diff --git a/web/src/api/types.ts b/web/src/api/types.ts
index 7dfb081..e6dc1f0 100644
--- a/web/src/api/types.ts
+++ b/web/src/api/types.ts
@@ -212,6 +212,38 @@ export interface UpdateBOMEntryRequest {
   metadata?: Record<string, unknown>;
 }
 
+// BOM Merge
+export interface MergeBOMResponse {
+  status: string;
+  diff: MergeBOMDiff;
+  warnings: MergeWarning[];
+  resolve_url: string;
+}
+
+export interface MergeBOMDiff {
+  added: MergeDiffEntry[];
+  removed: MergeDiffEntry[];
+  quantity_changed: MergeQtyChange[];
+  unchanged: MergeDiffEntry[];
+}
+
+export interface MergeDiffEntry {
+  part_number: string;
+  quantity: number | null;
+}
+
+export interface MergeQtyChange {
+  part_number: string;
+  old_quantity: number | null;
+  new_quantity: number | null;
+}
+
+export interface MergeWarning {
+  type: string;
+  part_number: string;
+  message: string;
+}
+
 // Schema properties
 export interface PropertyDef {
   type: string;
diff --git a/web/src/components/items/BOMTab.tsx b/web/src/components/items/BOMTab.tsx
index 3cffebf..b9f6352 100644
--- a/web/src/components/items/BOMTab.tsx
+++ b/web/src/components/items/BOMTab.tsx
@@ -46,6 +46,7 @@ export function BOMTab({ partNumber, isEditor }: BOMTabProps) {
   const unitCost = (e: BOMEntry) => Number(meta(e).unit_cost) || 0;
   const extCost = (e: BOMEntry) => unitCost(e) * (e.quantity ?? 0);
   const totalCost = entries.reduce((sum, e) => sum + extCost(e), 0);
+  const assemblyCount = entries.filter((e) => e.source === "assembly").length;
 
   const formToRequest = () => ({
     child_part_number: form.child_part_number,
@@ -139,12 +140,15 @@ export function BOMTab({ partNumber, isEditor }: BOMTabProps) {
         />
       </td>
       <td style={tdStyle}>
-        <input
+        <select
           value={form.source}
           onChange={(e) => setForm({ ...form, source: e.target.value })}
-          placeholder="Source"
           style={inputStyle}
-        />
+        >
+          <option value="">—</option>
+          <option value="manual">manual</option>
+          <option value="assembly">assembly</option>
+        </select>
       </td>
       <td style={tdStyle}>
         <input
@@ -247,6 +251,24 @@ export function BOMTab({ partNumber, isEditor }: BOMTabProps) {
         )}
       </div>
 
+      {isEditor && assemblyCount > 0 && (
+        <div
+          style={{
+            padding: "0.35rem 0.6rem",
+            marginBottom: "0.5rem",
+            borderRadius: "0.3rem",
+            backgroundColor: "rgba(148,226,213,0.1)",
+            border: "1px solid rgba(148,226,213,0.3)",
+            fontSize: "0.75rem",
+            color: "var(--ctp-subtext1)",
+          }}
+        >
+          {assemblyCount} assembly-sourced{" "}
+          {assemblyCount === 1 ? "entry" : "entries"}. Entries removed from the
+          FreeCAD assembly will remain here until manually deleted.
+        </div>
+      )}
+
       <div style={{ overflow: "auto" }}>
         <table
           style={{
@@ -289,7 +311,15 @@ export function BOMTab({ partNumber, isEditor }: BOMTabProps) {
                   >
                     {e.child_part_number}
                   </td>
-                  <td style={tdStyle}>{e.source ?? ""}</td>
+                  <td style={tdStyle}>
+                    {e.source === "assembly" ? (
+                      <span style={assemblyBadge}>assembly</span>
+                    ) : e.source === "manual" ? (
+                      <span style={manualBadge}>manual</span>
+                    ) : (
+                      "—"
+                    )}
+                  </td>
                   <td
                     style={{
                       ...tdStyle,
@@ -420,6 +450,25 @@ const saveBtnStyle: React.CSSProperties = {
   marginRight: "0.25rem",
 };
 
+const sourceBadgeBase: React.CSSProperties = {
+  padding: "0.1rem 0.4rem",
+  borderRadius: "1rem",
+  fontSize: "0.7rem",
+  fontWeight: 500,
+};
+
+const assemblyBadge: React.CSSProperties = {
+  ...sourceBadgeBase,
+  backgroundColor: "rgba(148,226,213,0.2)",
+  color: "var(--ctp-teal)",
+};
+
+const manualBadge: React.CSSProperties = {
+  ...sourceBadgeBase,
+  backgroundColor: "rgba(137,180,250,0.2)",
+  color: "var(--ctp-blue)",
+};
+
 const cancelBtnStyle: React.CSSProperties = {
   padding: "0.2rem 0.4rem",
   fontSize: "0.75rem",