xai-org/x-algorithm

This repository contains the core code that determines which posts a viewer sees in the For You feed on X. It combines in-network content (from accounts the viewer follows) with out-of-network content (discovered through ML-based retrieval and other mechanisms), filters content based on a variety of inputs, and ranks posts using a transformer model.

• Notable Updates
  • September 18th, 2026
  • August 14th, 2026
  • August 13th, 2026
• Overview
• System Architecture
  • Request Path
  • Labeling Path
• Components
• How It Works
  • Scoring and Ranking
  • Filtering
• Experiments and Configuration
• What's not in this repo?
• Under the Hood Label Transparency Tool
• Key Design Decisions
• License

• Under the Hood. Reports now include information about whether one's account or posts have had their visibility limited because of required compliance with law(s). For example, you'll be able to see if any of your posts were withheld from showing in a country following a legal demand — and which country.

Notable updates:

• How weights work. There's a common misconception about how weights related to actions (e.g. Like, Share, Block, Report, etc) work in ranking. The weights scale the predicted probabilities of such actions (or predicted continuous values, e.g. dwell time) — they do not scale the raw engagement counts, so e.g. it'd be incorrect to see that a report has 468 times higher weight than a like and conclude that e.g. "1 report cancels out 468 likes". The weights are a multiple on your own predicted probability of Liking, Reporting, etc, which is substantially driven by your own behavior. We've added comments to the code so that LLMs or people reading it are more likely to understand it correctly.
• Brazil 2026 Elections. As announced by X, in accordance with Brazilian electoral law, For You now runs Brazil2026ElectionFilter, which removes posts from accounts reported to Brazil's Electoral Court for the 2026 election, unless the viewer explicitly follows the account. (Account list updated October 13, 2026.) A benefit of open-source is that you can see that changes like this exist, and exactly how they work — take a look at the code.

This release:

• Adds key configuration parameters (including weights used to blend predicted action values into a score for a post)
• Adds code for systems that impact whether a post is filtered from the For You feed
• Replaces the Phoenix demonstration model with the code used to train the models the feed uses, as well as synthetic data generation code so one can run a proof-of-concept training run of Phoenix.

Among new systems included are:

1. Visibility filtering: visibility-filtering/ determines whether to show a post, drop it, or show it behind an interstitial.
2. The systems that produce labels that drive visibility filtering's responses: rules that apply labels (botmaker/, botmaker-rules/, scarecrow/), models that score accounts on various dimensions (agatha/, bdsm/, user-cred-v2/), models that examine images and video (media-model-proxy/, clip/), and enforcement (abuse-enforcement-service/).
3. Phoenix model code: phoenix/ now contains code that trains and runs the model, plus synthetic data generation.
4. SimClusters: simclusters/, an additional source of posts from accounts the viewer does not follow that is called in retrieval alongside Thunder and Phoenix retrieval.

This update is also paired with a new Under the Hood transparency tool that allows people to see aggregate statistics about the labels on their account and posts that can limit visibility.

The For You feed is assembled per request. Posts come from two places:

1. In-Network — thunder/ keeps recent posts from the accounts a viewer follows in memory
2. Out-of-Network — phoenix/ retrieval and simclusters/ find posts from accounts the viewer does not follow. popular_posts_source adds a shared list of popular recent posts.

Both are ranked together by the same model. Phoenix reads the viewer's recent engagement history and predicts, for each post, how likely the viewer is to take each action on it. Those predictions are combined into one score using weights held in the code — see Scoring and Ranking.

Two pipelines do the work. The Post Pipeline finds, ranks and filters posts. The Blending Pipeline wraps it and adds what the model does not rank: ads, Who to Follow recommendations, prompts.

Ranking sets the order. Whether a post can be shown at all is decided separately, by visibility-filtering/, from the viewer's own actions such as blocks and mutes and from labels that other systems here attach to posts and accounts.

┌──────────────────────────────────────────────────────────────────────────────────────────┐
│                                   FOR YOU FEED REQUEST                                   │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌──────────────────────────────── HOME MIXER   home-mixer/ ────────────────────────────────┐
│                                                                                          │
├───────────────────────  POST PIPELINE   PhoenixCandidatePipeline  ───────────────────────┤
│                                                                                          │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 1. QUERY HYDRATION                                                                 │  │
│  │    user action sequence — the viewer's recent engagements, and the                 │  │
│  │    main input to the model · following list · blocks and mutes · muted             │  │
│  │    keywords · posts already seen and served · followed topics, etc.                │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 2. CANDIDATE SOURCES — queried in parallel                                         │  │
│  │    ┌───────────────────────────────┐ ┌────────────────────────────────────────┐    │  │
│  │    │ IN-NETWORK                    │ │ OUT-OF-NETWORK                         │    │  │
│  │    │ Thunder                       │ │ Phoenix retrieval   retrieval model    │    │  │
│  │    │   recent posts from the       │ │ SimClusters         cluster similarity │    │  │
│  │    │   accounts the viewer follows │ │ Popular posts       shared list        │    │  │
│  │    └───────────────────────────────┘ └────────────────────────────────────────┘    │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 3. CANDIDATE HYDRATION                                                             │  │
│  │    post text and media · author details and account labels · quoted post ·         │  │
│  │    language · engagement counts · subscription status, etc.                        │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 4. PRE-SCORING FILTERS                                                             │  │
│  │    duplicates across sources · older than 48 hours · the viewer's own              │  │
│  │    posts · blocked and muted accounts · muted keywords · already seen              │  │
│  │    or served · subscriber-only posts the viewer cannot access, etc.                │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 5. SCORING                                                                         │  │
│  │    PhoenixScorer   a probability for each action the viewer might take             │  │
│  │    RankingScorer   weighted sum, then repeated-author decay, an                    │  │
│  │                    out-of-network discount, a new-author boost                     │  │
│  │    VMRanker        calls the reranking service in vm-ranker/                       │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 6. SELECTION — TopKScoreSelector                                                   │  │
│  │    sort by final score, keep the top K                                             │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ 7. POST-SELECTION FILTERS — after the order is fixed                               │  │
│  │    VFCandidateHydrator  asks visibility-filtering/ per post and viewer             │  │
│  │    VFFilter             removes the posts it said to drop                          │  │
│  │    DedupConversationFilter  collapses branches of one conversation                 │  │
│  │                         ◄── these labels come from the Labeling Path               │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                                                                          │
├─────────────────────  BLENDING PIPELINE   ForYouCandidatePipeline  ──────────────────────┤
│                                                                                          │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ the ranked posts are one source here; the others add non-post items:               │  │
│  │    ads · Who to Follow · prompts · push-to-home, etc.                              │  │
│  │                                                                                    │  │
│  │ BlenderSelector interleaves them. The default ads blender reorders                 │  │
│  │ posts for ad adjacency. Who to Follow and prompts go at fixed positions.           │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                            ▼                                             │
│  ┌────────────────────────────────────────────────────────────────────────────────────┐  │
│  │ SIDE EFFECTS — after the response is sent                                          │  │
│  │    record which posts were served · refresh the post cache · log ad                │  │
│  │    and client events, etc.                                                         │  │
│  └────────────────────────────────────────────────────────────────────────────────────┘  │
│                                                                                          │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌──────────────────────────────────────────────────────────────────────────────────────────┐
│                                 RANKED FOR YOU TIMELINE                                  │
└──────────────────────────────────────────────────────────────────────────────────────────┘

Stages can be switched on and off individually, with defaults in home-mixer/params/param.rs — see Experiments and Configuration for how those defaults relate to what runs in production.

┌───────  1. CONTENT UNDERSTANDING — happens continuously, not on the request path  ───────┐
│                                                                                          │
│    POSTS AND MEDIA                    ACCOUNTS                                           │
│    grox/          classifiers for     agatha/        blocks and reports                  │
│                   text and media                     relative to favorites               │
│    media-model-   image and video     bdsm/          inauthentic behavior                │
│      proxy/       models              user-cred-v2/  PageRank over follow                │
│    clip/          image and text                     and engagement edges                │
│                   embeddings the                                                         │
│                   media models use                                                       │
│                                                                                          │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌──────────────────────────────────  2. LABELING RULES  ───────────────────────────────────┐
│                                                                                          │
│    scarecrow/  reacts to events as they happen. Embeds botmaker/ as its                  │
│       rule engine and loads rules from botmaker-rules/scarecrow/. A rule                 │
│       reads: on this event, if these conditions hold, apply this label.                  │
│                                                                                          │
│    abuse-enforcement-service/  reads model scores about an account. Its                  │
│       rules label the account or its posts, challenge it, or suspend it.                 │
│       It does not re-apply an action overturned on appeal (abuse-ledger-service/).       │
│                                                                                          │
│    safety-label-user-agg/  labels an account for what its posts collected.               │
│                                                                                          │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌──────────────────────────────────────  3. STORAGE  ──────────────────────────────────────┐
│             labels are written to storage, and read back on the request path             │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌───────────────────  4. VISIBILITY FILTERING   visibility-filtering/  ────────────────────┐
│                                                                                          │
│    for each post and viewer, one of four answers:                                        │
│                                                                                          │
│       ALLOW          show the post normally                                              │
│       NOTICE         show the post with a notice attached to it                          │
│       INTERSTITIAL   show it behind an interstitial the viewer can tap                   │
│                      through, e.g. for adult or graphic media                            │
│       DROP           do not show it                                                      │
│                                                                                          │
│    the rules read the labels above, plus whether the viewer blocks, mutes                │
│    or follows the author, whether that account is protected, suspended or                │
│    deactivated, subscriber-only status, and the viewer's settings and                    │
│    country. Some rules drop a post only when it is a recommendation from                 │
│    an account the viewer does not follow — spam caught at high recall, for               │
│    instance. The same post is allowed to a follower.                                     │
│                                                                                          │
└──────────────────────────────────────────────────────────────────────────────────────────┘
                                             ▼
┌───────────────  5. POST-SELECTION FILTERS   VFFilter, AncillaryVFFilter  ────────────────┐
│                                                                                          │
│    drop  ──►  the post is removed after ranking, and so is any post whose                │
│               ancestor in the thread, quoted post or reposted post was                   │
│               itself dropped                                                             │
│    interstitial  ──►  the post stays in the feed; nothing in this                        │
│               repository draws the interstitial                                          │
│    notice  ──►  the post stays in the feed; nothing in this                              │
│               repository draws the notice                                                │
│                                                                                          │
└──────────────────────────────────────────────────────────────────────────────────────────┘

These produce the scores and labels that Visibility Filtering reads.

Phoenix predicts a probability for each action:

Engagement    favorite · reply · repost · quote · share · share via DM · share via copy link
Clicks        post · profile · link · photo expand · video open · quoted post
Attention     video quality view · dwell · dwell time · click dwell time · video continuation seconds · profile visit seconds
Author        follow author
Negative      not interested · mute author · block author · report · not dwelled

RankingScorer combines them:

Final Score = Σ (weight_i × P(action_i))

Positive actions carry positive weights, negative actions negative ones. The weights are in home-mixer/params/param.rs; the arithmetic is in xai-value-model/scoring.rs.

There is a common misconception to be aware of about the weights: they scale the predicted probabilities (or predicted continuous values, e.g. dwell time) — they do not scale the raw engagement counts, so e.g. it'd be incorrect to see that a report has 468 times higher weight than a like and conclude that e.g. "1 report cancels out 468 likes". The weights are a multiple on your own predicted probability of Liking, Reporting, etc, which is substantially driven by your own behavior.

Three adjustments follow:

• Author Diversity: each post after an author's first is multiplied by a decaying factor, down to a floor.
• Out-of-Network Discount: posts from accounts the viewer does not follow are multiplied by a factor below 1, as are replies and reposts from accounts the viewer does follow.
• New-Author Boost: posts from authors whose impressions are below a threshold are lifted toward a target position.

VMRanker then calls vm-ranker/, a separate service that reorders the result.

Pre-Scoring Filters (home-mixer/filters/), in order:

Already-seen posts are handled twice over: ThunderSource is passed the list and leaves them out, the other sources are not, so their repeats are caught by the filters above.

Post-Selection Filters:

Two things to know about how the rules run:

• The first rule that answers drop ends the evaluation.
• A further set of rules applies only when the post is a recommendation from an account the viewer does not follow, and those rules can only drop — spam caught at high recall, for instance. The same post is allowed to a follower. Both sets are listed in evaluation order in visibility-filtering/rules/registry.rs.

As we work to improve the algorithm we regularly run experiments on a small percentage of timeline traffic. Our aim is for experiments running at a notable share of traffic — e.g. 10% or more — to be visible in this repository.

To enable experimentation, many tunable values are read from a configuration system rather than written into the code. To help people understand the production defaults, we run cron scripts that set the defaults in this repository's code to be the primary production values, for example in home-mixer/params/param.rs.

docs/BIDIRECTIONAL_BOOST_CHANGE.md follows a widely-discussed timeline change, exemplifying what you would see as a param value changes over time.

We believe transparency is important for trust, and our aim is for the public to be able to understand how posts are distributed on X, so they can audit, critique or even help improve the system.

One challenge with making code that impacts post distribution public is that people could use it to try to game the system. To reduce the risk of this, there are a limited set of files not currently published in the repository, e.g.:

• Grox prompts. E.g. the j2 files with the specific LLM prompts used in Grox.
• Some botmaker rules

However, we still want the public to have insight into these systems. To accomplish that, we're piloting a new transparency tool that will show people the visibility-impacting labels that have been applied to their account and posts. This approach has multiple benefits:

• one can see the outcomes of these systems (and whether they affect their own account)
• one can see whether labels have been manually applied outside of automated systems
• one can match any labels present on their account to the code to understand if or how the visibility of their posts is affected, and critique it if desired

We believe the combination of code + transparent outputs is a powerful one for public transparency, and welcome feedback.

The focus of the repository is transparency into the code that affects post visibility in the For You timeline. All of the code here is inspectable, and some of the code is even designed to be runnable end-to-end — e.g. training and running the Phoenix scoring model. Where code is meant to be built and run, the relevant manifests are in the repo, e.g. phoenix/ ships a Cargo workspace, a pyproject.toml, a quickstart and synthetic data generation, so a small model can be trained and served end-to-end. Elsewhere, code may not necessarily include build- or deployment-related files or generally self-explanatory infrastructure imports (e.g. xai_service_runner or xai_kafka). If there's anything not here that you believe would help your understanding of the algorithm, please let us know.

We're piloting a new transparency tool that lets people see aggregate statistics about the visibility-impacting labels on their account and posts. Paired with the code in this repository, we believe this gives people valuable insight into the visibility of their posts.

The tool is available here — we'll be shaping it based on your feedback and expanding availability over time. The jobs and serving code that build the report are in under-the-hood/. The page that renders it is in under-the-hood/jetfuel/.

Rather than predicting a single "relevance" score, the model predicts probabilities for many actions. Combining them into one number is a separate, explicit step.

During transformer inference, candidates cannot attend to each other—only to the viewer context. This ensures the score for a post doesn't depend on which other posts are in the batch, making scores consistent and cacheable.

Both retrieval and ranking use multiple hash functions for embedding lookup, so there is no vocabulary to maintain and a new post is representable immediately.

Ranking decides the order. Visibility filtering decides whether a post can be shown at all. Different services, different inputs, different rules.

The candidate-pipeline crate provides a flexible framework for building recommendation pipelines with:

• Separation of pipeline execution and monitoring from business logic
• Parallel execution of independent stages and graceful error handling
• Easy addition of new sources, hydrations, filters, and scorers

Licensed under the Apache License 2.0. See LICENSE.