From Scoring to Spelling: Rebuilding Ads Retrieval at Instacart

What this post added

This post details the evolution of Instacart's ads retrieval system, moving from a BERT-based scoring model (Contextual Recommendations) to a generative model. The core technical innovation is the adoption of Instacart Semantic IDs (SIDs) as a new product vocabulary, replacing atomic product IDs. SIDs, generated by an RQ-VAE, compress product representations, provide coverage for new products, enable better generalization, and significantly reduce the model's embedding parameter space. The post also introduces a new training corpus format, the 'Context Template,' which enriches sessions with additional context tokens to improve the generative model's ability to understand user intent and recommend relevant products, especially for ads.