Serial verification gates are a core reliability primitive in LLM harnesses: a candidate answer is returned only if
k verifier calls all accept it. Under conditionally independent gates, the recent Odds Law (arXiv:2606.15712) shows that posterior log-odds grow linearly in
k, so failure decays exponentially, and states that "a tight theory of partially correlated verifier cascades remains open." This note gives a minimal such theory. Modeling the per-instance false-accept rate on the generator's own errors as a latent variable
α∼G (de Finetti), the exact cascade posterior is
ℓk=ℓ0−lnmk, with
mk the
k-th moment of
G. Then: (i)
ℓk is concave in
k for every non-degenerate
G -- the Odds Law is its tangent at the first gate and an upper bound; (ii) for Beta
(a,b) latents, failure decays polynomially,
1−rk≍k−b, with correlation parameter
ρv=1/(a+b+1); (iii) a blind-spot atom of mass
1−π at
α=1 caps the evidence extractable from any number of gates at
−ln(1−π) nats, so reliability saturates below 1; (iv) letting the true-accept rate also vary (
β∼H) yields a trichotomy -- gates eventually always help, plateau, or actively harm -- decided by the upper-tail exponents of
G and
H, with closed-form crossover
k†. The mechanism is survivorship: errors surviving gates are the high-
α ones. The theory is measurable:
R repeated verdicts per instance identify the first
R moments of
G, so two verdicts identify
ρv; beta-binomial likelihood and NPMLE recover the reliability curve and the ill-posed ceiling. In synthetic tests, independence-based extrapolation underestimates failure by 20x at
k=5 and ~3000x at
k=10; the correlated fit at
R=8 tracks held-out depths. The practical lever is decorrelation -- changing model family, modality, or evidence source -- not adding gates.