codebase-comprehension

Codebase Comprehension Protocol

EXTREMELY_IMPORTANT: This is a MANDATORY protocol, not a suggestion. Follow every step. Do not skip steps. Do not combine steps. Do not summarize. Work through each gate in order.

The expensive bugs are not in the line you change — they are in the caller three files away that depended on the behavior you just changed. This protocol forces you to map the blast radius before you touch anything.

STEP 1 — Locate entry points and the change site

Identify how execution reaches the code you intend to change: who calls it, from where, and under what conditions. If you cannot say how the code is reached, you cannot predict what your change affects.

STEP 2 — Trace the data flow through the change site

Follow the actual values through the code you will touch: what comes in, what is read or mutated, what goes out, and in what units/shape/type. State it explicitly — do not assume the values are what the names suggest.

STEP 3 — Enumerate every consumer of what you will change

List everything that depends on the current behavior or shape: callers, subscribers, tests, serialized formats, anything downstream. This set IS the blast radius. "It's just a small change" is a claim about this list — so produce the list.

STEP 4 — Name at least one thing that could break elsewhere

From the blast radius, state a concrete failure the change could cause OUTSIDE the code you are editing — a specific caller that passes values differently, a consumer relying on the old shape. "Nothing else is affected" is allowed only after STEP 3 produced an empty consumer set and you say so.

STEP 5 — Report

COMPREHENSION: [code to change]
├── Entry points: [how execution reaches it]
├── Data flow: [in → transform → out, with shapes/units]
├── Blast radius: [every consumer of current behavior/shape]
├── At-risk elsewhere: [≥1 concrete off-site failure the change could cause]
└── Confidence: [HIGH / MEDIUM / LOW]

Confidence: HIGH — full call graph and data flow traced, blast radius complete. MEDIUM — main paths traced, some dynamic/indirect callers unverified. LOW — the code is reached by paths you cannot fully see; widen the trace before editing.

Do NOT propose an edit until: - The entry points reaching the change site are named - The data flow through the change site is traced with shapes/units, not assumed - Every consumer of the current behavior/shape is listed (the blast radius) - At least one concrete off-site risk is named — or the consumer set is verified empty

Proposing an edit to unfamiliar code with no blast-radius trace is a protocol violation, even if the change "obviously just does one thing".

Red flags that this skill catches:

• "It's a one-line change, it's safe" — one-line changes to shared code have the widest blast radius precisely because they look safe. List the callers.
• "The function name tells me what it does" — names lie, especially in old code. Trace the values.
• "I'll find the breakage when tests fail" — if the consumer has no test, the breakage ships. Find the consumers first.
• "Nothing else uses this" — a claim, not a fact, until STEP 3 is done. Do STEP 3.

Handoff

Once the blast radius is mapped, make the edit, then verify each at-risk consumer. If the trace reveals a failing behavior, hand off to diagnostic-reasoning; if it reveals a structural problem, to architecture-reasoning.