Case Study: The Bidirectional Follow Boost
A real, dated 2026 algorithm change — told in full
The Bidirectional Follow Boost — A Real Algorithm Change
This is the flagship case study. It's a real, dated change that shows exactly how X's algorithm evolves — and it's documented in the repo itself.
Background: Base Weights
In the repo's base configuration (as of 2026-08-14):
- FavoriteWeight: 0.5
- ReplyWeight: 5.0
- QuoteWeight: 5.0
A reply is already weighted 10x a favorite before any boost.
The Change
In July 2026, X A/B-tested a "bidirectional follow boost" — a bonus applied only to original posts (not replies, not reposts) from accounts that mutually follow each other.
The boost is added on top of the base reply weight — it increases the weight on the *predicted probability* that you'll reply to a post from a mutual-follow author.
Timeline
| Date | What happened | Boost value |
|---|---|---|
| July 10, 2026 | A/B test begins — small % of users get values of 5, 10, 15, or 20 | Most users: 0 |
| July 13, 2026 | Broad launch — boost of 20 rolled out to many users | 20 |
| July 24, 2026 | After feedback (World Cup discussion crowded out), dialed back to 15 | 15 |
Current State (Aug 14, 2026)
BidirectionalFollowReplyWeightBoost = 15.0
This means: for an original post between two accounts that mutually follow each other, the reply weight is effectively 5.0 + 15.0 = 20.0 — 40x the base favorite weight.
Why This Matters
- 1.Mutual follows are the lever. The boost only applies to original posts between mutual followers. The relationship itself is the ranking signal.
- 2.Original posts only. Replies and reposts don't qualify — you need to post original content to your mutual followers.
- 3.The repo documents this. This isn't guesswork — it's in
docs/BIDIRECTIONAL_BOOST_CHANGE.mdwith the full timeline and code diffs. - 4.Algorithms change. The boost went from 20 → 15 after real-world feedback. This proves the algorithm is living code, not a fixed formula.
Check Your Understanding
1. What posts qualify for the bidirectional follow boost?
- A. All posts from mutual follows
- B. Only original posts (not replies/reposts) from mutual follows✓ correct
- C. Only replies between mutual follows
- D. Only posts with high engagement
Why: The boost applies only to original posts — not replies, not reposts — from accounts that mutually follow each other.
2. What was the boost value on July 13, and what is it now?
- A. Was 20, now 15✓ correct
- B. Was 15, now 20
- C. Was 10, now 5
- D. Was 5, now 0
Why: Launched broadly at +20 on July 13, dialed back to +15 on July 24 after World Cup feedback.
3. What is the effective reply weight for a mutual-follow original post?
- A. 5.0 (base only)
- B. 15.0 (boost only)
- C. 20.0 (5.0 base + 15.0 boost)✓ correct
- D. 25.0
Why: ReplyWeight (5.0) + BidirectionalFollowReplyWeightBoost (15.0) = 20.0. That's 40x the base FavoriteWeight (0.5).
Exercise
Identify 5 accounts in your niche that you follow but don't follow you back. Engage genuinely with their posts (replies, not just likes) for a week. If they follow back, your original posts to them now carry the bidirectional boost.
Hint: The boost is on the predicted probability of a reply — so posts that invite replies from mutual follows are doubly rewarded.