diff --git a/Code/consensus/src/executor.rs b/Code/consensus/src/executor.rs
index 029dd77c4..58a6e3c5b 100644
--- a/Code/consensus/src/executor.rs
+++ b/Code/consensus/src/executor.rs
@@ -27,20 +27,6 @@ where
     TimeoutElapsed(Timeout),
 }
 
-#[derive(Clone, Debug)]
-pub struct Executor<Ctx>
-where
-    Ctx: Context,
-{
-    height: Ctx::Height,
-    private_key: Secret<PrivateKey<Ctx>>,
-    address: Ctx::Address,
-    validator_set: Ctx::ValidatorSet,
-    pub round: Round,
-    votes: VoteKeeper<Ctx>,
-    round_states: BTreeMap<Round, RoundState<Ctx>>,
-}
-
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Message<Ctx>
 where
@@ -53,11 +39,37 @@ where
     NewRound(Round),
 }
 
-impl<Ctx> Executor<Ctx>
+pub trait Client<Ctx>
+where
+    Ctx: Context,
+{
+    fn get_value(&self) -> Ctx::Value;
+    fn validate_proposal(&self, proposal: &Ctx::Proposal) -> bool;
+}
+
+#[derive(Clone, Debug)]
+pub struct Executor<Ctx, Client>
 where
     Ctx: Context,
+    Client: self::Client<Ctx>,
+{
+    pub client: Client,
+    pub height: Ctx::Height,
+    pub private_key: Secret<PrivateKey<Ctx>>,
+    pub address: Ctx::Address,
+    pub validator_set: Ctx::ValidatorSet,
+    pub round: Round,
+    pub votes: VoteKeeper<Ctx>,
+    pub round_states: BTreeMap<Round, RoundState<Ctx>>,
+}
+
+impl<Ctx, Client> Executor<Ctx, Client>
+where
+    Ctx: Context,
+    Client: self::Client<Ctx>,
 {
     pub fn new(
+        client: Client,
         height: Ctx::Height,
         validator_set: Ctx::ValidatorSet,
         private_key: PrivateKey<Ctx>,
@@ -66,6 +78,7 @@ where
         let votes = VoteKeeper::new(validator_set.total_voting_power());
 
         Self {
+            client,
             height,
             private_key: Secret::new(private_key),
             address,
@@ -76,9 +89,12 @@ where
         }
     }
 
-    pub fn get_value(&self) -> Ctx::Value {
-        // TODO - add external interface to get the value
-        Ctx::DUMMY_VALUE
+    fn get_value(&self) -> Ctx::Value {
+        self.client.get_value()
+    }
+
+    fn validate_proposal(&self, proposal: &Ctx::Proposal) -> bool {
+        self.client.validate_proposal(proposal)
     }
 
     pub fn execute(&mut self, msg: Event<Ctx>) -> Option<Message<Ctx>> {
@@ -143,7 +159,9 @@ where
     }
 
     fn apply_proposal(&mut self, proposal: Ctx::Proposal) -> Option<RoundMessage<Ctx>> {
-        // TODO: Check for invalid proposal
+        if !self.validate_proposal(&proposal) {
+            todo!("invalid proposal");
+        }
 
         // Check that there is an ongoing round
         let Some(round_state) = self.round_states.get(&self.round) else {
diff --git a/Code/test/src/client.rs b/Code/test/src/client.rs
new file mode 100644
index 000000000..4e05c2345
--- /dev/null
+++ b/Code/test/src/client.rs
@@ -0,0 +1,24 @@
+use malachite_consensus::executor::Client;
+
+use crate::{Proposal, TestContext, Value};
+
+pub struct TestClient {
+    pub value: Value,
+    pub is_valid: fn(&Proposal) -> bool,
+}
+
+impl TestClient {
+    pub fn new(value: Value, is_valid: fn(&Proposal) -> bool) -> Self {
+        Self { value, is_valid }
+    }
+}
+
+impl Client<TestContext> for TestClient {
+    fn get_value(&self) -> Value {
+        self.value.clone()
+    }
+
+    fn validate_proposal(&self, proposal: &Proposal) -> bool {
+        (self.is_valid)(proposal)
+    }
+}
diff --git a/Code/test/src/lib.rs b/Code/test/src/lib.rs
index c39e1682a..fc979bccb 100644
--- a/Code/test/src/lib.rs
+++ b/Code/test/src/lib.rs
@@ -1,6 +1,7 @@
 #![forbid(unsafe_code)]
 #![deny(trivial_casts, trivial_numeric_casts)]
 
+mod client;
 mod context;
 mod height;
 mod proposal;
@@ -9,6 +10,7 @@ mod validator_set;
 mod value;
 mod vote;
 
+pub use crate::client::*;
 pub use crate::context::*;
 pub use crate::height::*;
 pub use crate::proposal::*;
diff --git a/Code/test/tests/consensus_executor.rs b/Code/test/tests/consensus_executor.rs
index b17cbca88..67d54afa9 100644
--- a/Code/test/tests/consensus_executor.rs
+++ b/Code/test/tests/consensus_executor.rs
@@ -3,7 +3,7 @@ use malachite_consensus::executor::{Event, Executor, Message};
 use malachite_round::state::{RoundValue, State, Step};
 
 use malachite_test::{
-    Address, Height, PrivateKey, Proposal, TestContext, Validator, ValidatorSet, Vote,
+    Address, Height, PrivateKey, Proposal, TestClient, TestContext, Validator, ValidatorSet, Vote,
 };
 use rand::rngs::StdRng;
 use rand::SeedableRng;
@@ -47,8 +47,8 @@ fn executor_steps_proposer() {
     let (my_sk, my_addr) = (sk1, addr1);
 
     let vs = ValidatorSet::new(vec![v1, v2.clone(), v3.clone()]);
-
-    let mut executor = Executor::new(Height::new(1), vs, my_sk.clone(), my_addr);
+    let client = TestClient::new(value.clone(), |_| true);
+    let mut executor = Executor::new(client, Height::new(1), vs, my_sk.clone(), my_addr);
 
     let proposal = Proposal::new(Height::new(1), Round::new(0), value.clone(), Round::new(-1));
 
@@ -229,7 +229,8 @@ fn executor_steps_not_proposer() {
     let (my_sk, my_addr) = (sk2, addr2);
 
     let vs = ValidatorSet::new(vec![v1.clone(), v2.clone(), v3.clone()]);
-    let mut executor = Executor::new(Height::new(1), vs, my_sk.clone(), my_addr);
+    let client = TestClient::new(value.clone(), |_| true);
+    let mut executor = Executor::new(client, Height::new(1), vs, my_sk.clone(), my_addr);
 
     let proposal = Proposal::new(Height::new(1), Round::new(0), value.clone(), Round::new(-1));
 
@@ -410,7 +411,8 @@ fn executor_steps_not_proposer_timeout_multiple_rounds() {
     let (my_sk, my_addr) = (sk3, addr3);
 
     let vs = ValidatorSet::new(vec![v1.clone(), v2.clone(), v3.clone()]);
-    let mut executor = Executor::new(Height::new(1), vs, my_sk.clone(), my_addr);
+    let client = TestClient::new(value.clone(), |_| true);
+    let mut executor = Executor::new(client, Height::new(1), vs, my_sk.clone(), my_addr);
 
     let steps = vec![
         // Start round 0, we, v3, are not the proposer