# Scripts: Deterministic Work Inside a Skill — Claude Code Skills & SKILL.md

Source: https://www.geekswithgeeks.com/en/claude-code-skills/f-scripts

> Put exact, repeatable logic in code and let Claude call it.

## Let code do what code is good at

Models are excellent at judgement and language, and unreliable at exact arithmetic, parsing and repetitive checks. A skill can bundle **scripts** (Python, Node, shell) that do such steps deterministically: validate a format, call an API, parse a log, compute a number, take a screenshot. Claude **runs** the script and reads its **output**; the script's source never needs to enter the context. Design scripts for an agent reader: accept **clear arguments**, print **concise, structured output** (JSON or short labelled lines), use **meaningful exit codes**, write **helpful error messages** that say how to fix the problem, avoid interactive prompts, and be **idempotent** when possible. List the exact command in SKILL.md, including the path relative to the skill folder, and say what the output means.

## A script a skill could call, run

I ran this with plain Python 3 (standard library only). The function checks a commit message against a team format and returns structured JSON with a list of problems. Valid messages return ok true; a missing type and a capitalised summary each produce a specific problem. A skill would tell Claude to run this instead of eyeballing the format.

```python
import json, re, sys

def check_commit_message(msg):
    """Deterministic helper a skill can call instead of asking the model to eyeball the format."""
    first = msg.splitlines()[0] if msg.strip() else ""
    problems = []
    m = re.match(r"^(feat|fix|docs|refactor|test|chore)(\([a-z0-9-]+\))?: (.+)$", first)
    if not m: problems.append("first line must look like 'type(scope): summary'")
    elif len(first) > 72: problems.append(f"first line is {len(first)} characters (max 72)")
    elif m.group(3)[0].isupper(): problems.append("summary should start lowercase")
    return {"ok": not problems, "problems": problems}

for msg in ("fix(cart): round discount to paise", "Fixed the bug", "feat: Add pagination"):
    print(json.dumps(check_commit_message(msg)))

```

Output:

```
{"ok": true, "problems": []}
{"ok": false, "problems": ["first line must look like 'type(scope): summary'"]}
{"ok": false, "problems": ["summary should start lowercase"]}
```

## Return errors Claude can act on

"Missing --repo: pass the repository as owner/name" helps; a bare stack trace does not. Print the fix, not just the failure.

**Quiz:** What does a skill's script give you that free-form model output does not?

- [x] Deterministic, repeatable results and context-free logic
- [ ] Better creative writing
- [ ] A larger context window
- [ ] Automatic approval of all actions

*Answer:* Deterministic, repeatable results and context-free logic. Code is predictable and its source does not consume context when run.