diff --git a/src/lib/medications/inventory/__tests__/summary.test.ts b/src/lib/medications/inventory/__tests__/summary.test.ts
index 965c4b417..c0d92b7f7 100644
--- a/src/lib/medications/inventory/__tests__/summary.test.ts
+++ b/src/lib/medications/inventory/__tests__/summary.test.ts
@@ -10,8 +10,13 @@
  */
 import { readFileSync } from "node:fs";
 import { join } from "node:path";
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 
+vi.mock("@/lib/logging/context", () => ({
+  annotate: vi.fn(),
+}));
+
+import { annotate } from "@/lib/logging/context";
 import {
   isAvailableSupply,
   summariseSupply,
@@ -85,6 +90,68 @@ describe("summariseSupply — expired stock is visible but never available", ()
   });
 });
 
+describe("summariseSupply — central negative-stock sanity gate (#31)", () => {
+  afterEach(() => {
+    vi.mocked(annotate).mockClear();
+  });
+
+  it("RCA: an available row whose capacity is NaN (corrupt Decimal) would surface a NaN headline pre-fix — now clamped to 0 + annotated", () => {
+    // The per-row availability predicate gates only on `unitsRemaining >
+    // 0`, so a row with a real remaining but a corrupt / legacy
+    // `unitsTotal` (a Decimal that deserialised to NaN) DOES pool in. The
+    // pre-fix `reduce` then produced a NaN `unitsTotal` / `dosesTotal`
+    // headline — the nonsensical Bestand the report described. The gate
+    // floors it to 0 and fires the underflow event.
+    const out = summariseSupply(
+      [{ state: "IN_USE", unitsTotal: Number.NaN, unitsRemaining: 3 }],
+      1,
+    );
+    expect(out.unitsRemaining).toBe(3);
+    expect(out.unitsTotal).toBe(0);
+    expect(out.dosesTotal).toBe(0);
+    expect(annotate).toHaveBeenCalledWith({
+      action: { name: "medication.inventory.underflow" },
+      meta: {
+        raw_units_remaining: 3,
+        raw_units_total: null,
+        clamped_units_remaining: 3,
+        available_count: 1,
+      },
+    });
+  });
+
+  it("sanity gate: a corrupt remaining (NaN) on an EXPIRED-suffix path floors the expired figure too", () => {
+    // The expired suffix is also pooled; a corrupt EXPIRED row must not
+    // surface a NaN suffix. EXPIRED never reaches the available pool, so
+    // no underflow event fires for it — but the expired figure floors.
+    const out = summariseSupply(
+      [{ state: "EXPIRED", unitsTotal: 4, unitsRemaining: Number.NaN }],
+      1,
+    );
+    expect(out.expiredUnits).toBe(0);
+    expect(out.unitsRemaining).toBe(0);
+    expect(annotate).not.toHaveBeenCalled();
+  });
+
+  it("fix: a healthy mixed inventory yields the correct pooled figure and never annotates", () => {
+    const out = summariseSupply(
+      [
+        { state: "IN_USE", unitsTotal: 4, unitsRemaining: 3 },
+        { state: "ACTIVE", unitsTotal: 4, unitsRemaining: 4 },
+      ],
+      2,
+    );
+    expect(out).toEqual({
+      unitsRemaining: 7,
+      unitsTotal: 8,
+      dosesRemaining: 3,
+      dosesTotal: 4,
+      expiredUnits: 0,
+    });
+    expect(annotate).not.toHaveBeenCalled();
+  });
+});
+
 describe("supply surfaces — both detail-page readouts ride the shared helper (source guard)", () => {
   const read = (rel: string) => readFileSync(join(process.cwd(), rel), "utf8");
 
diff --git a/src/lib/medications/inventory/summary.ts b/src/lib/medications/inventory/summary.ts
index 18017e33f..a55e60f0f 100644
--- a/src/lib/medications/inventory/summary.ts
+++ b/src/lib/medications/inventory/summary.ts
@@ -12,8 +12,24 @@
  * The Übersicht supply row and the Bestand summary both render from
  * this helper, so the surfaces cannot disagree about what "remaining"
  * means.
+ *
+ * v1.18.11 (#31) — this is also the single canonical sanity gate for
+ * the surfaced stock. A self-hoster reported the headline Bestand going
+ * nonsensically NEGATIVE. The container write paths floor at zero
+ * structurally (consumption never over-decrements, the stock-correction
+ * route clamps at `.min(0)` and re-runs the state machine), and the
+ * legacy running-sum ledger reads are clamped with `Math.max(0, …)` at
+ * their two call sites — but nothing guaranteed the POOLED figure this
+ * helper returns could not go negative from a single corrupt / legacy
+ * row carrying a negative `unitsRemaining` (or `unitsTotal`). Rather
+ * than leave each surface to re-clamp, the floor lives HERE, at the one
+ * point every readout flows through, and a raw-negative pool emits a
+ * `medication.inventory.underflow` wide-event so the underlying bug is
+ * observable if it ever recurs.
  */
 
+import { annotate } from "@/lib/logging/context";
+
 export type SupplyItemState = "ACTIVE" | "IN_USE" | "EXPIRED" | "USED_UP";
 
 export interface SupplyItem {
@@ -51,14 +67,47 @@ export function summariseSupply(
   // per dose) must stay fractional, else the dose-derived counts halve.
   const perDose = unitsPerDose > 0 ? unitsPerDose : 1;
   const available = items.filter(isAvailableSupply);
-  const unitsRemaining = available.reduce(
+  const rawUnitsRemaining = available.reduce(
     (sum, item) => sum + item.unitsRemaining,
     0,
   );
-  const unitsTotal = available.reduce((sum, item) => sum + item.unitsTotal, 0);
-  const expiredUnits = items
-    .filter((item) => item.state === "EXPIRED")
-    .reduce((sum, item) => sum + item.unitsRemaining, 0);
+  const rawUnitsTotal = available.reduce(
+    (sum, item) => sum + item.unitsTotal,
+    0,
+  );
+  const expiredUnits = clampNonNegative(
+    items
+      .filter((item) => item.state === "EXPIRED")
+      .reduce((sum, item) => sum + item.unitsRemaining, 0),
+  );
+
+  // v1.18.11 (#31) — central sanity gate. A NaN / negative pool can only
+  // come from a corrupt or legacy row that slipped past the per-row
+  // availability predicate; never surface it. Clamp to zero (the dose
+  // ran out) and emit an underflow wide-event so the data defect stays
+  // observable — `annotate` is a no-op outside a request context, so the
+  // pure helper stays safe to call from a server component.
+  const unitsRemaining = clampNonNegative(rawUnitsRemaining);
+  const unitsTotal = clampNonNegative(rawUnitsTotal);
+  if (
+    !Number.isFinite(rawUnitsRemaining) ||
+    rawUnitsRemaining < 0 ||
+    !Number.isFinite(rawUnitsTotal) ||
+    rawUnitsTotal < 0
+  ) {
+    annotate({
+      action: { name: "medication.inventory.underflow" },
+      meta: {
+        raw_units_remaining: Number.isFinite(rawUnitsRemaining)
+          ? rawUnitsRemaining
+          : null,
+        raw_units_total: Number.isFinite(rawUnitsTotal) ? rawUnitsTotal : null,
+        clamped_units_remaining: unitsRemaining,
+        available_count: available.length,
+      },
+    });
+  }
+
   return {
     unitsRemaining,
     unitsTotal,
@@ -67,3 +116,9 @@ export function summariseSupply(
     expiredUnits,
   };
 }
+
+/** Floor a pooled figure at zero, treating a non-finite value as zero
+ *  too — a corrupt row must never surface as `NaN` or a negative. */
+function clampNonNegative(value: number): number {
+  return Number.isFinite(value) && value > 0 ? value : 0;
+}