Explain Back

Explain Back

The failure mode this fixes

The most expensive bug in AI-assisted development isn't wrong code — it's code that looks right that nobody understands. The agent writes 80 lines, the human skims, says "looks good," and merges. Six weeks later it breaks and no one — human or agent — can say why it existed.

explain-back inverts review. Before code is trusted, the agent must explain it back in plain language until the human confirms the model is correct. If the agent can't explain a line, that line is suspect. If the human can't follow the explanation, the code is too clever. Either way, you find out now, not at 3am.

When to Use This Skill

• Right after you (the agent) generate a non-trivial change, before declaring it done
• When reviewing code an agent wrote and the diff "looks fine" but no one built a model of it
• When onboarding to an unfamiliar file before changing it
• When the human catches themselves about to approve something they can't explain
• Before building new logic on top of existing code whose behavior is assumed, not known

Don't use when: the change is a trivial rename, a config value, or a one-line typo fix. Reserve it for logic that someone will later have to reason about.

Instructions

Work through these phases in order. Do not skip to a fix or a "looks good" — the whole point is to surface the gap between what the code says and what everyone thinks it says.

1. Restate the intent (before reading the code closely)

State, in one sentence, what this code is supposed to do — from the requirement, the PR title, or the conversation. Do not look at the implementation yet. This is the contract you'll check the code against.

"This function is supposed to deduplicate incoming webhook events so we never process the same payment twice."

2. Explain it back, line group by line group

Walk the code in small chunks. For each chunk, say in plain language:

• What it does (the mechanic)
• Why it's there (the intent it serves)
• What breaks if it's wrong or removed

Use no jargon the human hasn't used first. If you cannot explain why a chunk exists — only what it does — flag it explicitly as UNJUSTIFIED. Unjustified code is the #1 source of silent bugs and is the highest-value thing this skill finds. If an UNJUSTIFIED chunk turns out to have no callers or no observable effect, hand it to delete-this; if the explanation keeps stumbling over what something is called, route the fix to name-things.

Scale depth to risk, not line count. On a large diff, chunk at the function level and drop to line-level only where the assumptions concentrate — state changes, boundaries, error paths, anything touching money or auth. An explanation nobody reads defeats the skill exactly the way "looks good" does.

3. Name the assumptions

List every assumption the code depends on that is not enforced in the code itself:

• "Assumes event.id is always present" — is it?
• "Assumes this runs single-threaded" — does it?
• "Assumes the upstream already validated the amount" — verified where?

Each unenforced assumption is a kill-risk. Rank them.

4. Find the divergence

Compare the explanation (steps 2–3) against the intent (step 1). Call out exactly where they diverge:

• Code does more than intended (scope creep / hidden side effects)
• Code does less than intended (missing cases)
• Code does something different (the bug)

5. Confirm or correct the model

Present the explanation to the human and ask them to restate the intent in their own words — not a yes/no. "Is this right?" invites a lazy "yep"; "In one line, what did you expect this to do?" surfaces the mismatch.

• If their restatement matches your explanation → the code is now trusted; proceed.
• If it diverges → you have found a real defect or a real misunderstanding. Stop and resolve it before writing any fix.

Running unattended (no human in the loop)? Do not silently proceed. Emit the full explanation, the ranked ASSUMPTIONS, and any UNJUSTIFIED or DIVERGENCE findings as an explicit, logged block, and treat each unconfirmed high-risk assumption as a blocker to flag — not something to wave through. The point of the skill survives without a human: the gap still gets written down where the next reader will see it.

Do not propose a fix until the model is confirmed. A fix built on a wrong model is just a faster bug.

If a CONTEXT.md exists at the repo root, explain in its vocabulary — translate the code into the project's real domain nouns, not generic ones.

Output format

INTENT
  <one sentence: what this is supposed to do>

EXPLANATION
  [chunk 1] what / why / breaks-if-wrong
  [chunk 2] ...
  UNJUSTIFIED: <any code whose "why" you cannot state>

ASSUMPTIONS (ranked by risk)
  1. <assumption> — enforced? <yes/no/where>
  2. ...

DIVERGENCE FROM INTENT
  <does more / does less / does different — be specific with line refs>

CONFIRM
  "In one line, what did you expect this to do?"  (restate, don't yes/no)

Anti-Patterns

Mental Model

Code you can't explain back is code you don't own yet. Explain it back until you do — before you trust it, not after it breaks.