diff --git a/.env.example b/.env.example
index cd30adc7..7f3e31af 100644
--- a/.env.example
+++ b/.env.example
@@ -143,6 +143,9 @@ YOLO_SUPPRESS_TILE_EDGE_DETECTIONS=true
 # Serving a promoted model at a different value means the runtime suppresses
 # detections its gate counted, so set this to the value the candidate was gated at.
 YOLO_CONTAINMENT_NMS_THRESHOLD=0.85
+# Segmentation carries its own value: masks and boxes overlap differently,
+# so one threshold need not fit both.
+SEGMENTATION_CONTAINMENT_NMS_THRESHOLD=0.85
 
 # Local segmentation models. GeoIntel never downloads model weights
 # automatically; point these to existing local files to enable inference.
diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index 4b94ab55..b1829ac6 100644
--- a/backend/app/core/config.py
+++ b/backend/app/core/config.py
@@ -415,6 +415,14 @@ class Settings(BaseSettings):
     )
     sam_model_version: str | None = Field(default=None, validation_alias="SAM_MODEL_VERSION")
     segmentation_max_masks_per_tile: int = Field(default=300, ge=1, validation_alias="SEGMENTATION_MAX_MASKS_PER_TILE")
+    # Masks and boxes overlap differently, so segmentation carries its own
+    # containment value rather than borrowing the detector's.
+    segmentation_containment_nms_threshold: float = Field(
+        default=0.85,
+        ge=0.0,
+        le=1.0,
+        validation_alias="SEGMENTATION_CONTAINMENT_NMS_THRESHOLD",
+    )
     segmentation_duplicate_iou_threshold: float = Field(
         default=0.5,
         ge=0.0,
diff --git a/backend/app/services/segmentation_service.py b/backend/app/services/segmentation_service.py
index 999a7c4d..22008b18 100644
--- a/backend/app/services/segmentation_service.py
+++ b/backend/app/services/segmentation_service.py
@@ -784,6 +784,7 @@ class SegmentationService:
         filtered_candidates = DetectionService._suppress_duplicate_candidates(
             candidates,
             iou_threshold=float(settings.segmentation_duplicate_iou_threshold),
+            containment_threshold=float(settings.segmentation_containment_nms_threshold),
         )
         persisted: list[Segmentation] = []
         for candidate in filtered_candidates:
@@ -834,6 +835,8 @@ class SegmentationService:
             "raw_segmentation_count": len(candidates),
             "suppressed_segmentation_count": len(candidates) - len(filtered_candidates),
             "duplicate_iou_threshold": float(settings.segmentation_duplicate_iou_threshold),
+            "containment_suppression_threshold": float(settings.segmentation_containment_nms_threshold),
+            "duplicate_iou_threshold": float(settings.segmentation_duplicate_iou_threshold),
             "tile_manifest_path": str(Path(tile_manifest_path or "").expanduser()),
             "runtime_model_provenance": runtime_model_provenance.as_dict(),
         }
diff --git a/backend/tests/test_segmentation_postprocessing_config.py b/backend/tests/test_segmentation_postprocessing_config.py
new file mode 100644
index 00000000..561efef0
--- /dev/null
+++ b/backend/tests/test_segmentation_postprocessing_config.py
@@ -0,0 +1,61 @@
+"""Segmentation must honour the same post-processing configuration as detection.
+
+Segmentation reuses the detection suppressor but passed only the IoU threshold,
+so it silently fell back to the hardcoded containment constant while detection
+read a configured one. A deployment tuning containment for a promoted model
+changed detection behaviour and left segmentation on the old value.
+"""
+
+from __future__ import annotations
+
+import pytest
+from shapely.geometry import box
+
+from app.core.config import Settings
+
+
+def test_segmentation_has_its_own_containment_setting() -> None:
+    settings = Settings(_env_file=None)
+
+    assert settings.segmentation_containment_nms_threshold == pytest.approx(0.85)
+
+
+def test_the_setting_is_independent_of_the_detection_one() -> None:
+    """Masks and boxes overlap differently; one value need not fit both."""
+
+    settings = Settings(
+        _env_file=None,
+        yolo_containment_nms_threshold=0.7,
+        segmentation_containment_nms_threshold=0.95,
+    )
+
+    assert settings.yolo_containment_nms_threshold == pytest.approx(0.7)
+    assert settings.segmentation_containment_nms_threshold == pytest.approx(0.95)
+
+
+def test_the_configured_value_reaches_the_suppressor() -> None:
+    from app.services.detection_service import DetectionService
+
+    def candidate(name: str, geometry, confidence: float):
+        return {
+            "class_name": "building",
+            "confidence": confidence,
+            "geometry": geometry,
+            "bbox": [0.0, 0.0, 1.0, 1.0],
+            "source_tile_path": f"/tiles/{name}.tif",
+            "properties": {"name": name},
+        }
+
+    outer = candidate("outer", box(0, 0, 10, 10), 0.9)
+    # Containment 0.9, IoU 0.09: only the containment rule can act on this pair.
+    mostly_nested = candidate("mostly", box(8.2, 1, 10.2, 6), 0.5)
+
+    strict = DetectionService._suppress_duplicate_candidates(
+        [outer, mostly_nested], iou_threshold=0.5, containment_threshold=0.95
+    )
+    loose = DetectionService._suppress_duplicate_candidates(
+        [outer, mostly_nested], iou_threshold=0.5, containment_threshold=0.7
+    )
+
+    assert len(strict) == 2
+    assert len(loose) == 1