diff --git a/node-graph/libraries/core-types/src/extent.rs b/node-graph/libraries/core-types/src/extent.rs
index 39b2117b9e..40d06fba11 100644
--- a/node-graph/libraries/core-types/src/extent.rs
+++ b/node-graph/libraries/core-types/src/extent.rs
@@ -75,4 +75,10 @@ impl LevelIn {
 	pub fn pushed(&self) -> bool {
 		self.level + 1 == self.depth
 	}
+
+	/// Whether the query targets the topmost level; `pushed` under the name a
+	/// non-creator (concat, remap) reads naturally.
+	pub fn top(&self) -> bool {
+		self.pushed()
+	}
 }
diff --git a/node-graph/libraries/core-types/src/gpoll.rs b/node-graph/libraries/core-types/src/gpoll.rs
index 071010fb16..fdd6692699 100644
--- a/node-graph/libraries/core-types/src/gpoll.rs
+++ b/node-graph/libraries/core-types/src/gpoll.rs
@@ -185,6 +185,16 @@ impl Extent {
 			_ => Extent::Free,
 		})
 	}
+
+	/// The sum of two extents, used to concatenate a level; a free operand
+	/// counts as one lane, so a scalar edge joins a concat as a single item.
+	pub fn sum(a: GPoll<Extent>, b: GPoll<Extent>) -> GPoll<Extent> {
+		let lanes = |extent| match extent {
+			Extent::Exactly(count) => count,
+			Extent::Free => 1,
+		};
+		a.zip(b).map(|(a, b)| Extent::Exactly(lanes(a) + lanes(b)))
+	}
 }
 
 /// A query over a node's nesting levels: one level, the product below or above
diff --git a/node-graph/libraries/core-types/src/node.rs b/node-graph/libraries/core-types/src/node.rs
index 3c289cbd64..f9fe7b8b3e 100644
--- a/node-graph/libraries/core-types/src/node.rs
+++ b/node-graph/libraries/core-types/src/node.rs
@@ -490,6 +490,15 @@ impl<'a, N> LazyInput<'a, N> {
 	{
 		self.cell.eval_input(self.input_index, self.node, ctx)
 	}
+
+	/// The edge's composite extent, for kernels that split or shift indices
+	/// over their sources.
+	pub fn extent<Input>(&self, ctx: &Input, at: Level) -> GPoll<Extent>
+	where
+		N: Node<Input>,
+	{
+		self.node.extent(ctx, at)
+	}
 }
 
 #[cfg(test)]
diff --git a/node-graph/nodes/gcore/src/record.rs b/node-graph/nodes/gcore/src/record.rs
index f5d9a60d73..4ede80920c 100644
--- a/node-graph/nodes/gcore/src/record.rs
+++ b/node-graph/nodes/gcore/src/record.rs
@@ -8,7 +8,7 @@
 use core_types::attribute::{Attr, Opacity, RemoveAttr};
 use core_types::context::{DeriveCtx, ExtractIndex, IndexLink, InjectIndex};
 use core_types::extent::{ExtentIn, LevelIn, ValueIn};
-use core_types::gpoll::{ErrorKind, Extent, GPoll, GraphError, Interrupt};
+use core_types::gpoll::{ErrorKind, Extent, GPoll, GraphError, Interrupt, Level};
 use core_types::Ctx;
 
 core_types::attribute! {
@@ -132,6 +132,39 @@ fn repeat_faded_extent(content: ExtentIn<'_>, count: ValueIn<'_, u32>, level: Le
 	}
 }
 
+/// Rank-model Extend: the output's top level is `base`'s lanes followed by
+/// `new`'s, each side evaluated within its own index range.
+#[node_macro::node(category("Test"), extent(extend_extent))]
+fn extend<T>(
+	ctx: impl Ctx + ExtractIndex + InjectIndex + Copy,
+	base: impl Node<Context<'_>, Output = T>,
+	new: impl Node<Context<'_>, Output = T>,
+) -> Result<T, Interrupt> {
+	let split = match base.extent(ctx, Level::Total) {
+		GPoll::Final(Extent::Exactly(count)) => count as u64,
+		GPoll::Pending => return Err(Interrupt::Pending),
+		_ => return Err(GraphError::new("extend over a non-exact base extent").into()),
+	};
+	let lane = ctx.innermost_index();
+	match lane < split {
+		true => base.eval(ctx),
+		false => {
+			let mut shifted = *ctx;
+			shifted.set_index(lane - split);
+			new.eval(&shifted)
+		}
+	}
+}
+
+/// The top level sums both sides; inner levels forward the base's, which the
+/// new side must match (rectangular).
+fn extend_extent(base: ExtentIn<'_>, new: ExtentIn<'_>, level: LevelIn) -> GPoll<Extent> {
+	match level.top() {
+		true => Extent::sum(base.at(level), new.at(level)),
+		false => base.at(level),
+	}
+}
+
 #[node_macro::node(category("Test"))]
 fn source_opacity(_: impl Ctx, _: (), element: f64, opacity: f64) -> (f64, Attr<Opacity>) {
 	(element, Attr(opacity))
@@ -264,6 +297,37 @@ mod tests {
 		}
 	}
 
+	struct LeveledSourceNode {
+		layout: Layout,
+		elements: Vec<f64>,
+		field: Option<(usize, f64)>,
+	}
+
+	impl<'e> Node<ContextImpl<'e>> for LeveledSourceNode {
+		type Output = RecordValue<'e>;
+
+		fn eval(&self, input: &ContextImpl<'e>) -> GPoll<RecordValue<'e>> {
+			let element = self.elements[input.innermost_index() as usize % self.elements.len()];
+			let dst = stack::push(self.layout.frame_bytes());
+			unsafe {
+				dst.cast::<f64>().write(element);
+				if let Some((offset, value)) = self.field {
+					dst.add(offset).cast::<f64>().write(value);
+				}
+			}
+			stack::pop(dst);
+			GPoll::Final(RecordValue::spilled(unsafe { Rec::new(dst.cast_const()) }))
+		}
+
+		fn extent_at(&self, _input: &ContextImpl<'e>, _level: u8) -> GPoll<Extent> {
+			GPoll::Final(Extent::Exactly(self.elements.len()))
+		}
+
+		fn layout(&self) -> &Layout {
+			&self.layout
+		}
+	}
+
 	struct IndexSourceNode {
 		layout: Layout,
 	}
@@ -597,6 +661,82 @@ mod tests {
 		}
 	}
 
+	#[test]
+	fn extend_concatenates_the_top_level_and_fills_the_union() {
+		let arena = Arena::new(1024).unwrap();
+		let generations = [];
+		let scope = scope_fixture(&generations, &arena);
+		let ctx = ContextImpl::root(&scope);
+
+		let leveled_f64_layout = |names: &[&'static str]| {
+			let writes: Vec<core_types::record::FieldWrite> = names
+				.iter()
+				.map(|name| core_types::record::FieldWrite {
+					name,
+					level: 0,
+					size: 8,
+					align: 8,
+					read_erased: <Opacity as AttributeMarker>::read_erased,
+					repark: None,
+				})
+				.collect();
+			Layout::default().with_writes(1, core_types::record::element_write::<f64>(), &writes)
+		};
+		let base_layout = leveled_f64_layout(&[Opacity::NAME]);
+		let new_layout = leveled_f64_layout(&[Length::NAME]);
+		let union = Layout::union(&[&base_layout, &new_layout]);
+		reserve_for(&[&base_layout, &new_layout, &union]);
+
+		let base = LeveledSourceNode {
+			layout: base_layout.clone(),
+			elements: vec![10., 11.],
+			field: Some((base_layout.offset_of(Opacity::NAME, 0).unwrap(), 0.5)),
+		};
+		let new = LeveledSourceNode {
+			layout: new_layout.clone(),
+			elements: vec![100., 101., 102.],
+			field: Some((new_layout.offset_of(Length::NAME, 0).unwrap(), 7.)),
+		};
+		let meta = core_types::record::LayoutMeta {
+			sources: vec![0, 1],
+			reads: vec![],
+			element: core_types::record::ElementSpec::Carried,
+			writes: vec![],
+			removes: vec![],
+			level_delta: 0,
+		};
+		let node = install(
+			ExtendNode::new(RecordSource::new(base, &base_layout, &union), RecordSource::new(new, &new_layout, &union), &union),
+			meta,
+			&[Some(&base_layout), Some(&new_layout)],
+		);
+		let out = Node::<ContextImpl>::layout(&node).clone();
+		assert_eq!(out.depth, 1);
+		assert_eq!(node.extent_at(&ctx, 0), GPoll::Final(Extent::Exactly(5)), "the top level sums both sides");
+
+		let head = ctx.index_head();
+		let expected = [10., 11., 100., 101., 102.];
+		for (lane, &element) in expected.iter().enumerate() {
+			let mark = stack::sp();
+			let scoped = ctx.promoted(&head, lane as u64);
+			let GPoll::Final(value) = node.eval(&scoped) else {
+				panic!("expected a final record");
+			};
+			let rec = out.rec(&value);
+			assert_eq!(unsafe { rec.element::<f64>() }, element);
+			let opacity = unsafe { rec.read::<f64>(out.offset_of(Opacity::NAME, 0).unwrap()) };
+			let length = unsafe { rec.read::<f64>(out.offset_of(Length::NAME, 0).unwrap()) };
+			match lane < 2 {
+				// The base side wrote its opacity; length fills from the census.
+				true => assert_eq!((opacity, length), (0.5, 0.)),
+				// The new side wrote its length; opacity fills from the census.
+				false => assert_eq!((opacity, length), (1., 7.)),
+			}
+			// SAFETY: the element and attrs were read out above, so no borrow into this lane's frames remains.
+			unsafe { stack::rewind(mark) };
+		}
+	}
+
 	#[test]
 	fn reducer_folds_a_repeated_level() {
 		let arena = Arena::new(1024).unwrap();