diff --git a/main_test.go b/main_test.go
index 6dc2e75..3a70645 100644
--- a/main_test.go
+++ b/main_test.go
@@ -64,6 +64,7 @@ func TestScripts(t *testing.T) {
 			env.Vars = append(env.Vars,
 				"GOPROXY="+proxyURL,
 				"GONOSUMDB=*",
+				"GOFLAGS=-mod=readonly", // TODO(mvdan): remove once we switch to Go 1.16
 				"gofullversion="+runtime.Version(),
 			)
 			bindir := filepath.Join(env.WorkDir, ".bin")
diff --git a/testdata/scripts/asm.txt b/testdata/scripts/asm.txt
index a92a0bf..47b5b9e 100644
--- a/testdata/scripts/asm.txt
+++ b/testdata/scripts/asm.txt
@@ -18,6 +18,8 @@ cmp stderr main.stderr
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/basic.txt b/testdata/scripts/basic.txt
index 4f86b65..f6b6562 100644
--- a/testdata/scripts/basic.txt
+++ b/testdata/scripts/basic.txt
@@ -57,6 +57,8 @@ binsubstr main$exe 'main.go' 'globalVar' 'globalFunc' $gofullversion
 
 -- go.mod --
 module test/mainfoo
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/cgo.txt b/testdata/scripts/cgo.txt
index f46a1e1..99369e2 100644
--- a/testdata/scripts/cgo.txt
+++ b/testdata/scripts/cgo.txt
@@ -18,6 +18,8 @@ cmp stderr main.stderr
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/debugdir.txt b/testdata/scripts/debugdir.txt
index 45dffaf..530a3b6 100644
--- a/testdata/scripts/debugdir.txt
+++ b/testdata/scripts/debugdir.txt
@@ -17,6 +17,8 @@ garble -debugdir ./test1 build -v
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/goprivate.txt b/testdata/scripts/goprivate.txt
index 8dff35b..6f41eaf 100644
--- a/testdata/scripts/goprivate.txt
+++ b/testdata/scripts/goprivate.txt
@@ -13,6 +13,8 @@ garble build -o=out ./standalone
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- standalone/main.go --
 package main
 
diff --git a/testdata/scripts/implement.txt b/testdata/scripts/implement.txt
index ba4b10b..44a7a6d 100644
--- a/testdata/scripts/implement.txt
+++ b/testdata/scripts/implement.txt
@@ -8,6 +8,8 @@ cmp stderr main.stderr
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/imports.txt b/testdata/scripts/imports.txt
index edd5e11..b247f45 100644
--- a/testdata/scripts/imports.txt
+++ b/testdata/scripts/imports.txt
@@ -1,7 +1,10 @@
 # Note that this is the only test with a module where we rely on the detection
 # of GOPRIVATE.
+# Also note that, since this is the only test using "real" external modules
+# fetched via GOPROXY, go.mod and go.sum should declare the dependencies.
 
 garble build -tags buildtag
+exec cat go.sum
 exec ./main
 cmp stdout main.stdout
 
@@ -29,6 +32,17 @@ cmp stdout main.stdout
 
 -- go.mod --
 module test/main
+
+go 1.15
+
+require rsc.io/quote v1.5.2
+-- go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
 -- main.go --
 package main
 
diff --git a/testdata/scripts/init.txt b/testdata/scripts/init.txt
index 2612062..9145f98 100644
--- a/testdata/scripts/init.txt
+++ b/testdata/scripts/init.txt
@@ -12,6 +12,8 @@ cmp stderr main.stderr
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/ldflags.txt b/testdata/scripts/ldflags.txt
index 31cbcaf..1121447 100644
--- a/testdata/scripts/ldflags.txt
+++ b/testdata/scripts/ldflags.txt
@@ -20,6 +20,8 @@ binsubstr main$exe 'unexportedVersion'
 
 -- go.mod --
 module domain.test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/literals.txt b/testdata/scripts/literals.txt
index a816c52..6a7d944 100644
--- a/testdata/scripts/literals.txt
+++ b/testdata/scripts/literals.txt
@@ -51,6 +51,8 @@ grep '^\s+type \w+ func\(byte\) \w+$' .obf-src/main/extraLiterals.go
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/modinfo.txt b/testdata/scripts/modinfo.txt
index 8d7cc09..614f2dc 100644
--- a/testdata/scripts/modinfo.txt
+++ b/testdata/scripts/modinfo.txt
@@ -14,6 +14,8 @@ binsubstr main$exe '(devel)'
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/plugin.txt b/testdata/scripts/plugin.txt
index 08d1ea4..11a677d 100644
--- a/testdata/scripts/plugin.txt
+++ b/testdata/scripts/plugin.txt
@@ -28,6 +28,8 @@ cmp stderr main.stderr
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- plugin/main.go --
 package main
 
diff --git a/testdata/scripts/seed.txt b/testdata/scripts/seed.txt
index ccd8592..4dfd755 100644
--- a/testdata/scripts/seed.txt
+++ b/testdata/scripts/seed.txt
@@ -39,6 +39,8 @@ stderr .
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main
 
diff --git a/testdata/scripts/syntax.txt b/testdata/scripts/syntax.txt
index 8e45c42..83e1300 100644
--- a/testdata/scripts/syntax.txt
+++ b/testdata/scripts/syntax.txt
@@ -1,6 +1,7 @@
 env GOPRIVATE='test/main,private.source/*'
 
 garble build -tags directives
+exec cat go.mod
 exec ./main$exe
 cmp stderr main.stderr
 
@@ -19,6 +20,8 @@ binsubstr main$exe 'globalVar' # 'globalType' only matches on go < 1.15
 
 -- extra/go.mod --
 module "private.source/extra"
+
+go 1.15
 -- extra/extra.go --
 package extra
 
@@ -28,8 +31,12 @@ func Func() string {
 -- go.mod --
 module test/main
 
+go 1.15
+
 // We include an extra module to obfuscate, included in the same original source
 // code via a replace directive.
+require private.source/extra v0.0.0-00010101000000-000000000000
+
 replace private.source/extra => ./extra
 -- main.go --
 package main
diff --git a/testdata/scripts/test.txt b/testdata/scripts/test.txt
index 80a1d39..beff578 100644
--- a/testdata/scripts/test.txt
+++ b/testdata/scripts/test.txt
@@ -19,6 +19,8 @@ stdout 'PASS.*TestFoo'
 
 -- go.mod --
 module test/bar
+
+go 1.15
 -- bar.go --
 package bar
 
diff --git a/testdata/scripts/tiny.txt b/testdata/scripts/tiny.txt
index c1df88b..e19f3df 100644
--- a/testdata/scripts/tiny.txt
+++ b/testdata/scripts/tiny.txt
@@ -22,6 +22,8 @@ stderr 'panic: oh noes'
 
 -- go.mod --
 module test/main
+
+go 1.15
 -- main.go --
 package main