diff --git a/.github/workflows/macos.yml b/.github/workflows/macos.yml
index 4c1a1f5..01f3982 100644
--- a/.github/workflows/macos.yml
+++ b/.github/workflows/macos.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        LLVM_VERSION: [18, 19]
+        LLVM_VERSION: [18, 19, 20, 21]
 
     steps:
       - uses: actions/checkout@v4
diff --git a/tests/Mutations/CallRemovalTests.cpp b/tests/Mutations/CallRemovalTests.cpp
index 31b58cd..8578327 100644
--- a/tests/Mutations/CallRemovalTests.cpp
+++ b/tests/Mutations/CallRemovalTests.cpp
@@ -43,8 +43,12 @@ TEST(CallRemoval, canMutate) {
       voidFunctionType, llvm::Function::InternalLinkage, "call_void", module);
 
   auto ptrType = llvm::PointerType::get(context, 0);
-#if LLVM_VERSION_MAJOR >= 19
-  auto intrinsicFunction = llvm::Intrinsic::getDeclaration(&module, llvm::Intrinsic::vastart, {ptrType});
+#if LLVM_VERSION_MAJOR >= 20
+  auto intrinsicFunction =
+      llvm::Intrinsic::getOrInsertDeclaration(&module, llvm::Intrinsic::vastart, { ptrType });
+#elif LLVM_VERSION_MAJOR >= 19
+  auto intrinsicFunction =
+      llvm::Intrinsic::getDeclaration(&module, llvm::Intrinsic::vastart, { ptrType });
 #else
   auto intrinsicFunction = llvm::Intrinsic::getDeclaration(&module, llvm::Intrinsic::vastart);
 #endif
diff --git a/tests/Mutations/InstrinsicReplacementTests.cpp b/tests/Mutations/InstrinsicReplacementTests.cpp
index 01c6369..e9b4f7b 100644
--- a/tests/Mutations/InstrinsicReplacementTests.cpp
+++ b/tests/Mutations/InstrinsicReplacementTests.cpp
@@ -33,10 +33,21 @@ TEST(IntrinsicReplacement, canMutate) {
   auto basicBlock = llvm::BasicBlock::Create(context, "entry", function);
 
   auto intrinsicType = llvm::Type::getInt8Ty(context);
+#if LLVM_VERSION_MAJOR >= 20
+  auto sadd = llvm::Intrinsic::getOrInsertDeclaration(
+      &module, llvm::Intrinsic::sadd_with_overflow, { intrinsicType });
+#else
   auto sadd = llvm::Intrinsic::getDeclaration(
       &module, llvm::Intrinsic::sadd_with_overflow, { intrinsicType });
+#endif
+
+#if LLVM_VERSION_MAJOR >= 20
+  auto ssub = llvm::Intrinsic::getOrInsertDeclaration(
+      &module, llvm::Intrinsic::ssub_with_overflow, { intrinsicType });
+#else
   auto ssub = llvm::Intrinsic::getDeclaration(
       &module, llvm::Intrinsic::ssub_with_overflow, { intrinsicType });
+#endif
 
   auto op1 = llvm::ConstantInt::get(intrinsicType, 5, false);
   auto op2 = llvm::ConstantInt::get(intrinsicType, 40, false);
@@ -58,8 +69,14 @@ TEST(IntrinsicReplacement, mutate) {
   auto basicBlock = llvm::BasicBlock::Create(context, "entry", function);
 
   auto intrinsicType = llvm::Type::getInt8Ty(context);
+
+#if LLVM_VERSION_MAJOR >= 20
+  auto sadd = llvm::Intrinsic::getOrInsertDeclaration(
+      &module, llvm::Intrinsic::sadd_with_overflow, { intrinsicType });
+#else
   auto sadd = llvm::Intrinsic::getDeclaration(
       &module, llvm::Intrinsic::sadd_with_overflow, { intrinsicType });
+#endif
 
   auto op1 = llvm::ConstantInt::get(intrinsicType, 5, false);
   auto op2 = llvm::ConstantInt::get(intrinsicType, 40, false);