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.
npx agentmods add skills/rvantonder/execute_code_py/code-only-envnpx skills add rvantonder/execute_code_py --skill code-only-envgit clone --depth 1 https://github.com/rvantonder/execute_code_pyWhat 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 | $0.00044 | $0.01707 |
| Opus 5 | $0.00022 | $0.00853 |
| Sonnet 5 | $0.00009 | $0.00341 |
| Haiku 4.5 | $0.00004 | $0.00171 |
Grade A, and why
code-only-env 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 2d 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 — 262 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Code-Only Environment Skill
Overview
You are operating in a code-only execution environment. This plugin restricts your capabilities to ensure you can ONLY execute Python code. All other tools have been blocked by PreToolUse hooks.
Your Only Tool
execute_code - Execute Python code and capture results
- What it does: Runs Python code in a local virtual environment (
.venv/) - Returns: Execution results, stdout, stderr, and any errors
- Result variable: Set
result = ...to return a value - Large outputs: Automatically saved to temp files (>5000 chars)
Example Usage
# Simple calculation
result = 2 + 2
# List comprehension
result = [i**2 for i in range(10)]
# With output
print("Processing...")
result = sum([1, 2, 3, 4, 5])
# Data processing
import json
data = {"name": "test", "value": 42}
result = json.dumps(data, indent=2)
What You CANNOT Do
The following tools are blocked and will be denied by the PreToolUse hook:
❌ Read - Cannot read files directly ❌ Write - Cannot write files directly ❌ Edit - Cannot edit files ❌ Bash - Cannot run shell commands ❌ Grep - Cannot search file contents ❌ Glob - Cannot search for files ❌ Task - Cannot spawn sub-agents ❌ WebFetch - Cannot fetch web content ❌ WebSearch - Cannot search the web ❌ All other tools - Completely blocked
What This Means
When a user asks you to:
- "Read the file config.json" → You must explain you cannot
- "Write a new file" → You must explain you cannot
- "Run git status" → You must explain you cannot
- "Search for functions" → You must explain you cannot
How to Work Effectively
1. Explain Your Constraints Clearly
When you cannot fulfill a request:
I'm operating in a code-only environment where I can ONLY execute Python code
via the execute_code tool. I cannot read files, write files, or run shell
commands. All other tools have been blocked.
However, I can help you by:
- Writing Python code that you can save and run
- Calculating results and processing data
- Explaining what code would be needed
- Providing code snippets for you to execute
What ships with it
3 files 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.
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.
- 2d ago First seen · 262 lines · 44 tokens per session scan A b4a59d6ec157
code-only-env is a skill published in the GitHub repository rvantonder/execute_code_py (58 stars, last pushed 7mo ago), licensed MIT. It adds 44 tokens to every session and 1,707 once invoked, about $0.0002 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 2026-08-30.
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