diff --git a/shared/tests/test_force_filters.py b/shared/tests/test_force_filters.py
index 2538225..98ff3c7 100644
--- a/shared/tests/test_force_filters.py
+++ b/shared/tests/test_force_filters.py
@@ -95,7 +95,41 @@ def test_reset_clears_the_memory_between_sessions():
     )
 
 
-def test_settings_round_trip_through_a_preferences_mapping():
+def test_the_chain_reproduces_the_sdk_filter_exactly():
+    """Replacing the SDK's low-pass with ours must not move a single number.
+
+    ``Sensor._apply_force_filter`` filters ``_force_raw`` with
+    ``orisys.signal_processing.LowpassFilter`` (Butterworth, ``sosfilt`` with
+    persistent state, steady-state reset) and then applies the device
+    calibration. Our chain uses the same recipe, so at the same settings the
+    recorded values stay identical — this test is what makes that a guarantee
+    rather than a hope.
+    """
+    try:
+        from orisys.signal_processing import LowpassFilter
+    except Exception:  # pragma: no cover - the SDK is not installed here
+        return
+
+    rng = np.random.default_rng(3)
+    signal = np.cumsum(rng.normal(0.0, 1.0, (400, 3)), axis=0) * 10.0 + 500.0
+
+    sdk = LowpassFilter(cutoff_hz=10.0, sample_rate_hz=30.0, order=2, n_channels=3)
+    sdk.configure(cutoff_hz=10.0, sample_rate_hz=30.0, order=2)
+    sdk.reset(signal[0])
+    sdk_out = np.array([sdk.process(row) for row in signal])
+
+    chain = ForceFilterChain(
+        FilterSettings(lowpass_enabled=True, lowpass_hz=10.0, sample_rate_hz=30.0)
+    )
+    # The chain carries six channels (raw + newtons); the first three are the
+    # raw counts the SDK filters, so give all six the same steady state.
+    chain.reset(list(signal[0]) * 2)
+    chain_out = np.array([chain.process(list(row) * 2)[:3] for row in signal])
+
+    assert np.array_equal(sdk_out, chain_out), (
+        f"the chain differs from the SDK filter by {np.abs(sdk_out - chain_out).max()}"
+    )
+
     settings = settings_from_mapping(
         {
             "median_enabled": True,
@@ -111,3 +145,14 @@ def test_settings_round_trip_through_a_preferences_mapping():
     # An empty mapping is the documented default, not a disabled chain.
     default = settings_from_mapping(None)
     assert default.as_dict()["lowpass_hz"] == FilterSettings().lowpass_hz
+
+
+def test_settings_round_trip_through_a_preferences_mapping():
+    settings = settings_from_mapping({
+        "median_enabled": True, "median_window": 7,
+        "lowpass_enabled": True, "lowpass_hz": 12.5, "show_unfiltered": True,
+    })
+    assert settings.median_enabled and settings.median_window == 7
+    assert settings.lowpass_hz == 12.5 and settings.show_unfiltered
+    assert settings.describe() == "median(7) + lowpass(12.5Hz,order2)"
+    assert settings_from_mapping(None).as_dict()["lowpass_hz"] == FilterSettings().lowpass_hz