diff --git a/testdata/script/gotoolchain.txtar b/testdata/script/gotoolchain.txtar
index 5f8af1d..29c16af 100644
--- a/testdata/script/gotoolchain.txtar
+++ b/testdata/script/gotoolchain.txtar
@@ -1,20 +1,33 @@
-# Test that garble works with transparent upgrades via GOTOOLCHAIN.
+# We want the upgraded toolchain to match the garble binary
+# which we will exec below, as otherwise garble will complain.
+# Tell GOTOOLCHAIN to upgrade to the same version thanks to `go mod init`
+# setting up a `go` directive with its Go version.
+cd mod
+go mod init test
+exec cat go.mod
+cd ..
+go env GOVERSION
+setenvfile GOVERSION_UPGRADE stdout
+
+# To test that garble works with transparent upgrades via GOTOOLCHAIN,
+# use a relatively old version, but still new enough to support GOTOOLCHAIN.
+env GOVERSION_BASE=go1.23.0
+setup-go ${GOVERSION_BASE}
+
 # We want to use the real GOPROXY so that we can download the newer
 # toolchain, and we use the host's GOMODCACHE so we can reuse it.
-setup-go go1.23.7
 env GOPROXY=proxy.golang.org
 env GOMODCACHE=${HOST_GOMODCACHE}
 
 # Verify that we are using an older version of Go.
 exec go version
-stdout 'go version go1\.23\.7'
-
-cd mod
+stdout 'go version '${GOVERSION_BASE@R}
 
 # The builds inside the module use the upgraded toolchain.
+cd mod
 
 exec go run .
-stderr 'hello from go1\.24\.1'
+stderr 'hello from '${GOVERSION_UPGRADE@R}
 
 # Note that garble hides runtime.Version by design, but we know that it requires
 # the Go toolchain version to match the exact same version that built garble,
@@ -22,10 +35,6 @@ stderr 'hello from go1\.24\.1'
 exec garble run .
 stderr 'hello from unknown'
 
--- mod/go.mod --
-module test
-
-go 1.24.1
 -- mod/main.go --
 package main