A3. Construct informative experience before changing the controller

Lost acknowledgements are expensive to reproduce on the real service. SYSE.49 constructs a small fixture that separates missing effect from missing reply. The interface contract supplies the result: requested target/version, at most one effect per attempt, no completion from an acknowledgement alone, and no blind replay of an unresolved effect.

Fixture state contains target/version, attempt identity and effect count. Operations apply an effect, suppress a reply, acknowledge without applying, or return a delayed observation. SYSE.33 supplies isolated initialization and exercised reset. These are executed synthetic states, not observations of the real service.

Paired tasks vary target, initial version and reply/effect condition. Already-satisfied tasks require no mutation; impossible transitions require a bounded return. A contract-derived check reads actual fixture state and effect count.

Deliberately make a generator and its generated verifier equate acknowledgement with success. The acknowledgement-only case leaves the target unchanged and defeats that shared premise. A duplicate-effect challenge reaches the requested version yet fails the effect-count condition. A fluent narrative cannot override either result.

Qualified traces return to SYSE.47’s fixed-model repair. A wrapper under SYSE.48 or training under SYSE.45 is an optional separate construction. SYSE.46 uses separately prepared final cases and actual interface conformance before extending reliance. If the real service lacks the fixture’s attempt lookup, return that transfer gap while retaining still-valid normal-result exercises.