diff --git a/internal/api/bom_handlers.go b/internal/api/bom_handlers.go
index 633ed5d..83c6008 100644
--- a/internal/api/bom_handlers.go
+++ b/internal/api/bom_handlers.go
@@ -594,6 +594,56 @@ func (s *Server) HandleGetBOMCost(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// BOM merge request/response types
+
+// MergeBOMRequest represents a request to merge assembly BOM entries.
+type MergeBOMRequest struct {
+	Source  string          `json:"source"`
+	Entries []MergeBOMEntry `json:"entries"`
+}
+
+// MergeBOMEntry represents a single entry in a merge request.
+type MergeBOMEntry struct {
+	ChildPartNumber string   `json:"child_part_number"`
+	Quantity        *float64 `json:"quantity"`
+}
+
+// MergeBOMResponse represents the result of a BOM merge.
+type MergeBOMResponse struct {
+	Status     string         `json:"status"`
+	Diff       MergeBOMDiff   `json:"diff"`
+	Warnings   []MergeWarning `json:"warnings"`
+	ResolveURL string         `json:"resolve_url"`
+}
+
+// MergeBOMDiff categorizes changes from a merge operation.
+type MergeBOMDiff struct {
+	Added           []MergeDiffEntry `json:"added"`
+	Removed         []MergeDiffEntry `json:"removed"`
+	QuantityChanged []MergeQtyChange `json:"quantity_changed"`
+	Unchanged       []MergeDiffEntry `json:"unchanged"`
+}
+
+// MergeDiffEntry represents an added, removed, or unchanged BOM entry.
+type MergeDiffEntry struct {
+	PartNumber string   `json:"part_number"`
+	Quantity   *float64 `json:"quantity"`
+}
+
+// MergeQtyChange represents a BOM entry whose quantity changed.
+type MergeQtyChange struct {
+	PartNumber  string   `json:"part_number"`
+	OldQuantity *float64 `json:"old_quantity"`
+	NewQuantity *float64 `json:"new_quantity"`
+}
+
+// MergeWarning represents a warning generated during merge.
+type MergeWarning struct {
+	Type       string `json:"type"`
+	PartNumber string `json:"part_number"`
+	Message    string `json:"message"`
+}
+
 // BOM CSV headers matching the user-specified format.
 var bomCSVHeaders = []string{
 	"Item", "Level", "Source", "PN", "Seller Description",
@@ -976,3 +1026,197 @@ func (s *Server) HandleImportBOMCSV(w http.ResponseWriter, r *http.Request) {
 
 	writeJSON(w, http.StatusOK, result)
 }
+
+// HandleMergeBOM merges assembly-derived BOM entries into the server's BOM.
+// Added entries are created, quantity changes are applied, and entries present
+// in the server but missing from the request are flagged as warnings (not deleted).
+func (s *Server) HandleMergeBOM(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	partNumber := chi.URLParam(r, "partNumber")
+
+	parent, err := s.items.GetByPartNumber(ctx, partNumber)
+	if err != nil {
+		s.logger.Error().Err(err).Msg("failed to get parent item")
+		writeError(w, http.StatusInternalServerError, "internal_error", "Failed to get parent item")
+		return
+	}
+	if parent == nil {
+		writeError(w, http.StatusNotFound, "not_found", "Parent item not found")
+		return
+	}
+
+	var req MergeBOMRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid_json", err.Error())
+		return
+	}
+	if len(req.Entries) == 0 {
+		writeError(w, http.StatusBadRequest, "invalid_request", "entries must not be empty")
+		return
+	}
+
+	// Fetch existing BOM (includes Source field)
+	existing, err := s.relationships.GetBOM(ctx, parent.ID)
+	if err != nil {
+		s.logger.Error().Err(err).Msg("failed to get existing BOM")
+		writeError(w, http.StatusInternalServerError, "internal_error", "Failed to get existing BOM")
+		return
+	}
+
+	// Build lookup map by child part number
+	existingMap := make(map[string]*db.BOMEntry, len(existing))
+	for _, e := range existing {
+		existingMap[e.ChildPartNumber] = e
+	}
+
+	var username *string
+	if user := auth.UserFromContext(ctx); user != nil {
+		username = &user.Username
+	}
+
+	diff := MergeBOMDiff{
+		Added:           make([]MergeDiffEntry, 0),
+		Removed:         make([]MergeDiffEntry, 0),
+		QuantityChanged: make([]MergeQtyChange, 0),
+		Unchanged:       make([]MergeDiffEntry, 0),
+	}
+	var warnings []MergeWarning
+
+	// Process incoming entries
+	for _, entry := range req.Entries {
+		if entry.ChildPartNumber == "" {
+			continue
+		}
+
+		child, err := s.items.GetByPartNumber(ctx, entry.ChildPartNumber)
+		if err != nil {
+			s.logger.Error().Err(err).Str("child", entry.ChildPartNumber).Msg("failed to look up child")
+			warnings = append(warnings, MergeWarning{
+				Type:       "error",
+				PartNumber: entry.ChildPartNumber,
+				Message:    fmt.Sprintf("Error looking up item: %s", err.Error()),
+			})
+			continue
+		}
+		if child == nil {
+			warnings = append(warnings, MergeWarning{
+				Type:       "not_found",
+				PartNumber: entry.ChildPartNumber,
+				Message:    fmt.Sprintf("Item '%s' not found in database", entry.ChildPartNumber),
+			})
+			continue
+		}
+
+		if ex, ok := existingMap[entry.ChildPartNumber]; ok {
+			// Entry already exists — check quantity
+			oldQty := ex.Quantity
+			newQty := entry.Quantity
+			if quantitiesEqual(oldQty, newQty) {
+				diff.Unchanged = append(diff.Unchanged, MergeDiffEntry{
+					PartNumber: entry.ChildPartNumber,
+					Quantity:   newQty,
+				})
+			} else {
+				// Update quantity
+				if err := s.relationships.Update(ctx, ex.RelationshipID, nil, newQty, nil, nil, nil, nil, username); err != nil {
+					s.logger.Error().Err(err).Str("child", entry.ChildPartNumber).Msg("failed to update quantity")
+					warnings = append(warnings, MergeWarning{
+						Type:       "error",
+						PartNumber: entry.ChildPartNumber,
+						Message:    fmt.Sprintf("Failed to update quantity: %s", err.Error()),
+					})
+				} else {
+					diff.QuantityChanged = append(diff.QuantityChanged, MergeQtyChange{
+						PartNumber:  entry.ChildPartNumber,
+						OldQuantity: oldQty,
+						NewQuantity: newQty,
+					})
+				}
+			}
+			delete(existingMap, entry.ChildPartNumber)
+		} else {
+			// New entry — create
+			rel := &db.Relationship{
+				ParentItemID: parent.ID,
+				ChildItemID:  child.ID,
+				RelType:      "component",
+				Quantity:     entry.Quantity,
+				Source:       "assembly",
+				CreatedBy:    username,
+			}
+			if err := s.relationships.Create(ctx, rel); err != nil {
+				if strings.Contains(err.Error(), "cycle") {
+					warnings = append(warnings, MergeWarning{
+						Type:       "cycle",
+						PartNumber: entry.ChildPartNumber,
+						Message:    fmt.Sprintf("Adding '%s' would create a cycle", entry.ChildPartNumber),
+					})
+				} else {
+					s.logger.Error().Err(err).Str("child", entry.ChildPartNumber).Msg("failed to create relationship")
+					warnings = append(warnings, MergeWarning{
+						Type:       "error",
+						PartNumber: entry.ChildPartNumber,
+						Message:    fmt.Sprintf("Failed to create: %s", err.Error()),
+					})
+				}
+				continue
+			}
+			diff.Added = append(diff.Added, MergeDiffEntry{
+				PartNumber: entry.ChildPartNumber,
+				Quantity:   entry.Quantity,
+			})
+		}
+	}
+
+	// Remaining entries in existingMap are not in the merge request
+	for pn, e := range existingMap {
+		if e.Source == "assembly" {
+			diff.Removed = append(diff.Removed, MergeDiffEntry{
+				PartNumber: pn,
+				Quantity:   e.Quantity,
+			})
+			warnings = append(warnings, MergeWarning{
+				Type:       "unreferenced",
+				PartNumber: pn,
+				Message:    "Present in server BOM but not in assembly",
+			})
+		}
+	}
+
+	resp := MergeBOMResponse{
+		Status:     "merged",
+		Diff:       diff,
+		Warnings:   warnings,
+		ResolveURL: fmt.Sprintf("/items/%s/bom", partNumber),
+	}
+
+	s.logger.Info().
+		Str("parent", partNumber).
+		Int("added", len(diff.Added)).
+		Int("updated", len(diff.QuantityChanged)).
+		Int("unchanged", len(diff.Unchanged)).
+		Int("unreferenced", len(diff.Removed)).
+		Int("warnings", len(warnings)).
+		Msg("BOM merge completed")
+
+	s.broker.Publish("bom.merged", mustMarshal(map[string]any{
+		"part_number":      partNumber,
+		"added":            len(diff.Added),
+		"quantity_changed": len(diff.QuantityChanged),
+		"unchanged":        len(diff.Unchanged),
+		"unreferenced":     len(diff.Removed),
+	}))
+
+	writeJSON(w, http.StatusOK, resp)
+}
+
+// quantitiesEqual compares two nullable float64 quantities.
+func quantitiesEqual(a, b *float64) bool {
+	if a == nil && b == nil {
+		return true
+	}
+	if a == nil || b == nil {
+		return false
+	}
+	return *a == *b
+}
diff --git a/internal/api/routes.go b/internal/api/routes.go
index 32d760a..0e8e215 100644
--- a/internal/api/routes.go
+++ b/internal/api/routes.go
@@ -165,6 +165,7 @@ func NewRouter(server *Server, logger zerolog.Logger) http.Handler {
 					r.Put("/thumbnail", server.HandleSetItemThumbnail)
 					r.Post("/bom", server.HandleAddBOMEntry)
 					r.Post("/bom/import", server.HandleImportBOMCSV)
+					r.Post("/bom/merge", server.HandleMergeBOM)
 					r.Put("/bom/{childPartNumber}", server.HandleUpdateBOMEntry)
 					r.Delete("/bom/{childPartNumber}", server.HandleDeleteBOMEntry)
 				})