diff --git a/cmd/constable/main.go b/cmd/constable/main.go
index 1ea8f60..f732742 100644
--- a/cmd/constable/main.go
+++ b/cmd/constable/main.go
@@ -6,6 +6,7 @@ import (
 	"github.com/nnutter/constable/internal/analysis/methodical"
 	"github.com/nnutter/constable/internal/analysis/nonmutating"
 	"github.com/nnutter/constable/internal/analysis/predicates"
+	"github.com/nnutter/constable/internal/analysis/sumtype"
 	"github.com/nnutter/constable/internal/analysis/testify"
 	"github.com/nnutter/constable/internal/driver"
 )
@@ -24,5 +25,6 @@ func main() {
 		testify.Analyzer,
 		complexity.Analyzer,
 		errcheck.Analyzer,
+		sumtype.Analyzer,
 	)
 }
diff --git a/go.mod b/go.mod
index 0b51c97..a736bb3 100644
--- a/go.mod
+++ b/go.mod
@@ -3,6 +3,7 @@ module github.com/nnutter/constable
 go 1.27.3
 
 require (
+	github.com/alecthomas/go-check-sumtype v0.5.0
 	github.com/kisielk/errcheck v1.30.0
 	github.com/stretchr/testify v1.12.1
 	golang.org/x/tools v0.52.0
diff --git a/go.sum b/go.sum
index 0ecfdac..0f56789 100644
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,5 @@
+github.com/alecthomas/go-check-sumtype v0.5.0 h1:yIFGZYGU5xv64lXp0llRhji/d/kDfS6AaqOI+wAxTqM=
+github.com/alecthomas/go-check-sumtype v0.5.0/go.mod h1:z2jV5/4nQ3XRzKG7PMrJ46DE2NtdnMjQA/fzfzWH81Q=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
 github.com/kisielk/errcheck v1.30.0 h1:ZIxNDAfAd4larXHTceoZP/dYPAB90dHg1OYhA2sVh5k=
diff --git a/internal/analysis/sumtype/sumtype.go b/internal/analysis/sumtype/sumtype.go
new file mode 100644
index 0000000..f883a1a
--- /dev/null
+++ b/internal/analysis/sumtype/sumtype.go
@@ -0,0 +1,16 @@
+// Package sumtype adapts the go-check-sumtype analyzer to constable.
+//
+// constable embeds the analyzer from another module instead of reimplementing
+// it, so this package holds the places where constable's own policy differs
+// from the upstream default.
+package sumtype
+
+import (
+	gochecksumtype "github.com/alecthomas/go-check-sumtype"
+
+	"github.com/nnutter/constable/internal/generated"
+)
+
+// Analyzer reports a type switch over a sum type that omits a variant. It is
+// the upstream analyzer with constable's generated-file policy applied.
+var Analyzer = generated.Skip(gochecksumtype.Analyzer)
diff --git a/internal/analysis/sumtype/sumtype_test.go b/internal/analysis/sumtype/sumtype_test.go
new file mode 100644
index 0000000..19b5e80
--- /dev/null
+++ b/internal/analysis/sumtype/sumtype_test.go
@@ -0,0 +1,13 @@
+package sumtype_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+
+	"github.com/nnutter/constable/internal/analysis/sumtype"
+)
+
+func TestAnalyzer(t *testing.T) {
+	analysistest.Run(t, analysistest.TestData(), sumtype.Analyzer, "st")
+}
diff --git a/internal/analysis/sumtype/testdata/src/st/st.go b/internal/analysis/sumtype/testdata/src/st/st.go
new file mode 100644
index 0000000..86cd397
--- /dev/null
+++ b/internal/analysis/sumtype/testdata/src/st/st.go
@@ -0,0 +1,19 @@
+// want package:`sumTypeFact\{Shape \[Circle Square\]\}`
+package st
+
+//sumtype:decl
+type Shape interface{ shape() }
+
+type Circle struct{}
+
+func (*Circle) shape() {}
+
+type Square struct{}
+
+func (*Square) shape() {}
+
+func F(s Shape) {
+	switch s.(type) { // want `missing cases for Square`
+	case *Circle:
+	}
+}
diff --git a/internal/analysis/sumtype/testdata/src/st/zz_generated.go b/internal/analysis/sumtype/testdata/src/st/zz_generated.go
new file mode 100644
index 0000000..eb96e70
--- /dev/null
+++ b/internal/analysis/sumtype/testdata/src/st/zz_generated.go
@@ -0,0 +1,9 @@
+// Code generated by hand. DO NOT EDIT.
+
+package st
+
+func Generated(s Shape) {
+	switch s.(type) {
+	case *Circle:
+	}
+}