diff --git a/internal/cli/cli.go b/internal/cli/cli.go
index 4460b60..3043bb1 100644
--- a/internal/cli/cli.go
+++ b/internal/cli/cli.go
@@ -4,8 +4,8 @@ package cli
 
 import (
 	"fmt"
-	"io"
-	"os"
+
+	"github.com/spf13/cobra"
 )
 
 // appname is the name the program reports in its logs and its version.
@@ -13,23 +13,22 @@ const appname = "simplexcalc"
 
 // Main runs the command named on the command line and returns the exit
 // status for main to pass to os.Exit.
+//
+// It leaves cobra's output streams at their defaults. cobra writes the
+// usage and an unknown help topic to stderr, but SetOut moves them onto
+// the stream it is given.
 func Main(version string) int {
-	return execute(version, os.Args[1:], os.Stdout, os.Stderr)
+	return execute(rootCmd(version))
 }
 
-// execute is Main with the arguments and the output passed in, so that
-// a test can supply them.
-func execute(version string, args []string, stdout, stderr io.Writer) int {
-	root := rootCmd(version)
-	root.SetArgs(args)
-	root.SetOut(stdout)
-	root.SetErr(stderr)
-
+// execute runs root and returns the exit status. It takes the command
+// so that a test can set its arguments and output streams first.
+func execute(root *cobra.Command) int {
 	err := root.Execute()
 	if err != nil {
 		// cobra has already printed the error; this only sets the exit
 		// status, which is what a supervisor and a shell script read.
-		_, _ = fmt.Fprintln(stderr, "exiting: "+err.Error())
+		_, _ = fmt.Fprintln(root.ErrOrStderr(), "exiting: "+err.Error())
 
 		return 1
 	}
diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
index ad82d34..e27d5bd 100644
--- a/internal/cli/cli_test.go
+++ b/internal/cli/cli_test.go
@@ -14,7 +14,12 @@ func TestVersionPrintsVersion(t *testing.T) {
 
 	var stdout, stderr bytes.Buffer
 
-	status := cli.Execute("1.2.3", []string{"version"}, &stdout, &stderr)
+	root := cli.RootCmd("1.2.3")
+	root.SetArgs([]string{"version"})
+	root.SetOut(&stdout)
+	root.SetErr(&stderr)
+
+	status := cli.Execute(root)
 	if status != 0 {
 		t.Fatalf("status = %d, want 0; stderr: %q", status, stderr.String())
 	}
@@ -30,7 +35,12 @@ func TestUnknownCommandFails(t *testing.T) {
 
 	var stdout, stderr bytes.Buffer
 
-	status := cli.Execute("1.2.3", []string{"frobnicate"}, &stdout, &stderr)
+	root := cli.RootCmd("1.2.3")
+	root.SetArgs([]string{"frobnicate"})
+	root.SetOut(&stdout)
+	root.SetErr(&stderr)
+
+	status := cli.Execute(root)
 	if status == 0 {
 		t.Fatal("status = 0, want non-zero")
 	}
diff --git a/internal/cli/export_test.go b/internal/cli/export_test.go
index fc75527..5d8b0fe 100644
--- a/internal/cli/export_test.go
+++ b/internal/cli/export_test.go
@@ -1,7 +1,11 @@
 package cli
 
-// Execute is execute, exported for the external test package, which
-// needs to supply the arguments and read the output.
+// RootCmd and Execute are rootCmd and execute, exported for the external
+// test package, which builds the command tree itself so that it can
+// supply the arguments and read the output.
 //
-//nolint:gochecknoglobals // a test seam, not mutable state.
-var Execute = execute
+//nolint:gochecknoglobals // test seams, not mutable state.
+var (
+	RootCmd = rootCmd
+	Execute = execute
+)