debug-systematically

A step-by-step guide for finding the actual cause of a software bug. It emphasizes reproducing the failure, testing one hypothesis at a time, and explaining the mechanism of the fix.

In plain words
What is it for?
Use it to reproduce failures, inspect exact errors, test focused hypotheses, add temporary diagnostics, narrow the search with a bisect, and verify the final fix.
Why use it?
It prevents speculative fixes that only hide symptoms or create new problems. The original failing command is run again to confirm the result.

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/zfinix/aster/debug-systematically
Any agent
npx skills add Zfinix/aster --skill debug-systematically
Clone the repo
git clone --depth 1 https://github.com/Zfinix/aster

Made for: Claude Code, Codex.

Per session 46 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 334 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 $0.00046 $0.00334
Opus 5 $0.00023 $0.00167
Sonnet 5 $0.00009 $0.00067
Haiku 4.5 $0.00005 $0.00033

Measured 2d ago against content hash f7bf6b576242, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

debug-systematically 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.

crates/aster-skills/optional-skills/debug-systematically/SKILL.md · 26 lines

What it actually says

Debug systematically

  1. Reproduce before touching anything. Find the exact command or input that shows the failure and run it. A bug you cannot reproduce is a bug you cannot claim to have fixed.
  2. Read the actual error, not the vibe of it. The message, the file, the line. Half of debugging is refusing to skim.
  3. One hypothesis at a time. State it ("the config is read before the env is set"), pick the cheapest observation that would falsify it, run that. Never stack three speculative fixes in one edit.
  4. Instrument close to the failure. A print or assert at the suspect line beats re-reading the whole module. Remove the instrumentation after.
  5. Bisect when lost. Halve the search space: does the failure survive with half the input, the previous commit, the feature flag off? git bisect exists for exactly this.
  6. When the fix lands, explain the mechanism in one sentence. "The cache key ignored the locale, so the first locale won." If you cannot say it, you patched a symptom.
  7. Re-run the original reproduction last. The fix is proven by the same command that showed the bug, plus the neighboring tests.
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. 2d ago First seen · 26 lines · 46 tokens per session scan A f7bf6b576242

Subscribe to this mod's changes

debug-systematically is a skill published in the GitHub repository Zfinix/aster (40 stars, last pushed 7d ago), licensed Apache-2.0. It adds 46 tokens to every session and 334 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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