diff --git a/Cargo.toml b/Cargo.toml
index 129a22dc..e5e1a23c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,7 +5,7 @@ resolver = "2"
 [workspace.package]
 version = "0.10.1"
 edition = "2021"
-rust-version = "1.81"
+rust-version = "1.85"
 authors = ["init4"]
 license = "MIT OR Apache-2.0"
 homepage = "https://github.com/init4tech/signet-sdk"
diff --git a/crates/bundle/src/send/bundle.rs b/crates/bundle/src/send/bundle.rs
index ee5baf08..bb031dbb 100644
--- a/crates/bundle/src/send/bundle.rs
+++ b/crates/bundle/src/send/bundle.rs
@@ -39,8 +39,12 @@ pub struct SignetEthBundle {
     pub bundle: EthSendBundle,
     /// Host fills to be applied with the bundle, represented as a signed
     /// permit2 fill.
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub host_fills: Option<SignedFill>,
+
+    /// Host transactions to be included in the host bundle.
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub host_txs: Vec<Bytes>,
 }
 
 impl SignetEthBundle {
@@ -194,6 +198,7 @@ mod test {
                     chainId: 100,
                 }],
             }),
+            host_txs: vec![b"host_tx1".into(), b"host_tx2".into()],
         };
 
         let serialized = serde_json::to_string(&bundle).unwrap();
@@ -202,6 +207,39 @@ mod test {
         assert_eq!(bundle, deserialized);
     }
 
+    #[test]
+    fn send_bundle_ser_roundtrip_no_host_no_fills() {
+        let bundle = SignetEthBundle {
+            bundle: EthSendBundle {
+                txs: vec![b"tx1".into(), b"tx2".into()],
+                block_number: 1,
+                min_timestamp: Some(2),
+                max_timestamp: Some(3),
+                reverting_tx_hashes: vec![B256::repeat_byte(4), B256::repeat_byte(5)],
+                replacement_uuid: Some("uuid".to_owned()),
+                ..Default::default()
+            },
+            host_fills: None,
+            host_txs: vec![],
+        };
+
+        let serialized = serde_json::to_string(&bundle).unwrap();
+        let deserialized: SignetEthBundle = serde_json::from_str(&serialized).unwrap();
+
+        assert_eq!(bundle, deserialized);
+    }
+
+    #[test]
+    fn test_deser_bundle_no_host_no_fills() {
+        let json = r#"
+        {"txs":["0x747831","0x747832"],"blockNumber":"0x1","minTimestamp":2,"maxTimestamp":3,"revertingTxHashes":["0x0404040404040404040404040404040404040404040404040404040404040404","0x0505050505050505050505050505050505050505050505050505050505050505"],"replacementUuid":"uuid"}"#;
+
+        let deserialized: SignetEthBundle = serde_json::from_str(json).unwrap();
+
+        assert!(deserialized.host_fills.is_none());
+        assert!(deserialized.host_txs.is_empty());
+    }
+
     #[test]
     fn send_bundle_resp_ser_roundtrip() {
         let resp = SignetEthBundleResponse { bundle_hash: B256::repeat_byte(1) };
diff --git a/crates/extract/src/block.rs b/crates/extract/src/block.rs
index a0d7d0fa..a17616ca 100644
--- a/crates/extract/src/block.rs
+++ b/crates/extract/src/block.rs
@@ -46,7 +46,7 @@ impl<'a, C: Extractable> HostEvents<'a, C> {
     }
 
     /// Add a [`Zenith::BlockSubmitted`] event to the host events.
-    pub fn ingest_block_submitted(
+    pub const fn ingest_block_submitted(
         &mut self,
         event: ExtractedEvent<'a, C::Receipt, Zenith::BlockSubmitted>,
     ) {
diff --git a/crates/sim/src/built.rs b/crates/sim/src/built.rs
index 890e4c1a..a957a78f 100644
--- a/crates/sim/src/built.rs
+++ b/crates/sim/src/built.rs
@@ -17,14 +17,20 @@ use tracing::{error, trace};
 pub struct BuiltBlock {
     /// The host fill actions.
     pub(crate) host_fills: Vec<SignedFill>,
+
+    /// The host transactions to be included in a resulting bundle.
+    pub(crate) host_txns: Vec<Bytes>,
+
     /// Transactions in the block.
     pub(crate) transactions: Vec<TxEnvelope>,
-    /// The block number for the block.
+
+    /// The block number for the Signet block.
     pub(crate) block_number: u64,
 
     /// The amount of gas used by the block so far
     pub(crate) gas_used: u64,
 
+    // -- Memoization fields --
     /// Memoized raw encoding of the block.
     pub(crate) raw_encoding: OnceLock<Bytes>,
     /// Memoized hash of the block.
@@ -47,6 +53,7 @@ impl BuiltBlock {
     pub const fn new(block_number: u64) -> Self {
         Self {
             host_fills: Vec::new(),
+            host_txns: Vec::new(),
             transactions: Vec::new(),
             block_number,
             gas_used: 0,
@@ -87,6 +94,12 @@ impl BuiltBlock {
         &self.host_fills
     }
 
+    /// Get the current list of host transactions included in this block.
+    #[allow(clippy::missing_const_for_fn)] // false positive, const deref
+    pub fn host_txns(&self) -> &[Bytes] {
+        &self.host_txns
+    }
+
     /// Unseal the block
     pub(crate) fn unseal(&mut self) {
         self.raw_encoding.take();
@@ -124,6 +137,7 @@ impl BuiltBlock {
             // extend the transactions with the decoded transactions.
             // As this builder does not provide bundles landing "top of block", its fine to just extend.
             self.transactions.extend(txs);
+            self.host_txns.extend(bundle.host_txs);
 
             if let Some(host_fills) = bundle.host_fills {
                 self.host_fills.push(host_fills);
diff --git a/crates/sim/src/cache.rs b/crates/sim/src/cache.rs
index 2d0a1aad..d728e4ed 100644
--- a/crates/sim/src/cache.rs
+++ b/crates/sim/src/cache.rs
@@ -307,6 +307,7 @@ mod test {
                 ..Default::default()
             },
             host_fills: None,
+            host_txs: vec![],
         }
     }
 
diff --git a/crates/sim/src/env.rs b/crates/sim/src/env.rs
index 167d0922..fc62116e 100644
--- a/crates/sim/src/env.rs
+++ b/crates/sim/src/env.rs
@@ -189,7 +189,7 @@ impl<Db, Insp> SimEnv<Db, Insp> {
     }
 
     /// Get a reference to the database.
-    pub fn db_mut(&mut self) -> &mut InnerDb<Db> {
+    pub const fn db_mut(&mut self) -> &mut InnerDb<Db> {
         &mut self.db
     }
 
@@ -219,7 +219,7 @@ impl<Db, Insp> SimEnv<Db, Insp> {
     }
 
     /// Set the execution timeout.
-    pub fn set_finish_by(&mut self, timeout: std::time::Instant) {
+    pub const fn set_finish_by(&mut self, timeout: std::time::Instant) {
         self.finish_by = timeout;
     }
 }
diff --git a/crates/test-utils/src/specs/mod.rs b/crates/test-utils/src/specs/mod.rs
index 918243aa..dd5aafda 100644
--- a/crates/test-utils/src/specs/mod.rs
+++ b/crates/test-utils/src/specs/mod.rs
@@ -94,5 +94,6 @@ pub fn simple_bundle<'a>(
             extra_fields: Default::default(),
         },
         host_fills,
+        host_txs: vec![],
     }
 }
diff --git a/crates/types/src/agg/fill.rs b/crates/types/src/agg/fill.rs
index 5233a828..a604656f 100644
--- a/crates/types/src/agg/fill.rs
+++ b/crates/types/src/agg/fill.rs
@@ -224,7 +224,7 @@ impl AggregateFills {
     }
 
     /// Mutably borrow the current fill mapping
-    pub fn fills_mut(&mut self) -> &mut HashMap<(u64, Address), HashMap<Address, U256>> {
+    pub const fn fills_mut(&mut self) -> &mut HashMap<(u64, Address), HashMap<Address, U256>> {
         &mut self.fills
     }
 
diff --git a/crates/zenith/src/block.rs b/crates/zenith/src/block.rs
index 8ad4f0dc..30f31fae 100644
--- a/crates/zenith/src/block.rs
+++ b/crates/zenith/src/block.rs
@@ -182,7 +182,7 @@ where
     }
 
     /// Mutable access to the header.
-    pub fn header_mut(&mut self) -> &mut ZenithHeader {
+    pub const fn header_mut(&mut self) -> &mut ZenithHeader {
         &mut self.header
     }