fettle-learn

fettle-learn is a command for coding agents from MilindGaharwar/fettle. It costs 10 tokens per session (323 once invoked), scanned A, original, Apache-2.0.

A command that drafts a Semgrep rule from a description of a bug or security problem. Semgrep is a tool that searches code for specified patterns.

In plain words
What is it for?
Use it to describe the failed code pattern, generate proposed rules and test fixtures, verify their matches, and promote approved rules into active checks.
Why use it?
It helps turn lessons from incidents into repeatable checks while keeping new rules quarantined until a person approves them.

Command

Part of the fettle plugin — 34 commands, 4 hooks shipped together

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 commands/milindgaharwar/fettle/fettle-learn
Clone the repo
git clone --depth 1 https://github.com/MilindGaharwar/fettle

Or install fettle, the plugin that ships this one along with the rest of its 34 commands, 4 hooks.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for fettle-learn

README.md
[![agentmods](https://agentmods.dev/badge/commands/milindgaharwar/fettle/fettle-learn.svg)](https://agentmods.dev/commands/milindgaharwar/fettle/fettle-learn)
Your own site
<a href="https://agentmods.dev/commands/milindgaharwar/fettle/fettle-learn"><img src="https://agentmods.dev/badge/commands/milindgaharwar/fettle/fettle-learn.svg" alt="Measured on agentmods" height="20"></a>
Per session 10 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 323 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.

ModelPer sessionOnce invoked
Fable 5 $0.00010 $0.00323
Opus 5 $0.00005 $0.00161
Sonnet 5 $0.00002 $0.00065
Haiku 4.5 $0.00001 $0.00032

Measured 4d ago against content hash 7e294f2186b0, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

fettle-learn 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 4d ago.

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.

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

  • learn — 95% identical, 5 lines differ
.opencode/commands/fettle-learn.md · 53 lines

What it actually says

/fettle:learn

Generate a semgrep rule from an incident description.

Usage

When the user invokes /fettle:learn, ask for the incident details:

  1. What failed? (the bug or vulnerability)
  2. What code pattern caused it?
  3. What should it look like instead?

Then run:

fettle learn --incident "INCIDENT_TEXT" --auto-save

The drafted rule lands in the rules/proposed/ quarantine (status: proposed) with:

  • Semgrep pattern
  • Citation (incident reference)
  • Violating fixture (tests/fixtures/learned/)
  • Clean fixture (tests/fixtures/learned/)

Proposals are NEVER loaded by gates. Promotion to rules/learned/ is an explicit human step.

Verification

After generating, verify the rule works against its fixtures:

semgrep --config rules/proposed/<rule-id>.yml tests/fixtures/learned/<rule-id>_violation.py

Should match the violation fixture and NOT match the clean fixture.

Approval

Review the quarantined proposals and promote the good ones:

fettle rules list
fettle rules promote <rule-id>

Promotion moves the rule to rules/learned/ (status: learned), where it can be loaded via .fettle.toml. fettle rules promote refuses proposals with an empty pattern.

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.

  1. 4d ago First seen · 53 lines · 10 tokens per session scan A 7e294f2186b0

Subscribe to this mod's changes

fettle-learn is a command published in the GitHub repository MilindGaharwar/fettle (2 stars, last pushed today), licensed Apache-2.0. It adds 10 tokens to every session and 323 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 2026-08-31.