diff --git a/evm/tests/basic_smart_contract.rs b/evm/tests/basic_smart_contract.rs
index 89aacd2c..28b5be0b 100644
--- a/evm/tests/basic_smart_contract.rs
+++ b/evm/tests/basic_smart_contract.rs
@@ -27,7 +27,7 @@ type C = KeccakGoldilocksConfig;
 
 /// Test a simple token transfer to a new address.
 #[test]
-#[ignore] // Too slow to run on CI.
+// #[ignore] // Too slow to run on CI.
 fn test_basic_smart_contract() -> anyhow::Result<()> {
     init_logger();
 
diff --git a/plonky2/Cargo.toml b/plonky2/Cargo.toml
index c5de1e63..367b99d0 100644
--- a/plonky2/Cargo.toml
+++ b/plonky2/Cargo.toml
@@ -16,6 +16,7 @@ gate_testing = []
 parallel = ["hashbrown/rayon", "plonky2_maybe_rayon/parallel"]
 std = ["anyhow/std", "rand/std", "itertools/use_std"]
 timing = ["std"]
+mock = []
 
 [dependencies]
 ahash = { version = "0.8.3", default-features = false, features = ["compile-time-rng"] } # NOTE: Be sure to keep this version the same as the dependency in `hashbrown`.
diff --git a/plonky2/src/fri/oracle.rs b/plonky2/src/fri/oracle.rs
index dca3202a..a7182e15 100644
--- a/plonky2/src/fri/oracle.rs
+++ b/plonky2/src/fri/oracle.rs
@@ -36,9 +36,32 @@ pub struct PolynomialBatch<F: RichField + Extendable<D>, C: GenericConfig<D, F =
     pub blinding: bool,
 }
 
+impl<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize> Default
+    for PolynomialBatch<F, C, D>
+{
+    fn default() -> Self {
+        PolynomialBatch {
+            polynomials: Vec::new(),
+            merkle_tree: MerkleTree::default(),
+            degree_log: 0,
+            rate_bits: 0,
+            blinding: false,
+        }
+    }
+}
+
 impl<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize>
     PolynomialBatch<F, C, D>
 {
+    pub fn default() -> Self {
+        Self {
+            polynomials: Vec::new(),
+            merkle_tree: MerkleTree::default(),
+            degree_log: 0,
+            rate_bits: 0,
+            blinding: false,
+        }
+    }
     /// Creates a list polynomial commitment for the polynomials interpolating the values in `values`.
     pub fn from_values(
         values: Vec<PolynomialValues<F>>,
diff --git a/plonky2/src/hash/merkle_tree.rs b/plonky2/src/hash/merkle_tree.rs
index b3b0e0cb..ab7e3958 100644
--- a/plonky2/src/hash/merkle_tree.rs
+++ b/plonky2/src/hash/merkle_tree.rs
@@ -17,6 +17,12 @@ use crate::util::log2_strict;
 // TODO: Change H to GenericHashOut<F>, since this only cares about the hash, not the hasher.
 pub struct MerkleCap<F: RichField, H: Hasher<F>>(pub Vec<H::Hash>);
 
+impl<F: RichField, H: Hasher<F>> Default for MerkleCap<F, H> {
+    fn default() -> Self {
+        Self(Vec::new())
+    }
+}
+
 impl<F: RichField, H: Hasher<F>> MerkleCap<F, H> {
     pub fn len(&self) -> usize {
         self.0.len()
@@ -54,6 +60,16 @@ pub struct MerkleTree<F: RichField, H: Hasher<F>> {
     pub cap: MerkleCap<F, H>,
 }
 
+impl<F: RichField, H: Hasher<F>> Default for MerkleTree<F, H> {
+    fn default() -> Self {
+        Self {
+            leaves: Vec::new(),
+            digests: Vec::new(),
+            cap: MerkleCap::default(),
+        }
+    }
+}
+
 fn capacity_up_to_mut<T>(v: &mut Vec<T>, len: usize) -> &mut [MaybeUninit<T>] {
     assert!(v.capacity() >= len);
     let v_ptr = v.as_mut_ptr().cast::<MaybeUninit<T>>();
diff --git a/plonky2/src/iop/generator.rs b/plonky2/src/iop/generator.rs
index 478c5ff5..10293339 100644
--- a/plonky2/src/iop/generator.rs
+++ b/plonky2/src/iop/generator.rs
@@ -16,7 +16,7 @@ use crate::util::serialization::{Buffer, IoResult, Read, Write};
 
 /// Given a `PartitionWitness` that has only inputs set, populates the rest of the witness using the
 /// given set of generators.
-pub(crate) fn generate_partial_witness<
+pub fn generate_partial_witness<
     'a,
     F: RichField + Extendable<D>,
     C: GenericConfig<D, F = F>,
diff --git a/plonky2/src/lookup_test.rs b/plonky2/src/lookup_test.rs
index c575417e..c8b084cc 100644
--- a/plonky2/src/lookup_test.rs
+++ b/plonky2/src/lookup_test.rs
@@ -469,6 +469,51 @@ mod tests {
         Ok(())
     }
 
+    #[cfg(feature = "mock")]
+    #[test]
+    fn test_circuit_build_mock() {
+        // This code is taken from examples/fibonacci.rs
+        use crate::field::types::Field;
+        use crate::iop::generator::generate_partial_witness;
+        use crate::iop::witness::{PartialWitness, Witness, WitnessWrite};
+        use crate::plonk::circuit_builder::CircuitBuilder;
+        use crate::plonk::circuit_data::CircuitConfig;
+        use crate::plonk::config::{GenericConfig, PoseidonGoldilocksConfig};
+
+        const D: usize = 2;
+        type C = PoseidonGoldilocksConfig;
+        type F = <C as GenericConfig<D>>::F;
+
+        let config = CircuitConfig::standard_recursion_config();
+        let mut builder = CircuitBuilder::<F, D>::new(config);
+
+        // The arithmetic circuit.
+        let initial_a = builder.add_virtual_target();
+        let initial_b = builder.add_virtual_target();
+        let mut prev_target = initial_a;
+        let mut cur_target = initial_b;
+        for _ in 0..99 {
+            let temp = builder.add(prev_target, cur_target);
+            prev_target = cur_target;
+            cur_target = temp;
+        }
+
+        // Public inputs are the two initial values (provided below) and the result (which is generated).
+        builder.register_public_input(initial_a);
+        builder.register_public_input(initial_b);
+        builder.register_public_input(cur_target);
+
+        // Provide initial values.
+        let mut pw = PartialWitness::new();
+        pw.set_target(initial_a, F::ZERO);
+        pw.set_target(initial_b, F::ONE);
+
+        let data = builder.build::<C>();
+        let partition_witness = generate_partial_witness(pw, &data.prover_only, &data.common);
+        let result = partition_witness.try_get_target(cur_target).unwrap();
+        assert_eq!(result, F::from_canonical_u64(3736710860384812976));
+    }
+
     fn init_logger() -> anyhow::Result<()> {
         let mut builder = env_logger::Builder::from_default_env();
         builder.format_timestamp(None);
diff --git a/plonky2/src/plonk/circuit_builder.rs b/plonky2/src/plonk/circuit_builder.rs
index 82d4ee89..89ec0903 100644
--- a/plonky2/src/plonk/circuit_builder.rs
+++ b/plonky2/src/plonk/circuit_builder.rs
@@ -1023,15 +1023,19 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         let max_fft_points = 1 << (degree_bits + max(rate_bits, log2_ceil(quotient_degree_factor)));
         let fft_root_table = fft_root_table(max_fft_points);
 
-        let constants_sigmas_vecs = [constant_vecs, sigma_vecs.clone()].concat();
-        let constants_sigmas_commitment = PolynomialBatch::<F, C, D>::from_values(
-            constants_sigmas_vecs,
-            rate_bits,
-            PlonkOracle::CONSTANTS_SIGMAS.blinding,
-            cap_height,
-            &mut timing,
-            Some(&fft_root_table),
-        );
+        let mut constants_sigmas_commitment = PolynomialBatch::<F, C, D>::default();
+        #[cfg(not(feature = "mock"))]
+        {
+            let constants_sigmas_vecs = [constant_vecs, sigma_vecs.clone()].concat();
+            constants_sigmas_commitment = PolynomialBatch::<F, C, D>::from_values(
+                constants_sigmas_vecs,
+                rate_bits,
+                PlonkOracle::CONSTANTS_SIGMAS.blinding,
+                cap_height,
+                &mut timing,
+                Some(&fft_root_table),
+            );
+        }
 
         // Map between gates where not all generators are used and the gate's number of used generators.
         let incomplete_gates = self