A repeatable process for making Fallow’s code analysis run faster while preserving correct results. It compares the same benchmark before and after one bounded optimization.

Use it to select a stable benchmark, measure a baseline, inspect the slow path, make one optimization, rerun checks, and keep the change only when the result is reproducible.

It prevents unreliable performance claims and helps detect when a speed improvement changes behavior or breaks a contract.

perf-loop — Skill by fallow-rs

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add skills/fallow-rs/fallow/perf-loop

Any agent

npx skills add fallow-rs/fallow --skill perf-loop

Clone the repo

git clone --depth 1 https://github.com/fallow-rs/fallow

Made for: Claude Code, Codex.

Per session 22 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 131 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5	$0.00022	$0.00131
Opus 5	$0.00011	$0.00066
Sonnet 5	$0.00004	$0.00026
Haiku 4.5	$0.00002	$0.00013

Measured yesterday against content hash 1e57ef6b5a38, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

perf-loop scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.agents/skills/perf-loop/SKILL.md · 19 lines

What it actually says

Performance loop

Choose a stable benchmark and preserve its identity and workload.
Record a statistically useful baseline.
Profile the hot path before editing.
Implement one bounded optimization.
Re-run the same benchmark and correctness checks.
Keep the change only when the improvement is reproducible and no contract regresses.
Use a new benchmark identifier for a materially different workload.
Run review.

Do not report performance gains from debug builds or incomparable fixtures.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

yesterday First seen · 19 lines · 22 tokens per session scan A 1e57ef6b5a38

Subscribe to this mod's changes

perf-loop is a skill published in the GitHub repository fallow-rs/fallow (4,429 stars, last pushed yesterday), licensed MIT. It adds 22 tokens to every session and 131 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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