diff --git a/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt b/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt
new file mode 100644
index 0000000000..0491acb872
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt
@@ -0,0 +1,106 @@
+# Because 'go mod' subcommands ignore build constraints, they can encounter
+# package-import cycles that are not possible in an ordinary build. This test
+# verifies that such cycles are handled even when they cross module boundaries.
+
+# First, verify that the import graph depends on build tags as expected.
+go list -deps example.com/left
+stdout '^example.com/right$'
+go list -deps example.com/right
+! stdout left
+
+env GOFLAGS=-tags=mirror
+go list -deps example.com/left
+! stdout right
+go list -deps example.com/right
+stdout '^example.com/left$'
+env GOFLAGS=''
+
+# 'go mod why' should be agnostic to build tags.
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+
+env GOFLAGS='-tags=mirror'
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+env GOFLAGS=''
+
+# 'go mod tidy' should successfully handle the cycle.
+env GOFLAGS=-mod=readonly
+go mod tidy
+
+# 'go mod vendor' should copy in both packages without crashing.
+go mod vendor
+exists vendor/example.com/left/default.go
+exists vendor/example.com/left/mirror.go
+exists vendor/example.com/right/default.go
+exists vendor/example.com/right/mirror.go
+
+-- go.mod --
+module example.com/chiral
+
+go 1.14
+
+require (
+	example.com/left v0.1.0
+	example.com/right v0.1.0
+)
+
+replace (
+	example.com/left => ./left
+	example.com/right => ./right
+)
+-- chiral.go --
+// Package chiral imports packages in an order that depends on build tags.
+package chiral
+-- default.go --
+// +build !mirror
+
+package chiral
+
+import _ "example.com/left"
+-- mirror.go --
+// +build mirror
+
+package chiral
+
+import _ "example.com/right"
+-- left/go.mod --
+module example.com/left
+
+go 1.14
+
+require example.com/right v0.1.0
+
+replace example.com/right v0.1.0 => ../right
+-- left/default.go --
+// +build !mirror
+
+package left
+
+import _ "example.com/right"
+-- left/mirror.go --
+// +build mirror
+
+package left
+-- right/go.mod --
+module example.com/right
+
+go 1.14
+
+require example.com/left v0.1.0
+
+replace example.com/left v0.1.0 => ../left
+-- right/default.go --
+// +build !mirror
+
+package right
+-- right/mirror.go --
+// +build mirror
+
+package right
+
+import _ "example.com/left"