diff --git a/src/lib/nightscout/__tests__/client.test.ts b/src/lib/nightscout/__tests__/client.test.ts
index b48d95bce..2fea3bc27 100644
--- a/src/lib/nightscout/__tests__/client.test.ts
+++ b/src/lib/nightscout/__tests__/client.test.ts
@@ -73,8 +73,12 @@ describe("parseSgvEntries", () => {
 });
 
 describe("mapSgvEntryToMeasurement", () => {
-  it("maps an SGV entry to a BLOOD_GLUCOSE mg/dL measurement", () => {
-    const m = mapSgvEntryToMeasurement(SAMPLE[0]!);
+  it("maps a parsed SGV entry to a BLOOD_GLUCOSE mg/dL measurement", () => {
+    const m = mapSgvEntryToMeasurement({
+      id: "abc123",
+      sgv: 112,
+      date: 1718000000000,
+    });
     expect(m.type).toBe("BLOOD_GLUCOSE");
     expect(m.unit).toBe("mg/dL");
     expect(m.value).toBe(112);
@@ -82,15 +86,16 @@ describe("mapSgvEntryToMeasurement", () => {
   });
 
   it("derives a stable externalId so re-sync is idempotent", () => {
-    const a = mapSgvEntryToMeasurement(SAMPLE[0]!);
-    const b = mapSgvEntryToMeasurement(SAMPLE[0]!);
+    const entry = { id: "abc123", sgv: 112, date: 1718000000000 };
+    const a = mapSgvEntryToMeasurement(entry);
+    const b = mapSgvEntryToMeasurement(entry);
     expect(a.externalId).toBe(b.externalId);
-    expect(a.externalId).toContain("abc123");
+    expect(a.externalId).toBe("ns:abc123");
   });
 
-  it("falls back to the timestamp when the entry has no _id", () => {
+  it("falls back to the timestamp when the entry has no id", () => {
     const m = mapSgvEntryToMeasurement({
-      type: "sgv",
+      id: null,
       sgv: 90,
       date: 1718000000000,
     });
diff --git a/src/lib/nightscout/client.ts b/src/lib/nightscout/client.ts
index daf2ab22c..e40f8b785 100644
--- a/src/lib/nightscout/client.ts
+++ b/src/lib/nightscout/client.ts
@@ -135,34 +135,33 @@ export function parseSgvEntries(payload: unknown): ParsedSgvEntry[] {
   return out;
 }
 
-/**
- * Map one SGV entry to a `BLOOD_GLUCOSE` mg/dL measurement. The `externalId`
- * is `ns:<id>` when the entry carries Nightscout's `_id`, falling back to
- * `ns:date:<epochMs>` — both stable per reading, so a re-sync collapses onto
- * the existing row via the `(userId, type, source, externalId)` unique. mg/dL
- * is stored canonical (Nightscout always reports mg/dL; the display unit is a
- * user preference resolved elsewhere).
- */
-export function mapSgvEntryToMeasurement(
-  entry: NightscoutSgvEntry,
-): NightscoutMeasurement {
-  const externalId = entry._id
-    ? `ns:${entry._id}`
-    : `ns:date:${entry.date ?? 0}`;
-  return {
-    type: "BLOOD_GLUCOSE",
-    value: entry.sgv as number,
-    unit: NIGHTSCOUT_GLUCOSE_UNIT,
-    measuredAt: new Date(entry.date as number),
-    externalId,
-  };
-}
-
 /** Externalid for a parsed entry — the canonical idempotency key. */
 export function externalIdFor(entry: ParsedSgvEntry): string {
   return entry.id ? `ns:${entry.id}` : `ns:date:${entry.date}`;
 }
 
+/**
+ * Map one parsed SGV entry to a `BLOOD_GLUCOSE` mg/dL measurement. The
+ * `externalId` is `ns:<_id>` when the entry carries Nightscout's `_id`, falling
+ * back to `ns:date:<epochMs>` (via `externalIdFor`) — both stable per reading,
+ * so a re-sync collapses onto the existing row via the
+ * `(userId, type, source, externalId)` unique. mg/dL is stored canonical
+ * (Nightscout always reports mg/dL; the display unit is a user preference
+ * resolved elsewhere). This is the SINGLE entry→measurement mapping — the sync
+ * write path consumes it, so the shape can never drift between two copies.
+ */
+export function mapSgvEntryToMeasurement(
+  entry: ParsedSgvEntry,
+): NightscoutMeasurement {
+  return {
+    type: "BLOOD_GLUCOSE",
+    value: entry.sgv,
+    unit: NIGHTSCOUT_GLUCOSE_UNIT,
+    measuredAt: new Date(entry.date),
+    externalId: externalIdFor(entry),
+  };
+}
+
 /**
  * Raised when an instance answers non-2xx or is unreachable. The connect
  * route + sync classifier switch on `status` to tell "wrong token" (401/403)
diff --git a/src/lib/nightscout/sync.ts b/src/lib/nightscout/sync.ts
index 7d0c438db..df29b9989 100644
--- a/src/lib/nightscout/sync.ts
+++ b/src/lib/nightscout/sync.ts
@@ -33,10 +33,9 @@ import {
 } from "@/lib/rollups/measurement-rollups";
 import { invalidateStatusInsightsForTypes } from "@/lib/insights/comprehensive-generate";
 import {
-  externalIdFor,
   fetchSgvEntries,
+  mapSgvEntryToMeasurement,
   NightscoutApiError,
-  NIGHTSCOUT_GLUCOSE_UNIT,
   type ParsedSgvEntry,
 } from "./client";
 import { getUserNightscoutCredentials } from "./credentials";
@@ -119,8 +118,7 @@ export async function upsertNightscoutEntries(
   const touched: Array<{ type: MeasurementType; measuredAt: Date }> = [];
 
   for (const entry of entries) {
-    const externalId = externalIdFor(entry);
-    const measuredAt = new Date(entry.date);
+    const mapped = mapSgvEntryToMeasurement(entry);
     try {
       await prisma.measurement.upsert({
         where: {
@@ -128,23 +126,23 @@ export async function upsertNightscoutEntries(
             userId,
             type,
             source: "NIGHTSCOUT",
-            externalId,
+            externalId: mapped.externalId,
           },
         },
         create: {
           userId,
           type,
           source: "NIGHTSCOUT",
-          value: entry.sgv,
-          unit: NIGHTSCOUT_GLUCOSE_UNIT,
-          measuredAt,
-          externalId,
+          value: mapped.value,
+          unit: mapped.unit,
+          measuredAt: mapped.measuredAt,
+          externalId: mapped.externalId,
         },
         // First-write-wins: a CGM sample is immutable, so a re-sync of the
         // same reading must not rewrite the stored value or measuredAt.
         update: {},
       });
-      touched.push({ type, measuredAt });
+      touched.push({ type, measuredAt: mapped.measuredAt });
       imported++;
     } catch (err) {
       getEvent()?.addWarning(