diff --git a/config/infer/icarus/modifier/charge_scale/mod_charge_scale_260813.yaml b/config/infer/icarus/modifier/charge_scale/mod_charge_scale_260813.yaml
new file mode 100644
index 0000000..bd79adb
--- /dev/null
+++ b/config/infer/icarus/modifier/charge_scale/mod_charge_scale_260813.yaml
@@ -0,0 +1,25 @@
+# ICARUS charge-scale detector variation
+# Revision date: August 13, 2026
+
+__meta__:
+  kind: mod
+  name: "charge_scale"
+  version: "260813"
+  date: "2026-08-13"
+  description: "Charge-scale detector variation"
+  applies_to: [mc]
+  compatible_with:
+    model: ">=250303"
+
+override:
+  model.modules.calibration.response:
+    # Apply the response variation before charge calibrations.
+    priority: 10
+    response_func: 1.0279*x - 0.000287*x*x + 2.4915e-7*x*x*x - 4.522989e-11*x*x*x*x
+  model.modules.calibration.smearing:
+    # Apply the smearing after the response function
+    priority: 9
+    distribution: normal
+    mode: multiplicative
+    mean: 1.0
+    scale: 0.14953
diff --git a/config/infer/sbnd/modifier/charge_scale/mod_charge_scale_260813.yaml b/config/infer/sbnd/modifier/charge_scale/mod_charge_scale_260813.yaml
index f05c8a0..5531419 100644
--- a/config/infer/sbnd/modifier/charge_scale/mod_charge_scale_260813.yaml
+++ b/config/infer/sbnd/modifier/charge_scale/mod_charge_scale_260813.yaml
@@ -20,6 +20,13 @@ override:
     response:
       # Apply the response variation before charge calibrations.
       priority: 10
-      response_func: 1.00582*x - 0.0001188*x*x - 4.4164e-8*x*x*x + 1.04071e-11*x*x*x*x
+      response_func: 0.9894425*x + 7.6032e-7*x*x - 2.552e-7*x*x*x + 6.74704e-11*x*x*x*x
+    smearing:
+      # Apply the smearing after the response function
+      priority: 9
+      distribution: normal
+      mode: multiplicative
+      mean: 1.0
+      scale: 0.0511
 
   post.apply_calibrations.depositions_source: depositions
diff --git a/tests/test_config_validation.py b/tests/test_config_validation.py
index 0c4b513..39b1aff 100644
--- a/tests/test_config_validation.py
+++ b/tests/test_config_validation.py
@@ -393,3 +393,64 @@ def test_sbnd_charge_scale_continues_from_varied_depositions(
         assert (
             config["post"]["apply_calibrations"]["depositions_source"] == "depositions"
         )
+
+    def test_icarus_charge_scale_only_adds_response_and_smearing(
+        self, config_infer_root, tmp_path
+    ):
+        """The ICARUS variation preserves calibration supplied by the base model."""
+        icarus_root = config_infer_root / "icarus"
+        modifier = (
+            icarus_root / "modifier" / "charge_scale" / "mod_charge_scale_260813.yaml"
+        )
+        modifier_config = yaml.safe_load(modifier.read_text(encoding="utf-8"))
+        assert modifier_config["__meta__"]["compatible_with"] == {"model": ">=250303"}
+        assert set(modifier_config["override"]) == {
+            "model.modules.calibration.response",
+            "model.modules.calibration.smearing",
+        }
+
+        composite = tmp_path / "composite.yaml"
+        composite.write_text(
+            yaml.safe_dump(
+                {
+                    "include": [
+                        str(icarus_root / "full_chain_co_260501.yaml"),
+                        str(modifier),
+                    ]
+                }
+            ),
+            encoding="utf-8",
+        )
+        config = load_config_with_includes(composite)
+        calibration = config["model"]["modules"]["calibration"]
+        assert calibration["stage"] == "segmentation"
+        assert calibration["gain"]["gain"] == 79.9169
+        assert "recombination" in calibration
+        assert "lifetime" in calibration
+        assert "transparency" in calibration
+        assert calibration["response"]["priority"] == 10
+        assert calibration["smearing"]["priority"] == 9
+
+    def test_icarus_charge_scale_accepts_first_in_chain_calibration(
+        self, config_infer_root, tmp_path
+    ):
+        """The first ICARUS model with in-chain calibration supports the modifier."""
+        icarus_root = config_infer_root / "icarus"
+        modifier = (
+            icarus_root / "modifier" / "charge_scale" / "mod_charge_scale_260813.yaml"
+        )
+        composite = tmp_path / "composite.yaml"
+        composite.write_text(
+            yaml.safe_dump(
+                {
+                    "include": [
+                        str(icarus_root / "full_chain_co_250303.yaml"),
+                        str(modifier),
+                    ]
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        config = load_config_with_includes(composite)
+        assert "response" in config["model"]["modules"]["calibration"]