feat(explore): reach dial to escape dense clusters + anti-revisit (#1476)

The Explore walk got stuck in dense signatures — neighbours all too similar, so
forward-arrow couldn't escape and Random was the only exit. Root cause: MMR only
diversifies WITHIN the nearest ~400 pool; in a dense cluster that whole pool is
near-identical, so there's no escape route in it.

- gallery_service.similar(reach=0.0, exclude_ids=None): reach>0 widens the pool
  (cap 400→1000) and _reach_sample strides across an outward-growing distance span
  so the set handed to MMR spans near→mid-far (guaranteed escape routes), not just
  the tight cluster. exclude_ids drops already-walked images. Gallery 'more like
  this' (reach=0) is unchanged.
- api/gallery similar: parse reach + exclude_ids.
- explore store: default reach 0.4 (auto-diversifies without touching the dial),
  pass the breadcrumb as exclude_ids, setReach action.
- ExploreView: a Near↔Far reach slider in the trail.
- tests: _reach_sample math (deeper ranks with higher reach, near kept); similar
  exclude_ids drops walked + reach path runs clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/gallery.py

@@ -148,6 +148,17 @@ async def similar():
     # Explore passes exclude_wip=1 to also drop work-in-progress from the
     # rabbit-hole; the gallery's own "similar" button omits it (keeps wip, #1274).
     exclude_wip = request.args.get("exclude_wip") in ("1", "true", "True")
+    # Explore reach (#1476): 0 = nearest (gallery default), →1 reaches into farther
+    # distance bands so the walk can escape a dense cluster. exclude_ids = the
+    # breadcrumb, so already-walked images aren't re-served as neighbours.
+    try:
+        reach = max(0.0, min(1.0, float(request.args.get("reach", "0"))))
+    except ValueError:
+        reach = 0.0
+    exclude_ids = [
+        int(x) for x in request.args.get("exclude_ids", "").split(",")
+        if x.strip().isdigit()
+    ] or None
     # post_id is the exclusive post-detail view — not a similarity scope.
     # include_hidden is a gallery-browse flag; similar() has its OWN presentation
     # exclusion (a similarity-quality concern, #1274), so drop it here (#141).
@@ -158,7 +169,8 @@ async def similar():
         svc = GalleryService(session)
         try:
             images = await svc.similar(
-                image_id=similar_to, limit=limit, exclude_wip=exclude_wip, **scope)
+                image_id=similar_to, limit=limit, exclude_wip=exclude_wip,
+                reach=reach, exclude_ids=exclude_ids, **scope)
         except ValueError as exc:
             return jsonify({"error": str(exc)}), 400
     if images is None:

backend/app/services/gallery_service.py

@@ -396,6 +396,25 @@ def _diversify_similar(src, rows, limit, *, dup_threshold=8, lam=0.40):
     return [kept[i] for i in order]
 
 
+def _reach_sample(rows, limit, reach):
+    """From a distance-sorted candidate pool (nearest first), pick a spread of ranks
+    that MIXES near (tag the current cluster) and mid-far (escape it) BEFORE dedup +
+    MMR — the Explore "reach" dial (#1476).
+
+    reach in (0, 1]: the sampled span grows outward from the anchor (0.25→1.0 of the
+    pool), evenly strided from rank 0 so the nearest are still represented. In a
+    dense signature the nearest ranks are near-identical, so reaching farther is the
+    only way to hand MMR genuinely different content — MMR alone can't escape a pool
+    that's already all-near. reach<=0 or a small pool passes through unchanged."""
+    n = len(rows)
+    want = max(limit * 8, 100)
+    if reach <= 0 or n <= want:
+        return rows
+    span = int(min(1.0, 0.25 + 0.75 * reach) * n)
+    idx = sorted({min(int(i * span / want), n - 1) for i in range(want)})
+    return [rows[i] for i in idx]
+
+
 async def _artists_for(session, image_ids: list[int]) -> dict[int, dict]:
     """Map image_id -> {"name","slug"} via the canonical
     image_record.artist_id (FC-2d-vii-c). Bounded by page size."""
@@ -717,6 +736,7 @@ class GalleryService:
         untagged: bool = False, no_artist: bool = False,
         date_from: datetime | None = None, date_to: datetime | None = None,
         exclude_wip: bool = False,
+        reach: float = 0.0, exclude_ids: list[int] | None = None,
     ) -> list[GalleryImage] | None:
         """Visual "more like this": images near `image_id`'s SigLIP embedding
         (pgvector, HNSW-indexed — alembic 0036), then DIVERSIFIED so the result
@@ -745,7 +765,13 @@ class GalleryService:
         # wide pool there's nothing but the near-dupes to choose from. Widened
         # (5×→8×, cap 200→400) so the stronger MMR has genuinely distinct
         # neighbourhoods to reach into for more variance (operator, 2026-07-01).
-        pool_n = min(400, max(limit * 8, 100))
+        # Explore's reach>0 (#1476) widens it a LOT more: in a dense signature the
+        # nearest few hundred are all near-identical, so far-enough candidates only
+        # exist deeper in the ranked pool. _reach_sample then strides across them.
+        if reach > 0:
+            pool_n = min(1000, max(limit * 25, 100))
+        else:
+            pool_n = min(400, max(limit * 8, 100))
         distance = ImageRecord.siglip_embedding.cosine_distance(src.siglip_embedding)
         eff = _effective_date_col()
         stmt = select(ImageRecord, Post.post_date, eff.label("eff"))
@@ -773,6 +799,10 @@ class GalleryService:
             ImageRecord.id != image_id,
             ImageRecord.id.not_in(presentation),
         )
+        # Anti-revisit (#1476): the Explore walk passes its breadcrumb so already-
+        # walked images aren't re-served as neighbours — → can't loop you back in.
+        if exclude_ids:
+            stmt = stmt.where(ImageRecord.id.not_in(exclude_ids))
         stmt = _apply_scope(
             stmt, tag_ids=tag_ids, post_id=None,
             artist_id=artist_id, media_type=media_type,
@@ -782,6 +812,10 @@ class GalleryService:
         )
         stmt = stmt.order_by(distance.asc()).limit(pool_n)
         rows = (await self.session.execute(stmt)).all()
+        # Explore reach: stride across an outward-growing distance span so the pool
+        # handed to MMR spans near→mid-far, not just the tight cluster (#1476).
+        if reach > 0:
+            rows = _reach_sample(rows, limit, reach)
         rows = _diversify_similar(src, rows, limit)
         artists = await _artists_for(self.session, [r[0].id for r in rows])
         return _gallery_images(rows, artists)

frontend/src/stores/explore.js

@@ -25,6 +25,10 @@ export const useExploreStore = defineStore('explore', () => {
   const cursor = ref(-1)
   const loading = ref(false)
   const error = ref(null)
+  // Reach (#1476): how far the walk reaches past the anchor's immediate cluster.
+  // 0 = nearest (can get stuck in a dense signature); ~0.4 default mixes in
+  // mid-far escape routes so the walk diversifies without hitting "Random image".
+  const reach = ref(0.4)
 
   const inflight = useInflightToken()
 
@@ -46,7 +50,13 @@ export const useExploreStore = defineStore('explore', () => {
         const body = await api.get('/api/gallery/similar', {
           // exclude_wip: keep work-in-progress out of the Explore rabbit-hole
           // (the gallery's own "similar" button still shows it) — operator 2026-07-08.
-          params: { similar_to: numId, limit: NEIGHBOR_LIMIT, exclude_wip: 1 },
+          // reach + exclude_ids (#1476): reach past the dense cluster + never re-serve
+          // an already-walked image, so the walk keeps moving instead of getting stuck.
+          params: {
+            similar_to: numId, limit: NEIGHBOR_LIMIT, exclude_wip: 1,
+            reach: reach.value,
+            exclude_ids: breadcrumb.value.map((c) => c.id).join(','),
+          },
         })
         if (!t.isCurrent()) return
         neighbors.value = body.images || []
@@ -113,6 +123,14 @@ export const useExploreStore = defineStore('explore', () => {
     loading.value = false
   }
 
+  // Change how far the walk reaches and re-fetch the current anchor's neighbours
+  // with the new setting (the anchor + trail are unchanged — only the grid varies).
+  function setReach (v) {
+    reach.value = Math.max(0, Math.min(1, Number(v)))
+    const id = anchor.value?.id
+    if (id != null) anchorOn(id)
+  }
+
   // --- TagPanel "host" surface ---------------------------------------------
   // The anchor IS the current image (same /api/gallery/image/<id> payload the
   // modal uses), so these mirror the modal store's tag-CRUD, targeting the
@@ -189,8 +207,8 @@ export const useExploreStore = defineStore('explore', () => {
   function close () {}
 
   return {
-    anchor, neighbors, breadcrumb, cursor, loading, error, NEIGHBOR_LIMIT,
+    anchor, neighbors, breadcrumb, cursor, loading, error, NEIGHBOR_LIMIT, reach,
-    anchorOn, reset, backTarget, forwardTarget,
+    anchorOn, reset, backTarget, forwardTarget, setReach,
     // host surface
     current, currentImageId,
     reloadTags, addExistingTag, removeTag, createAndAdd, close,

frontend/src/views/ExploreView.vue

@@ -31,6 +31,20 @@
           <img :src="c.thumbnail_url" alt="" loading="lazy" />
         </button>
         <div class="fc-ex__trail-actions">
+          <!-- Reach (#1476): how far each step reaches past the anchor's cluster.
+               Raise it to break out of a dense signature without hitting Random. -->
+          <div
+            class="fc-ex__reach"
+            title="How far each step reaches — raise it to escape a dense cluster without going fully random"
+          >
+            <v-icon size="16" color="accent">mdi-map-marker-distance</v-icon>
+            <v-slider
+              :model-value="store.reach" @end="store.setReach"
+              :min="0" :max="1" :step="0.2" hide-details density="compact"
+              color="accent" class="fc-ex__reach-slider"
+            />
+            <span class="fc-muted fc-ex__reach-label">{{ reachLabel }}</span>
+          </div>
           <!-- Active retrain right where you tag: fold the +/- you just gave
                into the heads without a trip to Settings (the nightly beat is the
                passive cadence). -->
@@ -153,6 +167,12 @@ const modal = useModalStore()
 
 const anchorId = computed(() => route.params.imageId || null)
 const isVideo = computed(() => !!store.anchor?.mime?.startsWith('video/'))
+const reachLabel = computed(() => {
+  const r = store.reach
+  if (r <= 0.15) return 'Near'
+  if (r <= 0.55) return 'Varied'
+  return 'Far'
+})
 
 // #1206: hovering a suggestion in the rail highlights the crop it came from on
 // the anchor image (same provide/inject as the modal viewer).
@@ -296,6 +316,14 @@ onUnmounted(() => {
 .fc-ex__trail-actions {
   margin-left: auto; display: flex; align-items: center; gap: 4px; flex: 0 0 auto;
 }
+.fc-ex__reach {
+  display: flex; align-items: center; gap: 6px;
+  margin-right: 8px;
+}
+.fc-ex__reach-slider { width: 96px; }
+.fc-ex__reach-label {
+  font-size: 12px; min-width: 44px; text-align: left;
+}
 
 /* The three panes fill the remaining height; each scrolls on its own.
    grid-template-rows: minmax(0, 1fr) BOUNDS the single row to the container

tests/test_gallery_similar.py

@@ -170,6 +170,23 @@ async def test_similar_respects_limit(db):
     assert len(res) == 2
 
 
+@pytest.mark.asyncio
+async def test_similar_exclude_ids_drops_walked(db):
+    """Explore passes its breadcrumb as exclude_ids so already-walked images
+    aren't re-served as neighbours (#1476). reach>0 runs cleanly too (small pool
+    → the sampler passes through)."""
+    src = await _img(db, 1, _vec(1, 0))
+    walked = await _img(db, 2, _vec(1, 0.05))
+    fresh = await _img(db, 3, _vec(1, 0.3))
+    svc = GalleryService(db)
+    res = await svc.similar(src.id, limit=10, exclude_ids=[walked.id])
+    ids = {i.id for i in res}
+    assert walked.id not in ids
+    assert fresh.id in ids
+    res_reach = await svc.similar(src.id, limit=10, reach=1.0)
+    assert fresh.id in {i.id for i in res_reach}
+
+
 # --- API ---
 
 @pytest.mark.asyncio

tests/test_reach_sample.py

@@ -0,0 +1,32 @@
+"""Explore reach sampler (#1476) — pure unit tests for `_reach_sample`, which picks
+a distance-rank spread so the neighbour pool handed to MMR spans near→mid-far and
+the walk can escape a dense cluster. No DB. `_reach_sample` only indexes rows, so
+plain ints stand in for the (ImageRecord, ...) tuples."""
+from backend.app.services.gallery_service import _reach_sample
+
+
+def test_reach_zero_or_negative_passes_through():
+    rows = list(range(1000))
+    assert _reach_sample(rows, 40, 0.0) is rows
+    assert _reach_sample(rows, 40, -1.0) is rows
+
+
+def test_small_pool_passes_through():
+    # n <= want (limit*8 = 320) → nothing to reach into.
+    rows = list(range(50))
+    assert _reach_sample(rows, 40, 1.0) is rows
+
+
+def test_higher_reach_reaches_deeper_ranks():
+    rows = list(range(1000))
+    near = _reach_sample(rows, 40, 0.2)
+    far = _reach_sample(rows, 40, 1.0)
+    # Both keep the nearest rank (stride starts at 0) so you can still tag the cluster.
+    assert near[0] == 0
+    assert far[0] == 0
+    # But higher reach samples genuinely farther ranks.
+    assert max(far) > max(near)
+    assert max(far) >= 900          # reach=1 spans (almost) the whole pool
+    assert max(near) <= 550         # reach=0.2 stays in the near half
+    # Never runs past the pool.
+    assert max(far) <= len(rows) - 1