diff --git a/javascript/lib.cpp b/javascript/lib.cpp
index b259f0b0..81f5c2d7 100644
--- a/javascript/lib.cpp
+++ b/javascript/lib.cpp
@@ -44,6 +44,7 @@ class CompiledIndex : public Napi::ObjectWrap<CompiledIndex> {
     Napi::Value Count(Napi::CallbackInfo const& ctx);
 
     std::unique_ptr<index_dense_t> native_;
+    std::mutex mtx;
 };
 
 Napi::Object CompiledIndex::Init(Napi::Env env, Napi::Object exports) {
@@ -165,13 +166,23 @@ void CompiledIndex::Add(Napi::CallbackInfo const& ctx) {
 
     // Create an instance of the executor with the default number of threads
     auto run_parallel = [&](auto vectors) {
+        std::string error = "";
         executor_stl_t executor;
         executor.fixed(tasks, [&](std::size_t /*thread_idx*/, std::size_t task_idx) {
             add_result_t result = native_->add(static_cast<default_key_t>(keys[task_idx]),
                                                vectors + task_idx * native_->dimensions());
-            if (!result)
-                Napi::Error::New(ctx.Env(), result.error.release()).ThrowAsJavaScriptException();
+            if (!result) {
+                std::lock_guard<std::mutex> lock(mtx);
+                error
+                    .append("<key:")
+                    .append(std::to_string(keys[task_idx]))
+                    .append(" message:")
+                    .append(result.error.release())
+                    .append(">");
+            }
         });
+        if (error.size() > 0)
+            Napi::Error::New(ctx.Env(), error).ThrowAsJavaScriptException();
     };
 
     Napi::TypedArray vectors = ctx[1].As<Napi::TypedArray>();
diff --git a/javascript/usearch.test.js b/javascript/usearch.test.js
index 5210a2bd..482c994f 100644
--- a/javascript/usearch.test.js
+++ b/javascript/usearch.test.js
@@ -166,7 +166,28 @@ test('Invalid operations', async (t) => {
             () => index.add(42n, new Float32Array([0.2, 0.6, 0.4])),
             {
                 name: 'Error',
-                message: 'Duplicate keys not allowed in high-level wrappers'
+                message: '<key:42 message:Duplicate keys not allowed in high-level wrappers>'
+            }
+        );
+    });
+
+    await t.test('Batch add containing the same key', () => {
+        const index = new usearch.Index({
+            metric: "l2sq",
+            connectivity: 16,
+            dimensions: 3,
+        });
+        index.add(42n, new Float32Array([0.2, 0.6, 0.4]));
+        assert.throws(
+            () =>  {
+              index.add(
+                [41n, 42n, 43n],
+                [[0.1, 0.6, 0.4], [0.2, 0.6, 0.4], [0.3, 0.6, 0.4]]
+              );
+            },
+            {
+                name: 'Error',
+                message: '<key:42 message:Duplicate keys not allowed in high-level wrappers>'
             }
         );
     });
@@ -232,7 +253,7 @@ test('Serialization', async (t) => {
             () => index.add(43n, new Float32Array([0.2, 0.6, 0.4])),
             {
                 name: 'Error',
-                message: "Can't add to an immutable index"
+                message: "<key:43 message:Can't add to an immutable index>"
             }
         );
     });