systematic-debugging

A method for finding the real cause of a software bug through a repeatable investigation. It reproduces the failure, observes what happens, tests one explanation at a time, and narrows the search.

In plain words
What is it for?
Use it when a bug is not obvious, especially for flaky failures, incorrect data, cache problems, or issues crossing multiple layers of an application.
Why use it?
It prevents random code changes based on guesses and makes sure the fix explains the exact failure that was observed.

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/jnmetacode/skillet/systematic-debugging
Any agent
npx skills add jnMetaCode/skillet --skill systematic-debugging
Clone the repo
git clone --depth 1 https://github.com/jnMetaCode/skillet

Made for: Claude Code, Codex.

Per session 43 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 532 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. 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 $0.00043 $0.00532
Opus 5 $0.00022 $0.00266
Sonnet 5 $0.00009 $0.00106
Haiku 4.5 $0.00004 $0.00053

Measured yesterday against content hash 8d9f567283a4, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

systematic-debugging scanned grade A with 1 finding 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 yesterday.

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.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

line, an assertion, a debugger breakpoint, one curl.
skills/systematic-debugging/SKILL.md · 46 lines

What it actually says

systematic-debugging

Never fix what you can't reproduce; never explain what you haven't observed. The loop is: reproduce → observe → hypothesize → test the hypothesis → narrow. One variable at a time.

Procedure

  1. Reproduce first. Find the smallest, fastest command that shows the failure deterministically. If it's flaky, make the loop tight (while ./repro; do :; done) and treat flakiness itself as a clue (timing, ordering, shared state).
  2. Read the actual error. The full message, the first stack frame in your own code, and the line right before things went wrong. Resist pattern-matching to a familiar failure — verify this one.
  3. State a falsifiable hypothesis ("the cache returns stale entries after a restart") and pick the cheapest observation that could kill it: a log line, an assertion, a debugger breakpoint, one curl.
  4. Bisect the space, whichever axis is cheapest:
    • history: git bisect run ./repro
    • data: half the failing input, recurse
    • stack: confirm the bad value at the boundary between two layers, then descend into the guilty one only
  5. The fix must explain the symptom. Before writing it, say why this cause produces exactly this behavior. If the explanation is fuzzy, you've found a bug, maybe not the bug.
  6. Prove it: the repro from step 1 now passes, AND a new regression test fails without the fix. Then look for siblings — the same mistake usually exists elsewhere (grep for the pattern you just fixed).

Anti-patterns

  • Changing two things between observations — you learn nothing from the result.
  • "It works now" without knowing why it failed — it will be back.
  • Adding sleeps to fix timing issues — that's hiding the race, not fixing it.
  • Debugging through the framework before confirming your own code's inputs and outputs at the boundary.
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. yesterday First seen · 46 lines · 43 tokens per session scan A 8d9f567283a4

Subscribe to this mod's changes

systematic-debugging is a skill published in the GitHub repository jnMetaCode/skillet (1 stars, last pushed 2mo ago), licensed MIT. It adds 43 tokens to every session and 532 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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