D227 - Diagnostics that intervene say so at the verdict

Status: decided Date: 2026-09-26

Each diagnostic declares an Effect: it observes or it intervenes. A run under an intervening diagnostic that reports progress prints a caveat at the verdict, and a diagnostic asked for but applied zero times is printed at the verdict as having measured nothing. A setting may never intervene. A guard is not trusted until it has been made to fail.

Why: the tooling can report success it did not establish exactly as a stub can. A moved fault under an intervention may mean the guest accepted a wrong answer. A mechanism whose "did nothing" output matches its "changed nothing" output is eventually read as an elimination. The caveat belongs where the conclusion is drawn.

Rejected:

• A caveat on every instrumented run: noise people learn to skip.
• Counts in the conditions line only: read straight past.
• Listing effects separately from the registry: two lists that drift.