  Although the architectures of compact multiple-planet systems are
well-characterized, there has been little examination of their "outer edges",
or the locations of their outermost planets. Here we present evidence that the
observed high-multiplicity Kepler systems truncate at smaller orbital periods
than can be explained by geometric and detection biases alone. To show this, we
considered the existence of hypothetical planets orbiting beyond the observed
transiting planets with properties dictated by the "peas-in-a-pod" patterns of
intra-system radius and period ratio uniformity. We evaluated the detectability
of these hypothetical planets using (1) a novel approach for estimating the
mutual inclination dispersion of multi-transiting systems based on transit
chord length ratios and (2) a model of transit probability and detection
efficiency that accounts for the impacts of planet multiplicity on
completeness. Under the assumption that the "peas-in-a-pod" patterns continue
to larger orbital separations than observed, we find that $\gtrsim35\%$ of
Kepler compact multis should possess additional detected planets beyond the
known planets, constituting a $\sim7\sigma$ discrepancy with the lack of such
detections. These results indicate that the outer ($\sim100-300$ days) regions
of compact multis experience a truncation (i.e. an "edge-of-the-multis") or a
significant breakdown of the "peas-in-a-pod" patterns, in the form of
systematically smaller radii or larger period ratios. We outline future
observations that can distinguish these possibilities, and we discuss
implications for planet formation theories.
