X has released the core code for its For You feed, including candidate generation, ranking, and filtering. The code details a mechanism that integrates posts from followers (in-network) with posts discovered through ML-based search (out-of-network), which are then ranked using a transformer model.
X Releases Core Code for For You Feed, Including Transformer-based Ranking
This article is a translation. Read the Japanese original
Ranking is handled by a transformer model called Phoenix. Phoenix reads the viewer's recent engagement history and predicts the probability that the viewer will take specific actions for each post. These predicted values are integrated into a single score using weights maintained within the code.
The process consists of two pipelines: the Post Pipeline and the Blending Pipeline. The Post Pipeline handles candidate discovery, ranking, and filtering, while the Blending Pipeline adds advertisements (which are not ranked by the model), "Who to follow" recommendations, and prompts. A visibility filter operates separately from the ranking to determine whether a post should be displayed based on blocks, mutes, and labels.
Alongside this release, X has provided a transparency tool called "Under the Hood," which allows users to check aggregated statistics for labels assigned to accounts and posts.
Source: X For You Feed Algorithm (HN 125pt, 65 comments) (HN Search (backfill), 2026-01-20)