diff --git a/crates/chia-consensus/src/build_interned_block.rs b/crates/chia-consensus/src/build_interned_block.rs
new file mode 100644
index 000000000..140a644c6
--- /dev/null
+++ b/crates/chia-consensus/src/build_interned_block.rs
@@ -0,0 +1,203 @@
+use crate::consensus_constants::ConsensusConstants;
+use crate::error::Result;
+use crate::generator_cost::total_cost_from_tree;
+use chia_bls::Signature;
+use chia_protocol::SpendBundle;
+use clvmr::allocator::{Allocator, NodePtr};
+use clvmr::serde::{intern_tree_limited, node_from_bytes_backrefs, node_to_bytes_backrefs};
+use std::borrow::Borrow;
+
+#[cfg(feature = "py-bindings")]
+use pyo3::prelude::*;
+#[cfg(feature = "py-bindings")]
+use pyo3::types::PyList;
+
+const MAX_SKIPPED_ITEMS: u32 = 6;
+const MIN_COST_THRESHOLD: u64 = 6_000_000;
+
+#[derive(PartialEq)]
+pub enum BuildBlockResult {
+    KeepGoing,
+    Done,
+}
+
+fn result(num_skipped: u32) -> BuildBlockResult {
+    if num_skipped > MAX_SKIPPED_ITEMS {
+        BuildBlockResult::Done
+    } else {
+        BuildBlockResult::KeepGoing
+    }
+}
+
+/// Builds a block generator under the INTERNED_GENERATOR cost model.
+///
+/// Unlike `BlockBuilder`, serialization cost is not computed incrementally.
+/// Cost comes from `total_cost_from_tree(intern_tree_limited(...))` on the
+/// full quoted generator tree. Serialization happens once in `finalize()`.
+#[cfg_attr(feature = "py-bindings", pyclass)]
+pub struct InternedBlockBuilder {
+    allocator: Allocator,
+    signature: Signature,
+    spend_list: NodePtr,
+    block_cost: u64,
+    generator_cost: u64,
+    num_skipped: u32,
+}
+
+impl InternedBlockBuilder {
+    pub fn new() -> Result<Self> {
+        let a = Allocator::new();
+        let spend_list = a.nil();
+        Ok(Self {
+            allocator: a,
+            signature: Signature::default(),
+            spend_list,
+            block_cost: 20,
+            generator_cost: 0,
+            num_skipped: 0,
+        })
+    }
+
+    fn compute_generator_cost(allocator: &mut Allocator, spend_list: NodePtr) -> Result<u64> {
+        // Build (q . ((spend_list)))
+        let inner = allocator.new_pair(spend_list, allocator.nil())?;
+        let outer = allocator.new_pair(allocator.one(), inner)?;
+        let interned = intern_tree_limited(allocator, outer, usize::MAX)?;
+        Ok(total_cost_from_tree(&interned))
+    }
+
+    /// Add a batch of spend bundles. `cost` must be execution + conditions cost
+    /// only (no byte cost). Returns `(added, BuildBlockResult)`.
+    pub fn add_spend_bundles<T, S>(
+        &mut self,
+        bundles: T,
+        cost: u64,
+        constants: &ConsensusConstants,
+    ) -> Result<(bool, BuildBlockResult)>
+    where
+        T: IntoIterator<Item = S>,
+        S: Borrow<SpendBundle>,
+    {
+        if self.generator_cost + self.block_cost + MIN_COST_THRESHOLD
+            > constants.max_block_cost_clvm
+        {
+            self.num_skipped += 1;
+            return Ok((false, BuildBlockResult::Done));
+        }
+
+        if self.generator_cost + self.block_cost + cost > constants.max_block_cost_clvm {
+            self.num_skipped += 1;
+            return Ok((false, result(self.num_skipped)));
+        }
+
+        let saved_spend_list = self.spend_list;
+        let a = &mut self.allocator;
+
+        let mut cumulative_signature = Signature::default();
+        for bundle in bundles {
+            for spend in &bundle.borrow().coin_spends {
+                let solution = node_from_bytes_backrefs(a, spend.solution.as_ref())?;
+                let item = a.new_pair(solution, NodePtr::NIL)?;
+                let amount = a.new_number(spend.coin.amount.into())?;
+                let item = a.new_pair(amount, item)?;
+                let puzzle = node_from_bytes_backrefs(a, spend.puzzle_reveal.as_ref())?;
+                let item = a.new_pair(puzzle, item)?;
+                let parent_id = a.new_atom(&spend.coin.parent_coin_info)?;
+                let item = a.new_pair(parent_id, item)?;
+                self.spend_list = a.new_pair(item, self.spend_list)?;
+            }
+            cumulative_signature.aggregate(&bundle.borrow().aggregated_signature);
+        }
+
+        let new_generator_cost =
+            Self::compute_generator_cost(&mut self.allocator, self.spend_list)?;
+
+        if new_generator_cost + self.block_cost + cost > constants.max_block_cost_clvm {
+            // Restore: the allocator is not reset (dead nodes are acceptable).
+            self.spend_list = saved_spend_list;
+            self.num_skipped += 1;
+            return Ok((false, result(self.num_skipped)));
+        }
+
+        self.generator_cost = new_generator_cost;
+        self.block_cost += cost;
+        self.signature.aggregate(&cumulative_signature);
+
+        let done = self.generator_cost + self.block_cost + MIN_COST_THRESHOLD
+            > constants.max_block_cost_clvm;
+        Ok((
+            true,
+            if done {
+                BuildBlockResult::Done
+            } else {
+                BuildBlockResult::KeepGoing
+            },
+        ))
+    }
+
+    pub fn cost(&self) -> u64 {
+        self.generator_cost + self.block_cost
+    }
+
+    /// Serialize the generator once and return `(bytes, signature, total_cost)`.
+    pub fn finalize(mut self, constants: &ConsensusConstants) -> Result<(Vec<u8>, Signature, u64)> {
+        let inner = self
+            .allocator
+            .new_pair(self.spend_list, self.allocator.nil())?;
+        let root = self.allocator.new_pair(self.allocator.one(), inner)?;
+
+        let serialized = node_to_bytes_backrefs(&self.allocator, root)?;
+
+        let generator_cost = Self::compute_generator_cost(&mut self.allocator, self.spend_list)?;
+        let total_cost = generator_cost + self.block_cost;
+
+        assert!(total_cost <= constants.max_block_cost_clvm);
+        Ok((serialized, self.signature, total_cost))
+    }
+}
+
+#[cfg(feature = "py-bindings")]
+#[pymethods]
+impl InternedBlockBuilder {
+    #[new]
+    pub fn py_new() -> PyResult<Self> {
+        Ok(Self::new()?)
+    }
+
+    #[pyo3(name = "add_spend_bundles")]
+    pub fn py_add_spend_bundle(
+        &mut self,
+        bundles: &Bound<'_, PyList>,
+        cost: u64,
+        constants: &ConsensusConstants,
+    ) -> PyResult<(bool, bool)> {
+        let (added, result) = self.add_spend_bundles(
+            bundles.iter().map(|item| {
+                item.extract::<Bound<'_, SpendBundle>>()
+                    .expect("spend bundle")
+                    .get()
+                    .clone()
+            }),
+            cost,
+            constants,
+        )?;
+        let done = matches!(result, BuildBlockResult::Done);
+        Ok((added, done))
+    }
+
+    #[pyo3(name = "cost")]
+    pub fn py_cost(&self) -> u64 {
+        self.cost()
+    }
+
+    #[pyo3(name = "finalize")]
+    pub fn py_finalize(
+        &mut self,
+        constants: &ConsensusConstants,
+    ) -> PyResult<(Vec<u8>, Signature, u64)> {
+        let mut temp = InternedBlockBuilder::new()?;
+        std::mem::swap(self, &mut temp);
+        let (generator, sig, cost) = temp.finalize(constants)?;
+        Ok((generator, sig, cost))
+    }
+}
diff --git a/crates/chia-consensus/src/lib.rs b/crates/chia-consensus/src/lib.rs
index a7df7b7ec..044234211 100644
--- a/crates/chia-consensus/src/lib.rs
+++ b/crates/chia-consensus/src/lib.rs
@@ -4,6 +4,7 @@
 pub mod additions_and_removals;
 pub mod allocator;
 pub mod build_compressed_block;
+pub mod build_interned_block;
 pub mod check_time_locks;
 mod coin_id;
 mod condition_sanitizers;