The Stacks project

Lemma 4.42.5. Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $F : \mathcal{X} \to \mathcal{Y}$ be a $1$-morphism. Make a choice of pullbacks for $\mathcal{Y}$. Assume

1. each functor $F_ U : \mathcal{X}_ U \longrightarrow \mathcal{Y}_ U$ between fibre categories is faithful, and

2. for each $U$ and each $y \in \mathcal{Y}_ U$ the presheaf

   \[ (f : V \to U) \longmapsto \{ (x, \phi ) \mid x \in \mathcal{X}_ V, \phi : f^*y \to F(x)\} /\cong \]

   is a representable presheaf on $\mathcal{C}/U$.

Then $F$ is representable.