From 360a68e07876b5bff9cd27d6df6c3cdcd93ce930 Mon Sep 17 00:00:00 2001
From: harrierpigeon <college.joseph.robertson@gmail.com>
Date: Fri, 2 Oct 2026 01:11:59 -0500
Subject: [PATCH] Belt: warn when the purge tower is enabled but the project
 has no tower object

The purge tower is a model object the GUI creates and sizes, and libslic3r
only purges into one that exists. A multi-filament belt project sliced from
the CLI without it changed filament with nowhere to purge, silently.

Raised in Hanif Koh's review of #14394.
---
 src/libslic3r/Print.cpp        | 15 +++++++++++++++
 tests/fff_print/test_print.cpp | 30 ++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index 5bfdfe8eac..444bf7cde6 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -1553,6 +1553,21 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
         add_warning(warningtemp);
     }
 
+    // The purge tower is a model object the GUI creates and sizes; libslic3r only purges
+    // into one that exists. A project sliced without it (the CLI on a project saved before
+    // the tower was generated) changes filament with nowhere to purge.
+    if (m_config.belt_printer.value && m_config.enable_belt_purge_tower.value
+        && m_config.print_sequence != PrintSequence::ByObject
+        && ! m_config.spiral_mode.value && extruders.size() > 1 && ! this->has_belt_purge_tower()) {
+        StringObjectException warningtemp;
+        warningtemp.string     = L("The belt purge tower is enabled but the project has no purge tower object; "
+                                   "filament changes will not be purged. Open the project in the application "
+                                   "to generate the tower.");
+        warningtemp.opt_key    = "enable_belt_purge_tower";
+        warningtemp.is_warning = true;
+        add_warning(warningtemp);
+    }
+
     if (m_config.belt_printer.value && m_config.enable_belt_purge_tower.value) {
         const size_t prism_count = std::count_if(m_objects.begin(), m_objects.end(), [](const PrintObject *object) {
             return object->config().belt_purge_tower_object.value;
diff --git a/tests/fff_print/test_print.cpp b/tests/fff_print/test_print.cpp
index 6d8d3d2ec9..f9c9528461 100644
--- a/tests/fff_print/test_print.cpp
+++ b/tests/fff_print/test_print.cpp
@@ -336,6 +336,36 @@ TEST_CASE("Belt purge planning requires its managed purge object", "[Print][Purg
     CHECK_FALSE(print.has_wipe_tower());
 }
 
+// The GUI creates the purge tower object; a project sliced without one (the CLI) must say
+// that its filament changes go unpurged.
+TEST_CASE("Belt purge tower enabled without a tower object warns", "[Print][PurgeTower][belt]")
+{
+    DynamicPrintConfig config = multifilament_config(2, {
+        { "belt_printer",            1 },
+        { "enable_belt_purge_tower", 1 },
+        { "layer_change_gcode",      "G92 E0\n" }
+    });
+    auto purge_warnings = [](Print &print) {
+        std::vector<StringObjectException> warnings;
+        print.validate(&warnings);
+        return std::count_if(warnings.begin(), warnings.end(), [](const StringObjectException &w) {
+            return w.opt_key == "enable_belt_purge_tower";
+        });
+    };
+
+    Model model;
+    Print print;
+    build_cubes(model, print, config, /*n=*/2, /*overlap=*/false);
+    model.objects[1]->config.set_key_value("extruder", new ConfigOptionInt(2));
+    print.apply(model, config);
+    REQUIRE(print.extruders().size() > 1);
+    CHECK(purge_warnings(print) == 1);
+
+    model.objects.front()->config.set_key_value("belt_purge_tower_object", new ConfigOptionBool(true));
+    print.apply(model, config);
+    CHECK(purge_warnings(print) == 0);
+}
+
 TEST_CASE("Belt purge rejects multiple managed purge objects", "[Print][PurgeTower][Regression]")
 {
     DynamicPrintConfig config = multifilament_config(2, {