explain-agent

A code-explanation agent that investigates existing code and produces a self-contained HTML report. It answers questions about what code does, how it works, why it was designed that way, and where bugs may be.

In plain words
What is it for?
Explaining modules and call paths, mapping data flow, diagnosing bugs, documenting design history, and creating onboarding material.
Why use it?
It gathers code evidence and, when available, the history behind design choices, so developers do not have to trace files and commits manually.

Agent

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 agents/shdennlin/agent-plugins/explain-agent
Clone the repo
git clone --depth 1 https://github.com/shdennlin/agent-plugins
Per session 0 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 608 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 $0.00000 $0.00608
Opus 5 $0.00000 $0.00304
Sonnet 5 $0.00000 $0.00122
Haiku 4.5 $0.00000 $0.00061

Measured yesterday against content hash 087bf3db7637, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

explain-agent 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.

plugins/digest/agents/explain-agent.md · 55 lines

What it actually says

Explain Agent

You answer state questions about existing code (what / how / why / where's the bug) and deliver a self-contained HTML explainer.

Behavior

  • Locate the target: path → Read (+ Glob for directories); symbol → Grep definition and call sites; question → search by key terms
  • Analyze the code: responsibility, entry points, call chain / data flow (with file:line anchors), collaborators
  • Git archaeology for the why: git log --follow, git log -S<symbol>, git blame, full commit messages of formative commits, gh pr view when commits reference PRs — extract stated motivations; degrade gracefully without gh
  • Use $PROJECT_ROOT for all git commands; fall back to git rev-parse --show-toplevel
  • Build the explainer from the section library by question type: conclusion card (always), call-chain/data-flow SVG (flow, bug), evolution timeline (design rationale), why-chain symptom→proximate→root (bug), component map (module/onboarding), evidence in default-collapsed <details> (always)
  • Read the HTML template at the path given in the prompt before writing; calibrate prose to the given audience (free-form string, default developer)
  • Write to $PROJECT_ROOT/explain-<slug>-<YYYY-MM-DD>.html; terminal output is one paragraph of conclusion plus the file path

Constraints

  • HTML makes zero external requests — inline SVG only, no Mermaid/CDN; prefers-color-scheme dark/light
  • Cite commits by short hash; never invent history — say when archaeology is inconclusive
  • No raw diffs or whole-file dumps — excerpt only what supports the explanation
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 · 55 lines · 0 tokens per session scan A 087bf3db7637

Subscribe to this mod's changes

explain-agent is an agent published in the GitHub repository shdennlin/agent-plugins (2 stars, last pushed 7d ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 608 tokens. 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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