diff --git a/src/lib/fhir/__tests__/build-bundle.test.ts b/src/lib/fhir/__tests__/build-bundle.test.ts
index 98378b987..985b7a263 100644
--- a/src/lib/fhir/__tests__/build-bundle.test.ts
+++ b/src/lib/fhir/__tests__/build-bundle.test.ts
@@ -398,6 +398,44 @@ describe("buildFhirDocumentBundle", () => {
     expect(flights?.valueQuantity?.code).toBe("{flights}");
   });
 
+  it("routes HealthKit placeholder codes onto the custom CodeSystem, never LOINC", () => {
+    const bundle = buildFhirDocumentBundle(
+      makeData({
+        measurements: {
+          WEIGHT: [{ value: 80, measuredAt: "2026-04-30T08:00:00.000Z" }],
+          FLIGHTS_CLIMBED: [
+            { value: 12, measuredAt: "2026-04-30T08:00:00.000Z" },
+          ],
+        },
+      }),
+      { insuranceNumber: null },
+      FIXED_NOW,
+    );
+    const codings = observationsOf(bundle).flatMap((o) => o.code.coding ?? []);
+
+    // A real LOINC metric keeps the LOINC namespace.
+    const weight = codings.find((c) => c.code === "29463-7");
+    expect(weight?.system).toBe("http://loinc.org");
+
+    // The HealthKit placeholder moves to the shared custom CodeSystem with the
+    // raw HK identifier as the code (byte-aligned with the iOS exporter).
+    const flightsCoding = codings.find(
+      (c) => c.code === "HKQuantityTypeIdentifierFlightsClimbed",
+    );
+    expect(flightsCoding?.system).toBe(
+      "https://healthlog.dev/fhir/CodeSystem/healthkit",
+    );
+
+    // Conformance invariant: no HealthKit identifier is ever emitted under the
+    // LOINC namespace anywhere in the bundle.
+    const hkUnderLoinc = codings.filter(
+      (c) =>
+        c.system === "http://loinc.org" &&
+        c.code?.startsWith("HKQuantityTypeIdentifier"),
+    );
+    expect(hkUnderLoinc).toHaveLength(0);
+  });
+
   it("discriminates glucose LOINC by context (random/fasting/afterMeal)", () => {
     const bundle = buildFhirDocumentBundle(
       makeData({
diff --git a/src/lib/fhir/build-bundle.ts b/src/lib/fhir/build-bundle.ts
index 229e5d827..f816843de 100644
--- a/src/lib/fhir/build-bundle.ts
+++ b/src/lib/fhir/build-bundle.ts
@@ -21,6 +21,7 @@ import type { DoctorReportData } from "@/lib/doctor-report-data";
 import { resolveGlucoseUnit, convertGlucose } from "@/lib/glucose";
 import {
   LOINC_SYSTEM,
+  HEALTHKIT_CODESYSTEM,
   UCUM_SYSTEM,
   MEASUREMENT_LOINC,
   BP_PANEL_LOINC,
@@ -66,8 +67,15 @@ function escapeXml(value: string): string {
 
 function codeableFromMapping(m: LoincMapping): FhirCodeableConcept {
   if (m.loinc) {
+    // HealthKit placeholder codes have no published LOINC term; they must not
+    // sit under the LOINC namespace (a non-LOINC code there is a conformance
+    // violation). Route them onto the shared custom CodeSystem instead —
+    // byte-aligned with the iOS exporter.
+    const system = m.loinc.startsWith("HKQuantityTypeIdentifier")
+      ? HEALTHKIT_CODESYSTEM
+      : LOINC_SYSTEM;
     return {
-      coding: [{ system: LOINC_SYSTEM, code: m.loinc, display: m.display }],
+      coding: [{ system, code: m.loinc, display: m.display }],
       text: m.display,
     };
   }
diff --git a/src/lib/fhir/loinc-map.ts b/src/lib/fhir/loinc-map.ts
index 2cbae62df..5e662a244 100644
--- a/src/lib/fhir/loinc-map.ts
+++ b/src/lib/fhir/loinc-map.ts
@@ -12,6 +12,15 @@
 
 export const LOINC_SYSTEM = "http://loinc.org";
 export const UCUM_SYSTEM = "http://unitsofmeasure.org";
+/**
+ * Shared custom CodeSystem for HealthKit placeholder metrics that have no
+ * published LOINC term. A non-LOINC code under `http://loinc.org` is a FHIR
+ * conformance violation, so these route here instead. Byte-aligned with the
+ * iOS exporter (confirmed 2026-06-01) — both clients emit the identical
+ * `system` + raw `HKQuantityTypeIdentifier…` `code`.
+ */
+export const HEALTHKIT_CODESYSTEM =
+  "https://healthlog.dev/fhir/CodeSystem/healthkit";
 
 export type FhirObservationCategory =
   | "vital-signs"
@@ -145,10 +154,12 @@ export const MEASUREMENT_LOINC: Record<string, LoincMapping> = {
     unit: "kg",
     category: "vital-signs",
   },
-  // HK-placeholder codes — no published LOINC term. iOS emits the HealthKit
-  // identifier STRING as the `code` (in the LOINC system slot); the server
-  // must emit the identical placeholder, not invent a LOINC. The `loinc`
-  // field below carries that placeholder string verbatim.
+  // HK-placeholder codes — no published LOINC term. Both iOS and the server
+  // emit the HealthKit identifier STRING as the `code` under the shared
+  // `HEALTHKIT_CODESYSTEM` (NOT the LOINC namespace — a non-LOINC code under
+  // http://loinc.org fails FHIR conformance). The `loinc` field below carries
+  // the placeholder string verbatim; the builder routes any
+  // `HKQuantityTypeIdentifier…` code onto the custom system.
   WALKING_DOUBLE_SUPPORT: {
     loinc: "HKQuantityTypeIdentifierWalkingDoubleSupportPercentage",
     display: "Walking double support percentage",