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 skills add saski/arnesto --skill modern-cli-designgit clone --depth 1 https://github.com/saski/arnestoWrote 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/skills/saski/arnesto/modern-cli-design)<a href="https://agentmods.dev/skills/saski/arnesto/modern-cli-design"><img src="https://agentmods.dev/badge/skills/saski/arnesto/modern-cli-design/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/skills/saski/arnesto/modern-cli-design"><img src="https://agentmods.dev/badge/skills/saski/arnesto/modern-cli-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector warn
SkillSpector: 2 findings, up to medium
These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →
- medium Excessive Agency · line 125 Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.Fix: Add human-in-the-loop confirmation for destructive, irreversible, or high-impact operations. Never auto-execute commands that modify files, send data, or alter system state.
- medium Rogue Agent · line 171 Skill establishes unauthorized persistence across sessions via cron jobs, startup scripts, or state files. Session persistence allows an attacker to maintain access beyond the current interaction.Fix: Remove any persistence mechanisms (cron jobs, startup scripts, state files). Skills should not maintain state across sessions without explicit user consent.
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 | $0.00051 | $0.03258 |
| Opus 5.5 | $0.00020 | $0.01303 |
| Sonnet 5.5 | $0.00010 | $0.00652 |
| Haiku 4.5 | $0.00005 | $0.00326 |
Grade A, and why
modern-cli-design 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 — 278 lines — stays where its author put it; the contents beside it link to each section on GitHub.
STARTER_CHARACTER = 🔧⚡
Modern CLI Design
Specialized concerns (long-running operations, credentials, testing) live in references/ — see the loading guidance at the end of this file.
Core principle: stdout is for data, stderr is for everything else
stdout carries the product the user asked for — results, JSON, records. stderr carries everything else — progress, spinners, warnings, errors, debug, prompts.
A single spinner character on stdout breaks every downstream pipe. mycli list | jq . must never see a progress indicator on stdout.
Buffering matters. stdout is line-buffered on a TTY and block-buffered when piped (~2x faster than stderr due to fewer write syscalls). stderr is unbuffered — every write is a syscall, but error messages appear immediately even on crash.
Check TTY on stdout and stderr independently. A user running mycli run 2>/dev/tty | jq . has stdout piped and stderr on a terminal — spinners on stderr are correct in that case.
In Go, write data with os.Stdout.Write / fmt.Fprintln(os.Stdout, ...) and diagnostics with fmt.Fprintln(os.Stderr, ...). Use github.com/mattn/go-isatty to detect TTY per file descriptor.
Format flag hierarchy
Four output modes with different contracts:
- Default (human): colors, tables, formatted text, progress on stderr. Not a contract — may change between versions.
--plain: one record per line, no colors, flat rows with no borders. Stable across minor versions. Enablesmycli list --plain | grep error | wc -l.--json: structured JSON on stdout. stdout contains ONLY valid JSON — zero spinners, zero colors, zero progress. Schema is versioned.--jsonimplies non-interactive regardless of TTY.--format ndjson: newline-delimited JSON for streaming. Each line is one parseable object. Include atypefield per record to multiplex events. Final line may be a summary record.
JSON envelope is consistent across every command:
{"ok": true, "data": {...}}
{"ok": false, "error": {"code": "...", "message": "...", "fix": "...", "transient": false}}
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.
- 1mo ago First seen · 278 lines · 51 tokens per session scan A affe9a31f803
modern-cli-design is a skill published in the GitHub repository saski/arnesto (6 stars, last pushed 4d ago), licensed Unlicense. It adds 51 tokens to every session and 3,258 once invoked, about $0.0002 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 2026-09-03.
Other skills, from other repositories
mockery-suite-tests
Use when writing Go tests that need test doubles or cover multiple cases - detect the repo's mocking approach (Mockery v3, gomock, hand-written testify mocks, fakes), table-driven cases, testify suites when the repo uses them, and real-Postgres tests for repository adapters.
fiber-rest-api
Use when adding or changing a Go HTTP endpoint — Fiber v2 or v3 (detect which), thin handlers, central error mapping, boundary validation; the same rules apply to net/http, chi, echo or gin.
go-hexagonal-architecture
Use when adding a use case, port, repository or adapter in a Go service — where each piece goes, how errors and transactions cross layers, and how to check the import direction.
go-security-pitfalls
Use when reviewing Go changes - net/http and Fiber handlers, database/sql and GORM queries, os/exec, filepath and os.Root, html/template, crypto, TLS and SSH clients, JSON binding.
sql-query-performance
Use when writing or changing a query against PostgreSQL from Go — avoiding N+1 loops, reading EXPLAIN output, indexing, and keyset pagination.
zerolog-observability
Use when adding logging to Go code - the repository's logger with structured fields, request-scoped correlation, correct levels, no secrets or double-logging.