systematic-debugging

A four-phase method for diagnosing unexpected errors during software development. It requires observing the failure, forming a testable explanation, testing it, and then applying a fix.

In plain words
What is it for?
Use it when tests fail unexpectedly, an error is unclear, or repeated fixes have not solved the problem.
Why use it?
It reduces trial-and-error debugging by requiring evidence before changes are made.

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/aegntic/compound-engineering/systematic-debugging
Any agent
npx skills add aegntic/compound-engineering --skill systematic-debugging
Clone the repo
git clone --depth 1 https://github.com/aegntic/compound-engineering

Made for: Claude Code, Codex.

Per session 25 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,340 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 $0.00025 $0.01340
Opus 5 $0.00013 $0.00670
Sonnet 5 $0.00005 $0.00268
Haiku 4.5 $0.00003 $0.00134

Measured yesterday against content hash ca3110e4660a, 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 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 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.

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.

skills/systematic-debugging/SKILL.md · 141 lines

How it starts

The opening of the file, as written. The whole thing — 141 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Systematic Debugging

A structured methodology for debugging unexpected errors encountered during implementation. Prevents trial-and-error guessing by enforcing a disciplined observe-hypothesize-test-fix cycle.

Announce at start: "I'm using the systematic-debugging skill to diagnose this issue."

When to Use

  • A test fails unexpectedly during implementation
  • An error occurs that you did not anticipate
  • A subagent has retried 2+ times without success
  • Something "should work" but does not
  • You are tempted to make random changes hoping something fixes it

When NOT to Use

  • The error message is clear and the fix is obvious (just fix it)
  • You are validating a reported bug (use the bug-reproduction-validator agent instead)
  • The failure is a known issue documented in docs/solutions/

The 4-Phase Process

Phase 1: Observe

Gather evidence before forming any theories. Do not skip this phase.

  1. Reproduce the error -- Run the exact command that failed. Confirm you see the same error.
  2. Read the full error -- Read the COMPLETE error message, stack trace, and any surrounding output. Do not skim.
  3. Check the context -- What changed recently? What was the last thing that worked? What is different now?
  4. Gather state -- Check relevant logs, database state, environment variables, configuration files, or any other state that could be relevant.
  5. Note the symptoms -- Write down exactly what you observe:
    • What happens (the error)
    • What should happen (expected behavior)
    • When it happens (always, sometimes, under specific conditions)

Output of Phase 1: A clear, factual description of the symptoms with no theories attached.

Phase 2: Hypothesize

Form structured theories about the root cause. Do not jump to fixing yet.

  1. List 3 possible root causes ranked by likelihood:
    • Most likely: [hypothesis based on error message and context]
    • Possible: [alternative explanation]
    • Less likely but worth checking: [edge case or environmental cause]

Read the full file on GitHub · 141 lines

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 · 141 lines · 25 tokens per session scan A ca3110e4660a

Subscribe to this mod's changes

systematic-debugging is a skill published in the GitHub repository aegntic/compound-engineering (2 stars, last pushed 2mo ago), licensed MIT. It adds 25 tokens to every session and 1,340 once invoked, about $0.0001 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 2026-08-31.

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