From 075b03975f066199e4104ae0bc681577fceef6dd Mon Sep 17 00:00:00 2001
From: HMITCHR <265609730+HMITCHR@users.noreply.github.com>
Date: Mon, 12 Oct 2026 18:55:02 -0600
Subject: [PATCH] Read the initial filament start G-code through get_at()
 (#16080)

A filament_start_gcode list shorter than the filament count made the Bambu start-of-export path throw std::out_of_range. Use get_at() like the other filament_start_gcode reads, and add a regression test.
---
 src/libslic3r/GCode.cpp              |  2 +-
 tests/fff_print/test_gcodewriter.cpp | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/src/libslic3r/GCode.cpp b/src/libslic3r/GCode.cpp
index b79a386324..f423681c9d 100644
--- a/src/libslic3r/GCode.cpp
+++ b/src/libslic3r/GCode.cpp
@@ -4262,7 +4262,7 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
             config.set_key_value("nozzle_diameter_at_nozzle_id", new ConfigOptionFloats(get_nozzle_diameters_by_nozzle_id(group_result.get())));
             config.set_key_value("nozzle_volume_types", new ConfigOptionStrings(get_nozzle_volume_types_by_nozzle_id(group_result.get())));
             config.set_key_value("layer_num", new ConfigOptionInt(m_layer_index));
-            std::string filament_start_gcode = this->placeholder_parser_process("filament_start_gcode", print.config().filament_start_gcode.values.at(initial_non_support_extruder_id), initial_non_support_extruder_id,&config);
+            std::string filament_start_gcode = this->placeholder_parser_process("filament_start_gcode", print.config().filament_start_gcode.get_at(initial_non_support_extruder_id), initial_non_support_extruder_id,&config);
             file.writeln(filament_start_gcode);
             // Mark the first filament used in print. Multi-nozzle printers (H2C) get ";VT%d H%d" where
             // H = dynamic ? nozzle_id : -1; existing single-nozzle printers keep the bare ";VT%d" so their
diff --git a/tests/fff_print/test_gcodewriter.cpp b/tests/fff_print/test_gcodewriter.cpp
index 170dfabae3..d491527db6 100644
--- a/tests/fff_print/test_gcodewriter.cpp
+++ b/tests/fff_print/test_gcodewriter.cpp
@@ -1748,3 +1748,29 @@ TEST_CASE("Percent accelerations resolve against the option they are a percentag
         REQUIRE(accelerations_by_role[role_name] == std::set<int>{ value });
     }
 }
+
+TEST_CASE("Initial filament start G-code tolerates a list shorter than the filament count", "[GCodeWriter]")
+{
+    // Two filaments but only one filament_start_gcode entry, and the only object prints on
+    // filament 2, so the BBL export starts on a filament past the end of the list.
+    DynamicPrintConfig config = multifilament_config(2);
+    config.set_key_value("filament_start_gcode", new ConfigOptionStrings({"; filament start"}));
+
+    Model model;
+    auto *obj = model.add_object();
+    obj->add_volume(cube(20));
+    obj->add_instance();
+    obj->config.set_key_value("extruder", new ConfigOptionInt(2));
+
+    Print print;
+    print.is_BBL_printer() = true;
+    arrange_objects_on_test_bed(model, config);
+    for (auto *mo : model.objects) {
+        mo->ensure_on_bed();
+        print.auto_assign_extruders(mo);
+    }
+    print.apply(model, config);
+    print.validate();
+
+    REQUIRE_NOTHROW(Slic3r::Test::gcode(print));
+}