diff --git a/build-system b/build-system
index 1a8c6c121173..5d3ed11e8017 160000
--- a/build-system
+++ b/build-system
@@ -1 +1 @@
-Subproject commit 1a8c6c121173172691a08e2ecd5dfb210418400a
+Subproject commit 5d3ed11e8017e3daf574e3b287e62c7a99c24bb4
diff --git a/circuits/cpp/barretenberg b/circuits/cpp/barretenberg
index af91bc8c1228..e8ed76b87eb4 160000
--- a/circuits/cpp/barretenberg
+++ b/circuits/cpp/barretenberg
@@ -1 +1 @@
-Subproject commit af91bc8c12286d5a5abc16d4d79f4b5e2c3e7ecc
+Subproject commit e8ed76b87eb4f0be55d7b02628d90561db8f8529
diff --git a/circuits/cpp/src/aztec3/circuits/abis/.test.cpp b/circuits/cpp/src/aztec3/circuits/abis/.test.cpp
index 0bee851c6627..e18331ea8eaa 100644
--- a/circuits/cpp/src/aztec3/circuits/abis/.test.cpp
+++ b/circuits/cpp/src/aztec3/circuits/abis/.test.cpp
@@ -9,7 +9,7 @@
 
 namespace {
 // Builder
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 
 // Types
 using CT = aztec3::utils::types::CircuitTypes<Builder>;
diff --git a/circuits/cpp/src/aztec3/circuits/apps/.test.cpp b/circuits/cpp/src/aztec3/circuits/apps/.test.cpp
index ab4ca379df42..c4ba2b5dab37 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/.test.cpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/.test.cpp
@@ -25,7 +25,7 @@
 
 namespace {
 // Builder
-using C = UltraCircuitBuilder;
+using C = CircuitSimulatorBN254;
 
 // Types
 using CT = aztec3::utils::types::CircuitTypes<C>;
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/basic_contract_deployment/init.hpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/basic_contract_deployment/init.hpp
index 4b193f601d7d..ea51024d5772 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/basic_contract_deployment/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/basic_contract_deployment/init.hpp
@@ -11,10 +11,13 @@
 #include "aztec3/utils/types/convert.hpp"
 #include "aztec3/utils/types/native_types.hpp"
 
+#include "barretenberg/stdlib/primitives/circuit_builders/circuit_builders_fwd.hpp"
+
 namespace aztec3::circuits::apps::test_apps::basic_contract_deployment {
 
 // Builder
-using C = UltraCircuitBuilder;
+
+using C = CircuitSimulatorBN254;
 
 // Native and circuit types
 using CT = aztec3::utils::types::CircuitTypes<C>;
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/.test.cpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/.test.cpp
index 5e68463937ed..1253947f8030 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/.test.cpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/.test.cpp
@@ -58,7 +58,7 @@ TEST_F(escrow_tests, circuit_deposit)
     // info("variables: ", builder.variables);
     info("failed?: ", builder.failed());
     info("err: ", builder.err());
-    info("n: ", builder.num_gates);
+    // info("n: ", builder.num_gates);
 }
 
 TEST_F(escrow_tests, circuit_transfer)
@@ -83,7 +83,7 @@ TEST_F(escrow_tests, circuit_transfer)
     // info("variables: ", builder.variables);
     info("failed?: ", builder.failed());
     info("err: ", builder.err());
-    info("n: ", builder.num_gates);
+    // info("n: ", builder.num_gates);
 }
 
 TEST_F(escrow_tests, circuit_withdraw)
@@ -107,7 +107,7 @@ TEST_F(escrow_tests, circuit_withdraw)
     // info("variables: ", builder.variables);
     info("failed?: ", builder.failed());
     info("err: ", builder.err());
-    info("n: ", builder.num_gates);
+    // info("n: ", builder.num_gates);
 }
 
 }  // namespace aztec3::circuits::apps::test_apps::escrow
\ No newline at end of file
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/init.hpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/init.hpp
index 30d5da1dbbcb..0f6f6a38610d 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/escrow/init.hpp
@@ -11,10 +11,14 @@
 #include "aztec3/utils/types/convert.hpp"
 #include "aztec3/utils/types/native_types.hpp"
 
+#include "barretenberg/proof_system/circuit_builder/circuit_simulator.hpp"
+#include "barretenberg/stdlib/primitives/circuit_builders/circuit_builders_fwd.hpp"
+
 namespace aztec3::circuits::apps::test_apps::escrow {
 
 // Builder
-using C = UltraCircuitBuilder;
+// WORKTODO: this is coupled to the circuit builder in the private kernel.
+using C = CircuitSimulatorBN254;
 
 // Native and circuit types
 using CT = aztec3::utils::types::CircuitTypes<C>;
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/.test.cpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/.test.cpp
index ba7acefda62f..f689eb318939 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/.test.cpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/.test.cpp
@@ -56,7 +56,8 @@ TEST(private_to_private_function_call_tests, circuit_private_to_private_function
     // info("variables: ", fn1_builder.variables);
     info("failed?: ", fn1_builder.failed());
     info("err: ", fn1_builder.err());
-    info("n: ", fn1_builder.num_gates);
+    // info("n: ", fn1_builder.num_gates);
+    // WORKTODO: this test leads to a failing composer, but there are no expectations in the test.
 }
 
 }  // namespace aztec3::circuits::apps::test_apps::private_to_private_function_call
\ No newline at end of file
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/function_2_1.cpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/function_2_1.cpp
index ac424411667e..7197abd6d9d4 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/function_2_1.cpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/function_2_1.cpp
@@ -41,7 +41,7 @@ void function_2_1(FunctionExecutionContext& exec_ctx, std::vector<NT::fr> const&
 
     auto product = a * b * c;
 
-    CT::address const unique_person_who_may_initialise = 999999;
+    CT::address const unique_person_who_may_initialise = { msg_sender.context_, 999999 };
 
     unique_person_who_may_initialise.assert_equal(msg_sender);
 
diff --git a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/init.hpp b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/init.hpp
index 7f2e42a70a47..129156199952 100644
--- a/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/apps/test_apps/private_to_private_function_call/init.hpp
@@ -13,7 +13,7 @@
 namespace aztec3::circuits::apps::test_apps::private_to_private_function_call {
 
 // Builder
-using C = UltraCircuitBuilder;
+using C = CircuitSimulatorBN254;
 
 // Native and circuit types
 using CT = aztec3::utils::types::CircuitTypes<C>;
diff --git a/circuits/cpp/src/aztec3/circuits/kernel/private/c_bind.cpp b/circuits/cpp/src/aztec3/circuits/kernel/private/c_bind.cpp
index 43e45c2e2401..e4e1a57bc795 100644
--- a/circuits/cpp/src/aztec3/circuits/kernel/private/c_bind.cpp
+++ b/circuits/cpp/src/aztec3/circuits/kernel/private/c_bind.cpp
@@ -12,7 +12,7 @@
 #include <array>
 
 namespace {
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 using NT = aztec3::utils::types::NativeTypes;
 using DummyBuilder = aztec3::utils::DummyCircuitBuilder;
 using aztec3::circuits::abis::PreviousKernelData;
diff --git a/circuits/cpp/src/aztec3/circuits/kernel/private/init.hpp b/circuits/cpp/src/aztec3/circuits/kernel/private/init.hpp
index 81866dedde7a..c1690fb476d8 100644
--- a/circuits/cpp/src/aztec3/circuits/kernel/private/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/kernel/private/init.hpp
@@ -7,9 +7,11 @@
 #include "aztec3/utils/types/convert.hpp"
 #include "aztec3/utils/types/native_types.hpp"
 
+#include "barretenberg/stdlib/primitives/circuit_builders/circuit_builders_fwd.hpp"
+
 namespace aztec3::circuits::kernel::private_kernel {
 
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 
 using Aggregator = aztec3::circuits::recursion::Aggregator;
 
diff --git a/circuits/cpp/src/aztec3/circuits/kernel/public/c_bind.cpp b/circuits/cpp/src/aztec3/circuits/kernel/public/c_bind.cpp
index b15896d162d1..2286aad080b6 100644
--- a/circuits/cpp/src/aztec3/circuits/kernel/public/c_bind.cpp
+++ b/circuits/cpp/src/aztec3/circuits/kernel/public/c_bind.cpp
@@ -12,7 +12,7 @@
 #include <barretenberg/barretenberg.hpp>
 
 namespace {
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 using NT = aztec3::utils::types::NativeTypes;
 using DummyBuilder = aztec3::utils::DummyCircuitBuilder;
 using aztec3::circuits::abis::KernelCircuitPublicInputs;
diff --git a/circuits/cpp/src/aztec3/circuits/kernel/public/init.hpp b/circuits/cpp/src/aztec3/circuits/kernel/public/init.hpp
index 5a7e60acb778..8b877095910a 100644
--- a/circuits/cpp/src/aztec3/circuits/kernel/public/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/kernel/public/init.hpp
@@ -10,7 +10,7 @@
 
 namespace aztec3::circuits::kernel::public_kernel {
 
-using Builder = proof_system::UltraCircuitBuilder;
+using Builder = proof_system::CircuitSimulatorBN254;
 
 using Aggregator = aztec3::circuits::recursion::Aggregator;
 
diff --git a/circuits/cpp/src/aztec3/circuits/recursion/aggregator.hpp b/circuits/cpp/src/aztec3/circuits/recursion/aggregator.hpp
index 343ba12e79e7..9c621bf77239 100644
--- a/circuits/cpp/src/aztec3/circuits/recursion/aggregator.hpp
+++ b/circuits/cpp/src/aztec3/circuits/recursion/aggregator.hpp
@@ -14,7 +14,7 @@ class Aggregator {
         const CT::AggregationObject& previous_aggregation_output = CT::AggregationObject())
     {
         CT::AggregationObject result =
-            verify_proof<plonk::flavor::Ultra>(builder, vk, proof, previous_aggregation_output);
+            plonk::stdlib::recursion::verify_proof<Flavor>(builder, vk, proof, previous_aggregation_output);
 
         return result;
     }
diff --git a/circuits/cpp/src/aztec3/circuits/recursion/init.hpp b/circuits/cpp/src/aztec3/circuits/recursion/init.hpp
index 427d25f502da..fdf857dc94b7 100644
--- a/circuits/cpp/src/aztec3/circuits/recursion/init.hpp
+++ b/circuits/cpp/src/aztec3/circuits/recursion/init.hpp
@@ -3,11 +3,14 @@
 #include "aztec3/utils/types/convert.hpp"
 #include "aztec3/utils/types/native_types.hpp"
 
+#include "barretenberg/plonk/flavor/flavor.hpp"
 #include <barretenberg/barretenberg.hpp>
 
 namespace aztec3::circuits::recursion {
 // Builder
-using Builder = UltraCircuitBuilder;
+using Flavor = proof_system::plonk::flavor::SimulatorBN254;
+// using Flavor = proof_system::plonk::flavor::Ultra;
+using Builder = typename Flavor::CircuitBuilder;
 
 // Generic types:
 using CT = aztec3::utils::types::CircuitTypes<Builder>;
@@ -15,7 +18,6 @@ using NT = aztec3::utils::types::NativeTypes;
 using aztec3::utils::types::to_ct;
 
 // Recursion types and methods
-using plonk::stdlib::recursion::verify_proof;
 using transcript::Manifest;
 
 }  // namespace aztec3::circuits::recursion
\ No newline at end of file
diff --git a/circuits/cpp/src/aztec3/circuits/rollup/base/c_bind.cpp b/circuits/cpp/src/aztec3/circuits/rollup/base/c_bind.cpp
index 0b20a881e403..69c59d27800d 100644
--- a/circuits/cpp/src/aztec3/circuits/rollup/base/c_bind.cpp
+++ b/circuits/cpp/src/aztec3/circuits/rollup/base/c_bind.cpp
@@ -11,7 +11,7 @@
 #include <barretenberg/barretenberg.hpp>
 
 namespace {
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 using NT = aztec3::utils::types::NativeTypes;
 using DummyBuilder = aztec3::utils::DummyCircuitBuilder;
 using aztec3::circuits::abis::BaseOrMergeRollupPublicInputs;
diff --git a/circuits/cpp/src/aztec3/circuits/rollup/root/c_bind.cpp b/circuits/cpp/src/aztec3/circuits/rollup/root/c_bind.cpp
index 9bc1e7b66f7a..e05bbc5bf342 100644
--- a/circuits/cpp/src/aztec3/circuits/rollup/root/c_bind.cpp
+++ b/circuits/cpp/src/aztec3/circuits/rollup/root/c_bind.cpp
@@ -9,7 +9,7 @@
 #include <barretenberg/barretenberg.hpp>
 
 namespace {
-using Builder = UltraCircuitBuilder;
+using Builder = CircuitSimulatorBN254;
 using NT = aztec3::utils::types::NativeTypes;
 using DummyBuilder = aztec3::utils::DummyCircuitBuilder;
 using aztec3::circuits::rollup::native_root_rollup::root_rollup_circuit;