Fix todo update dropping untouched fields

internal/commands/todos.go

@@ -1164,7 +1164,29 @@ Clear a field by passing its --no- flag or an empty value:
 					return convertSDKError(err)
 				}
 			} else {
-				req := &basecamp.UpdateTodoRequest{}
+				// Fetch existing todo so we can preserve fields the user
+				// didn't change. The BC3 API clears fields by omission,
+				// so a partial PUT would wipe untouched fields.
+				existingTodo, err := app.Account().Todos().Get(cmd.Context(), todoID)
+				if err != nil {
+					return convertSDKError(err)
+				}
+
+				req := &basecamp.UpdateTodoRequest{
+					Content:     existingTodo.Content,
+					Description: existingTodo.Description,
+					DueOn:       existingTodo.DueOn,
+					StartsOn:    existingTodo.StartsOn,
+				}
+				if len(existingTodo.Assignees) > 0 {
+					ids := make([]int64, len(existingTodo.Assignees))
+					for i, a := range existingTodo.Assignees {
+						ids[i] = a.ID
+					}
+					req.AssigneeIDs = ids
+				}
+
+				// Override with user-provided values.
 				if effectiveTitle != "" {
 					req.Content = effectiveTitle
 				}

internal/commands/todos_test.go

@@ -1810,6 +1810,53 @@ func TestTodosUpdateClearPreservesAssignees(t *testing.T) {
 	assert.Equal(t, float64(42), ids[0])
 }
 
+func TestTodosUpdateDueDatePreservesExistingFields(t *testing.T) {
+	transport := &mockTodoUpdateTransport{}
+	app := setupTodoUpdateApp(t, transport)
+
+	cmd := NewTodosCmd()
+	err := executeTodosCommand(cmd, app, "update", "999", "--due", "2026-04-12")
+	require.NoError(t, err)
+	require.NotEmpty(t, transport.capturedBody, "expected PUT body but got none")
+
+	var body map[string]any
+	err = json.Unmarshal(transport.capturedBody, &body)
+	require.NoError(t, err)
+
+	// The new due date is applied.
+	assert.Equal(t, "2026-04-12", body["due_on"])
+
+	// BC3 API clears fields by omission, so untouched fields from the
+	// existing todo must be preserved in the request body.
+	assert.Equal(t, "Test todo", body["content"])
+	assert.Equal(t, "Existing desc", body["description"])
+	assert.Equal(t, "2026-03-25", body["starts_on"])
+
+	ids, ok := body["assignee_ids"].([]any)
+	require.True(t, ok, "assignee_ids must be preserved")
+	require.Len(t, ids, 1)
+	assert.Equal(t, float64(42), ids[0])
+}
+
+func TestTodosUpdateTitlePreservesExistingFields(t *testing.T) {
+	transport := &mockTodoUpdateTransport{}
+	app := setupTodoUpdateApp(t, transport)
+
+	cmd := NewTodosCmd()
+	err := executeTodosCommand(cmd, app, "update", "999", "New title")
+	require.NoError(t, err)
+	require.NotEmpty(t, transport.capturedBody)
+
+	var body map[string]any
+	err = json.Unmarshal(transport.capturedBody, &body)
+	require.NoError(t, err)
+
+	assert.Equal(t, "New title", body["content"])
+	assert.Equal(t, "Existing desc", body["description"])
+	assert.Equal(t, "2026-04-01", body["due_on"])
+	assert.Equal(t, "2026-03-25", body["starts_on"])
+}
+
 // =============================================================================
 // --assignee filtering tests (single-list path)
 // =============================================================================