Before you change a function, you want to know what depends on it. vg impact answers exactly that: what breaks if you change this node? It computes a deterministic structural blast radius so you can plan a change with eyes open.
How it works
vg impact uses reverse reachability: starting from the node you are changing, it walks backward through the call graph to find everything that could be affected. It applies decay confidence so that nearer, more direct dependents are weighted more strongly than distant ones.
Because it is structural and deterministic, the same code and node produce the same blast radius every time — a reliable basis for planning.
Prerequisites
Build (or rebuild) the graph so impact reflects current code:
vg build
Run impact analysis
vg impact
The output lists the nodes reachable backward from your target — the dependents that may need attention if you change it.
How to use the result
- A small blast radius means the change is relatively contained.
- A large blast radius is a signal to slow down, split the change, or add tests.
- The confidence weighting helps you prioritize: focus first on the closest, highest-confidence dependents.
Surface the tests to run
Impact analysis pairs naturally with testing. Use the tests flag to surface exactly the tests you should run before shipping:
vg impact --tests
This turns a risk estimate into an action: run these tests, then ship with confidence.
Related
See the dedicated articles on test impact with --tests and on interpreting impact output. To trace exactly how a dependent connects back to your node, use vg path.