diff --git a/book/tests/test_narrative_invariants.py b/book/tests/test_narrative_invariants.py
new file mode 100644
index 000000000..1fa915b45
--- /dev/null
+++ b/book/tests/test_narrative_invariants.py
@@ -0,0 +1,113 @@
+# test_narrative_invariants.py
+# This test suite protects the "truth" of the textbook.
+# It ensures that the mathematical conclusions drawn in the prose
+# remain valid even as underlying hardware constants are updated.
+
+import pytest
+from calc.constants import (
+    ureg,
+    SPEED_OF_LIGHT_FIBER_KM_S,
+    NETWORK_10G_BW,
+    CLOUD_EGRESS_PER_GB,
+    CLOUD_ELECTRICITY_PER_KWH,
+    PHONE_BATTERY_WH,
+    OBJECT_DETECTOR_POWER_W,
+    RESNET50_FLOPs,
+    A100_MEM_BW,
+    MOBILE_NPU_MEM_BW,
+    VIDEO_1080P_WIDTH, VIDEO_1080P_HEIGHT, VIDEO_BYTES_PER_PIXEL_RGB, VIDEO_FPS_STANDARD,
+    NETWORK_5G_ENERGY_PER_MB_MJ, ENERGY_MOBILENET_INF_MJ,
+    MB, GB
+)
+from calc.formulas import (
+    calc_network_latency_ms,
+    calc_monthly_egress_cost,
+    calc_fleet_tco,
+    calc_bottleneck
+)
+
+def test_distance_penalty_precludes_cloud():
+    """
+    Narrative: "Physics has made Cloud ML impossible for this application."
+    Invariant: Round-trip time for 1500km must exceed the 10ms safety budget.
+    """
+    distance_km = 1500
+    safety_budget_ms = 10
+    
+    rtt_ms = calc_network_latency_ms(distance_km)
+    
+    assert rtt_ms > safety_budget_ms, \
+        f"Narrative Violation: Cloud latency ({rtt_ms:.1f}ms) is now faster than safety budget ({safety_budget_ms}ms)!"
+
+def test_bandwidth_makes_cloud_streaming_impossible():
+    """
+    Narrative: "You need 15x more bandwidth than exists."
+    Invariant: 100 cameras require significantly more bandwidth than a 10Gbps link.
+    """
+    num_cameras = 100
+    raw_bytes_per_sec = (VIDEO_1080P_WIDTH * VIDEO_1080P_HEIGHT * VIDEO_BYTES_PER_PIXEL_RGB * VIDEO_FPS_STANDARD)
+    total_bandwidth = num_cameras * raw_bytes_per_sec
+    
+    # Check that we exceed 10Gbps link
+    assert total_bandwidth > NETWORK_10G_BW, \
+        "Narrative Violation: 10Gbps link is now sufficient for 100 raw streams!"
+        
+    # Check the "15x" claim (allow some drift, but it should be >10x)
+    ratio = total_bandwidth / NETWORK_10G_BW
+    assert ratio > 10, \
+        f"Narrative Violation: Bandwidth shortage dropped to {ratio:.1f}x (text implies ~15x)"
+
+def test_energy_gap_favors_edge():
+    """
+    Narrative: "Transmitting data is 1000x more energy-expensive than processing it locally."
+    Invariant: Transmission energy >> Inference energy.
+    """
+    # Energy for 1MB
+    data_size = 1 * MB
+    tx_energy = NETWORK_5G_ENERGY_PER_MB_MJ * data_size
+    npu_energy = ENERGY_MOBILENET_INF_MJ
+    
+    ratio = tx_energy / npu_energy
+    
+    assert ratio > 500, \
+        f"Narrative Violation: Energy gap dropped to {ratio:.1f}x (text says 1000x)!"
+
+def test_edge_tco_cheaper_than_cloud():
+    """
+    Narrative: "Coral Dev Board ... meets requirements at 1/4 the cost of Jetson... vs cloud inference at ~$500K"
+    Invariant: Edge TCO < Cloud TCO for high-volume scenario.
+    """
+    stores = 500
+    cameras = 20
+    fps = 15
+    years = 3
+    
+    # Cloud Cost Reference (from text approximation)
+    # 1M inferences/day was $36k/year.
+    # Here we have 500 stores * 20 cams * 15 fps = 150,000 inf/sec
+    # This volume is massive. Just check Edge vs Edge relative costs first.
+    
+    coral_cost = 150 * ureg.dollar
+    coral_power = 2 * ureg.watt
+    
+    jetson_cost = 600 * ureg.dollar
+    jetson_power = 25 * ureg.watt
+    
+    coral_tco = calc_fleet_tco(coral_cost, coral_power, stores, years, CLOUD_ELECTRICITY_PER_KWH)
+    jetson_tco = calc_fleet_tco(jetson_cost, jetson_power, stores, years, CLOUD_ELECTRICITY_PER_KWH)
+    
+    assert coral_tco < jetson_tco, "Narrative Violation: Coral is no longer cheaper than Jetson!"
+    
+    ratio = jetson_tco / coral_tco
+    assert ratio > 2, f"Narrative Violation: Jetson/Coral cost ratio is {ratio:.1f}x (text implies ~3-4x)"
+
+def test_battery_tax_depletes_phone():
+    """
+    Narrative: "Your single feature has just consumed 30% [or 100%] of the budget."
+    Invariant: A 2W continuous load drains a 15Wh battery in < 1 day.
+    """
+    runtime = PHONE_BATTERY_WH / OBJECT_DETECTOR_POWER_W
+    day = 24 * ureg.hour
+    
+    assert runtime < day, "Narrative Violation: Battery now lasts more than a day with continuous ML!"
+    assert runtime < (8 * ureg.hour), "Narrative Violation: Battery lasts too long (>8h) for the 'few hours' narrative."
diff --git a/book/tools/dependencies/requirements.txt b/book/tools/dependencies/requirements.txt
index 3d32b7a18..9088dd0e2 100644
--- a/book/tools/dependencies/requirements.txt
+++ b/book/tools/dependencies/requirements.txt
@@ -34,3 +34,4 @@ black>=23.0.0
 #faiss-cpu>=1.7.4
 #scikit-learn>=1.3.2
 #torch>=2.0.0
+pint>=0.23