backend/app/services/discord_grouping.py

@@ -59,14 +59,23 @@ from __future__ import annotations
 
 import logging
 import math
+from collections import Counter
 from dataclasses import dataclass, field
 from datetime import UTC, datetime, timedelta
 
-from sqlalchemy import Select, func, select, update
+from sqlalchemy import Select, delete, func, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from ..models import ImageProvenance, ImageRecord, MLSettings, Post, Source
+from ..models import (
+    ImageProvenance,
+    ImageRecord,
+    MLSettings,
+    Post,
+    PostAssociation,
+    Source,
+)
+from .post_naming import FAMILY_MAX_POSTS, leading_name, rarity, token_frequencies
 
 log = logging.getLogger(__name__)
 
@@ -600,6 +609,344 @@ async def join_open_groups(
     return joined
 
 
+# ---------------------------------------------------------------------------
+# #4390: a trickle is one drop — later drops of the same piece merge in.
+# ---------------------------------------------------------------------------
+#
+# Operator, 2026-09-24, on a feed of "Grouped from 1 Discord message" cards:
+# *"the groups are still single image even when they can clearly be seen as
+# group"*. 665 of Yellowroom's 714 drops were one message.
+#
+# Both paths above join on cosine distance to the group's SEED, and the stages
+# of one piece fail it: `svtt_wip4` did not join `svtt_wip3` from the day
+# before. A creator trickles a piece out as sketch -> wip -> wip -> release, and
+# each stage is nearest to the one before it, not to the first.
+#
+# Measured on artist 8 before any of this was written (#4390 log):
+#
+# * phash cannot see it. Stages of one piece sit 68-134 bits apart; unrelated
+#   pieces by the same artist sit at a median of 126, p5 110. Lesson #4400.
+# * The embedding's NEAREST neighbour can. Every stage of three real trickles
+#   had a sibling stage as its single nearest image in the artist's whole
+#   library, while siblings further along ranked 40-100 — which is exactly why
+#   seed distance fails. Negative control over all 137 recent Discord images:
+#   where the nearest neighbour was another message within 7 days, the two
+#   carried the same working name 53 times out of 53. Disagreements start past
+#   7 days.
+# * The working name sees it directly, when there is one.
+#
+# So a later drop merges into an earlier one when the two are within
+# `discord_group_close_after_hours` of each other (168h — the measured 7 days)
+# AND either they share a gated LEADING working name, or one's image has the
+# other's image as its nearest neighbour. A drop reaching SEVERAL earlier drops
+# pulls them all together — unless two of them are named as different pieces,
+# in which case nothing moves (see `_compatible`): leaving a drop alone is
+# recoverable, a wrong merge asserts that unrelated art belongs together.
+#
+# Chaining is permitted here and was forbidden above, deliberately. The seed
+# rule exists because tiny steps can drift from one piece to another; the
+# measured precision of nearest-neighbour inside 7 days is what bounds drift
+# for this route, and each link is between neighbours in time, never across a
+# quiet week.
+
+# How many unchecked drops one sweep examines per source. A first run over an
+# established library drains over successive sweeps, oldest first, rather than
+# issuing one nearest-neighbour query per image of the whole history at once.
+TRICKLE_BATCH = 300
+
+
+@dataclass
+class _Drop:
+    post: Post
+    members: set[int]
+    first_at: datetime
+    last_at: datetime
+    names: set[str]
+    images: list[int]
+    nearest: set[int] | None
+
+
+async def _nearest_message(
+    session: AsyncSession, *, artist_id: int, image_id: int, exclude: set[int],
+) -> int | None:
+    """The post that owns the nearest image in the artist's whole library.
+
+    The whole LIBRARY, not this source, because that is what was measured: a
+    neighbour that turns out to be a Patreon re-post simply yields no Discord
+    drop to merge into, which errs toward leaving things alone. `exclude` is
+    the drop's own messages — an image is always nearest to its own siblings
+    in the same drop, which says nothing.
+    """
+    embedding = (await session.execute(
+        select(ImageRecord.siglip_embedding).where(ImageRecord.id == image_id)
+    )).scalar_one_or_none()
+    if embedding is None:
+        return None
+    stmt = (
+        select(ImageRecord.primary_post_id)
+        .where(
+            ImageRecord.artist_id == artist_id,
+            ImageRecord.id != image_id,
+            ImageRecord.siglip_embedding.is_not(None),
+            ImageRecord.primary_post_id.is_not(None),
+        )
+        .order_by(ImageRecord.siglip_embedding.cosine_distance(embedding))
+        .limit(1)
+    )
+    if exclude:
+        stmt = stmt.where(ImageRecord.primary_post_id.not_in(exclude))
+    return (await session.execute(stmt)).scalar_one_or_none()
+
+
+async def _load_drops(session: AsyncSession, source: Source) -> list[_Drop]:
+    """Every live drop of this source, with what the merge rule reads, oldest first."""
+    posts = (await session.execute(
+        select(Post).where(
+            Post.source_id == source.id,
+            Post.synthesized_by == DROP_GROUPER,
+            Post.absorbed_by_post_id.is_(None),
+        )
+    )).scalars().all()
+    if not posts:
+        return []
+    by_id = {p.id: p for p in posts}
+
+    msg_at = func.coalesce(Post.post_date, Post.downloaded_at)
+    members: dict[int, set[int]] = {pid: set() for pid in by_id}
+    times: dict[int, list[datetime]] = {pid: [] for pid in by_id}
+    for mid, owner, at in (await session.execute(
+        select(Post.id, Post.absorbed_by_post_id, msg_at)
+        .where(Post.absorbed_by_post_id.in_(list(by_id)))
+    )).all():
+        members[owner].add(mid)
+        times[owner].append(at)
+
+    owner_of = {m: d for d, ms in members.items() for m in ms}
+    names: dict[int, set[str]] = {pid: set() for pid in by_id}
+    images: dict[int, list[int]] = {pid: [] for pid in by_id}
+    if owner_of:
+        for iid, primary, path in (await session.execute(
+            select(ImageRecord.id, ImageRecord.primary_post_id, ImageRecord.path)
+            .where(ImageRecord.primary_post_id.in_(list(owner_of)))
+            .order_by(ImageRecord.id)
+        )).all():
+            drop = owner_of[primary]
+            images[drop].append(iid)
+            if (name := leading_name(path)) is not None:
+                names[drop].add(name)
+
+    out = []
+    for pid, post in by_id.items():
+        if not times[pid]:
+            continue
+        stored = (post.synthesis_details or {}).get("nearest_message_ids")
+        out.append(_Drop(
+            post=post, members=members[pid],
+            first_at=min(times[pid]), last_at=max(times[pid]),
+            names=names[pid], images=images[pid],
+            nearest=set(stored) if stored is not None else None,
+        ))
+    return sorted(out, key=lambda d: (d.first_at, d.post.id))
+
+
+async def _name_posts(session: AsyncSession, artist_id: int) -> Counter[str]:
+    """Post-span counts of the artist's working names — the same corpus the
+    teaser card and the announcement matcher count against."""
+    by_post: dict[int, list[str]] = {}
+    for pid, path in (await session.execute(
+        select(ImageRecord.primary_post_id, ImageRecord.path).where(
+            ImageRecord.artist_id == artist_id,
+            ImageRecord.primary_post_id.is_not(None),
+        )
+    )).all():
+        by_post.setdefault(pid, []).append(path)
+    return token_frequencies(by_post.values())
+
+
+async def _repoint_associations(
+    session: AsyncSession, *, from_id: int, to_id: int,
+) -> None:
+    """Move announcement links from a drop about to merge onto the one it joins.
+
+    Without this the merge would silently undo a teaser link: the association's
+    payload FK cascades on delete. Where the teaser already points at the
+    surviving drop, the stronger claim is kept — a link over a proposal over a
+    dismissal — and the duplicate goes.
+    """
+    rank = {"linked": 2, "pending": 1, "dismissed": 0}
+    moving = (await session.execute(
+        select(PostAssociation).where(PostAssociation.payload_post_id == from_id)
+    )).scalars().all()
+    for a in moving:
+        existing = (await session.execute(
+            select(PostAssociation).where(
+                PostAssociation.announcement_post_id == a.announcement_post_id,
+                PostAssociation.payload_post_id == to_id,
+            )
+        )).scalar_one_or_none()
+        if existing is None:
+            a.payload_post_id = to_id
+            continue
+        if rank.get(a.status, 0) > rank.get(existing.status, 0):
+            existing.status = a.status
+            existing.linked_by = a.linked_by
+        await session.delete(a)
+    await session.flush()
+
+
+async def merge_trickles(
+    session: AsyncSession,
+    source: Source,
+    *,
+    gap: timedelta,
+    min_images: int,
+    cooldown: timedelta,
+    batch: int = TRICKLE_BATCH,
+) -> int:
+    """Fold later drops of the same piece into the earlier one. Returns merges."""
+    drops = await _load_drops(session, source)
+    if len(drops) < 2:
+        return 0
+    name_posts = await _name_posts(session, source.artist_id)
+
+    def gated(names: set[str]) -> set[str]:
+        return {n for n in names if rarity(name_posts.get(n, 0), FAMILY_MAX_POSTS) > 0}
+
+    alive: list[_Drop] = []
+    merged = 0
+    checked = 0
+    for drop in drops:
+        details = drop.post.synthesis_details or {}
+        if details.get("trickle_checked"):
+            alive.append(drop)
+            continue
+        if checked >= batch:
+            # Unchecked and out of budget: still a candidate for LATER drops'
+            # reverse edges, just not examined itself this run.
+            alive.append(drop)
+            continue
+        checked += 1
+
+        if drop.nearest is None:
+            found: set[int] = set()
+            for iid in drop.images:
+                pid = await _nearest_message(
+                    session, artist_id=source.artist_id, image_id=iid,
+                    exclude=drop.members,
+                )
+                if pid is not None:
+                    found.add(pid)
+            drop.nearest = found
+
+        mine = gated(drop.names)
+        targets: dict[int, tuple[_Drop, str]] = {}
+        for earlier in alive:
+            if drop.first_at - earlier.last_at > gap:
+                continue
+            shared = mine & gated(earlier.names)
+            if shared:
+                targets[earlier.post.id] = (earlier, f"name:{min(shared)}")
+            elif drop.nearest & earlier.members or (earlier.nearest or set()) & drop.members:
+                targets[earlier.post.id] = (earlier, "nearest")
+
+        record = dict(details)
+        record["nearest_message_ids"] = sorted(drop.nearest)
+        record["trickle_checked"] = True
+        drop.post.synthesis_details = record
+
+        if not targets or not _compatible(
+            [gated(t.names) for t, _route in targets.values()] + [mine]
+        ):
+            alive.append(drop)
+            continue
+
+        # Every target is the same piece as this drop, so they are the same
+        # piece as each other: fold them all into the earliest, then this drop.
+        ordered = sorted(targets.values(), key=lambda tr: (tr[0].first_at, tr[0].post.id))
+        into = ordered[0][0]
+        for other, route in ordered[1:]:
+            await _merge_drop(
+                session, into=into, drop=other, route=route,
+                min_images=min_images, cooldown=cooldown,
+            )
+            alive.remove(other)
+            merged += 1
+        await _merge_drop(
+            session, into=into, drop=drop, route=ordered[0][1],
+            min_images=min_images, cooldown=cooldown,
+        )
+        merged += 1
+    return merged
+
+
+def _compatible(name_sets: list[set[str]]) -> bool:
+    """May drops carrying these working names become one post?
+
+    Refused only when two of them are NAMED AS DIFFERENT PIECES — both carry a
+    gated name, and they share none. An unnamed drop (a canvas screenshot)
+    fits anywhere, which is the whole of the Marin case: two early stages both
+    nearest to the same later one are one trickle, not an ambiguity.
+
+    What it does NOT refuse is a drop the creator made two pieces in
+    themselves. Measured on artist 8: one November message carries both
+    `AdL01_wip4` and `Year_20k_wip_z4`, so its drop holds both names, and a
+    later drop of either piece joins it on its own name. That is the creator's
+    co-posting carried forward — Discord shows those two together too — not a
+    bridge FC built.
+    """
+    named = [n for n in name_sets if n]
+    return all(a & b for i, a in enumerate(named) for b in named[i + 1:])
+
+
+async def _merge_drop(
+    session: AsyncSession,
+    *,
+    into: _Drop,
+    drop: _Drop,
+    route: str,
+    min_images: int,
+    cooldown: timedelta,
+) -> None:
+    """Absorb `drop`'s messages into `into`, carry its links over, delete it.
+
+    Growth is stamped at the merged messages' OWN time, not the wall clock.
+    Merging history must not drag a two-year-old drop to the top of the feed,
+    and the time the group actually grew is when those messages arrived.
+    """
+    await _repoint_associations(session, from_id=drop.post.id, to_id=into.post.id)
+    grew_before = into.post.last_grew_at
+    await _absorb_into(
+        session, group=into.post, member_ids=sorted(drop.members),
+        source_id=into.post.source_id, now=drop.last_at,
+        min_images=min_images, cooldown=cooldown,
+    )
+    # Never backwards: a group that already grew later than these messages
+    # keeps that later date.
+    if grew_before is not None and grew_before > drop.last_at:
+        into.post.last_grew_at = grew_before
+    details = dict(into.post.synthesis_details or {})
+    if grew_before is not None and grew_before > drop.last_at:
+        details["last_grew_at"] = grew_before.isoformat()
+    # The honesty rule, extended: a grouping FC invented says what it was
+    # built from, and a merge says WHY — "name:svtt" or "nearest".
+    details["merged"] = [
+        *details.get("merged", []),
+        {"post_id": drop.post.id, "route": route, "message_ids": sorted(drop.members)},
+    ]
+    details["nearest_message_ids"] = sorted((into.nearest or set()) | (drop.nearest or set()))
+    into.post.synthesis_details = details
+
+    into.members |= drop.members
+    into.names |= drop.names
+    into.images += drop.images
+    into.nearest = (into.nearest or set()) | (drop.nearest or set())
+    into.last_at = max(into.last_at, drop.last_at)
+
+    await session.execute(delete(ImageProvenance).where(ImageProvenance.post_id == drop.post.id))
+    await session.delete(drop.post)
+    await session.flush()
+
+
 async def sweep(session: AsyncSession, *, now: datetime | None = None) -> dict:
     """Group every enabled Discord source. No-op when the switch is off.
 
@@ -612,6 +959,7 @@ async def sweep(session: AsyncSession, *, now: datetime | None = None) -> dict:
     if not settings.discord_grouping_enabled:
         return {
             "enabled": False, "sources": 0, "posts_created": 0, "images_joined": 0,
+            "drops_merged": 0,
         }
 
     sources = (await session.execute(
@@ -626,6 +974,7 @@ async def sweep(session: AsyncSession, *, now: datetime | None = None) -> dict:
 
     created = 0
     joined = 0
+    merged = 0
     for source in sources:
         joined += await join_open_groups(
             session, source,
@@ -644,12 +993,20 @@ async def sweep(session: AsyncSession, *, now: datetime | None = None) -> dict:
             window_minutes=window_minutes,
             now=now,
         )
+        # Last, so the drops the two passes above just wrote are merged in
+        # the same sweep rather than showing as singletons for an hour.
+        merged += await merge_trickles(
+            session, source,
+            gap=timedelta(hours=float(settings.discord_group_close_after_hours)),
+            min_images=int(settings.discord_group_resurface_min_images),
+            cooldown=timedelta(hours=float(settings.discord_group_resurface_cooldown_hours)),
+        )
     log.info(
         "discord drop grouping: %d source(s), %d synthetic post(s) created, "
-        "%d image(s) joined to open groups",
+        "%d image(s) joined to open groups, %d trickle drop(s) merged",
-        len(sources), created, joined,
+        len(sources), created, joined, merged,
     )
     return {
         "enabled": True, "sources": len(sources),
-        "posts_created": created, "images_joined": joined,
+        "posts_created": created, "images_joined": joined, "drops_merged": merged,
     }

backend/app/services/post_naming.py

@@ -165,6 +165,19 @@ MAX_TOKEN_POSTS = 6
 # four-post band, where conto's `illustration9` and `maid` also sit.
 IDENTITY_FLOOR = 0.75
 
+# A LEADING name spanning this many of ONE ARTIST's posts is a habit — a
+# character the creator returns to — not one piece's trickle. Used wherever a
+# name gathers a FAMILY: the teaser card's variants (#4401) and the Discord
+# grouper's trickle merge (#4390), so the two cannot disagree about what a
+# family is.
+#
+# Its own value rather than MAX_TOKEN_POSTS (6), which is calibrated for
+# PAIRING two posts and measured too tight for a family. On artist 8,
+# `tentacooler` spans 6 posts over 7 days and `0-k1` 6 posts over 10: both real
+# families, both gated out at 6. At 8, `anya` (7) passes the cap — and has no
+# pair inside any family window, which is what the window is for.
+FAMILY_MAX_POSTS = 8
+
 
 def _strip_prefixes(stem: str) -> str:
     """Remove the framing each platform's importer adds around the real name."""

backend/app/services/post_unification.py

@@ -58,17 +58,7 @@ from ..utils.text import html_to_plain, truncate_at_word
 from .discord_grouping import PLATFORM as DISCORD
 from .gallery_service import thumbnail_url
 from .post_association_service import DUPLICATE_MAX_DISTANCE
-from .post_naming import leading_name, rarity, token_frequencies
+from .post_naming import FAMILY_MAX_POSTS, leading_name, rarity, token_frequencies
-
-# A leading name spanning this many of ONE ARTIST's posts is a habit — a
-# character the creator returns to — not one piece's trickle.
-#
-# Its own value rather than post_naming.MAX_TOKEN_POSTS (6), which is
-# calibrated for PAIRING two posts, and measured too tight for a family. On
-# artist 8, `tentacooler` spans 6 posts over 7 days and `0-k1` 6 posts over 10:
-# both real families, both gated out at 6. At 8, `anya` (7) passes the cap —
-# and has no pair inside the family window, which is what the window is for.
-FAMILY_MAX_POSTS = 8
 
 # The text each referenced post contributes to the card, per post. The card
 # clamps it again; this keeps a long Discord thread from making the feed

backend/app/tasks/maintenance.py

@@ -1175,7 +1175,7 @@ def group_discord_drops() -> str:
         return "disabled"
     return (
         f"sources={res['sources']} created={res['posts_created']} "
-        f"joined={res['images_joined']}"
+        f"joined={res['images_joined']} merged={res['drops_merged']}"
     )
 
 

tests/test_discord_grouping.py

@@ -374,6 +374,7 @@ async def test_the_sweep_is_a_no_op_when_the_switch_is_off(db):
     # `images_joined` broke this assertion, which is exactly what it is for.
     assert result == {
         "enabled": False, "sources": 0, "posts_created": 0, "images_joined": 0,
+        "drops_merged": 0,
     }
 
 

tests/test_discord_trickles.py

@@ -0,0 +1,253 @@
+"""#4390: a creator's trickle is one drop, not a card per message.
+
+Operator, 2026-09-24, on a feed of "Grouped from 1 Discord message" cards:
+*"the groups are still single image even when they can clearly be seen as
+group"*. 665 of Yellowroom's 714 drops were one message, because both join
+paths demand closeness to a drop's FIRST image and a piece's stages drift away
+from it — each is nearest the one before, not the first.
+
+The merge pass joins a later drop to an earlier one within 7 days when they
+share a working name, or when one's image is the other's nearest neighbour in
+the artist's library. Vectors here are built at stated angles (see
+test_discord_grouping._vec) so each test says exactly which image is nearest
+to which, rather than hoping.
+"""
+import math
+from datetime import UTC, datetime, timedelta
+
+import pytest
+from sqlalchemy import select
+
+from backend.app.models import (
+    Artist,
+    ImageRecord,
+    Post,
+    PostAssociation,
+    Source,
+)
+from backend.app.services.discord_grouping import (
+    DROP_GROUPER,
+    _compatible,
+    group_source,
+    merge_trickles,
+)
+
+pytestmark = pytest.mark.integration
+
+DIM = 1152
+GAP = timedelta(hours=168)
+T0 = datetime.now(UTC) - timedelta(days=30)
+
+
+def _vec(angle: float) -> list[float]:
+    v = [0.0] * DIM
+    v[0] = math.cos(angle)
+    v[1] = math.sin(angle)
+    return v
+
+
+# --- the compatibility rule, pure --------------------------------------------
+
+
+def test_unnamed_stages_fit_with_anything():
+    """The Marin case: canvas screenshots carry no name, and two early stages
+    both nearest to one later stage are one trickle, not an ambiguity."""
+    assert _compatible([set(), set(), set()])
+    assert _compatible([set(), {"svtt"}])
+
+
+def test_two_differently_named_pieces_never_meet():
+    assert not _compatible([{"alpha"}, {"beta"}, set()])
+
+
+def test_pieces_sharing_a_name_are_one_piece():
+    assert _compatible([{"year", "20k"}, {"year"}])
+
+
+# --- end to end --------------------------------------------------------------
+
+
+async def _seed(db, name):
+    artist = Artist(name=name, slug=name)
+    db.add(artist)
+    await db.flush()
+    source = Source(artist_id=artist.id, platform="discord",
+                    url=f"https://discord.com/channels/1/{name}", enabled=True)
+    db.add(source)
+    await db.flush()
+    return artist, source
+
+
+_n = iter(range(1, 100_000))
+
+
+async def _message(db, artist, source, *, at, angle, name=None, text=None):
+    n = next(_n)
+    post = Post(source_id=source.id, artist_id=artist.id,
+                external_post_id=f"msg-{n}", post_date=at, description=text)
+    db.add(post)
+    await db.flush()
+    # gallery-dl's Discord shape, so the prefix strips and NAME is the working
+    # name. Unnamed messages are canvas screenshots, which carry none.
+    stem = name or f"Screenshot_2026-09-08_{n:06d}"
+    db.add(ImageRecord(
+        path=f"/images/{artist.slug}/20260901_{1234567890000 + n}_01_{stem}.png",
+        sha256=f"{n:064d}", size_bytes=10, mime="image/png", width=10, height=10,
+        origin="downloaded", primary_post_id=post.id, artist_id=artist.id,
+        siglip_embedding=_vec(angle),
+    ))
+    await db.flush()
+    return post
+
+
+async def _group_then_merge(db, source):
+    """E2 makes the singletons exactly as it does live; then the merge pass."""
+    await group_source(db, source, max_distance=0.10, window_minutes=60)
+    merged = await merge_trickles(
+        db, source, gap=GAP, min_images=2, cooldown=timedelta(hours=24),
+    )
+    await db.commit()
+    return merged
+
+
+async def _drops(db, source):
+    return (await db.execute(
+        select(Post).where(
+            Post.source_id == source.id,
+            Post.synthesized_by == DROP_GROUPER,
+            Post.absorbed_by_post_id.is_(None),
+        ).order_by(Post.post_date)
+    )).scalars().all()
+
+
+@pytest.mark.asyncio
+async def test_stages_a_day_apart_become_one_drop(db):
+    """The screenshot, measured shape: each stage 0.12 from the next and 0.46
+    from the first, so seed distance splits them and nearest-neighbour joins
+    them."""
+    artist, source = await _seed(db, "trickle-artist")
+    a = await _message(db, artist, source, at=T0, angle=0.0, text="Very early Marin.")
+    b = await _message(db, artist, source, at=T0 + timedelta(hours=17), angle=0.5,
+                       text="Might get mirrored.")
+    c = await _message(db, artist, source, at=T0 + timedelta(hours=20), angle=1.0,
+                       text="Got there eventually.")
+    await db.commit()
+
+    assert await _group_then_merge(db, source) == 2
+
+    (drop,) = await _drops(db, source)
+    assert set(drop.synthesis_details["member_post_ids"]) == {a.id, b.id, c.id}
+    assert drop.synthesis_details["message_count"] == 3
+    # The body is every message's text, in arrival order.
+    assert drop.description.index("Very early") < drop.description.index("Got there")
+    # And it says why — a grouping FC invented has to be checkable.
+    assert {m["route"] for m in drop.synthesis_details["merged"]} == {"nearest"}
+
+
+@pytest.mark.asyncio
+async def test_a_shared_working_name_joins_across_days(db):
+    """`svtt_wip3` did not join `svtt_wip4` from the day before on the live
+    instance. Orthogonal vectors here, so only the name can do it — and the
+    route it records says so."""
+    artist, source = await _seed(db, "named-artist")
+    await _message(db, artist, source, at=T0, angle=0.0, name="svtt_wip3")
+    await _message(db, artist, source, at=T0 + timedelta(days=2), angle=math.pi / 2,
+                   name="svtt_drench_b")
+    await db.commit()
+
+    await _group_then_merge(db, source)
+
+    (drop,) = await _drops(db, source)
+    assert drop.synthesis_details["merged"][0]["route"] == "name:svtt"
+
+
+@pytest.mark.asyncio
+async def test_nothing_joins_across_a_quiet_week(db):
+    """Measured: where the nearest neighbour was another message within 7 days
+    the names agreed 53 times of 53; past 7 days they begin to disagree."""
+    artist, source = await _seed(db, "quiet-artist")
+    await _message(db, artist, source, at=T0, angle=0.0, name="alpha_wip1")
+    await _message(db, artist, source, at=T0 + timedelta(days=8), angle=0.05,
+                   name="alpha_base")
+    await db.commit()
+
+    assert await _group_then_merge(db, source) == 0
+    assert len(await _drops(db, source)) == 2
+
+
+@pytest.mark.asyncio
+async def test_two_named_pieces_meeting_through_a_third_stay_apart(db):
+    """C's image is nearest A's; B's image is nearest C's. A is `alpha`, B is
+    `beta` — so C reaches two different pieces and nothing moves."""
+    artist, source = await _seed(db, "bridge-artist")
+    await _message(db, artist, source, at=T0, angle=0.0, name="alpha")
+    await _message(db, artist, source, at=T0 + timedelta(hours=3), angle=0.6, name="beta")
+    await _message(db, artist, source, at=T0 + timedelta(days=1), angle=0.25)
+    await db.commit()
+
+    assert await _group_then_merge(db, source) == 0
+    assert len(await _drops(db, source)) == 3
+
+
+@pytest.mark.asyncio
+async def test_a_teaser_link_survives_its_drop_being_merged(db):
+    """The association's payload FK cascades. Merging the drop a teaser was
+    linked to must carry the link across, or it silently undoes #4402."""
+    artist, source = await _seed(db, "linked-artist")
+    patreon = Source(artist_id=artist.id, platform="patreon",
+                     url="https://patreon.com/linked-artist", enabled=True)
+    db.add(patreon)
+    await db.flush()
+    teaser = Post(source_id=patreon.id, artist_id=artist.id, external_post_id="teaser",
+                  post_date=T0 + timedelta(days=1, hours=2))
+    db.add(teaser)
+    await _message(db, artist, source, at=T0, angle=0.0, name="svtt_wip3")
+    await _message(db, artist, source, at=T0 + timedelta(days=1), angle=math.pi / 2,
+                   name="svtt_drench_b")
+    await db.commit()
+    await group_source(db, source, max_distance=0.10, window_minutes=60)
+    later = (await _drops(db, source))[-1]
+    db.add(PostAssociation(announcement_post_id=teaser.id, payload_post_id=later.id,
+                           score=1.0, status="linked", linked_by="fc"))
+    await db.commit()
+
+    await merge_trickles(db, source, gap=GAP, min_images=2, cooldown=timedelta(hours=24))
+    await db.commit()
+
+    (drop,) = await _drops(db, source)
+    link = (await db.execute(select(PostAssociation))).scalar_one()
+    assert (link.payload_post_id, link.status, link.linked_by) == (drop.id, "linked", "fc")
+
+
+@pytest.mark.asyncio
+async def test_merging_history_does_not_drag_it_to_the_top_of_the_feed(db):
+    """Growth is stamped at the merged messages' own time. A merge the first
+    sweep makes over two-year-old drops must not read as news today."""
+    artist, source = await _seed(db, "history-artist")
+    old = datetime.now(UTC) - timedelta(days=700)
+    await _message(db, artist, source, at=old, angle=0.0, name="svtt_wip1")
+    await _message(db, artist, source, at=old + timedelta(days=1), angle=0.5, name="svtt_wip2")
+    await _message(db, artist, source, at=old + timedelta(days=2), angle=1.0, name="svtt_base")
+    await db.commit()
+
+    await _group_then_merge(db, source)
+
+    (drop,) = await _drops(db, source)
+    assert drop.last_grew_at <= old + timedelta(days=2, minutes=1)
+    assert drop.resurfaced_at is None or drop.resurfaced_at <= old + timedelta(days=2, minutes=1)
+
+
+@pytest.mark.asyncio
+async def test_a_checked_drop_is_not_examined_again(db):
+    """Each drop costs one nearest-neighbour query per image, once. The flag is
+    what stops every hourly sweep repeating the whole history."""
+    artist, source = await _seed(db, "checked-artist")
+    await _message(db, artist, source, at=T0, angle=0.0, name="alpha")
+    await _message(db, artist, source, at=T0 + timedelta(days=3), angle=math.pi / 2,
+                   name="beta")
+    await db.commit()
+    await _group_then_merge(db, source)
+
+    for drop in await _drops(db, source):
+        assert drop.synthesis_details["trickle_checked"] is True
+        assert "nearest_message_ids" in drop.synthesis_details

tests/test_post_unification.py

@@ -29,11 +29,8 @@ from backend.app.models import (
 from backend.app.services.discord_grouping import DROP_GROUPER
 from backend.app.services.post_association_service import PostAssociationService
 from backend.app.services.post_feed_service import PostFeedService
-from backend.app.services.post_unification import (
+from backend.app.services.post_naming import FAMILY_MAX_POSTS
-    FAMILY_MAX_POSTS,
+from backend.app.services.post_unification import Candidate, family
-    Candidate,
-    family,
-)
 
 T0 = datetime(2026, 9, 1, 12, 0, tzinfo=UTC)
 WINDOW = timedelta(days=60)