diff --git a/README.md b/README.md
index 5d3300f..d988268 100644
--- a/README.md
+++ b/README.md
@@ -19,8 +19,6 @@ To build and preview the `probe-viewer` web-app locally:
 ```bash
 cd apps/probe-viewer
 uv run build.py
-# build
-npm run build
-# run
-npx vite preview
+# build & run
+npm run build && npx vite preview
 ```
diff --git a/apps/probe-viewer/build.py b/apps/probe-viewer/build.py
index 77f2e90..5c6c6ac 100755
--- a/apps/probe-viewer/build.py
+++ b/apps/probe-viewer/build.py
@@ -52,6 +52,7 @@ class ManifestEntry:
     json_url: str
     contact_count: int
     shank_count: int
+    num_sides: int
     has_3d_geometry: bool
     annotations: dict
 
@@ -74,12 +75,18 @@ def load_probe_metadata(json_path: Path) -> ManifestEntry:
     if not probes:
         raise ValueError(f"No probes found in {json_path}")
 
+    if len(probes) > 1:
+        raise ValueError(f"Multiple probes found in {json_path}; expected one")
+    probe = probes[0]
+
     manufacturer = json_path.parents[1].name
     model = json_path.parent.name
     probe_id = f"{manufacturer}:{model}"
 
-    total_contacts = sum(len(probe.get("contact_positions", [])) for probe in probes)
-    shank_count = max(len(set(probe.get("shank_ids") or [None])) for probe in probes)
+    total_contacts = len(probe.get("contact_positions", []))
+    shank_count = len(set(probe.get("shank_ids") or [None]))
+    num_sides = 1 if probe.get("contact_sides") is None else len(set(probe.get("contact_sides")))
+
     has_3d = any(probe.get("ndim") == 3 for probe in probes)
     annotations = probes[0].get("annotations") or {}
     display_name = annotations.get("model_name") or model
@@ -92,6 +99,7 @@ def load_probe_metadata(json_path: Path) -> ManifestEntry:
         json_url=json_path.name,
         contact_count=total_contacts,
         shank_count=shank_count,
+        num_sides=num_sides,
         has_3d_geometry=has_3d,
         annotations=annotations,
     )
diff --git a/apps/probe-viewer/src/components/Sidebar.tsx b/apps/probe-viewer/src/components/Sidebar.tsx
index e90bb69..91a99cb 100644
--- a/apps/probe-viewer/src/components/Sidebar.tsx
+++ b/apps/probe-viewer/src/components/Sidebar.tsx
@@ -26,6 +26,19 @@ export function Sidebar() {
   const navigate = useNavigate();
   const searchQuery = useAppStore((state) => state.searchQuery);
   const setSearchQuery = useAppStore((state) => state.setSearchQuery);
+  const sideFilter = useAppStore((state) => state.sideFilter);
+  const setSideFilter = useAppStore((state) => state.setSideFilter);
+
+  // Only manufacturers with at least one multi-sided probe show the "sides"
+  // count; when every probe of a manufacturer is single-sided the metric is
+  // noise, so it's omitted for all of that manufacturer's items.
+  const multiSideManufacturers = useMemo(() => {
+    const set = new Set<string>();
+    manifest.forEach((entry) => {
+      if (entry.numSides > 1) set.add(entry.manufacturer);
+    });
+    return set;
+  }, [manifest]);
 
   const manufacturers = useMemo(() => {
     const unique = new Set<string>();
@@ -41,12 +54,30 @@ export function Sidebar() {
     }
   }, [manufacturers, selectedManufacturer, selectManufacturer]);
 
+  // Distinct side counts among the selected manufacturer's probes. The side
+  // dropdown only appears when this has more than one value (e.g. a catalog
+  // mixing single- and double-sided probes).
+  const availableSideCounts = useMemo(() => {
+    const counts = new Set<number>();
+    manifest.forEach((entry) => {
+      if (!selectedManufacturer || entry.manufacturer === selectedManufacturer) {
+        counts.add(entry.numSides);
+      }
+    });
+    return Array.from(counts.values()).sort((a, b) => a - b);
+  }, [manifest, selectedManufacturer]);
+
+  const showSideFilter = availableSideCounts.length > 1;
+
   const filteredEntries = useMemo(() => {
     const query = searchQuery.trim().toLowerCase();
     return manifest.filter((entry) => {
       if (selectedManufacturer && entry.manufacturer !== selectedManufacturer) {
         return false;
       }
+      if (sideFilter !== null && entry.numSides !== sideFilter) {
+        return false;
+      }
       if (!query) {
         return true;
       }
@@ -55,7 +86,7 @@ export function Sidebar() {
         entry.displayName.toLowerCase().includes(query)
       );
     });
-  }, [manifest, selectedManufacturer, searchQuery]);
+  }, [manifest, selectedManufacturer, searchQuery, sideFilter]);
 
   useEffect(() => {
     // Re-pick a probe only when a stale one is selected (e.g. after switching
@@ -111,6 +142,8 @@ export function Sidebar() {
       <span className="sidebar-item-name">{entry.displayName}</span>
       <span className="sidebar-item-meta">
         {entry.contactCount} contacts · {entry.shankCount} shanks
+        {multiSideManufacturers.has(entry.manufacturer) &&
+          ` · ${entry.numSides} ${entry.numSides === 1 ? "side" : "sides"}`}
       </span>
     </button>
   );
@@ -226,6 +259,31 @@ export function Sidebar() {
         />
       </div>
 
+      {showSideFilter && (
+        <div className="sidebar-control">
+          <label className="sidebar-label" htmlFor="side-select">
+            Number of sides
+          </label>
+          <select
+            id="side-select"
+            value={sideFilter === null ? "" : String(sideFilter)}
+            onChange={(event) =>
+              setSideFilter(
+                event.target.value === "" ? null : Number(event.target.value),
+              )
+            }
+            disabled={manifestStatus !== "success"}
+          >
+            <option value="">All sides</option>
+            {availableSideCounts.map((count) => (
+              <option key={count} value={String(count)}>
+                {count} {count === 1 ? "side" : "sides"}
+              </option>
+            ))}
+          </select>
+        </div>
+      )}
+
       <div className="sidebar-list" role="list">
         {manifestStatus === "loading" && (
           <p className="sidebar-hint">Loading manifest…</p>
diff --git a/apps/probe-viewer/src/services/manifest.ts b/apps/probe-viewer/src/services/manifest.ts
index b80864d..d2bafce 100644
--- a/apps/probe-viewer/src/services/manifest.ts
+++ b/apps/probe-viewer/src/services/manifest.ts
@@ -11,6 +11,7 @@ function normalizeEntry(raw: RawManifestEntry): ManifestEntry {
   jsonUrl: `${import.meta.env.BASE_URL}${raw.json_url}`,
   contactCount: raw.contact_count,
   shankCount: raw.shank_count,
+  numSides: raw.num_sides,
   has3dGeometry: raw.has_3d_geometry,
   annotations: raw.annotations ?? {},
 };
diff --git a/apps/probe-viewer/src/state/useAppStore.ts b/apps/probe-viewer/src/state/useAppStore.ts
index 2904f13..64c5124 100644
--- a/apps/probe-viewer/src/state/useAppStore.ts
+++ b/apps/probe-viewer/src/state/useAppStore.ts
@@ -31,6 +31,9 @@ interface AppState {
   selectedManufacturer?: string;
   selectedProbeId?: string;
   searchQuery: string;
+  // Filter the list to probes with this many sides; null = show all. Only
+  // meaningful for manufacturers whose catalog mixes side counts.
+  sideFilter: number | null;
   probeCache: Record<string, ProbeInterfaceFile>;
   probeStatus: Record<string, ProbeLoadState>;
   view: ViewState;
@@ -42,6 +45,7 @@ interface AppState {
   loadManifest: () => Promise<void>;
   selectManufacturer: (manufacturer?: string) => void;
   setSearchQuery: (query: string) => void;
+  setSideFilter: (sides: number | null) => void;
   selectProbe: (probeId?: string) => void;
   ensureProbeLoaded: (probeId: string) => Promise<ProbeInterfaceFile | undefined>;
   setZoom: (zoom: number) => void;
@@ -88,6 +92,7 @@ export const useAppStore = create<AppState>((set, get) => ({
   selectedManufacturer: undefined,
   selectedProbeId: undefined,
   searchQuery: "",
+  sideFilter: null,
   probeCache: {},
   probeStatus: {},
   view: INITIAL_VIEW_STATE,
@@ -121,10 +126,15 @@ export const useAppStore = create<AppState>((set, get) => ({
     }
   },
 
-  selectManufacturer: (manufacturer) => set({ selectedManufacturer: manufacturer }),
+  // Switching manufacturer clears the side filter — it's specific to whichever
+  // catalog was showing and rarely applies to the next one.
+  selectManufacturer: (manufacturer) =>
+    set({ selectedManufacturer: manufacturer, sideFilter: null }),
 
   setSearchQuery: (query) => set({ searchQuery: query }),
 
+  setSideFilter: (sides) => set({ sideFilter: sides }),
+
   selectProbe: (probeId) =>
     set((state) => {
       if (!probeId) {
diff --git a/apps/probe-viewer/src/types/probe.ts b/apps/probe-viewer/src/types/probe.ts
index 9172e40..efccde7 100644
--- a/apps/probe-viewer/src/types/probe.ts
+++ b/apps/probe-viewer/src/types/probe.ts
@@ -6,6 +6,7 @@ export interface RawManifestEntry {
   json_url: string;
   contact_count: number;
   shank_count: number;
+  num_sides: number;
   has_3d_geometry: boolean;
   annotations: Record<string, unknown>;
 }
@@ -18,6 +19,7 @@ export interface ManifestEntry {
   jsonUrl: string;
   contactCount: number;
   shankCount: number;
+  numSides: number;
   has3dGeometry: boolean;
   annotations: Record<string, unknown>;
 }