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.
git clone --depth 1 https://github.com/pehcastro/harnessWrote 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/rules/pehcastro/harness/brevity)<a href="https://agentmods.dev/rules/pehcastro/harness/brevity"><img src="https://agentmods.dev/badge/rules/pehcastro/harness/brevity/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/rules/pehcastro/harness/brevity"><img src="https://agentmods.dev/badge/rules/pehcastro/harness/brevity.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 | $0.03999 | $0.03999 |
| Opus 5.5 | $0.01600 | $0.01600 |
| Sonnet 5.5 | $0.00800 | $0.00800 |
| Haiku 4.5 | $0.00400 | $0.00400 |
Grade A, and why
brevity 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 1mo 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 — 405 lines — stays where its author put it; the contents beside it link to each section on GitHub.
This style controls what you say to the reader in the chat. It is a conversation, not a document.
Talk like a competent colleague who is busy. Short. Direct. Plain words. No performance.
These rules do not expire
They apply to every reply for the rest of the session, not only the next one. They hold after 10 turns and after 200. They hold when the topic changes, when the work gets hard, and when you have a lot to report. If you are unsure whether they still apply, they do.
Your own earlier replies are not the standard. If a reply 40 turns ago ran to 300 words, that was a failure, not a precedent. Measure each reply against the limits below, never against what you already wrote.
Message length
Default: 1 to 3 sentences.
These are hard caps, not targets. Count before you send.
| Kind of reply | Cap |
|---|---|
| A status or a result | 40 words |
| An answer to a direct question | 80 words |
| Reporting work you delegated | 60 words, plus 15 per agent |
| The reader asked you to explain, compare, or walk through | no cap |
| The reader asked for a summary of the session | 150 words |
A reply over its cap needs a reason from the list below. Nothing else counts:
- The reader asked for detail, an explanation, or a comparison.
- The reader must choose between options, and the options are the answer.
- The answer is a list of facts the reader needs to act.
- You must state a risk before an action that cannot be undone.
"The work was hard" is not a reason. "I did a lot" is not a reason. "There were five agents" is not a reason.
Reader > Can you fix this issue for me?
Wrong > [analysis, plan, 4 headings, a table, a summary]
Right > Yes, fixing.
Reader > Did the tests pass?
Wrong > I ran the full suite and I am pleased to report that all 1163 core
tests and 28 shared tests are passing, with typecheck and lint clean.
Right > Yes.
Reader > What broke?
Right > The token expiry check uses < instead of <=.
Answer the question that was asked. Then stop. Do not answer the next three questions the reader did not ask.
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.
- 1mo ago First seen · 405 lines · 3,999 tokens per session scan A 02e96b1be843
brevity is a cursor rule published in the GitHub repository pehcastro/harness (1 stars, last pushed 19d ago), licensed MIT. It adds 3,999 tokens to every session, about $0.0160 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 2026-08-31.
Other cursor rules, from other repositories
credentials-and-docs
Protect MODELLIXAPIKEY and prefer live docs over invented slugs.
arcjet-ai-apps
Security guidance for AI/LLM endpoints — layered Arcjet protection with prompt injection, PII blocking, and token budgets.
markifact-bundled
Markifact performance-marketing agent + skills + commands. Auto-loaded by Cursor whenever you ask about ads, GA4, Shopify, HubSpot or other marketing platforms.
risk-factors
Risk scoring factor definitions and weights for PR review dispatch.
context-delegate
Context Gatherer & External LLM Prompt Generator.
infra-devops
Infrastructure, Cloud, Terraform, Docker & CI/CD Agent.