diff --git a/src/engine/sparqlExpressions/GroupConcatExpression.cpp b/src/engine/sparqlExpressions/GroupConcatExpression.cpp
index 6ef036239f..29d98ddc0b 100644
--- a/src/engine/sparqlExpressions/GroupConcatExpression.cpp
+++ b/src/engine/sparqlExpressions/GroupConcatExpression.cpp
@@ -33,8 +33,20 @@ sparqlExpression::GroupConcatExpression::evaluate(
       result.reserve(20000);
       bool firstIteration = true;
       for (auto& inp : generator) {
-        auto literal = detail::LiteralValueGetterWithoutStrFunction{}(
-            std::move(inp), context);
+        // For GROUP_CONCAT, we need to:
+        // 1. Convert IRIs to string literals (implicit STR() behavior)
+        // 2. Preserve language tags on literals (for mergeLanguageTags)
+        //
+        // LiteralValueGetterWithoutStrFunction preserves language tags but
+        // returns nullopt for IRIs. LiteralValueGetterWithStrFunction handles
+        // IRIs but strips language tags. We try WithoutStr first, and only
+        // fall back to WithStr for IRIs (when WithoutStr returns nullopt).
+        auto literal =
+            detail::LiteralValueGetterWithoutStrFunction{}(inp, context);
+        if (!literal.has_value()) {
+          // This is an IRI (or other non-literal). Use WithStr to convert it.
+          literal = detail::LiteralValueGetterWithStrFunction{}(inp, context);
+        }
         if (firstIteration) {
           firstIteration = false;
           detail::pushLanguageTag(langTag, literal);
diff --git a/test/engine/GroupConcatExpressionTest.cpp b/test/engine/GroupConcatExpressionTest.cpp
index 5d3efc0398..f8929fc8b7 100644
--- a/test/engine/GroupConcatExpressionTest.cpp
+++ b/test/engine/GroupConcatExpressionTest.cpp
@@ -130,6 +130,50 @@ TEST(GroupConcatExpression, concatenationWithLanguageTags) {
                                   lit("\"a;a\""));
 }
 
+// _____________________________________________________________________________
+// Test that IRIs are converted to strings (like implicit STR() application).
+// This tests vocabulary IRIs, local vocab IRIs, and literals mixed together.
+TEST(GroupConcatExpression, concatenationWithIris) {
+  auto* qec = ad_utility::testing::getQec();
+  auto getId = ad_utility::testing::makeGetId(qec->getIndex());
+
+  auto iri = [](std::string_view s) {
+    return tc::LiteralOrIri{tc::Iri::fromIriref(s)};
+  };
+
+  LocalVocab localVocab;
+  IdTable input{1, ad_utility::makeUnlimitedAllocator<Id>()};
+
+  // 1. Add a vocabulary IRI.
+  Id xId = getId("<x>");
+  input.push_back({xId});
+  expectIdsAreConcatenatedTo(false, input,
+                             IdOrLiteralOrIri{LocalVocabEntry{lit("\"x\"")}});
+
+  // 2. Add a local vocab IRI.
+  auto localIriIdx = localVocab.getIndexAndAddIfNotContained(
+      LocalVocabEntry{iri("<http://example.org/foo>")});
+  input.push_back({Id::makeFromLocalVocabIndex(localIriIdx)});
+  expectIdsAreConcatenatedTo(
+      false, input,
+      IdOrLiteralOrIri{LocalVocabEntry{lit("\"x;http://example.org/foo\"")}});
+
+  // 3. Add a local vocab literal.
+  auto literalIdx =
+      localVocab.getIndexAndAddIfNotContained(LocalVocabEntry{lit("\"bar\"")});
+  input.push_back({Id::makeFromLocalVocabIndex(literalIdx)});
+  expectIdsAreConcatenatedTo(false, input,
+                             IdOrLiteralOrIri{LocalVocabEntry{
+                                 lit("\"x;http://example.org/foo;bar\"")}});
+
+  // 4. Add another vocabulary IRI.
+  Id labelId = getId("<label>");
+  input.push_back({labelId});
+  expectIdsAreConcatenatedTo(false, input,
+                             IdOrLiteralOrIri{LocalVocabEntry{lit(
+                                 "\"x;http://example.org/foo;bar;label\"")}});
+}
+
 // _____________________________________________________________________________
 TEST(GroupConcatExpression, getCacheKey) {
   Variable var{"?x"};