diff --git a/api/repositories/knowledge/segment_repository.py b/api/repositories/knowledge/segment_repository.py
index 0b478b331d833b..c9e3af3647443c 100644
--- a/api/repositories/knowledge/segment_repository.py
+++ b/api/repositories/knowledge/segment_repository.py
@@ -506,6 +506,32 @@ def get_indexing_snapshots(
                 for segment_id in segment_ids
             )
 
+    def get_exclusive_attachment_ids(self, segment_ref: SegmentRef, attachment_ids: Sequence[str]) -> tuple[str, ...]:
+        """Return candidates with no other segment binding in the same dataset.
+
+        Accepts a trusted segment reference and candidate upload IDs. This read
+        does not mutate bindings or uploads. Disabled segments still own their
+        attachments; references in other datasets do not own this dataset's
+        vector collection. The result reflects bindings at read time.
+        """
+        if not attachment_ids:
+            return ()
+        with self._session_factory() as session:
+            shared_ids = set(
+                session.scalars(
+                    select(SegmentAttachmentBinding.attachment_id).where(
+                        SegmentAttachmentBinding.tenant_id == segment_ref.document.dataset.tenant_id,
+                        SegmentAttachmentBinding.dataset_id == segment_ref.document.dataset.dataset_id,
+                        SegmentAttachmentBinding.attachment_id.in_(attachment_ids),
+                        or_(
+                            SegmentAttachmentBinding.document_id != segment_ref.document.document_id,
+                            SegmentAttachmentBinding.segment_id != segment_ref.segment_id,
+                        ),
+                    )
+                )
+            )
+        return tuple(attachment_id for attachment_id in attachment_ids if attachment_id not in shared_ids)
+
     def replace_attachments(self, segment_ref: SegmentRef, attachment_ids: Sequence[str]) -> None:
         with self._session_factory.begin() as session:
             if _get_segment(session, segment_ref) is None:
diff --git a/api/services/knowledge/segments/indexing.py b/api/services/knowledge/segments/indexing.py
index 462e19f5ecec4c..7768f8c77034af 100644
--- a/api/services/knowledge/segments/indexing.py
+++ b/api/services/knowledge/segments/indexing.py
@@ -249,16 +249,28 @@ def _attachment_documents(snapshot: SegmentIndexingSnapshot, files) -> list[Docu
 
     @with_credit_usage_created_by(CreditUsageCreatedBy.KNOWLEDGE_INDEXING)
     def update_attachments(self, segment_ref: SegmentRef, attachment_ids: Sequence[str]) -> None:
+        """Replace an existing segment's attachments using tenant-owned uploads.
+
+        Keep retained vectors and vectors referenced by another segment in this
+        dataset. Only removed, exclusively owned vectors can be deleted. External
+        index I/O completes before replacing bindings, so an index failure leaves
+        the previous bindings available for diagnosis and retry.
+        """
         snapshot = self._segments.get_indexing_snapshot(segment_ref)
         if snapshot.dataset.indexing_technique != "high_quality" or set(snapshot.attachment_ids) == set(attachment_ids):
             return
         files = self._uploads.get_files(workspace_id=snapshot.dataset.tenant_id, file_ids=attachment_ids)
         if snapshot.dataset.is_multimodal:
             vector = self._vector(snapshot)
-            if snapshot.attachment_ids:
-                vector.delete_by_ids(list(snapshot.attachment_ids))
-            if files:
-                vector.create_multimodal(self._attachment_documents(snapshot, files.values()), upload_files=files)
+            removed_ids = tuple(file_id for file_id in snapshot.attachment_ids if file_id not in files)
+            exclusive_ids = self._segments.get_exclusive_attachment_ids(segment_ref, removed_ids)
+            if exclusive_ids:
+                vector.delete_by_ids(list(exclusive_ids))
+            added_files = {file_id: file for file_id, file in files.items() if file_id not in snapshot.attachment_ids}
+            if added_files:
+                vector.create_multimodal(
+                    self._attachment_documents(snapshot, added_files.values()), upload_files=added_files
+                )
         self._segments.replace_attachments(segment_ref, tuple(files))
 
     def update_summary(self, segment_ref: SegmentRef, *, summary: str | None, content_changed: bool) -> None:
diff --git a/api/tests/unit_tests/services/knowledge/segments/test_adapters.py b/api/tests/unit_tests/services/knowledge/segments/test_adapters.py
index 67e457c65a2b49..91b8a9486e55de 100644
--- a/api/tests/unit_tests/services/knowledge/segments/test_adapters.py
+++ b/api/tests/unit_tests/services/knowledge/segments/test_adapters.py
@@ -821,6 +821,119 @@ def test_segment_update_preserves_or_replaces_attachments(
     assert ("image-1" in probe.deleted_ids) is removed_original
 
 
+@pytest.mark.parametrize("other_document_id", ["document-1", "document-2"])
+@pytest.mark.parametrize("other_enabled", [True, False])
+def test_attachment_update_preserves_shared_vector_until_last_binding(
+    segment_update: SegmentUpdateEntry,
+    indexing_probe: IndexingProbe,
+    sqlite_session_factory: sessionmaker[Session],
+    other_document_id: str,
+    other_enabled: bool,
+) -> None:
+    app, repository, probe = indexing_probe
+    context = RequestContext("request", None, "author", "workspace-1")
+    with sqlite_session_factory.begin() as session:
+        session.add(_upload("image-1", "workspace-1", "shared.png"))
+        dataset = session.get(Dataset, "dataset-1")
+        assert dataset is not None
+        dataset.is_multimodal = True
+        if other_document_id != "document-1":
+            session.add(_document(other_document_id, "workspace-1", "dataset-1"))
+    first = app.create_segment(
+        context,
+        dataset_id="dataset-1",
+        document_id="document-1",
+        values={"content": "first", "attachment_ids": ["image-1"]},
+    )
+    second = app.create_segment(
+        context,
+        dataset_id="dataset-1",
+        document_id=other_document_id,
+        values={"content": "second", "attachment_ids": ["image-1"]},
+    )
+    second_ref = DatasetRef("workspace-1", "dataset-1").document(other_document_id).segment(second.data.id)
+    repository.save_segment(second_ref, {"enabled": other_enabled})
+    segment_update(
+        context,
+        dataset_id="dataset-1",
+        document_id="document-1",
+        segment_id=first.data.id,
+        values={"content": "first", "attachment_ids": []},
+    )
+    assert "image-1" not in probe.deleted_ids
+    assert repository.get_indexing_snapshot(second_ref).attachment_ids == ("image-1",)
+    repository.save_segment(second_ref, {"enabled": True})
+    segment_update(
+        context,
+        dataset_id="dataset-1",
+        document_id=other_document_id,
+        segment_id=second.data.id,
+        values={"content": "second", "attachment_ids": []},
+    )
+    assert probe.deleted_ids.count("image-1") == 1
+    with sqlite_session_factory() as session:
+        assert session.get(UploadFile, "image-1") is not None
+
+
+def test_attachment_update_does_not_rewrite_retained_vector(
+    segment_update: SegmentUpdateEntry,
+    indexing_probe: IndexingProbe,
+    sqlite_session_factory: sessionmaker[Session],
+) -> None:
+    app, _, probe = indexing_probe
+    context = RequestContext("request", None, "author", "workspace-1")
+    with sqlite_session_factory.begin() as session:
+        session.add_all([_upload("image-1", "workspace-1", "one.png"), _upload("image-2", "workspace-1", "two.png")])
+        dataset = session.get(Dataset, "dataset-1")
+        assert dataset is not None
+        dataset.is_multimodal = True
+    created = app.create_segment(
+        context,
+        dataset_id="dataset-1",
+        document_id="document-1",
+        values={"content": "first", "attachment_ids": ["image-1", "image-2"]},
+    )
+    writes_before = probe.writes
+    segment_update(
+        context,
+        dataset_id="dataset-1",
+        document_id="document-1",
+        segment_id=created.data.id,
+        values={"content": "first", "attachment_ids": ["image-1"]},
+    )
+    assert probe.deleted_ids == ["image-2"]
+    assert probe.writes == writes_before
+
+
+def test_attachment_reference_query_respects_vector_collection_scope(
+    sqlite_session_factory: sessionmaker[Session],
+) -> None:
+    with sqlite_session_factory.begin() as session:
+        for workspace, dataset, document, segment, image in [
+            ("workspace-1", "dataset-1", "document-1", "segment-1", "own"),
+            ("workspace-1", "dataset-2", "document-2", "segment-2", "other-dataset"),
+            ("workspace-2", "dataset-1", "document-2", "segment-2", "other-tenant"),
+            ("workspace-1", "dataset-1", "document-2", "segment-2", "shared"),
+        ]:
+            session.add(
+                SegmentAttachmentBinding(
+                    tenant_id=workspace,
+                    dataset_id=dataset,
+                    document_id=document,
+                    segment_id=segment,
+                    attachment_id=image,
+                )
+            )
+    repository = SQLAlchemySegmentRepository(session_factory=sqlite_session_factory)
+    ref = DatasetRef("workspace-1", "dataset-1").document("document-1").segment("segment-1")
+    assert repository.get_exclusive_attachment_ids(ref, ["own", "other-dataset", "other-tenant", "shared"]) == (
+        "own",
+        "other-dataset",
+        "other-tenant",
+    )
+    assert repository.get_exclusive_attachment_ids(ref, []) == ()
+
+
 def test_segment_indexing_and_summary_use_closed_read_sessions(
     segment_update: SegmentUpdateEntry, indexing_probe: IndexingProbe, sqlite_session_factory: sessionmaker[Session]
 ) -> None: