Borrowing it
Nothing to install: this file belongs to GregorBiswanger/featherspec. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/GregorBiswanger/featherspec/main/AGENTS.mdgit clone --depth 1 https://github.com/GregorBiswanger/featherspecWrote 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.
[](https://agentmods.dev/instructions/gregorbiswanger/featherspec/agents-md)<a href="https://agentmods.dev/instructions/gregorbiswanger/featherspec/agents-md"><img src="https://agentmods.dev/badge/instructions/gregorbiswanger/featherspec/agents-md/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/instructions/gregorbiswanger/featherspec/agents-md"><img src="https://agentmods.dev/badge/instructions/gregorbiswanger/featherspec/agents-md.svg" alt="Reviewed on agentmods" width="80" height="20"></a>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 5.1 | $0.02637 | $0.02637 |
| Opus 5.5 | $0.01055 | $0.01055 |
| Sonnet 5.5 | $0.00527 | $0.00527 |
| Haiku 4.5 | $0.00264 | $0.00264 |
Grade A, and why
featherspec AGENTS.md 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 29d 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.
How it starts
The opening of the file, as written. The whole thing — 201 lines — stays where its author put it; the contents beside it link to each section on GitHub.
FeatherSpec — Constitution (Spec-Driven Development)
You are the Spec-Driven Development (SDD) assistant for this repository. Work spec-first for anything that changes behaviour:
- Specify — clarify goals, constraints, and testable acceptance criteria.
- Plan — propose a small, verifiable plan before changing code.
- Act — implement with minimal diffs, verify, and update docs in the same change set.
Single source of truth (do not duplicate)
Everything mutable lives here in AGENTS.md only: DocLanguage, FeatherSpecVersion,
IssueTracker, the architecture: snapshot, and the Style & Output Preferences section.
Commands that update those write to this file. The thin loader file(s) may not hold a copy.
A command may restate a rule when it must be in front of the model at the moment it acts.
Such a restatement must say that it is one and name its source (AGENTS.md or the owning
.claude/rules/* file) as authoritative. Anything else is a copy, and copies drift.
Declared exceptions: README.md mirrors constitution content for humans; frontmatter
description/argument-hint lines mirror the command table; instructions-loader applyTo
globs mirror the rules' paths: globs. On divergence this file wins.
Repository Settings (managed by /sdd-setup and /sdd-featherspec-update)
DocLanguage: English # template default until /sdd-setup asks; governs docs and dialogue, wiring stays English.
FeatherSpecVersion: 1.9.0 # managed by /sdd-featherspec-update; do not edit by hand
IssueTracker: none # none | github | jira | <other> — set by /sdd-setup
Non-negotiables
Always — prefer small, testable steps over large refactors; keep changes consistent with
the architecture: snapshot below.
Ask first — name the action and wait for a yes: adding or upgrading a dependency · schema or data migrations · deleting or moving files you did not create in this task · any git write (commit, branch, reset, push) · running a command that reaches the network.
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.
- 29d ago Changed · +6 tokens per session 7ed7f91d4b01
- 1mo ago Changed · -2 lines · -11 tokens per session 910e554ffda9
- 1mo ago Changed · +1 lines · +33 tokens per session b0ad086becc0
- 1mo ago First seen · 202 lines · 2,609 tokens per session scan A f4e6bad64701
featherspec AGENTS.md is an instructions file published in the GitHub repository GregorBiswanger/featherspec (69 stars, last pushed 1mo ago), licensed MIT. It adds 2,637 tokens to every session, about $0.0105 per session on Opus 5.5. 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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