For You Feed Design

Collaborative filtering feed for Grain, reverse-engineered from the Bluesky For You feed playground by spacecowboy17.

Algorithm

Personalized mode (user has favorites)

1. Seed: Get user's 500 most recent favorites
2. Co-likers: Find other users who also favorited those same galleries
3. Candidates: Collect galleries those co-likers favorited (excluding user's own favorites and own galleries)
4. Score each candidate:

For each path (co-liker → candidate gallery):
  path_score = 1 / (co_liker_total_likes ^ divisor_power)
  path_score *= (1 - corater_decay) ^ (items_from_this_corater - 1)

base_score = sum(path_scores)
smoothed = base_score * (num_paths ^ smoothing_factor)
time_factor = 0.5 ^ (age_hours / half_life)    // half_life=0 disables decay
final_score = smoothed * time_factor / (total_favorites ^ popularity_penalty)

5. Rank by final_score descending, paginate with cursor

Cold start fallback (no favorites)

Return galleries ordered by favorite count descending, filtered to last 30 days.

Default Parameters

Files

• server/feeds/foryou.ts — feed generator with scoring logic
• app/lib/queries.ts — add forYouFeedQuery
• app/routes/feeds/for-you/+page.ts — page load
• app/routes/feeds/for-you/+page.svelte — feed UI
• app/lib/preferences.ts — add to DEFAULT_PINNED

Pagination

Score-based cursor: encode score:rkey as cursor string. Since scores are computed in JS (not SQL), fetch all candidates, score, sort, then slice by cursor position. For v1, use offset-based cursor to keep it simple.

Filters

• Exclude galleries user already favorited
• Exclude user's own galleries
• Respect takedown status and hidden labels
• Require gallery to have at least 1 item