diff --git a/plonky2/src/fri/oracle.rs b/plonky2/src/fri/oracle.rs
index dca3202a..8642a6c5 100644
--- a/plonky2/src/fri/oracle.rs
+++ b/plonky2/src/fri/oracle.rs
@@ -36,6 +36,20 @@ 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>
 {
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/lookup_test.rs b/plonky2/src/lookup_test.rs
index c575417e..165d5dfe 100644
--- a/plonky2/src/lookup_test.rs
+++ b/plonky2/src/lookup_test.rs
@@ -469,6 +469,49 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_circuit_build_mock() {
+        // This code is taken from examples/fibonacci.rs
+        use crate::field::types::Field;
+        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.mock_build::<C>();
+        let partition_witness = data.generate_witness(pw);
+        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..bd436f2c 100644
--- a/plonky2/src/plonk/circuit_builder.rs
+++ b/plonky2/src/plonk/circuit_builder.rs
@@ -40,8 +40,8 @@ use crate::iop::generator::{
 use crate::iop::target::{BoolTarget, Target};
 use crate::iop::wire::Wire;
 use crate::plonk::circuit_data::{
-    CircuitConfig, CircuitData, CommonCircuitData, ProverCircuitData, ProverOnlyCircuitData,
-    VerifierCircuitData, VerifierCircuitTarget, VerifierOnlyCircuitData,
+    CircuitConfig, CircuitData, CommonCircuitData, MockCircuitData, ProverCircuitData,
+    ProverOnlyCircuitData, VerifierCircuitData, VerifierCircuitTarget, VerifierOnlyCircuitData,
 };
 use crate::plonk::config::{AlgebraicHasher, GenericConfig, GenericHashOut, Hasher};
 use crate::plonk::copy_constraint::CopyConstraint;
@@ -916,7 +916,10 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
     }
 
     /// Builds a "full circuit", with both prover and verifier data.
-    pub fn build<C: GenericConfig<D, F = F>>(mut self) -> CircuitData<F, C, D> {
+    pub fn build_with_options<C: GenericConfig<D, F = F>>(
+        mut self,
+        commit_to_sigma: bool,
+    ) -> CircuitData<F, C, D> {
         let mut timing = TimingTree::new("preprocess", Level::Trace);
 
         #[cfg(feature = "std")]
@@ -1023,15 +1026,19 @@ 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 constants_sigmas_commitment = if commit_to_sigma {
+            let constants_sigmas_vecs = [constant_vecs, sigma_vecs.clone()].concat();
+            PolynomialBatch::<F, C, D>::from_values(
+                constants_sigmas_vecs,
+                rate_bits,
+                PlonkOracle::CONSTANTS_SIGMAS.blinding,
+                cap_height,
+                &mut timing,
+                Some(&fft_root_table),
+            )
+        } else {
+            PolynomialBatch::<F, C, D>::default()
+        };
 
         // Map between gates where not all generators are used and the gate's number of used generators.
         let incomplete_gates = self
@@ -1151,6 +1158,17 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         }
     }
 
+    pub fn build<C: GenericConfig<D, F = F>>(self) -> CircuitData<F, C, D> {
+        self.build_with_options(true)
+    }
+
+    pub fn mock_build<C: GenericConfig<D, F = F>>(self) -> MockCircuitData<F, C, D> {
+        let circuit_data = self.build_with_options(false);
+        MockCircuitData {
+            prover_only: circuit_data.prover_only,
+            common: circuit_data.common,
+        }
+    }
     /// Builds a "prover circuit", with data needed to generate proofs but not verify them.
     pub fn build_prover<C: GenericConfig<D, F = F>>(self) -> ProverCircuitData<F, C, D> {
         // TODO: Can skip parts of this.
diff --git a/plonky2/src/plonk/circuit_data.rs b/plonky2/src/plonk/circuit_data.rs
index 6e4e0638..c93de8cb 100644
--- a/plonky2/src/plonk/circuit_data.rs
+++ b/plonky2/src/plonk/circuit_data.rs
@@ -24,9 +24,9 @@ use crate::gates::selectors::SelectorsInfo;
 use crate::hash::hash_types::{HashOutTarget, MerkleCapTarget, RichField};
 use crate::hash::merkle_tree::MerkleCap;
 use crate::iop::ext_target::ExtensionTarget;
-use crate::iop::generator::WitnessGeneratorRef;
+use crate::iop::generator::{generate_partial_witness, WitnessGeneratorRef};
 use crate::iop::target::Target;
-use crate::iop::witness::PartialWitness;
+use crate::iop::witness::{PartialWitness, PartitionWitness};
 use crate::plonk::circuit_builder::CircuitBuilder;
 use crate::plonk::config::{GenericConfig, Hasher};
 use crate::plonk::plonk_common::PlonkOracle;
@@ -111,6 +111,22 @@ impl CircuitConfig {
     }
 }
 
+/// Mock circuit data to only do witness generation without generating a proof.
+#[derive(Eq, PartialEq, Debug)]
+pub struct MockCircuitData<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize>
+{
+    pub prover_only: ProverOnlyCircuitData<F, C, D>,
+    pub common: CommonCircuitData<F, D>,
+}
+
+impl<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize>
+    MockCircuitData<F, C, D>
+{
+    pub fn generate_witness(&self, inputs: PartialWitness<F>) -> PartitionWitness<F> {
+        generate_partial_witness::<F, C, D>(inputs, &self.prover_only, &self.common)
+    }
+}
+
 /// Circuit data required by the prover or the verifier.
 #[derive(Eq, PartialEq, Debug)]
 pub struct CircuitData<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize> {