Retention Rights Determine What Budget-Matched Agent-Memory Comparisons Establish

Valid controller attribution requires equal retention rights, not only equal memory and compute budgets. A controller that can pin rare constraints or replay-validate promotions acts over a larger feasible action set than one limited to eviction and ranking. We introduce capability alignment, a paired intervention that imposes a common retention-right profile before attribution, and full-versus-aligned estimands that quantify the resulting attribution shift. RetainScope, a validated consolidation controller for mutable tool traces, makes the protocol executable. In five-seed, identity-paired audits using Llama-3.1-70B-Instruct and a 4.0 GB cap, the RetainScope–MemRouter gap on LongToolBench v2.1 contracts from $+1.3$ to $+0.2$ pp (51.2 vs. 51.0), an $84.6$ shrinkage with direct 95% CI [0.3, 1.9] and $p=0.012$. On LOCOMO v2, $+2.8$ pp contracts to $+0.3$ pp (47.6 $\pm$ 1.9 vs. 47.3 $\pm$ 1.8), an 89.3% shrinkage with direct 95% CI [1.2, 3.8] and $p=0.006$. Both aligned residual intervals overlap zero, while paging/self-edit and frequency-based comparators yield the same attribution decision and gaps contract in 64 of 72 profile-grid pairs. RetainScope also supplies a strong systems operating point: 51.9% LongToolBench success, 52% fewer regressions than Store-all, 2.6 GB median end-to-end memory, and 1.63 s p95 retrieval latency. These results establish retention-right parity as the comparison control for selector attribution and full-system rows as complementary evidence for deployment choice.