Scaling Conditional Learned Retrieval for Pinterest Home Feed

This post details the evolution of Pinterest's Conditional Learned Retrieval (CLR) system for home feed candidate generation. Key contributions include: expanding CLR to support Interest, Pin, and Board conditions; scaling model foundations with a Conditioned User Sequence Transformer and integrating the PinFM Foundation Model for deeper user sequence understanding; and redesigning the serving infrastructure with a Slot Architecture (condition_os, condition_id, condition_type_id) for condition-agnostic routing, replacing bespoke pipelines with a unified model. Pin tower improvements include using large ID embedding tables and semantic IDs derived from quantized content embeddings.