diff --git a/backend/app/api/sources.py b/backend/app/api/sources.py
index e7c2e15..4438c4c 100644
--- a/backend/app/api/sources.py
+++ b/backend/app/api/sources.py
@@ -7,7 +7,9 @@ from ..extensions import get_session
 from ..models import DownloadEvent, MembershipSync, PlatformMembership, Source
 from ..services.artist_membership_service import ArtistMembershipService
 from ..services.artist_membership_service import rescan as membership_rescan
-from ..services.membership_roster import roster_is_fresh
+from ..services.artist_service import ArtistService
+from ..services.membership_reconcile import reconcile_all
+from ..services.membership_roster import roster_is_fresh, source_for_membership
 from ..services.scheduler_service import active_platform_cooldowns, scheduler_status
 from ..services.source_service import (
     KNOWN_PLATFORMS,
@@ -386,3 +388,68 @@ async def rescan_membership_suggestions():
         result = await membership_rescan(session)
         await session.commit()
     return jsonify(result)
+
+
+# --- #387 C4: reconciling the roster against what FC actually tracks -------
+#
+# Asymmetric on purpose. The "you subscribe but FC doesn't follow it" direction
+# carries a per-row action, because adding a source is the reversible half. The
+# "FC follows it but your roster doesn't show it" direction is REPORT ONLY by
+# the operator's decision (2026-09-11): it says what it sees and links to the
+# Subscriptions row, and offers no one-click disable.
+
+
+@sources_bp.route("/reconciliation", methods=["GET"])
+async def reconciliation():
+    async with get_session() as session:
+        return jsonify(await reconcile_all(session))
+
+
+@sources_bp.route("/reconciliation/adopt", methods=["POST"])
+async def adopt_membership():
+    """Start tracking a creator the roster says the account already pays for.
+
+    One row, one click, never a sweep side effect: adding a source commits disk,
+    worker time and rate budget, and unwinding it means deleting files.
+    """
+    body = await request.get_json()
+    if not isinstance(body, dict):
+        return _bad("invalid_body", status=400)
+    membership_id = body.get("membership_id")
+    if not isinstance(membership_id, int):
+        return _bad("membership_id_required", status=400)
+
+    async with get_session() as session:
+        membership = await session.get(PlatformMembership, membership_id)
+        if membership is None:
+            return _bad("membership_not_found", status=404)
+        if not membership.url:
+            return _bad("membership_has_no_url", status=400)
+
+        existing = await source_for_membership(session, membership)
+        if existing is not None:
+            # The operator got there by another route between the page load and
+            # the click. That is them being ahead of us, not an error.
+            return jsonify({"already_tracked": existing.id})
+
+        # The sweep already captured the creator's real display name, so the
+        # artist gets its true name with NO lookup on the request path. Task
+        # #1293 asked for `resolve_display_name` here; the roster satisfies that
+        # concern earlier in the pipeline than #1293 expected, which also keeps
+        # this route off the network entirely (rule 164). The vanity is the
+        # fallback, never the preferred value.
+        name = membership.display_name or membership.vanity_or_none()
+        if not name:
+            return _bad("membership_has_no_name", status=400)
+
+        artist, _created = await ArtistService(session).find_or_create(name)
+        try:
+            record = await SourceService(session).create(
+                artist_id=artist.id,
+                platform=membership.platform,
+                url=membership.url,
+            )
+        except DuplicateSourceError as exc:
+            return jsonify({"already_tracked": exc.existing_id})
+        artist_id = artist.id
+    return jsonify({"source_id": record.id, "artist_id": artist_id}), 201
diff --git a/backend/app/services/artist_membership_service.py b/backend/app/services/artist_membership_service.py
index 3d42b74..020c673 100644
--- a/backend/app/services/artist_membership_service.py
+++ b/backend/app/services/artist_membership_service.py
@@ -75,6 +75,7 @@ from ..models import (
 )
 from ..utils.slug import slugify
 from ..utils.text import html_to_plain
+from .membership_roster import source_for_membership
 
 log = logging.getLogger(__name__)
 
@@ -204,6 +205,14 @@ class ArtistMembershipService:
         membership = await self.session.get(PlatformMembership, membership_id)
         if membership is None:
             return 0
+        # The shared identity join (C4), used here as the NEGATIVE check. A
+        # membership FC already has a source for is tracked — whoever it happens
+        # to be filed under — and proposing it to some OTHER artist would be
+        # exactly the wrong link this service exists to avoid making.
+        # `_candidate_artists` only knows whether a GIVEN artist has a source on
+        # the platform, which cannot see a source sitting under someone else.
+        if await source_for_membership(self.session, membership) is not None:
+            return 0
         already = await self._decided(membership_id)
 
         made = 0
diff --git a/backend/app/services/membership_reconcile.py b/backend/app/services/membership_reconcile.py
new file mode 100644
index 0000000..cd0df5e
--- /dev/null
+++ b/backend/app/services/membership_reconcile.py
@@ -0,0 +1,243 @@
+"""Reconciling the learned roster against the sources FC actually tracks.
+
+Milestone 387, step C4. The step the operator asked for; C0-C3 are what make it
+trustworthy enough to act on.
+
+## The buckets
+
+1. `subscribed_not_tracked` — you pay for this and FC does not follow it. The
+   adoption win, and the only bucket carrying an action.
+2. `tracked_not_subscribed` — FC follows this and the roster does not show you
+   paying for it. REPORT ONLY, by the operator's decision (2026-09-11): it says
+   what it sees and links to the existing Subscriptions row, and offers no
+   one-click disable.
+3. `matched` — the healthy set. Counted, not listed loudly.
+4. `unidentified` — sources this join cannot speak to at all. Reported as
+   exactly that, because the alternative is filing them under a verdict.
+
+## Why absence is the dangerous direction
+
+Bucket 1 is safe to be wrong about: the cost of offering a source the operator
+does not want is one ignored row. Bucket 2 is not. It is computed from an
+ABSENCE — no membership matched — and three different things produce that
+absence: the subscription genuinely lapsed, the sweep failed, or the creator
+renamed and this source has never been walked so no exact id was ever cached.
+
+Two guards follow from that, and they are the substance of this module:
+
+* the whole bucket is gated on `roster_is_fresh`, so a failed or never-run sweep
+  yields an empty list rather than a confident accusation (C3 built the state
+  this reads);
+* every row carries the BASIS for its claim, so "your membership says former
+  patron" and "we know this creator's id and it is not in your roster" and "we
+  only have a URL handle to go on" are three different sentences rather than one
+  overconfident one.
+
+`has_paid_access` returning None is honoured throughout: unknown is never
+rendered as lapsed. That is the whole reason it returns a tri-state.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from ..models import Artist, MembershipSync, PlatformMembership, Source
+from .membership_roster import (
+    get_sync_state,
+    has_paid_access,
+    identity_keys_for_source,
+    match_kind,
+    roster_is_fresh,
+    url_tail,
+)
+
+# Why a source appears in `tracked_not_subscribed`. Ordered strongest first —
+# the UI renders a different sentence per basis, because collapsing them into
+# one would make the weakest claim sound like the strongest.
+BASIS_LAPSED = "lapsed"           # a matched membership says access ended
+BASIS_ABSENT_EXACT = "absent_exact"    # exact id known, not in a fresh roster
+BASIS_ABSENT_HANDLE = "absent_handle"  # only a URL handle to go on
+
+
+def _membership_row(m: PlatformMembership) -> dict:
+    return {
+        "id": m.id,
+        "platform": m.platform,
+        "external_campaign_id": m.external_campaign_id,
+        "display_name": m.display_name or m.vanity_or_none(),
+        "url": m.url,
+        "vanity": m.vanity_or_none(),
+        "status": m.status,
+        "tier_names": m.tier_names,
+        "amount_cents": m.amount_cents,
+        "currency": m.currency,
+        "paid_access": has_paid_access(
+            m.platform, m.status,
+            is_free_member=bool((m.details or {}).get("is_free_member")),
+        ),
+    }
+
+
+def _source_row(source: Source, artist: Artist) -> dict:
+    return {
+        "id": source.id,
+        "platform": source.platform,
+        "url": source.url,
+        "enabled": source.enabled,
+        "artist": {"id": artist.id, "name": artist.name, "slug": artist.slug},
+    }
+
+
+async def reconcile(
+    session: AsyncSession, *, platform: str, now: datetime | None = None,
+) -> dict:
+    """Sort one platform's memberships and sources into the four buckets.
+
+    Always returns the COMPLETE shape, including when the roster is not fresh —
+    a caller reading `len(result["tracked_not_subscribed"])` must not have to
+    check which keys exist first. `fresh` is what says whether the emptiness
+    means anything.
+    """
+    state = await get_sync_state(session, platform)
+    fresh = roster_is_fresh(state, now=now)
+
+    memberships = (await session.execute(
+        select(PlatformMembership).where(PlatformMembership.platform == platform)
+    )).scalars().all()
+    rows = (await session.execute(
+        select(Source, Artist)
+        .join(Artist, Artist.id == Source.artist_id)
+        .where(Source.platform == platform)
+    )).all()
+
+    # Nested loop rather than a SQL join: the match is a predicate over a JSON
+    # blob and a derived URL handle, neither of which is indexable, and both
+    # sides are tens of rows on any real library. Keeping it in Python means ONE
+    # definition of identity (`match_kind`) instead of a second one in SQL that
+    # could drift from it.
+    pairs: dict[int, tuple[PlatformMembership, str]] = {}
+    matched_membership_ids: set[int] = set()
+    for source, _artist in rows:
+        for m in memberships:
+            kind = match_kind(source, m)
+            if kind:
+                pairs[source.id] = (m, kind)
+                matched_membership_ids.add(m.id)
+                break
+
+    subscribed_not_tracked = []
+    for m in memberships:
+        if m.id in matched_membership_ids:
+            continue
+        paid = has_paid_access(
+            m.platform, m.status,
+            is_free_member=bool((m.details or {}).get("is_free_member")),
+        )
+        # A membership FC knows has ENDED is not an adoption opportunity —
+        # adding it would start a walk that can only fetch what is already
+        # public. Unknown (None) is still offered: the operator can judge it,
+        # and refusing to show it would hide a real subscription behind a word
+        # this code has not been taught.
+        if paid is False:
+            continue
+        subscribed_not_tracked.append(_membership_row(m))
+
+    tracked_not_subscribed = []
+    matched = []
+    unidentified = []
+    for source, artist in rows:
+        pair = pairs.get(source.id)
+        if pair is not None:
+            m, kind = pair
+            paid = has_paid_access(
+                m.platform, m.status,
+                is_free_member=bool((m.details or {}).get("is_free_member")),
+            )
+            if paid is False:
+                if not source.enabled:
+                    # Already off. Reporting a source the operator has already
+                    # stopped following is noise, not a finding.
+                    continue
+                row = _source_row(source, artist)
+                row["basis"] = BASIS_LAPSED
+                row["matched_by"] = kind
+                row["membership"] = _membership_row(m)
+                tracked_not_subscribed.append(row)
+            else:
+                row = _source_row(source, artist)
+                row["matched_by"] = kind
+                row["membership"] = _membership_row(m)
+                matched.append(row)
+            continue
+
+        # No membership matched. Whether that MEANS anything depends entirely on
+        # how well this source can be identified at all.
+        has_exact = bool(identity_keys_for_source(source))
+        if not has_exact and url_tail(source.url) is None:
+            # Nothing to match on — a sidecar anchor or a URL with no handle.
+            # Reported as unidentified rather than silently dropped, so the
+            # counts add up to the source list the operator can see.
+            unidentified.append(_source_row(source, artist))
+            continue
+        if not source.enabled:
+            # Already off. Telling the operator to stop following something they
+            # have stopped following is noise, not a finding.
+            continue
+        row = _source_row(source, artist)
+        row["basis"] = BASIS_ABSENT_EXACT if has_exact else BASIS_ABSENT_HANDLE
+        row["matched_by"] = None
+        row["membership"] = None
+        tracked_not_subscribed.append(row)
+
+    # THE GATE. Everything above computed the bucket; this decides whether it may
+    # be shown. A stale or never-run roster makes every absence meaningless, and
+    # an absence rendered as a verdict is how this feature would tell the
+    # operator to cancel something they are still paying for.
+    if not fresh:
+        tracked_not_subscribed = []
+
+    return {
+        "platform": platform,
+        "fresh": fresh,
+        # How many sources exist on this platform at all. The UI needs it to
+        # decide whether an untrustworthy roster is worth mentioning: with no
+        # sources here there is nothing to reconcile, and a stale-roster warning
+        # would be noise on an install that simply has not started yet (that
+        # empty-install case is C6's, not this card's).
+        "tracked_total": len(rows),
+        "last_success_at": (
+            state.last_success_at.isoformat()
+            if state is not None and state.last_success_at else None
+        ),
+        "subscribed_not_tracked": subscribed_not_tracked,
+        "tracked_not_subscribed": tracked_not_subscribed,
+        "matched": matched,
+        "unidentified": unidentified,
+    }
+
+
+async def reconcile_all(session: AsyncSession, now: datetime | None = None) -> dict:
+    """Every platform the roster knows about, in one payload for the UI.
+
+    The platform list is the UNION of platforms with memberships and platforms
+    with sync state, not just the former. A sweep that has never succeeded has
+    recorded zero memberships, and deriving the list from memberships alone
+    would drop exactly that platform from the payload — making a broken
+    credential indistinguishable from a platform FC was never asked about. That
+    distinction is the whole reason C3 records sync state.
+    """
+    with_memberships = (await session.execute(
+        select(PlatformMembership.platform).distinct()
+    )).scalars().all()
+    with_state = (await session.execute(
+        select(MembershipSync.platform)
+    )).scalars().all()
+    platforms = set(with_memberships) | set(with_state)
+    return {
+        "platforms": [
+            await reconcile(session, platform=p, now=now) for p in sorted(platforms)
+        ]
+    }
diff --git a/backend/app/services/membership_roster.py b/backend/app/services/membership_roster.py
index e5d7568..8e9c43e 100644
--- a/backend/app/services/membership_roster.py
+++ b/backend/app/services/membership_roster.py
@@ -34,7 +34,7 @@ from sqlalchemy import func, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from ..models import MembershipSync, PlatformMembership
+from ..models import MembershipSync, PlatformMembership, Source
 
 log = logging.getLogger(__name__)
 
@@ -275,3 +275,96 @@ async def sync_platform(
     await session.commit()
     log.info("membership sync ok for %s: %d membership(s)", platform, len(memberships))
     return {"platform": platform, "ok": True, "count": len(memberships)}
+
+
+# ---------------------------------------------------------------------------
+# Membership <-> Source identity (#387 C4)
+# ---------------------------------------------------------------------------
+#
+# "Is this membership already tracked?" is asked by TWO features — C4's
+# reconciliation buckets and E4's creator suggestions — and it lives here, once,
+# on purpose. Built inline in C4 it would have looked finished while leaving E4
+# matching on name similarity alone, so the two would answer the same question
+# differently and only one of them would be right.
+#
+# E4 and C4 use it from opposite sides: E4 as the NEGATIVE check (propose only
+# where nothing matches) and C4 as the join itself.
+
+# Any platform that caches its creator id does so under this suffix; see
+# `download_service._phase3_persist`, which writes `patreon_campaign_id`.
+_CAMPAIGN_KEY_SUFFIX = "_campaign_id"
+
+
+def identity_keys_for_source(source: Source) -> set[str]:
+    """Every platform-side creator id cached on this source.
+
+    Reads ANY `<platform>_campaign_id` override rather than naming Patreon's,
+    so a second platform participates by caching its id under the same suffix —
+    no registry, no `if platform ==` branch (rule 169). A source that has never
+    been walked has cached nothing and simply contributes no exact key, which is
+    what makes the handle fallback below necessary rather than merely tolerated.
+    """
+    keys = set()
+    for name, value in (source.config_overrides or {}).items():
+        if name.endswith(_CAMPAIGN_KEY_SUFFIX) and isinstance(value, str) and value:
+            keys.add(value)
+    return keys
+
+
+def url_tail(url: str | None) -> str | None:
+    """The creator handle at the end of a source URL, lowercased.
+
+    Deliberately the same derivation as `PlatformMembership.vanity_or_none`'s
+    own fallback, so both sides of the comparison reduce a URL to a handle the
+    same way. Query strings and fragments are stripped first; Patreon's `/c/`
+    and `/cw/` forms both end in the vanity, so they need no special case (the
+    missing-`/c/` regex is what broke creator detection in #1485).
+
+    Returns None for the pre-0030 `sidecar:` synthetic anchors, which are not
+    feeds and must never match anything.
+    """
+    if not url or url.startswith("sidecar:"):
+        return None
+    cleaned = url.split("?", 1)[0].split("#", 1)[0]
+    tail = cleaned.rstrip("/").rsplit("/", 1)[-1]
+    return tail.lower() or None
+
+
+def match_kind(source: Source, membership: PlatformMembership) -> str | None:
+    """How this source and this membership are known to be the same creator.
+
+    Returns "campaign" for an exact platform-id match, "vanity" for agreeing URL
+    handles, or None for no evidence.
+
+    THE ORDER MUST NOT BE INVERTED. The campaign id is exact and the handle is
+    not, but the id is only written AFTER a source has been walked at least once
+    — so checking the handle first would let a stale or renamed URL outvote the
+    authoritative id on every source FC has actually polled.
+    """
+    if source.platform != membership.platform:
+        return None
+    if membership.external_campaign_id in identity_keys_for_source(source):
+        return "campaign"
+    vanity = membership.vanity_or_none()
+    tail = url_tail(source.url)
+    if vanity and tail and vanity.strip().lower() == tail:
+        return "vanity"
+    return None
+
+
+async def source_for_membership(
+    session: AsyncSession, membership: PlatformMembership,
+) -> Source | None:
+    """The source FC already tracks for this membership, if there is one.
+
+    Scoped to the membership's own platform, so a creator tracked on Discord and
+    subscribed to on Patreon does not read as already-tracked — that pairing is
+    E4's suggestion to make, not an identity.
+    """
+    rows = (await session.execute(
+        select(Source).where(Source.platform == membership.platform)
+    )).scalars().all()
+    for source in rows:
+        if match_kind(source, membership):
+            return source
+    return None
diff --git a/frontend/src/components/subscriptions/MembershipReconcileCard.vue b/frontend/src/components/subscriptions/MembershipReconcileCard.vue
new file mode 100644
index 0000000..5c7f511
--- /dev/null
+++ b/frontend/src/components/subscriptions/MembershipReconcileCard.vue
@@ -0,0 +1,144 @@
+<template>
+  <!-- Renders nothing when there is nothing to say — same posture as
+       NeedsAttentionCard. A reconciliation card that always shows would train
+       the operator to scroll past it. -->
+  <v-card v-if="anythingToSay" variant="tonal" class="mb-4 fc-recon">
+    <v-card-text>
+      <div v-for="p in interesting" :key="p.platform" class="fc-recon__platform">
+        <!-- Bucket 1: the adoption win. The only direction with an action,
+             because adding a source is the reversible half. -->
+        <template v-if="p.subscribed_not_tracked.length">
+          <div class="fc-recon__head">
+            <v-icon icon="mdi-account-plus-outline" size="small" class="me-2" />
+            <strong>
+              {{ p.subscribed_not_tracked.length }}
+              {{ p.platform }}
+              {{ p.subscribed_not_tracked.length === 1 ? 'creator' : 'creators' }}
+              you subscribe to but don't follow here
+            </strong>
+          </div>
+          <div v-for="m in p.subscribed_not_tracked" :key="m.id" class="fc-recon__row">
+            <div class="fc-recon__body">
+              <strong>{{ m.display_name }}</strong>
+              <span v-if="m.tier_names?.length" class="fc-recon__dim">
+                · {{ m.tier_names.join(', ') }}
+              </span>
+              <!-- A membership whose status this build has not been taught is
+                   still offered, and says so rather than being presented as a
+                   confirmed active subscription. -->
+              <span v-if="m.paid_access === null" class="fc-recon__dim">
+                · status not recognised ({{ m.status }})
+              </span>
+            </div>
+            <v-btn size="small" variant="tonal" @click="store.adopt(m.id)">
+              Add subscription
+            </v-btn>
+          </div>
+        </template>
+
+        <!-- Bucket 2: REPORT ONLY. No disable control here by design — the
+             source list on this same page is where that decision belongs. -->
+        <template v-if="p.tracked_not_subscribed.length">
+          <div class="fc-recon__head">
+            <v-icon icon="mdi-help-circle-outline" size="small" class="me-2" />
+            <strong>
+              {{ p.tracked_not_subscribed.length }}
+              {{ p.platform }}
+              {{ p.tracked_not_subscribed.length === 1 ? 'source' : 'sources' }}
+              your roster doesn't account for
+            </strong>
+          </div>
+          <div v-for="s in p.tracked_not_subscribed" :key="s.id" class="fc-recon__row">
+            <div class="fc-recon__body">
+              <strong>{{ s.artist.name }}</strong>
+              <div class="fc-recon__dim">{{ reasonFor(s) }}</div>
+            </div>
+          </div>
+          <div class="fc-recon__dim fc-recon__note">
+            Nothing has been changed. If you want one of these to stop checking,
+            disable it in the list below.
+          </div>
+        </template>
+
+        <!-- The roster could not be trusted, so bucket 2 was not computed. Said
+             in words: an empty list here must never read as "all clear". -->
+        <div v-if="!p.fresh" class="fc-recon__dim fc-recon__note">
+          <v-icon icon="mdi-clock-alert-outline" size="small" class="me-1" />
+          <template v-if="!p.last_success_at">
+            Your {{ p.platform }} roster has never synced, so FC can't tell which
+            sources you still subscribe to.
+          </template>
+          <template v-else>
+            Your {{ p.platform }} roster last synced
+            {{ formatRelative(p.last_success_at) }}, which is too old to draw
+            conclusions from.
+          </template>
+          Sync it from Settings → Subscription roster.
+        </div>
+      </div>
+    </v-card-text>
+  </v-card>
+</template>
+
+<script setup>
+import { computed, onMounted } from 'vue'
+
+import { formatRelative } from '../../utils/date.js'
+import { useMembershipReconcileStore } from '../../stores/membershipReconcile.js'
+
+// #387 C4. Two directions, deliberately unequal: subscriptions FC doesn't
+// follow get a one-click add, while sources the roster doesn't account for are
+// REPORTED ONLY (operator decision, 2026-09-11). The asymmetry is the point —
+// adding a source is trivially undone, and "you no longer subscribe to this" is
+// computed from an absence that has three possible causes.
+const store = useMembershipReconcileStore()
+
+// An untrustworthy roster is only worth mentioning when there is something it
+// would have reconciled — otherwise a failed sweep on a platform with no
+// sources yet would put a warning on a page with nothing to warn about.
+const interesting = computed(() => store.platforms.filter(
+  p => p.subscribed_not_tracked.length
+    || p.tracked_not_subscribed.length
+    || (!p.fresh && p.tracked_total)
+))
+const anythingToSay = computed(() => interesting.value.length > 0)
+
+function reasonFor (s) {
+  if (s.basis === 'lapsed') {
+    return `Your membership reads "${s.membership?.status}" — you're not a paying supporter of this creator right now.`
+  }
+  if (s.basis === 'absent_exact') {
+    return "FC knows this creator's id on the platform, and it isn't in your roster."
+  }
+  // absent_handle — the weakest claim, and it says so. A renamed creator looks
+  // exactly like this, which is why it is not phrased as a conclusion.
+  return "No membership matched this source's address. It may simply have been renamed."
+}
+
+onMounted(() => { store.load() })
+</script>
+
+<style scoped>
+.fc-recon__platform + .fc-recon__platform {
+  margin-top: 16px;
+}
+.fc-recon__head {
+  display: flex;
+  align-items: center;
+  margin: 8px 0 4px;
+}
+.fc-recon__row {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 0;
+  border-top: 1px solid rgba(var(--v-theme-on-surface), 0.12);
+}
+.fc-recon__body { flex: 1; min-width: 0; }
+/* Rule 96: dim text is the theme token, never bare opacity. */
+.fc-recon__dim {
+  color: rgb(var(--v-theme-on-surface-variant));
+  font-size: 0.8125rem;
+}
+.fc-recon__note { margin-top: 8px; }
+</style>
diff --git a/frontend/src/components/subscriptions/SubscriptionsTab.vue b/frontend/src/components/subscriptions/SubscriptionsTab.vue
index b5ef2b2..16597e4 100644
--- a/frontend/src/components/subscriptions/SubscriptionsTab.vue
+++ b/frontend/src/components/subscriptions/SubscriptionsTab.vue
@@ -7,6 +7,9 @@
          nothing to say. -->
     <NeedsAttentionCard />
     <RecentArrivalsCard />
+    <!-- #387 C4: what you pay for vs. what FC follows. Renders nothing when
+         the two agree. -->
+    <MembershipReconcileCard />
 
     <div class="fc-subs__bar">
       <v-btn color="accent" prepend-icon="mdi-plus" @click="openAddSource(null)">
@@ -327,6 +330,7 @@ import { usePlatformsStore } from '../../stores/platforms.js'
 import { useImportStore } from '../../stores/import.js'
 import NeedsAttentionCard from './NeedsAttentionCard.vue'
 import RecentArrivalsCard from './RecentArrivalsCard.vue'
+import MembershipReconcileCard from './MembershipReconcileCard.vue'
 import SourceRow from './SourceRow.vue'
 import SourceCard from './SourceCard.vue'
 import SourceHealthDot from './SourceHealthDot.vue'
diff --git a/frontend/src/stores/membershipReconcile.js b/frontend/src/stores/membershipReconcile.js
new file mode 100644
index 0000000..66d6a90
--- /dev/null
+++ b/frontend/src/stores/membershipReconcile.js
@@ -0,0 +1,48 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
+import { toast } from '../utils/toast.js'
+
+// Backs the reconciliation card (#387 C4). Two directions with deliberately
+// different weights: adopting a creator you already pay for is a per-row
+// action, while "you follow this and your roster doesn't show it" only ever
+// REPORTS — the operator disables from the Subscriptions row itself.
+//
+// `fresh` is the field that matters most here. A stale or never-run roster
+// returns an empty tracked_not_subscribed list, and the card must say WHY it is
+// empty rather than letting it read as "everything is fine".
+export const useMembershipReconcileStore = defineStore('membershipReconcile', () => {
+  const api = useApi()
+  const platforms = ref([])
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
+
+  async function load () {
+    await run(async () => {
+      const body = await api.get('/api/sources/reconciliation')
+      platforms.value = body.platforms || []
+    })
+  }
+
+  async function adopt (membershipId) {
+    try {
+      const res = await api.post('/api/sources/reconciliation/adopt', {
+        membership_id: membershipId
+      })
+      toast({
+        text: res.already_tracked
+          ? 'Already tracked — nothing to add'
+          : 'Now tracking this creator',
+        type: 'success'
+      })
+      // Reload rather than splice: adopting moves the row from one bucket to
+      // another, and the counts beside them have to move with it.
+      await load()
+    } catch (e) {
+      toast({ text: `Could not add: ${e?.body?.detail || e.message}`, type: 'error' })
+    }
+  }
+
+  return { platforms, loading, error, load, adopt }
+})
diff --git a/tests/test_artist_membership_suggestions.py b/tests/test_artist_membership_suggestions.py
index 2a9bbe6..6e3c7fd 100644
--- a/tests/test_artist_membership_suggestions.py
+++ b/tests/test_artist_membership_suggestions.py
@@ -166,6 +166,30 @@ async def test_an_artist_already_on_that_platform_is_never_proposed(db):
     assert await ArtistMembershipService(db).match_membership(m.id) == 0
 
 
+@pytest.mark.asyncio
+async def test_a_membership_already_tracked_elsewhere_is_never_proposed(db):
+    """C4's shared identity join, used here as the NEGATIVE check.
+
+    A membership FC already has a source for is tracked — even when that source
+    sits under a DIFFERENT artist — and proposing it to this one would be the
+    wrong link this service exists to avoid. `_candidate_artists` cannot see
+    this on its own: it only knows whether the artist in front of it has a
+    source on the platform, not whether someone else already does.
+    """
+    await _artist_with_discord(db, "Maewix", "maewix")
+    other = Artist(name="Someone Else", slug="someoneelse")
+    db.add(other)
+    await db.flush()
+    db.add(Source(
+        artist_id=other.id, platform="patreon",
+        url="https://www.patreon.com/maewix", enabled=True,
+    ))
+    m = await _membership(db, display_name="Maewix")
+    await db.commit()
+
+    assert await ArtistMembershipService(db).match_membership(m.id) == 0
+
+
 @pytest.mark.asyncio
 async def test_an_artist_with_no_sources_at_all_is_never_proposed(db):
     """Not a creator FC follows through another channel, which is the whole
diff --git a/tests/test_membership_reconcile.py b/tests/test_membership_reconcile.py
new file mode 100644
index 0000000..95bde7f
--- /dev/null
+++ b/tests/test_membership_reconcile.py
@@ -0,0 +1,320 @@
+"""Milestone 387 C4: what you pay for, against what FC actually follows.
+
+The buckets are easy; the honesty is the work. `tracked_not_subscribed` is
+computed from an ABSENCE — no membership matched this source — and three
+different things produce that absence: the subscription really lapsed, the
+sweep failed, or the creator renamed and this source has never been walked so
+no exact id was ever cached. Only the first means what the bucket's name says.
+
+So most of what follows pins refusals: the gate that empties the bucket when the
+roster cannot be trusted, the tri-state that keeps "unknown" out of "lapsed",
+and the per-row basis that stops the weakest claim sounding like the strongest.
+"""
+from datetime import UTC, datetime, timedelta
+
+import pytest
+
+from backend.app.models import Artist, MembershipSync, PlatformMembership, Source
+from backend.app.services.membership_reconcile import (
+    BASIS_ABSENT_EXACT,
+    BASIS_ABSENT_HANDLE,
+    BASIS_LAPSED,
+    reconcile,
+    reconcile_all,
+)
+from backend.app.services.membership_roster import ROSTER_STALE_AFTER
+
+pytestmark = pytest.mark.integration
+
+
+async def _artist(db, name="Maewix"):
+    a = Artist(name=name, slug=name.lower().replace(" ", ""))
+    db.add(a)
+    await db.flush()
+    return a
+
+
+async def _source(db, artist, *, url, platform="patreon", enabled=True, overrides=None):
+    s = Source(
+        artist_id=artist.id, platform=platform, url=url, enabled=enabled,
+        config_overrides=overrides,
+    )
+    db.add(s)
+    await db.flush()
+    return s
+
+
+async def _membership(
+    db, *, campaign="c1", platform="patreon", status="active_patron",
+    display_name="Maewix", url="https://www.patreon.com/maewix", details=None,
+):
+    m = PlatformMembership(
+        platform=platform, external_campaign_id=campaign, status=status,
+        display_name=display_name, url=url,
+        details={"campaign": {"vanity": "maewix"}} if details is None else details,
+    )
+    db.add(m)
+    await db.flush()
+    return m
+
+
+async def _synced(db, *, platform="patreon", ago=timedelta(hours=1)):
+    db.add(MembershipSync(platform=platform, last_success_at=datetime.now(UTC) - ago))
+    await db.flush()
+
+
+# --- the join --------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_a_cached_campaign_id_matches_regardless_of_the_url(db):
+    """The exact key doing the work. The URL deliberately does NOT agree, so a
+    match here can only have come from the cached id."""
+    a = await _artist(db)
+    await _source(
+        db, a, url="https://www.patreon.com/an-old-handle",
+        overrides={"patreon_campaign_id": "c1"},
+    )
+    await _membership(db, campaign="c1")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["matched"]) == 1
+    assert out["matched"][0]["matched_by"] == "campaign"
+    assert out["subscribed_not_tracked"] == []
+    assert out["tracked_not_subscribed"] == []
+
+
+@pytest.mark.asyncio
+async def test_the_handle_matches_when_nothing_has_been_cached_yet(db):
+    """The fallback that exists because the id is only written AFTER a source
+    has been walked once — a freshly added source has no id at all."""
+    a = await _artist(db)
+    await _source(db, a, url="https://www.patreon.com/maewix")
+    await _membership(db, campaign="c1")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["matched"]) == 1
+    assert out["matched"][0]["matched_by"] == "vanity"
+
+
+@pytest.mark.asyncio
+async def test_the_c_and_cw_url_forms_both_reduce_to_the_vanity(db):
+    """Patreon serves /c/<vanity> and /cw/<vanity> (#3886). A handle derivation
+    that missed these is the exact class of bug in #1485."""
+    a = await _artist(db)
+    await _source(db, a, url="https://www.patreon.com/c/maewix")
+    await _membership(db, campaign="c1")
+    await _synced(db)
+    await db.commit()
+
+    assert len((await reconcile(db, platform="patreon"))["matched"]) == 1
+
+
+@pytest.mark.asyncio
+async def test_the_same_id_on_another_platform_is_not_a_match(db):
+    """Nothing stops two platforms minting the same opaque id."""
+    a = await _artist(db)
+    await _source(
+        db, a, url="https://subscribestar.adult/maewix", platform="subscribestar",
+        overrides={"subscribestar_campaign_id": "c1"},
+    )
+    await _membership(db, campaign="c1", platform="patreon")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert out["matched"] == []
+    assert len(out["subscribed_not_tracked"]) == 1
+
+
+# --- the gate: an untrustworthy roster accuses nobody -----------------------
+
+
+@pytest.mark.asyncio
+async def test_a_stale_roster_produces_no_unaccounted_sources(db):
+    """THE property. A sweep that stopped working a week ago must not turn every
+    source into "you no longer subscribe to this"."""
+    a = await _artist(db)
+    await _source(
+        db, a, url="https://www.patreon.com/tracked",
+        overrides={"patreon_campaign_id": "not-in-the-roster"},
+    )
+    await _synced(db, ago=ROSTER_STALE_AFTER + timedelta(days=1))
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert out["fresh"] is False
+    assert out["tracked_not_subscribed"] == []
+
+
+@pytest.mark.asyncio
+async def test_a_never_synced_roster_produces_no_unaccounted_sources(db):
+    """Never-synced is not zero. With no MembershipSync row at all there is no
+    evidence of anything, and the bucket must stay empty."""
+    a = await _artist(db)
+    await _source(
+        db, a, url="https://www.patreon.com/tracked",
+        overrides={"patreon_campaign_id": "whatever"},
+    )
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert out["fresh"] is False
+    assert out["last_success_at"] is None
+    assert out["tracked_not_subscribed"] == []
+
+
+@pytest.mark.asyncio
+async def test_the_shape_is_complete_even_when_the_roster_is_stale(db):
+    """A caller must be able to read any bucket unconditionally — the E2 lesson
+    where a shorter disabled payload broke an exact-shape assertion."""
+    await _synced(db, ago=ROSTER_STALE_AFTER + timedelta(days=1))
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert set(out) == {
+        "platform", "fresh", "tracked_total", "last_success_at",
+        "subscribed_not_tracked", "tracked_not_subscribed", "matched",
+        "unidentified",
+    }
+
+
+# --- what lands in the report-only bucket, and why -------------------------
+
+
+@pytest.mark.asyncio
+async def test_a_lapsed_membership_reports_its_source_as_lapsed(db):
+    a = await _artist(db)
+    await _source(db, a, url="https://www.patreon.com/maewix")
+    await _membership(db, campaign="c1", status="former_patron")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["tracked_not_subscribed"]) == 1
+    assert out["tracked_not_subscribed"][0]["basis"] == BASIS_LAPSED
+    assert out["matched"] == []
+
+
+@pytest.mark.asyncio
+async def test_a_known_id_missing_from_a_fresh_roster_is_the_strong_claim(db):
+    a = await _artist(db)
+    await _source(
+        db, a, url="https://www.patreon.com/tracked",
+        overrides={"patreon_campaign_id": "not-in-the-roster"},
+    )
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert out["tracked_not_subscribed"][0]["basis"] == BASIS_ABSENT_EXACT
+
+
+@pytest.mark.asyncio
+async def test_a_handle_only_source_makes_the_weaker_claim(db):
+    """A renamed creator looks exactly like this, so it must not be phrased the
+    same way as the case where the id is known."""
+    a = await _artist(db)
+    await _source(db, a, url="https://www.patreon.com/some-old-name")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert out["tracked_not_subscribed"][0]["basis"] == BASIS_ABSENT_HANDLE
+
+
+@pytest.mark.asyncio
+async def test_an_already_disabled_source_is_not_reported(db):
+    """Telling the operator to stop following something they have already
+    stopped following is noise, not a finding."""
+    a = await _artist(db)
+    await _source(db, a, url="https://www.patreon.com/gone", enabled=False)
+    await _synced(db)
+    await db.commit()
+
+    assert (await reconcile(db, platform="patreon"))["tracked_not_subscribed"] == []
+
+
+@pytest.mark.asyncio
+async def test_a_sidecar_anchor_is_unidentified_not_unsubscribed(db):
+    """Pre-0030 synthetic anchors are not feeds. They cannot be matched, so they
+    are reported as unmatchable rather than filed under a verdict."""
+    a = await _artist(db)
+    await _source(db, a, url="sidecar:patreon:maewix", enabled=False)
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["unidentified"]) == 1
+    assert out["tracked_not_subscribed"] == []
+
+
+# --- what is offered for adoption, and what is withheld --------------------
+
+
+@pytest.mark.asyncio
+async def test_a_membership_with_no_source_is_offered(db):
+    await _membership(db, campaign="c1")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["subscribed_not_tracked"]) == 1
+    assert out["subscribed_not_tracked"][0]["paid_access"] is True
+
+
+@pytest.mark.asyncio
+async def test_a_lapsed_membership_is_not_offered_for_adoption(db):
+    """Adding it would start a walk that can only fetch what is already public."""
+    await _membership(db, campaign="c1", status="former_patron")
+    await _synced(db)
+    await db.commit()
+
+    assert (await reconcile(db, platform="patreon"))["subscribed_not_tracked"] == []
+
+
+@pytest.mark.asyncio
+async def test_an_unrecognised_status_is_still_offered(db):
+    """None is not False. Withholding a membership because this build has not
+    been taught its status word would hide a real subscription."""
+    await _membership(db, campaign="c1", status="a_word_nobody_characterised")
+    await _synced(db)
+    await db.commit()
+
+    out = await reconcile(db, platform="patreon")
+    assert len(out["subscribed_not_tracked"]) == 1
+    assert out["subscribed_not_tracked"][0]["paid_access"] is None
+
+
+@pytest.mark.asyncio
+async def test_a_free_follow_is_not_offered_as_a_subscription(db):
+    """`is_free_member` is a second axis: a CURRENT membership nobody pays for
+    is not a subscription to adopt."""
+    await _membership(
+        db, campaign="c1", status="active_patron",
+        details={"campaign": {"vanity": "maewix"}, "is_free_member": True},
+    )
+    await _synced(db)
+    await db.commit()
+
+    assert (await reconcile(db, platform="patreon"))["subscribed_not_tracked"] == []
+
+
+# --- the all-platforms payload ---------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_a_platform_whose_sweep_never_succeeded_still_appears(db):
+    """Deriving the platform list from memberships alone would drop exactly the
+    platform whose credential is broken — making it indistinguishable from one
+    FC was never asked about."""
+    db.add(MembershipSync(platform="patreon", last_error_type="PatreonAuthError"))
+    await db.commit()
+
+    out = await reconcile_all(db)
+    assert [p["platform"] for p in out["platforms"]] == ["patreon"]
+    assert out["platforms"][0]["fresh"] is False