diff --git a/pkg/agentic/prep.go b/pkg/agentic/prep.go
index c2545d1..cdde752 100644
--- a/pkg/agentic/prep.go
+++ b/pkg/agentic/prep.go
@@ -1041,6 +1041,10 @@ func (s *PrepSubsystem) buildPrompt(ctx context.Context, input PrepInput, branch
 // ensureWorkspaceTaskFile("/srv/.core/workspace/core/go-io/task-42")
 // keeps TODO.md present for the prompt and the local agent shell wrapper.
 func ensureWorkspaceTaskFile(workspaceDir string) error {
+	if workspaceDir == "" {
+		return core.E("prepWorkspace", "workspace dir is required", nil)
+	}
+
 	todoPath := core.JoinPath(workspaceDir, "TODO.md")
 	if readResult := fs.Read(todoPath); readResult.OK && core.Trim(readResult.Value.(string)) != "" {
 		return nil
@@ -1054,7 +1058,7 @@ func ensureWorkspaceTaskFile(workspaceDir string) error {
 		return core.E("prepWorkspace", "load TODO.md template", nil)
 	}
 
-	if writeResult := fs.Write(todoPath, templateResult.Value.(string)); !writeResult.OK {
+	if writeResult := fs.WriteAtomic(todoPath, templateResult.Value.(string)); !writeResult.OK {
 		if err, ok := writeResult.Value.(error); ok {
 			return core.E("prepWorkspace", "write TODO.md", err)
 		}
diff --git a/pkg/agentic/prep_test.go b/pkg/agentic/prep_test.go
index a2c43ac..d0e1e85 100644
--- a/pkg/agentic/prep_test.go
+++ b/pkg/agentic/prep_test.go
@@ -1311,3 +1311,9 @@ func TestPrep_TestPrepWorkspace_Ugly(t *testing.T) {
 	_, _, err := s.TestPrepWorkspace(context.Background(), PrepInput{Repo: ".."})
 	require.Error(t, err)
 }
+
+func TestPrep_EnsureWorkspaceTaskFile_Bad(t *testing.T) {
+	err := ensureWorkspaceTaskFile("")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "workspace dir is required")
+}