How X’s For You feed picks your posts, from the open-source code
Follow one stranger’s post through X’s For You: your taps, the sources, the filters, Phoenix’s guesses, the action weights and the top fifty.
Lesson summary
Your taps pick the pile, rules empty it, Phoenix guesses what you’d do, the weights add it up, and the top fifty reach your screen.
- X starts from your recent taps, not your name or bio: that tape is the model’s main input.
- Posts come from Thunder (people you follow) and from Phoenix retrieval (strangers nearest your taps): a few thousand per request.
- Rules remove before anything is judged: copies, posts over 48 hours old, posts you saw, accounts you blocked. A rule never weighs.
- Phoenix guesses your chance of each action for each post, and reads each post alone, so its neighbours never change its guess.
- Each chance is multiplied by a weight and summed: a reply 5, a like 0.5, a report −234. It is your chance, so mass reports can’t move it.
- Strangers are marked down to 0.75, a repeat author is halved down to a quarter, and the top fifty are kept.
What is the main thing X reads about you before ranking?
Your recent taps: replies, likes, long looks, skips. Not your name or your bio.
Where did the baker’s loaf come from, if you never followed the baker?
Phoenix retrieval: it found the posts nearest your taps, from accounts you don’t follow.
Can a filter push a post down the feed?
No. A filter only removes a post; ordering is the score’s job.
Would the loaf’s guess change if it were scored next to a viral post?
No. Each post is read alone against your taps, so its batch never changes its numbers.
Does one report cancel 468 likes?
No. The weights multiply your own predicted chance of each action, and your chance of reporting is tiny.
Why did your friend’s second photo drop so far?
A second post from the same author is halved, down to a floor of a quarter.
Stage 1 of 8
Why did X put this post on your screen?
You open X: at the top of your screen sits a loaf of bread, from a baker you have never followed.
You did not ask for bread, but there it sits, above your own friend's beach photo.
So who put a stranger's loaf there, at the very top?
Why did X put this post on your screen?
On X, each thing you scroll past is a post; here, a post is a small boat.
The baker's loaf is the warm one with a porthole face; your friend's beach photo flies a sail.
A few thousand more wait out in the harbour tonight, like a shouting football take or a cat in a box.
Your screen is this lit quay along the top of the wall; only the highest boats moor there.
For You is the lock below it: it lifts each boat to its own height, so the highest reach the quay.
The baker, your friend and their posts are made up for teaching, but the lock's fixed numbers come from X's open-source code.
So what lifted a stranger's loaf so high? It starts with you: with the things you tap.
Stage 2 of 8
What does X know about me when it starts?
Your taps go up here, on the harbour log by the quay: one signal flag for each thing you did.
Last week you replied to a loaf, liked a crumb shot, and watched a cat for a long time.
You replied to your friend too, and scrolled straight past a football post.
When you open X, that column is what the model reads, and its lamp swings to where your flags point: bread.
Your name and your bio hang on the board beside it.
So tell it outright: write I love football in your bio.
And… nothing. The log has no flag for a bio, so the lamp never moved.
It reads what you did, not who you say you are: no name, no bio, only your taps.
A name says who you are; a tap says what you will do next, and that is what it is guessing.
You never followed the baker. So can the lamp still find the baker’s loaf?
Yes. It follows your flags, not your follows: bread flags find bread boats, whoever sailed them.
Now a fresh log. Raise three flags so the lamp lands on the loaf before the tide turns. But how do strangers’ boats get into this harbour at all?
Stage 3 of 8
Where do strangers’ posts come from?
Two ways in, and only one is for strangers. First, your friend’s boats tie up at the home moorings the moment your friend posts.
That’s Thunder: a store, kept in memory, of everything new from the people you follow.
It hands over up to 1200 of them, and only from people you follow.
So unfollow everyone. With the home moorings empty, surely nothing is left to fill your screen?
The moorings emptied, yet the pile filled again, and the baker’s loaf came in with it.
Strangers come by sea. This pilot boat does retrieval, and it reads your flags.
It turns your flags and every post into points on one chart, then tows in the boats whose points sit nearest yours.
That’s up to 1000 strangers: the baker’s loaf, because you replied to a loaf, and the cat in a box.
You scrolled past football. So does the pilot tow in the shouting take?
No. It sails in on its own, with the popular posts, up to 500 of them.
Thunder, the pilot and the popular posts: a few thousand boats in the pile, gathered fresh each time you open X.
Now steer the pilot: two tows whose wake points where your flags do, before the tide turns. Then, does X weigh every boat?
Stage 4 of 8
Does it look at every post?
Not yet. Before anything is judged, every boat in the pile queues at one gate.
This is the filter: the harbourmaster’s sea gate, between the outer harbour and the lock.
The shouting take is next in line, and you would rather never see it.
Go on, push it back, and see what the gate does.
And… nothing. It sailed through: the gate has no scale, so it can only open or shut.
A filter never weighs a post; it only removes it.
So what does it remove? Here comes the loaf again, the same post from a second source: a duplicate, shut out.
Then a loaf from three days ago: anything older than 48 hours is shut out too.
Then a post you already saw, and one from someone you blocked: both shut out.
Four rules, and each one only says yes or no.
None of them asks whether you would like the post.
Now the cat in a box, from a stranger you never followed. Does the gate stop it?
No. Being a stranger breaks no rule, so the cat goes through, just like the baker’s loaf.
Of the nine boats here, five go on into the lock, all floating at one height.
Which one rises is decided later, inside the lock. Your turn: be the harbourmaster.
Shut the gate on each boat a rule removes; let the rest through. Shut it on the baker’s loaf, and it never reaches your screen.
Stage 5 of 8
How does it guess what I’d do?
The boats the gate let through sail into the lock, and each one gets a chamber of its own.
Over each chamber works the lock-keeper, Phoenix. It reads your log of taps, and the boat below.
From those it loads cargo, one piece for each thing you might do: like, reply, not interested.
Each piece’s size is your chance of doing it: for the loaf, a like is 0.22 and a reply 0.11.
Two chambers over, the shouting take gets its own: a bigger reply buoy, 0.14, and a fatter not-interested sack.
So slide the loaf into the chamber beside the take. Surely all that shouting drags its cargo down.
And… nothing. Every buoy and the sack stay exactly the size they were, alone or crowded.
That’s the rule: Phoenix reads each post alone, so its neighbours never change its chances.
The walls are how the model is built: each post may look at your taps, never at the post beside it.
So if the take’s reply buoy swelled, would the loaf’s shrink?
No. The loaf’s cargo comes from your log and its own boat, and nothing else.
That’s why X can read a few thousand boats at once and still give each one the same guess.
Your turn, before the gates open: feed the loaf’s chamber only what Phoenix reads. Then how do all its chances become one height?
Stage 6 of 8
So a report is worth 468 likes, and anyone can bury a post?
Phoenix’s guesses are in. Here they add up to one number: each chance is cargo, and each kind of cargo has a weight.
A like buoy lifts half a point per chance, a reply buoy five, and the sacks pull down.
Add them up, and the water stands at the score: the baker’s loaf comes to 0.47 and moors.
Now the scary one: the report anchor weighs minus 234, as much as 468 likes.
So sink the loaf: get a thousand strangers to report it.
And… nothing. The anchor’s chain stayed a hair, and the loaf still scores 0.47.
That chain is your own chance of reporting it, and their reports never touch your chance.
The weight is huge because your chance is tiny: a report is over a thousand times rarer than a like.
Now the shouting take, alone in its own chamber, carrying the biggest reply buoys of any post: plus 0.70.
Most replies, most lift. Does it reach your screen?
No. It sinks to minus 1.63, far below the loaf’s water.
The post you would reply to most can still sink, because your own chance of muting it is weighed too.
Why? You’d likely tap not interested on it, and that sack alone drags minus 1.43.
Even your tiny chance of reporting it hangs on the anchor: minus 0.70.
So your own taps set those chances, and strangers’ reports never reach them.
Add three taps to your log and keep the take below the loaf. But what stops one account filling your whole screen?
Stage 7 of 8
Why isn’t my feed all one account?
One score isn’t the last word. Look down from the quay: five boats, each floating at its score.
The baker’s loaf floats highest, at 0.47, and your friend’s beach photo is next, at 0.27.
But the baker is a stranger to you, and so are the cat and the shouting take.
So the keeper opens each stranger’s valve, the out-of-network discount, and a quarter of their water drains out.
The loaf drops from 0.47 to 0.35, three quarters of what it was, and it still floats highest.
Now, your friend posted twice this morning. Surely that earns your friend two good berths.
Go on, moor the second photo up beside the first.
And… it sank. Same friend, second boat: its water is halved, from 0.26 to 0.13.
A third boat from your friend would get a quarter, and never less: that’s the floor.
So one account can’t fill your screen just by posting more, and your friend still keeps a good berth.
After the valve and the halving, does the stranger’s loaf still top your screen?
Yes. Its 0.35 still beats your friend’s 0.27, even with a quarter of its water let out.
Along the quay are fifty berths, and the top fifty boats moor there, in order of their water; the rest never reach your screen.
The loaf takes the first berth, and the shouting take, at minus 1.22, sinks below every boat here.
Now you’re the lock-keeper. Mark down each stranger and the repeat before the tide turns.
Stage 8 of 8
So why the loaf?
Let’s walk back. First the context stop: the model reads your recent taps, not your name or your bio.
Then the sources stop: Thunder brings what the people you follow posted, and Phoenix retrieval brings strangers’ posts near your taps.
Then the filters stop: rules remove a copy, an old post, a seen one or a blocked one, and never weigh anything.
Then the phoenix stop: Phoenix guesses your chance of each action, reading each post alone.
Then the weights stop: each chance times its weight, summed, and since it is your own chance, mass reports cannot bury a post for you.
Then the mix stop: strangers count three quarters, a repeat author is halved, and the top fifty are kept.
So that’s the whole trick: your taps fill the pile, rules thin it, Phoenix guesses, the weights add it up, and the mix keeps fifty.
So who put the baker’s loaf at the top?
You never followed the baker. Your own taps did: last week you replied about bread, and here a reply is worth ten likes.