diff --git a/backend/tests/unit/test_scheduler_ams_mapping.py b/backend/tests/unit/test_scheduler_ams_mapping.py
index 006ee7805..39ebb052d 100644
--- a/backend/tests/unit/test_scheduler_ams_mapping.py
+++ b/backend/tests/unit/test_scheduler_ams_mapping.py
@@ -590,6 +590,91 @@ class TestExtractNozzleMappingFrom3mf:
assert result == {1: 1, 2: 0}
zf.close()
+ def test_single_active_extruder_maps_all_slots(self):
+ """When only one extruder has nozzles installed, all filaments map to it.
+
+ H2C scenario: left extruder has no nozzles (Standard#0|High Flow#0),
+ right extruder has one Standard nozzle (Standard#1). Print uses only
+ the right extruder, so all filaments should map to physical extruder 0.
+ """
+ slice_info = """
+
+
+
+
+
+ """
+ zf = _make_3mf_zip(
+ {
+ "physical_extruder_map": ["1", "0"],
+ "extruder_nozzle_stats": ["Standard#0|High Flow#0", "Standard#1"],
+ },
+ slice_info_xml=slice_info,
+ )
+ result = extract_nozzle_mapping_from_3mf(zf)
+ # Only extruder index 1 is active → physical_extruder_map[1] = 0 (RIGHT)
+ assert result == {1: 0, 2: 0}
+ zf.close()
+
+ def test_two_active_extruders_falls_through(self):
+ """When both extruders have nozzles, the single-active shortcut is skipped.
+
+ Should fall through to the normal group_id-based mapping.
+ """
+ slice_info = """
+
+
+
+
+
+ """
+ zf = _make_3mf_zip(
+ {
+ "physical_extruder_map": ["1", "0"],
+ "extruder_nozzle_stats": ["Standard#1", "High Flow#1"],
+ },
+ slice_info_xml=slice_info,
+ )
+ result = extract_nozzle_mapping_from_3mf(zf)
+ # Both active → normal group_id mapping: group 0 → phys 1, group 1 → phys 0
+ assert result == {1: 1, 2: 0}
+ zf.close()
+
+ def test_missing_extruder_nozzle_stats_falls_through(self):
+ """When extruder_nozzle_stats is absent, the single-active shortcut is skipped.
+
+ Should fall through to the normal group_id-based mapping.
+ """
+ slice_info = """
+
+
+
+
+
+ """
+ zf = _make_3mf_zip(
+ {
+ "physical_extruder_map": ["1", "0"],
+ },
+ slice_info_xml=slice_info,
+ )
+ result = extract_nozzle_mapping_from_3mf(zf)
+ # No extruder_nozzle_stats → normal group_id mapping
+ assert result == {1: 1, 2: 0}
+ zf.close()
+
+ def test_single_active_extruder_no_slice_info_returns_none(self):
+ """Single active extruder but no slice_info should return None."""
+ zf = _make_3mf_zip(
+ {
+ "physical_extruder_map": ["1", "0"],
+ "extruder_nozzle_stats": ["Standard#0", "Standard#1"],
+ },
+ )
+ result = extract_nozzle_mapping_from_3mf(zf)
+ assert result is None
+ zf.close()
+
class TestNozzleAwareMapping:
"""Test nozzle-aware filament matching in the print scheduler."""