diff --git a/cmd/cue/cmd/testdata/tasks/cmd_baddisplay.out b/cmd/cue/cmd/testdata/tasks/cmd_baddisplay.out
index 780db31..2d64bfb 100644
--- a/cmd/cue/cmd/testdata/tasks/cmd_baddisplay.out
+++ b/cmd/cue/cmd/testdata/tasks/cmd_baddisplay.out
@@ -1,3 +1,3 @@
 text: conflicting values 42 and string (mismatched types int and string):
-    ./testdata/tasks/task_tool.cue:29:9
+    ./testdata/tasks/task_tool.cue:31:9
     tool/cli:4:9
diff --git a/cmd/cue/cmd/testdata/tasks/task_tool.cue b/cmd/cue/cmd/testdata/tasks/task_tool.cue
index 0c6c737..228222b 100644
--- a/cmd/cue/cmd/testdata/tasks/task_tool.cue
+++ b/cmd/cue/cmd/testdata/tasks/task_tool.cue
@@ -1,5 +1,7 @@
 package home
 
+import "tool/exec"
+
 command run: runBase & {
 	task echo cmd: "echo \(message)"
 }
@@ -9,7 +11,7 @@
 }
 
 command errcode: {
-	task bad: {
+	task bad: exec.Run & {
 		kind:   "exec"
 		cmd:    "ls --badflags"
 		stderr: string // suppress error message
@@ -17,7 +19,7 @@
 
 // TODO: capture stdout and stderr for tests.
 command runRedirect: {
-	task echo: {
+	task echo: exec.Run & {
 		kind: "exec"
 		cmd:  "echo \(message)"
 	}
diff --git a/cue/builtins.go b/cue/builtins.go
index 7cbd04f..a055e9a 100644
--- a/cue/builtins.go
+++ b/cue/builtins.go
@@ -2871,7 +2871,7 @@
 		}
 		stdout:  *null | string | bytes
 		stderr:  *null | string | bytes
-		stdin?:  string | bytes
+		stdin:   *null | string | bytes
 		success: bool
 	}
 }`,
diff --git a/pkg/tool/exec/exec.cue b/pkg/tool/exec/exec.cue
index 78efc4e..ede6634 100644
--- a/pkg/tool/exec/exec.cue
+++ b/pkg/tool/exec/exec.cue
@@ -36,8 +36,9 @@
 	// stderr is like stdout, but for errors.
 	stderr: *null | string | bytes
 
-	// stdin specifies the input for the process.
-	stdin?: string | bytes
+	// stdin specifies the input for the process. If null, stdin of the current
+	// process is used.
+	stdin: *null | string | bytes
 
 	// success is set to true when the process terminates with with a zero exit
 	// code or false otherwise. The user can explicitly specify the value
diff --git a/pkg/tool/exec/exec.go b/pkg/tool/exec/exec.go
index f4b19b2..1bdca9a 100644
--- a/pkg/tool/exec/exec.go
+++ b/pkg/tool/exec/exec.go
@@ -78,16 +78,29 @@
 
 	cmd := exec.CommandContext(ctx.Context, bin, args...)
 
-	if v := v.Lookup("stdin"); v.Exists() {
+	stream := func(name string) (stream cue.Value, ok bool) {
+		c := v.Lookup(name)
+		// Although the schema defines a default versions, older implementations
+		// may not use it yet.
+		if !c.Exists() {
+			return
+		}
+		if err := c.Null(); err == nil {
+			return
+		}
+		return c, true
+	}
+
+	if v, ok := stream("stdin"); ok {
 		if cmd.Stdin, err = v.Reader(); err != nil {
 			return nil, fmt.Errorf("cue: %v", err)
 		}
 	}
-	captureOut := v.Lookup("stdout").Exists()
+	_, captureOut := stream("stdout")
 	if !captureOut {
 		cmd.Stdout = ctx.Stdout
 	}
-	captureErr := v.Lookup("stderr").Exists()
+	_, captureErr := stream("stderr")
 	if !captureErr {
 		cmd.Stderr = ctx.Stderr
 	}