diff --git a/src/libslic3r/GCode.cpp b/src/libslic3r/GCode.cpp
index eb54d1f18a..0de4c16944 100644
--- a/src/libslic3r/GCode.cpp
+++ b/src/libslic3r/GCode.cpp
@@ -2887,7 +2887,9 @@ static BambuBedType to_bambu_bed_type(BedType type)
     return bambu_bed_type;
 }
 
-// Orca IMEX: Returns the tool indices active in the current IMEX mode.
+// Orca IMEX: Returns the PHYSICAL tool indices the active IMEX mode drives, or an EMPTY
+// vector in primary mode -- a single tool, so there are no parallel carriages to list.
+// The mode's Primary head IS included in parallel modes; callers handle it themselves.
 // In copy/mirror parallel modes, secondary carriages never receive tool-change
 // commands — the firmware mirrors the primary's moves — so they don't appear in
 // tool_ordering.all_extruders(). This helper extracts the active mode's tool string
@@ -2906,6 +2908,10 @@ static std::vector<int> get_imex_active_tools(const Print& print)
     const std::string& raw_mode = print.objects().front()->config().imex_parallel_mode.value;
     const std::string  active_mode = raw_mode.empty() ? kImexPrimaryMode : raw_mode;
 
+    // Primary mode drives a single tool, so there are no parallel carriages to list.
+    if (active_mode == kImexPrimaryMode)
+        return active_tools;
+
     const auto* mode_names_opt = print.config().option<ConfigOptionStrings>("imex_mode_names");
     const auto* tools_opt      = print.config().option<ConfigOptionStrings>("imex_mode_active_tools");
 
diff --git a/tests/fff_print/test_multifilament.cpp b/tests/fff_print/test_multifilament.cpp
index 7f8f5b5c6f..0656193a6d 100644
--- a/tests/fff_print/test_multifilament.cpp
+++ b/tests/fff_print/test_multifilament.cpp
@@ -715,6 +715,7 @@ TEST_CASE("Multi-extruder slice stays in bounds with a short max_layer_height",
     REQUIRE_FALSE(print.objects().front()->layers().empty());
 }
 
+
 // Shared IMEX printer geometry: 7 logical extruders across 4 physical heads.
 // physical_extruder_map is only honoured when its length matches the nozzle count
 // (PrintApply feeds effective_physical_extruder_map the nozzle_diameter size), so the
@@ -779,6 +780,53 @@ TEST_CASE("Parallel-mode IMEX prints transition the printing head to its second-
     CHECK(gcode.find("M104 S240 T1") != std::string::npos);
 }
 
+// IMEX supplements is_extruder_used for the secondary carriages a parallel mode drives.
+// `primary` drives exactly one tool, so the supplement must not run: routing every region
+// to filament 6 puts the initial tool on physical head 2, while the mode's only declared
+// head is 0, which the unguarded supplement resolved back to filament slot 0.
+TEST_CASE("Primary-mode IMEX prints mark only the filament slot they print with",
+          "[MultiFilament][IMEX]")
+{
+    DynamicPrintConfig config = multifilament_config(7);
+    imex_7x4_printer(config);
+    all_regions_on_filament(config, 6); // filament 6 => logical slot 5 => physical head 2
+    config.set_deserialize_strict({
+        { "imex_parallel_mode", "primary" },
+        { "machine_start_gcode",
+          ";USED0:{if is_extruder_used[0]}1{else}0{endif}\n"
+          ";USED5:{if is_extruder_used[5]}1{else}0{endif}\n" },
+    });
+
+    const std::string gcode = slice({ cube(20) }, config);
+
+    CHECK(gcode.find(";USED5:1") != std::string::npos);
+    CHECK(gcode.find(";USED0:0") != std::string::npos);
+}
+
+// Guard rail for the fix above: the modes the supplement exists for must keep marking their
+// secondaries. `copy` declares heads 0 and 1; neither is the initial tool's head (filament 6
+// routes to head 2), so both are still enumerated. Head 0 -> slot 0, head 1 -> slot 4.
+TEST_CASE("Copy-mode IMEX prints still mark every secondary carriage's filament slot",
+          "[MultiFilament][IMEX]")
+{
+    DynamicPrintConfig config = multifilament_config(7);
+    imex_7x4_printer(config);
+    all_regions_on_filament(config, 6);
+    config.set_deserialize_strict({
+        { "imex_parallel_mode", "copy" },
+        { "machine_start_gcode",
+          ";USED0:{if is_extruder_used[0]}1{else}0{endif}\n"
+          ";USED4:{if is_extruder_used[4]}1{else}0{endif}\n"
+          ";USED5:{if is_extruder_used[5]}1{else}0{endif}\n" },
+    });
+
+    const std::string gcode = slice({ cube(20) }, config);
+
+    CHECK(gcode.find(";USED5:1") != std::string::npos);
+    CHECK(gcode.find(";USED0:1") != std::string::npos);
+    CHECK(gcode.find(";USED4:1") != std::string::npos);
+}
+
 // The two IMEX changes are coupled: get_imex_active_tools() now returns an empty roster in
 // primary mode, so if the temperature branch ever stopped excluding primary it would enter,
 // emit nothing, skip the standard path, and silently restore the bug the copy-mode case above