Open briefing
The For You algorithm, in one place.
xAI published the code that ranks X. This digest turns that dump into diagrams, the live weight table, a post scorer, and the Under the Hood label report.
Score a post
Phoenix predicts actions. Home Mixer multiplies them by published weights. Run that math on a post you describe.
Under the Hood
Aggregate stats on labels that can limit visibility for your posts and account last month.
The only formula that matters
Phoenix does not emit a relevance score. It emits a probability for each action. Ranking is an explicit sum.
01
Phoenix
P of each action
02
Weights
Published table
03
Sum
Σ w × P
04
Adjust
Diversity · OON ×0.75
05
VM Ranker
DPP variety
06
Order
Final slate
Weights that move the needle
All heads| Action | Weight | Why it matters |
|---|---|---|
| Share via copy link | +20 | Highest positive weight. Phoenix treats a copied link as a strong intent signal. |
| Reply | +5 | Conversation is valued far above a like. Mutual follows add +15 on original posts. |
| Like | +0.5 | Cheap, common, and therefore down-weighted. A like is not a strong ranking lever. |
| Mute author | -58.8 | Highest everyday negative besides report. Quiet rejection is treated as strong dislike. |
| Report | -234 | Nuclear. One predicted report wipes out almost any combination of positives. |
Two jobs, two paths
Ranking sets order. Visibility filtering decides whether a post can appear at all. They do not share logic.
- 1
Hydrate the viewer
History, follows, blocks, mutes, already-seen, topics
- 2
Fetch candidates in parallel
Same ranker scores all three. Being retrieved is not being shown.
In-network
Thunder
Recent posts from accounts you follow
Out-of-network
Phoenix retrieval
Nearest posts to your history
Out-of-network
SimClusters
Cluster overlap, not embeddings
- 3
Hydrate posts
Text, media, author, labels, quotes, engagement counts
- 4
Pre-score filters
Dupes, older than 48h, self posts, muted, already seen
- 5
Score
Phoenix heads → weighted sum → author decay → OON ×0.75 → VM variety
- 6
Keep top K
Sort by final score. Order is fixed after this.
- 7
Visibility filter
A separate service. First drop wins.
AllowShow normallyInterstitialBehind a warningDropRemove from the slate - 8
Blend and serve
Ads, Who to Follow, and prompts sit outside Phoenix
Architecture
Request path, labeling path, and how Home Mixer calls everyone else.
Phoenix
Two-tower retrieval and a transformer that isolates candidates.
Label catalog
Every visibility-impacting post and account label from Under the Hood.