blast-radius

Blast radius

Establishing what a change can break, before it breaks it.

Why this exists

The changes that cause incidents are almost never the ones that looked dangerous. They're the one-line additions to a shared DTO, the extra field in a response, the "obviously safe" nullable column. They cause incidents because the danger wasn't in the change — it was in the twenty-three things that depended on the thing being changed, and nobody enumerated them.

An FDE is unusually exposed here. An engineer who has worked on a system for five years carries an approximate blast radius in their head and will flinch at the right moments. You have no such instinct, and you cannot acquire one in time. Enumeration is the substitute, and done properly it is better than instinct — it produces a list somebody else can check.

When this applies

• Before any change beyond a genuinely local one
• Modifying anything shared, published, or widely called
• Any schema change
• "What breaks if we touch this?"
• Feeding release-readiness — this skill is the consumer list, not the go/no-go

When it doesn't

• Genuinely local change: private method, one caller, well covered by tests
• Net-new code nothing calls yet
• You need to know how something works rather than what depends on it — that's trace-the-flow

Prerequisites

• .fde/02-system-map.md — integration surface and module boundaries
• .fde/04-feasibility.md — the change sites you're assessing
• A trace helps where the change is on a request path, but isn't required

Procedure

Work outward. Each ring is harder to see than the last, and the outer rings are where incidents come from.

Ring 1 — Direct callers, in this repository

# Count before reading — see context-strategy
grep -rl "<SymbolName>" --include="*.<ext>" . | wc -l
grep -rn "<SymbolName>" --include="*.<ext>" . | head -40

Where the count is large, that is itself the finding: a symbol with sixty callers is not a local change regardless of how small the diff is. Stop reading ring-1 hits after about forty. Record the count, sample the rest, and move out — rings 3–6 are where incidents come from.