diff --git a/interactive/Cargo.toml b/interactive/Cargo.toml
index b3e3a028d..b730cf48d 100644
--- a/interactive/Cargo.toml
+++ b/interactive/Cargo.toml
@@ -14,7 +14,7 @@ workspace = true
 [dependencies]
 columnar = { workspace = true }
 # The columnar kernels for the interpreted backend, pinned by git rev.
-corgi = { git = "https://github.com/frankmcsherry/wip", rev = "c4626fce02288594c9806e9b747a19598d680e0a" }
+corgi = { git = "https://github.com/frankmcsherry/wip", rev = "cb26fbd29c223a97781891298a0cc7b5f94a5b2f" }
 differential-dataflow = { workspace = true }
 mimalloc = "0.1.48"
 serde = { version = "1.0", features = ["derive"] }
diff --git a/interactive/tests/corgi_backend.rs b/interactive/tests/corgi_backend.rs
index 31db6d137..ca50f5332 100644
--- a/interactive/tests/corgi_backend.rs
+++ b/interactive/tests/corgi_backend.rs
@@ -31,6 +31,9 @@ fn inputs_for(prog: &str) -> Vec<Vec<(Value, Value)>> {
         // scalar_ops: (key, a, b) triples; a values straddle the `> 2` and `= -5` tests.
         "scalar_ops" => vec![rows(&[&[1, 1, 9], &[1, 4, 8], &[2, 3, 7], &[3, -5, 6], &[3, 2, 5]])],
         "sum_ops" => vec![rows(&[&[1, 10], &[2, 20], &[2, 21]])],
+        // sum_skew: any keyed pairs — the skew is in the program, not the data.
+        "sum_skew" => vec![rows(&[&[1, 10], &[2, 20], &[2, 21], &[3, 30]])],
+        "sum_skew_compiled" => vec![rows(&[&[1, 10], &[2, 20], &[2, 21], &[3, 30]])],
         "case_ops" => vec![rows(&[&[1, 10], &[2, 20], &[3, 14], &[3, 30]])],
         // pair_keys: composite keys with overlap, fanout, and one-sided keys on both sides.
         "pair_keys" => vec![
@@ -76,6 +79,8 @@ fn assert_backends_agree(prog: &str) {
 #[test] fn join_fallback() { assert_backends_agree("join_fallback"); }
 #[test] fn scalar_ops() { assert_backends_agree("scalar_ops"); }
 #[test] fn sum_ops() { assert_backends_agree("sum_ops"); }
+#[test] fn sum_skew() { assert_backends_agree("sum_skew"); }
+#[test] fn sum_skew_compiled() { assert_backends_agree("sum_skew_compiled"); }
 #[test] fn case_ops() { assert_backends_agree("case_ops"); }
 #[test] fn tour() { assert_backends_agree("tour"); }
 #[test] fn pair_keys() { assert_backends_agree("pair_keys"); }
diff --git a/interactive/tests/programs/sum_skew.ddp b/interactive/tests/programs/sum_skew.ddp
new file mode 100644
index 000000000..2fa9dbb58
--- /dev/null
+++ b/interactive/tests/programs/sum_skew.ddp
@@ -0,0 +1,18 @@
+-- Two collections that commit DIFFERENT variant arms, concatenated into one arrangement.
+-- Neither side's shape names the whole variant universe: one infers `Sum([Some(_)])`
+-- (tag 0 only), the other `Sum([None, Some(_)])` (tag 1 only) — two arities for one DDIR
+-- type. corgi reads a differing Sum arity as a type error, so the two must be reconciled
+-- with uncommitted (⊥) lanes before anything compares or gathers them.
+--
+-- This is the ROW-WISE FALLBACK case: `hash` is a term corgi does not lower, so the shapes
+-- here come from `infer_shape_cols` scanning the data. See `sum_skew_compiled.ddp` for the
+-- same defect on the compiled path — both under-approximate, just from different sources.
+con Rare(1)   = 0;
+con Common(1) = 1;
+
+let pairs = input 0 | key($0[0] ; $0[1]);
+
+let onlyRare   = pairs | map( $0[0] ; Rare(hash(1000000, $1[0])) );
+let onlyCommon = pairs | map( $0[0] ; Common(hash(1000000, $1[0])) );
+
+export "result" = (onlyRare + onlyCommon) | arrange | inspect(total);
diff --git a/interactive/tests/programs/sum_skew_compiled.ddp b/interactive/tests/programs/sum_skew_compiled.ddp
new file mode 100644
index 000000000..2a3a0251c
--- /dev/null
+++ b/interactive/tests/programs/sum_skew_compiled.ddp
@@ -0,0 +1,16 @@
+-- `sum_skew.ddp` on the COMPILED path: no `hash`, so both maps lower to corgi logic.
+--
+-- The compiled path derives shape from the TERM rather than the data, but that is no less
+-- of an under-approximation: `infer_term_shape` gives `Inject(tag, _)` an arity of `tag + 1`,
+-- so `Rare(_)` compiles to one lane and `Common(_)` to two. A single term reconciles its own
+-- arms (`If` joins them), but two separate operators have nothing to reconcile them, and the
+-- declared universe (`con`) that would is discarded at parse.
+con Rare(1)   = 0;
+con Common(1) = 1;
+
+let pairs = input 0 | key($0[0] ; $0[1]);
+
+let onlyRare   = pairs | map( $0[0] ; Rare($1[0]) );
+let onlyCommon = pairs | map( $0[0] ; Common($1[0]) );
+
+export "result" = (onlyRare + onlyCommon) | arrange | inspect(total);