read-before-edit

A code-review habit for unfamiliar files and functions that makes the agent read the surrounding code before editing.

In plain words
What is it for?
It helps with modifying existing functions, matching local naming and error-handling patterns, checking callers, and reusing helpers already in the codebase.
Why use it?
It prevents changes based on guesses about what code does, which can break nearby logic or existing callers.

Skill for Claude CodeCodex

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/buildmoonshot/skillpacks/read-before-edit
Any agent
npx skills add buildmoonshot/skillpacks --skill read-before-edit
Clone the repo
git clone --depth 1 https://github.com/buildmoonshot/skillpacks

Made for: Claude Code, Codex.

Per session 55 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 358 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 $0.00055 $0.00358
Opus 5 $0.00028 $0.00179
Sonnet 5 $0.00011 $0.00072
Haiku 4.5 $0.00006 $0.00036

Measured 3d ago against content hash 586e3880fd95, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

read-before-edit 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 3d 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.

skills/engineering/beginner/read-before-edit/SKILL.md · 27 lines

What it actually says

Read Before You Edit

Before changing existing code, read enough of it to understand what you're touching. Editing blind is how agents break working software.

What to do

  1. Read the function/section you're changing — and the code immediately around it. Understand what it does and what depends on it before you touch a line.

  2. Find the existing patterns. How does this file name things, handle errors, structure logic? Match that, don't impose your own style.

  3. Check who calls it. A quick look at where a function is used tells you what you might break. Changing a signature or return value without checking callers is a classic self-inflicted bug.

  4. Reuse what's there. If the codebase already has a helper, a utility, or a pattern for what you need, use it instead of writing a parallel one. New code should look like it belongs.

The trap to avoid

Don't pattern-match on the name of a function and assume it does what you'd expect. Open it and confirm. The function called validateUser might also mutate state, log, or throw — assumptions about unread code are where bugs hide.

Why this matters

Most agent-introduced bugs aren't bad logic — they're good logic that didn't fit the existing code, broke an unseen caller, or duplicated something already there. Two minutes of reading prevents the kind of bug that takes an hour to trace.

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

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. 3d ago First seen · 27 lines · 55 tokens per session scan A 586e3880fd95

Subscribe to this mod's changes

read-before-edit is a skill published in the GitHub repository buildmoonshot/skillpacks (2 stars, last pushed 2mo ago), licensed MIT. It adds 55 tokens to every session and 358 once invoked, about $0.0003 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.

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