When phage restriction is not immunity: Immune gates, replication-niche barriers, and assay-limited phenotypes.

Antiphage discovery increasingly yields restrictive phenotypes before mechanisms are known. This Perspective argues that reduced plaque formation or low efficiency of plating should be classified phenotype-first, not automatically as immunity. I distinguish immune-gated defenses, replication-niche barriers, and assay-limited phenotypes, and propose a decision-tree workflow using native context, matched physiology, infection-stage readouts, triggers, and phage-mediated rescue. Clearer labels should broadly improve annotation, counter-defense interpretation, and phage-host engineering for therapy, biocontrol, and biotechnology in complex microbial settings.