diff --git a/pkg/tools/spawn.go b/pkg/tools/spawn.go
index 73d385cb0..8b166b41f 100644
--- a/pkg/tools/spawn.go
+++ b/pkg/tools/spawn.go
@@ -3,6 +3,7 @@ package tools
 import (
 	"context"
 	"fmt"
+	"strings"
 )
 
 type SpawnTool struct {
@@ -66,8 +67,8 @@ func (t *SpawnTool) SetAllowlistChecker(check func(targetAgentID string) bool) {
 
 func (t *SpawnTool) Execute(ctx context.Context, args map[string]any) *ToolResult {
 	task, ok := args["task"].(string)
-	if !ok {
-		return ErrorResult("task is required")
+	if !ok || strings.TrimSpace(task) == "" {
+		return ErrorResult("task is required and must be a non-empty string")
 	}
 
 	label, _ := args["label"].(string)
diff --git a/pkg/tools/spawn_test.go b/pkg/tools/spawn_test.go
new file mode 100644
index 000000000..0646c82a9
--- /dev/null
+++ b/pkg/tools/spawn_test.go
@@ -0,0 +1,79 @@
+package tools
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+func TestSpawnTool_Execute_EmptyTask(t *testing.T) {
+	provider := &MockLLMProvider{}
+	manager := NewSubagentManager(provider, "test-model", "/tmp/test", nil)
+	tool := NewSpawnTool(manager)
+
+	ctx := context.Background()
+
+	tests := []struct {
+		name string
+		args map[string]any
+	}{
+		{"empty string", map[string]any{"task": ""}},
+		{"whitespace only", map[string]any{"task": "   "}},
+		{"tabs and newlines", map[string]any{"task": "\t\n  "}},
+		{"missing task key", map[string]any{"label": "test"}},
+		{"wrong type", map[string]any{"task": 123}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := tool.Execute(ctx, tt.args)
+			if result == nil {
+				t.Fatal("Result should not be nil")
+			}
+			if !result.IsError {
+				t.Error("Expected error for invalid task parameter")
+			}
+			if !strings.Contains(result.ForLLM, "task is required") {
+				t.Errorf("Error message should mention 'task is required', got: %s", result.ForLLM)
+			}
+		})
+	}
+}
+
+func TestSpawnTool_Execute_ValidTask(t *testing.T) {
+	provider := &MockLLMProvider{}
+	manager := NewSubagentManager(provider, "test-model", "/tmp/test", nil)
+	tool := NewSpawnTool(manager)
+
+	ctx := context.Background()
+	args := map[string]any{
+		"task":  "Write a haiku about coding",
+		"label": "haiku-task",
+	}
+
+	result := tool.Execute(ctx, args)
+	if result == nil {
+		t.Fatal("Result should not be nil")
+	}
+	if result.IsError {
+		t.Errorf("Expected success for valid task, got error: %s", result.ForLLM)
+	}
+	if !result.Async {
+		t.Error("SpawnTool should return async result")
+	}
+}
+
+func TestSpawnTool_Execute_NilManager(t *testing.T) {
+	tool := NewSpawnTool(nil)
+
+	ctx := context.Background()
+	args := map[string]any{"task": "test task"}
+
+	result := tool.Execute(ctx, args)
+	if !result.IsError {
+		t.Error("Expected error for nil manager")
+	}
+	if !strings.Contains(result.ForLLM, "Subagent manager not configured") {
+		t.Errorf("Error message should mention manager not configured, got: %s", result.ForLLM)
+	}
+}