diff --git a/claudetool/bash.go b/claudetool/bash.go
index 882dddf..b24e700 100644
--- a/claudetool/bash.go
+++ b/claudetool/bash.go
@@ -167,6 +167,9 @@
 	cmd.Dir = WorkingDir(ctx)
 	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
 
+	// Set environment with SKETCH=1
+	cmd.Env = append(os.Environ(), "SKETCH=1")
+
 	var output bytes.Buffer
 	cmd.Stdin = nil
 	cmd.Stdout = &output
@@ -246,6 +249,9 @@
 	cmd.Dir = WorkingDir(ctx)
 	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
 
+	// Set environment with SKETCH=1
+	cmd.Env = append(os.Environ(), "SKETCH=1")
+
 	// Open output files
 	stdout, err := os.Create(stdoutFile)
 	if err != nil {
diff --git a/claudetool/bash_test.go b/claudetool/bash_test.go
index f39114e..3865383 100644
--- a/claudetool/bash_test.go
+++ b/claudetool/bash_test.go
@@ -131,6 +131,24 @@
 		}
 	})
 
+	// Test SKETCH=1 environment variable is set
+	t.Run("SKETCH Environment Variable", func(t *testing.T) {
+		req := bashInput{
+			Command: "echo $SKETCH",
+			Timeout: "5s",
+		}
+
+		output, err := executeBash(ctx, req)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		want := "1\n"
+		if output != want {
+			t.Errorf("Expected SKETCH=1, got %q", output)
+		}
+	})
+
 	// Test command with output to stderr
 	t.Run("Command with stderr", func(t *testing.T) {
 		req := bashInput{
@@ -195,7 +213,7 @@
 			Command    string `json:"command"`
 			Background bool   `json:"background"`
 		}{
-			Command:    "echo 'Hello from background'",
+			Command:    "echo 'Hello from background' $SKETCH",
 			Background: true,
 		}
 		inputJSON, err := json.Marshal(inputObj)
@@ -235,7 +253,7 @@
 		if err != nil {
 			t.Fatalf("Failed to read stdout file: %v", err)
 		}
-		expected := "Hello from background\n"
+		expected := "Hello from background 1\n"
 		if string(stdoutContent) != expected {
 			t.Errorf("Expected stdout content %q, got %q", expected, string(stdoutContent))
 		}