Respond with an error if request is invalid

app/controllers/v1/purls_controller.rb

@@ -20,10 +20,18 @@ def update
                 retry
               end
 
-      PurlCocinaUpdater.update(@purl, cocina_object)
-      write_public_files
-
-      render json: true, status: :accepted
+      begin
+        PurlCocinaUpdater.update(@purl, cocina_object)
+        write_public_files
+
+        render json: true, status: :accepted
+      rescue Cocina::Models::ValidationError => e
+        render json: {
+          errors: [
+            { title: 'bad request', detail: e.message }
+          ]
+        }, status: :bad_request
+      end
     end
 
     def destroy

spec/requests/v1/purls_controller_spec.rb

@@ -75,6 +75,23 @@
       end
     end
 
+    context 'with invalid cocina json' do
+      let(:data) { { invalid: true}.to_json }
+      before do
+        allow(PurlCocinaUpdater).to receive(:update)
+      end
+
+      context 'with a new item' do
+        it 'creates a new purl entry' do
+          expect do
+            post "/purls/#{druid}", params: data, headers:
+          end.to change(Purl, :count).by(1)
+          expect(response).to have_http_status(:bad_request)
+          expect(PurlCocinaUpdater).not_to have_received(:update)
+        end
+      end
+    end
+
     context 'when no authorization token is provided' do
       it 'returns 401' do
         post "/purls/#{druid}", params: data, headers: headers.except('Authorization')