diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 807ce8d5dc5..cd57d99de76 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -634,7 +634,7 @@ func MinReqs() mvs.Reqs {
 			direct = append(direct, m.Path)
 		}
 	}
-	min, err := mvs.Req(Target, buildList, direct, Reqs())
+	min, err := mvs.Req(Target, direct, Reqs())
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}
diff --git a/src/cmd/go/internal/mvs/mvs.go b/src/cmd/go/internal/mvs/mvs.go
index 4e7a828c24f..8855d44f21d 100644
--- a/src/cmd/go/internal/mvs/mvs.go
+++ b/src/cmd/go/internal/mvs/mvs.go
@@ -250,10 +250,15 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (m
 	return list, nil
 }
 
-// Req returns the minimal requirement list for the target module
-// that results in the given build list, with the constraint that all
-// module paths listed in base must appear in the returned list.
-func Req(target module.Version, list []module.Version, base []string, reqs Reqs) ([]module.Version, error) {
+// Req returns the minimal requirement list for the target module,
+// with the constraint that all module paths listed in base must
+// appear in the returned list.
+func Req(target module.Version, base []string, reqs Reqs) ([]module.Version, error) {
+	list, err := BuildList(target, reqs)
+	if err != nil {
+		return nil, err
+	}
+
 	// Note: Not running in parallel because we assume
 	// that list came from a previous operation that paged
 	// in all the requirements, so there's no I/O to overlap now.
diff --git a/src/cmd/go/internal/mvs/mvs_test.go b/src/cmd/go/internal/mvs/mvs_test.go
index 72d3ea95b79..e195e857b8f 100644
--- a/src/cmd/go/internal/mvs/mvs_test.go
+++ b/src/cmd/go/internal/mvs/mvs_test.go
@@ -280,6 +280,20 @@ D2:
 build A: A B1 C1 D1
 upgrade* A: A B2 C2 D2
 
+# Cycles with multiple possible solutions.
+# (golang.org/issue/34086)
+name: cycle3
+M: A1 C2
+A1: B1
+B1: C1
+B2: C2
+C1:
+C2: B2
+build M: M A1 B2 C2
+req M: A1 B2
+req M A: A1 B2
+req M C: A1 C2
+
 # Requirement minimization.
 
 name: req1
@@ -390,7 +404,15 @@ func Test(t *testing.T) {
 			fns = append(fns, func(t *testing.T) {
 				list, err := Upgrade(m(kf[1]), reqs, ms(kf[2:])...)
 				if err == nil {
-					list, err = Req(m(kf[1]), list, nil, reqs)
+					// Copy the reqs map, but substitute the upgraded requirements in
+					// place of the target's original requirements.
+					upReqs := make(reqsMap, len(reqs))
+					for m, r := range reqs {
+						upReqs[m] = r
+					}
+					upReqs[m(kf[1])] = list
+
+					list, err = Req(m(kf[1]), nil, upReqs)
 				}
 				checkList(t, key, list, err, val)
 			})
@@ -418,11 +440,7 @@ func Test(t *testing.T) {
 				t.Fatalf("req takes at least one argument: %q", line)
 			}
 			fns = append(fns, func(t *testing.T) {
-				list, err := BuildList(m(kf[1]), reqs)
-				if err != nil {
-					t.Fatal(err)
-				}
-				list, err = Req(m(kf[1]), list, kf[2:], reqs)
+				list, err := Req(m(kf[1]), kf[2:], reqs)
 				checkList(t, key, list, err, val)
 			})
 			continue
diff --git a/src/cmd/go/testdata/script/mod_tidy_cycle.txt b/src/cmd/go/testdata/script/mod_tidy_cycle.txt
new file mode 100644
index 00000000000..e46f37d7fa2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_cycle.txt
@@ -0,0 +1,75 @@
+# Regression test for https://golang.org/issue/34086:
+# 'go mod tidy' produced different go.mod file from other
+# subcommands when certain kinds of cycles were present
+# in the build graph.
+
+env GO111MODULE=on
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+# If the go.mod file is already tidy, 'go mod graph' should not modify it.
+go mod graph
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module root
+
+go 1.13
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	b v0.2.0 => ./b2
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+
+require (
+	a v0.1.0
+	b v0.2.0 // indirect
+)
+-- main.go --
+package main
+
+import _ "a"
+
+func main() {}
+
+-- a1/go.mod --
+module a
+
+go 1.13
+
+require b v0.1.0
+-- a1/a.go --
+package a
+
+import _ "c"
+-- b1/go.mod --
+module b
+
+go 1.13
+
+require c v0.1.0
+-- b2/go.mod --
+module b
+
+go 1.13
+
+require c v0.2.0
+-- c1/go.mod --
+module c
+
+go 1.13
+-- c2/c.go --
+package c
+-- c2/go.mod --
+module c
+
+go 1.13
+
+require b v0.2.0
+-- c2/c.go --
+package c