diagnosing-bugs

A structured method for finding hard bugs and performance slowdowns by reproducing, narrowing down, testing theories, measuring behavior, fixing the cause, and adding a regression test.

In plain words
What is it for?
Use it to investigate broken tests, errors, unexpected behavior, or performance regressions in a codebase.
Why use it?
It replaces guesswork with a repeatable process and a clear pass-or-fail signal.

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/stainless-code/codemap/diagnosing-bugs
Any agent
npx skills add stainless-code/codemap --skill diagnosing-bugs
Clone the repo
git clone --depth 1 https://github.com/stainless-code/codemap

Made for: Claude Code, Codex.

Per session 68 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,457 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 $0.00068 $0.02457
Opus 5 $0.00034 $0.01229
Sonnet 5 $0.00014 $0.00491
Haiku 4.5 $0.00007 $0.00246

Measured 2d ago against content hash a70800faa2c0, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

diagnosing-bugs 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.

The scan reads SKILL.md. This mod also ships 1 executable file (scripts/hitl-loop.template.sh), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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.

.agents/skills/diagnosing-bugs/SKILL.md · 142 lines

How it starts

The opening of the file, as written. The whole thing — 142 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Diagnosing bugs

A discipline for hard bugs. Skip phases only when explicitly justified.

When exploring the codebase, query codemap (the structural SQLite index) before reaching for Grep or Read per the codemap rule — symbol-shaped questions ("where is X defined?", "what calls X?") have direct answers in the symbols / calls tables. Read the relevant section of docs/architecture.md to ground the mental model of layering, and check docs/glossary.md for canonical domain terms (file types, recipe ids, schema columns).

Phase 1 — Build a feedback loop

This is the skill. Everything else is mechanical. If you have a fast, deterministic, agent-runnable pass/fail signal for the bug, you will find the cause — bisection, hypothesis-testing, and instrumentation all just consume that signal. If you don't have one, no amount of staring at code will save you.

Spend disproportionate effort here. Be aggressive. Be creative. Refuse to give up.

Ways to construct one — try them in roughly this order

  1. Failing test at whatever seam reaches the bug — unit, integration, e2e. Codemap convention: src/**/<name>.test.ts for unit + integration; fixtures/golden/ for query-shape regressions; bun test <file> runs them.
  2. CLI invocation with a fixture input, diffing stdout against a known-good snapshot. Examples: bun src/index.ts query --json … against fixtures/minimal/, golden runner under scripts/query-golden.ts.
  3. Replay a captured trace. Save a real .codemap/index.db / config / fixture file to disk; replay it through the code path in isolation.
  4. Throwaway harness. Spin up a minimal subset (one parser, one DB connection) that exercises the bug code path with a single function call.
  5. Property / fuzz loop. If the bug is "sometimes wrong output", run 1000 random inputs and look for the failure mode.
  6. Bisection harness. If the bug appeared between two known states (commit, dataset, version), automate "boot at state X, check, repeat" so you can git bisect run it.
  7. Differential loop. Run the same input through old-version vs new-version (or two configs) and diff outputs. The B.6 baseline machinery (codemap query --save-baseline / --baseline) is built for exactly this — use it.
  8. HITL bash script. Last resort. If a human must click or copy a value out of the IDE, drive them with scripts/hitl-loop.template.sh so the loop is still structured. Captured output feeds back to you.

Read the full file on GitHub · 142 lines

Files

What ships with it

1 file 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.

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 · 142 lines · 68 tokens per session scan A a70800faa2c0

Subscribe to this mod's changes

diagnosing-bugs is a skill published in the GitHub repository stainless-code/codemap (8 stars, last pushed 7d ago), licensed MIT. It adds 68 tokens to every session and 2,457 once invoked, about $0.0003 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.

Related

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