# Form a SINGLE hypothesis, test it MINIMALLY — do not bundle multiple fix attempts into one change

## The rule

When debugging, the agent forms exactly one hypothesis at a time ("I think X is the root cause because Y"), states it explicitly, and tests it with the smallest possible change. The agent does not change multiple variables at once; the agent does not "fix several obvious problems" while in the debugging flow; the agent does not "while I'm here" improve unrelated code.

Per `skills/systematic-debugging/SKILL.md:145-163` (Phase 3: Hypothesis and Testing):

> **Scientific method:**
>
> 1. **Form Single Hypothesis**
>    - State clearly: "I think X is the root cause because Y"
>    - Write it down
>    - Be specific, not vague
>
> 2. **Test Minimally**
>    - Make the SMALLEST possible change to test hypothesis
>    - One variable at a time
>    - Don't fix multiple things at once
>
> 3. **Verify Before Continuing**
>    - Did it work? Yes → Phase 4
>    - Didn't work? Form NEW hypothesis
>    - DON'T add more fixes on top

## Why

Bundled fixes are unauditable. If the test passes, the agent doesn't know WHICH of the bundled changes was responsible; if the test fails, the agent doesn't know which to revert. The next fix attempt becomes a search across multiple variables, which is exponentially slower than testing one at a time.

Stating the hypothesis explicitly is a discipline against "I just tried a bunch of things." If the agent cannot state the hypothesis in one sentence, the hypothesis is too vague to test — refine first, then test.

## What this means in practice

- Before each fix attempt, the agent writes: "I think [specific X] is the root cause because [specific Y]; I will test by [single change Z]."
- The change is single-line or single-parameter; not multi-file.
- If the change does not resolve the bug, the agent REVERTS the change (or notes it explicitly) and forms a new hypothesis. The revert is part of the protocol, not an option.
- "While I'm here" improvements are forbidden inside a debugging flow. The agent returns to the improvement in a separate commit after the bug is resolved.
- After three failed hypotheses (not three fix attempts in one commit; three SEPARATE hypotheses), the architecture question is reached (see `three_fix_failures_question_architecture`).
- The agent does not batch commits inside a debugging flow to "save time." Each hypothesis gets its own commit (atomic per hypothesis), with the test commit prepended when possible.