diff --git a/l4d2web/l4d2web/services/steam_workshop.py b/l4d2web/l4d2web/services/steam_workshop.py
index bebd7f4..549b710 100644
--- a/l4d2web/l4d2web/services/steam_workshop.py
+++ b/l4d2web/l4d2web/services/steam_workshop.py
@@ -140,6 +140,64 @@ def resolve_collection(collection_id: str) -> list[str]:
     return children
 
 
+def expand_collections(ids: list[str]) -> list[str]:
+    """Resolve a mix of item and collection IDs into a flat list of item IDs.
+
+    Performs one batched POST to GetCollectionDetails. For each input ID:
+      - If Steam returns result==1 with a children array, the ID is a
+        collection — replace it with its non-nested child item IDs in order.
+      - If Steam returns result==9 (k_EResultFileNotFound), the ID is not a
+        collection — pass it through unchanged.
+
+    Result preserves input order; collection children are inserted at the
+    position the collection ID held. Duplicates (across pass-throughs and
+    expanded children) are removed, keeping first occurrence.
+    """
+    if not ids:
+        return []
+    for sid in ids:
+        if not _NUMERIC_ID_RE.fullmatch(sid):
+            raise ValueError(f"steam id must be digits only: {sid!r}")
+
+    payload: dict[str, str | int] = {"collectioncount": len(ids)}
+    for index, sid in enumerate(ids):
+        payload[f"publishedfileids[{index}]"] = sid
+
+    response = _session().post(
+        GET_COLLECTION_DETAILS_URL,
+        data=payload,
+        timeout=REQUEST_TIMEOUT_SECONDS,
+    )
+    response.raise_for_status()
+    body = response.json()
+
+    by_id: dict[str, dict] = {
+        str(entry.get("publishedfileid", "")): entry
+        for entry in body.get("response", {}).get("collectiondetails", [])
+    }
+
+    expanded: list[str] = []
+    seen: set[str] = set()
+
+    def _add(sid: str) -> None:
+        if sid and sid not in seen:
+            seen.add(sid)
+            expanded.append(sid)
+
+    for sid in ids:
+        entry = by_id.get(sid)
+        if entry and entry.get("result") == 1 and "children" in entry:
+            for child in entry["children"]:
+                if child.get("filetype", 0) != 0:
+                    continue
+                child_id = child.get("publishedfileid")
+                if child_id is not None:
+                    _add(str(child_id))
+        else:
+            _add(sid)
+    return expanded
+
+
 def fetch_metadata_batch(
     steam_ids: list[str], *, mode: Literal["add", "refresh"]
 ) -> list[WorkshopMetadata]:
diff --git a/l4d2web/tests/test_steam_workshop.py b/l4d2web/tests/test_steam_workshop.py
index 69b7001..3656570 100644
--- a/l4d2web/tests/test_steam_workshop.py
+++ b/l4d2web/tests/test_steam_workshop.py
@@ -310,3 +310,122 @@ def test_refresh_all_uses_thread_pool_and_collects_errors(tmp_path: Path) -> Non
     assert report.downloaded == 2
     assert report.errors == 1
     assert "2" in report.per_item_errors
+
+
+def test_expand_collections_passes_through_items() -> None:
+    fake_response = MagicMock(status_code=200)
+    fake_response.raise_for_status = MagicMock()
+    fake_response.json.return_value = {
+        "response": {
+            "collectiondetails": [
+                {"publishedfileid": "1001", "result": 9},
+                {"publishedfileid": "1002", "result": 9},
+            ]
+        }
+    }
+    with patch.object(steam_workshop, "_session", return_value=MagicMock(post=MagicMock(return_value=fake_response))):
+        result = steam_workshop.expand_collections(["1001", "1002"])
+    assert result == ["1001", "1002"]
+
+
+def test_expand_collections_replaces_collection_with_children() -> None:
+    fake_response = MagicMock(status_code=200)
+    fake_response.raise_for_status = MagicMock()
+    fake_response.json.return_value = {
+        "response": {
+            "collectiondetails": [
+                {
+                    "publishedfileid": "555",
+                    "result": 1,
+                    "children": [
+                        {"publishedfileid": "1001", "filetype": 0},
+                        {"publishedfileid": "1002", "filetype": 0},
+                    ],
+                },
+            ]
+        }
+    }
+    with patch.object(steam_workshop, "_session", return_value=MagicMock(post=MagicMock(return_value=fake_response))):
+        result = steam_workshop.expand_collections(["555"])
+    assert result == ["1001", "1002"]
+
+
+def test_expand_collections_mixed_items_and_collections() -> None:
+    fake_response = MagicMock(status_code=200)
+    fake_response.raise_for_status = MagicMock()
+    fake_response.json.return_value = {
+        "response": {
+            "collectiondetails": [
+                {"publishedfileid": "1001", "result": 9},
+                {
+                    "publishedfileid": "555",
+                    "result": 1,
+                    "children": [
+                        {"publishedfileid": "2001", "filetype": 0},
+                        {"publishedfileid": "2002", "filetype": 0},
+                    ],
+                },
+                {"publishedfileid": "1003", "result": 9},
+            ]
+        }
+    }
+    with patch.object(steam_workshop, "_session", return_value=MagicMock(post=MagicMock(return_value=fake_response))):
+        result = steam_workshop.expand_collections(["1001", "555", "1003"])
+    # Input order preserved; collection's children inserted where the collection was.
+    assert result == ["1001", "2001", "2002", "1003"]
+
+
+def test_expand_collections_skips_nested_collections() -> None:
+    fake_response = MagicMock(status_code=200)
+    fake_response.raise_for_status = MagicMock()
+    fake_response.json.return_value = {
+        "response": {
+            "collectiondetails": [
+                {
+                    "publishedfileid": "555",
+                    "result": 1,
+                    "children": [
+                        {"publishedfileid": "1001", "filetype": 0},
+                        {"publishedfileid": "9999", "filetype": 1},
+                        {"publishedfileid": "1002", "filetype": 0},
+                    ],
+                },
+            ]
+        }
+    }
+    with patch.object(steam_workshop, "_session", return_value=MagicMock(post=MagicMock(return_value=fake_response))):
+        result = steam_workshop.expand_collections(["555"])
+    assert result == ["1001", "1002"]
+
+
+def test_expand_collections_deduplicates() -> None:
+    fake_response = MagicMock(status_code=200)
+    fake_response.raise_for_status = MagicMock()
+    fake_response.json.return_value = {
+        "response": {
+            "collectiondetails": [
+                {"publishedfileid": "1001", "result": 9},
+                {
+                    "publishedfileid": "555",
+                    "result": 1,
+                    "children": [
+                        {"publishedfileid": "1001", "filetype": 0},
+                        {"publishedfileid": "1002", "filetype": 0},
+                    ],
+                },
+            ]
+        }
+    }
+    with patch.object(steam_workshop, "_session", return_value=MagicMock(post=MagicMock(return_value=fake_response))):
+        result = steam_workshop.expand_collections(["1001", "555"])
+    assert result == ["1001", "1002"]
+
+
+def test_expand_collections_empty_input_returns_empty() -> None:
+    result = steam_workshop.expand_collections([])
+    assert result == []
+
+
+def test_expand_collections_rejects_non_numeric_ids() -> None:
+    with pytest.raises(ValueError):
+        steam_workshop.expand_collections(["1001", "not-a-number"])