Rust binary; RTK runs the real command, preserves its behavior, and emits a smaller observation using command-specific rules. It does not compress the model's native file/search tools or the conversation history.

Question and scope

Which shell observations can RTK v0.45.0 reduce deterministically, and which context sources remain outside its reach?

Method

The review fixes RTK to v0.45.0, inspects its command coverage and deterministic transforms, and distinguishes payload counters from independent task-level evidence.

Findings

Current facts

Metadata source: GitHub API. Current scope and caveats: README.

Current interception

agent asks shell tool to run command
       ↓
hook classifies and rewrites eligible command
       ↓
rtk runs the real command
       ↓
command-specific deterministic filter
       ↓
exit status + compact observation enter context

Because compression occurs before the new observation enters history, it is cache-safe by construction. No learned model can hallucinate the summary. The trade is reach: the current README states built-in Read, Grep, and Glob tools bypass the hook. RTK only helps when the agent invokes shell commands RTK recognizes or the user explicitly calls RTK wrappers.

Current release evidence

v0.45.0 adds Mistral Vibe hook support and multiline command rewriting. It also hardens GitHub Copilot hook installation/healing while preserving user configuration. These are integration improvements; the fundamental shell-only boundary remains.

Release source: v0.45.0 notes.

Current evidence

RTK provides current per-command examples and a local gain ledger. These establish mechanics, not end-to-end efficiency. No independent accepted-task benchmark explicitly testing v0.45.0 was found at the cutoff. Results from superseded versions are excluded.

Implications for jackin❯

Benefits

• Deterministic, inspectable rules; no ML model or inference latency.
• Small single artifact and low per-command overhead.
• Zero MCP tool-schema rent.
• Write-time cache safety.
• Broad current command catalog and custom filter support.
• Explicit raw command remains available.
• Current README correctly scopes headline percentages to Bash output.

Best fit

RTK is a good candidate when traces show supported shell output is a large fraction of fresh input: verbose tests, builds, Git status/diff, package managers, containers, or infrastructure CLIs. It is also the simplest option when deterministic behavior, minimal runtime, and zero model dependency dominate.

Do not install it as a generic token remedy when the agent mainly uses native reads/search, commands are already quiet, or prompt-cache reads—not new shell output—dominate consumption.

Current verdict

RTK v0.45.0 is the narrowest and easiest-to-reason-about input tool. That is both strength and limit. It is objectively good for large supported shell observations; there is no current evidence that it reduces every coding task. Status: use after a reach audit, not by headline.

Next: 04 — lean-ctx v3.9.18.

Limitations and unknowns

Costs and risks

• Structural reach ceiling: native agent file/search tools bypass it.
• Command rewriting can conflict with other PreToolUse hooks or shell wrappers.
• A successful command may still contain a detail removed by the filter; raw rerun is the recovery path.
• Filtering can cause the agent to issue follow-up commands, erasing gross savings.
• rtk gain estimates removed output using approximate token arithmetic; it cannot observe counterfactual turns, cache changes, or task quality.
• Current documentation conflicts on telemetry default: privacy copy says anonymous telemetry is opt-in and disabled by default, while DISCLAIMER.md says telemetry is enabled by default. Resolve from the exact installed build/config before deployment.
• Current README's verification example still names an older version, a documentation freshness defect.

Sources

Evidence and repository references are cited inline beside the claims they support.

Related work

• Token-optimization tools dossier
• Token-optimization techniques
• Context engine