Something changed that nobody meant to change

Point this at it

Which sensor depends on what moved. For a published interface, an OpenAPI spec, a gRPC service, a JVM library, use API compatibility: run openapi-diff on the old and new spec in CI, or revapi or MiMa on the old and new jar, and fail the build on any line classified BREAKING. That takes seconds, needs no deployment, and has to know nothing about who is calling.

For rendered output, HTML, a JSON body, a generated report, use snapshot tests: record one snapshot of the output the last surprise came from, with Jest or Vitest, commit it, and read the diff every time it fails before touching the update flag.

Reading it

• Red from compatibility: a removed endpoint or a changed type, named. It tells you what you did, and cannot tell you what it costs; go find out who is on the other end before deciding. Red from a snapshot: a diff. Decide intended or accidental, and if forty files moved, say the shared cause in one sentence or approve nothing.
• Green: the interface shape held, or the output matched the recording.
• It does not prove the meaning held. A flipped default or a field that now means something else passes the compatibility check, and a surface no spec describes produces no verdict at all. A snapshot cannot tell you the original recording was right.

If the doubt survives

If the shape held and the output matched and something downstream still broke, the pieces changed how they behave together rather than what they expose. That is emergent failure, and the next play covers it.