diff --git a/CHANGELOG.md b/CHANGELOG.md
index c097beae..8399c5a4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,15 @@ All notable changes to vouch are documented here. Format follows
   artifact the caller could not already retrieve, and it touches no write path.
 
 ### Fixed
+- **`kb.detect_themes` no longer leaks claim and session ids the viewer
+  cannot retrieve** (#657): the detector filtered claims on status and
+  `approved_by` but never on `ArtifactScope`, so a private or cross-project
+  claim contributed its id — and the session that produced it — to a
+  returned `ThemeCluster`. `propose_theme` writes both lists into the theme
+  page body, so the leak became committed yaml on approval rather than
+  stopping at a response. `detect_themes` now filters through
+  `scoping.is_visible` like `kb.recall` and the salience sidebar already do,
+  and takes an optional `viewer` for callers that carry one.
 - **security: empty-quote receipts no longer clear the auto-approve gate**
   (#513 reopened, root-caused): `verify_receipt` and `verify_evidence` both
   guarded only on `quote is None`, not an empty string. An `Evidence` with
diff --git a/src/vouch/themes.py b/src/vouch/themes.py
index 2e970f1b..b9c50200 100644
--- a/src/vouch/themes.py
+++ b/src/vouch/themes.py
@@ -19,6 +19,7 @@
 from .config_coerce import coerce_bool
 from .models import ClaimStatus, ProposalStatus
 from .proposals import ProposalError, propose_page
+from .scoping import ViewerContext, is_visible, viewer_from
 from .storage import KBStore
 
 logger = logging.getLogger(__name__)
@@ -96,11 +97,15 @@ def detect_themes(
     min_sessions: int | None = None,
     min_claims: int | None = None,
     top_k: int | None = None,
+    viewer: ViewerContext | None = None,
 ) -> DetectResult:
     """Detect recurring entity clusters across sessions.
 
     Pure read-only operation. Returns ranked clusters without persisting
-    anything. Excludes archived, superseded, redacted, and pending claims.
+    anything. Excludes archived, superseded, redacted, and pending claims,
+    and anything the viewer cannot retrieve — ``viewer`` defaults to the
+    config-resolved context, so a KB read with no explicit viewer reads as
+    its own project (see ``scoping.viewer_from``).
     """
     cfg = _load_theme_config(store)
     if not cfg["enabled"]:
@@ -113,11 +118,20 @@ def detect_themes(
     # Collect approved claims that reference entities and belong to sessions.
     claims = store.list_claims()
     # Also exclude pending (working) — only look at review-gated claims.
+    # Viewer-filtered for the same reason search/recall/salience are: a
+    # cluster is a list of claim ids and session ids, so an unfiltered scan
+    # hands a private or cross-project claim's id — and the session that
+    # produced it — to a viewer that cannot retrieve the claim itself. The
+    # leak is durable rather than advisory: `propose_theme` writes those ids
+    # into a theme page body that then lands in the KB on approval.
+    if viewer is None:
+        viewer = viewer_from(config_path=store.config_path)
     eligible = [
         c for c in claims
         if c.status not in _EXCLUDED_STATUSES
         and c.entities
         and c.approved_by is not None
+        and is_visible(c.scope, viewer)
     ]
 
     # Map each claim to its session(s) via decided proposals.
diff --git a/tests/test_themes.py b/tests/test_themes.py
index 2703fefa..88302e35 100644
--- a/tests/test_themes.py
+++ b/tests/test_themes.py
@@ -8,7 +8,9 @@
 
 from vouch import sessions as sess_mod
 from vouch import themes
+from vouch.models import ArtifactScope, Visibility
 from vouch.proposals import ProposalError, approve, propose_claim, propose_entity
+from vouch.scoping import ViewerContext, is_visible, viewer_from
 from vouch.storage import KBStore
 
 
@@ -123,6 +125,66 @@ def test_detect_themes_excludes_archived(store: KBStore) -> None:
     assert len(result.clusters) == 0
 
 
+def test_detect_themes_excludes_claims_the_viewer_cannot_see(
+    store: KBStore,
+) -> None:
+    """A claim outside the viewer's scope must not reach a cluster."""
+    _seed_multi_session_claims(store)
+    # Re-stamp every claim private to an agent nobody is reading as.
+    for claim in store.list_claims():
+        claim.scope = ArtifactScope(visibility=Visibility.PRIVATE, agent="alice")
+        store.update_claim(claim)
+
+    viewer = viewer_from(config_path=store.config_path)
+    assert not [c for c in store.list_claims() if is_visible(c.scope, viewer)]
+
+    result = themes.detect_themes(store, min_sessions=1, min_claims=1)
+    assert result.clusters == []
+
+
+def test_detect_themes_honors_explicit_viewer(store: KBStore) -> None:
+    """The claim's own agent still sees it; a different agent does not."""
+    _seed_multi_session_claims(store)
+    for claim in store.list_claims():
+        claim.scope = ArtifactScope(visibility=Visibility.PRIVATE, agent="alice")
+        store.update_claim(claim)
+
+    alice = themes.detect_themes(
+        store, min_sessions=2, min_claims=2,
+        viewer=ViewerContext(agent="alice"),
+    )
+    assert [c.entities for c in alice.clusters], "alice owns these claims"
+
+    bob = themes.detect_themes(
+        store, min_sessions=1, min_claims=1, viewer=ViewerContext(agent="bob"),
+    )
+    assert bob.clusters == []
+
+
+def test_detect_themes_scope_leak_does_not_reach_a_theme_page(
+    store: KBStore,
+) -> None:
+    """Regression: the detector fed private claim ids into a durable page.
+
+    `propose_theme` writes `cluster.claim_ids` and `cluster.session_ids` into
+    the page body, so an unfiltered scan leaked them past the review gate as
+    committed yaml rather than as an advisory read.
+    """
+    _seed_multi_session_claims(store)
+    private_ids = set()
+    for claim in store.list_claims():
+        if "session-mgmt" in claim.entities:
+            continue
+        claim.scope = ArtifactScope(visibility=Visibility.PRIVATE, agent="alice")
+        store.update_claim(claim)
+        private_ids.add(claim.id)
+    assert private_ids
+
+    result = themes.detect_themes(store, min_sessions=1, min_claims=1)
+    for cluster in result.clusters:
+        assert not private_ids & set(cluster.claim_ids)
+
+
 def test_detect_themes_disabled_config(store: KBStore) -> None:
     """When themes.enabled=false in config, returns empty."""
     import yaml