diff --git a/.coderabbit.yaml b/.coderabbit.yaml
index 770ba8d19..849ea7d60 100644
--- a/.coderabbit.yaml
+++ b/.coderabbit.yaml
@@ -25,3 +25,5 @@ reviews:
     # and they are the least likely place for a shipped bug. Re-include them
     # for a PR that is mostly test work.
     - "!test/**"
+  auto_review:
+    drafts: true
\ No newline at end of file
diff --git a/lib/ble/ble_engine.dart b/lib/ble/ble_engine.dart
index 3d7f07476..a14f61d17 100644
--- a/lib/ble/ble_engine.dart
+++ b/lib/ble/ble_engine.dart
@@ -307,6 +307,10 @@ Sample? sampleFromGen5Historical(Gen5HistoricalRecord? g) {
     // enum: a stored name freezes a meaning, a stored 2-bit code does not.
     // Corroboration only, never a stage — see Sample.bandSleepState.
     bandSleepState: g.sleepStateRawNibble,
+    // Raw candidate byte, never a percentage — the encoding is not pinned and
+    // no ground truth exists, so this is carried for diagnostics only and is
+    // not persisted (see Sample.spo2CandidateRaw).
+    spo2CandidateRaw: g.spo2CandidateRaw,
   );
 }
 
@@ -5778,6 +5782,12 @@ class BleEngine {
       // about a vessel: beat-detection quality, motion-artifact rejection,
       // corroborating the band's own RR intervals. build those, not a number.
       sample = decodeGen5HistoricalSample(frame.inner);
+      // The gen5 SpO2 candidate byte is never written to _log — FileLog lives in
+      // Android external storage, so a timestamped candidate/sleep line there
+      // would be readable by other apps (CWE-532). It is not persisted as a
+      // decoded column either; only whole UNDECODABLE frames reach raw_archive,
+      // which can contain the byte (kept for the commit-before-ACK invariant).
+      // In-memory carrier: Sample.spo2CandidateRaw — raw, never a percentage.
     } else if (recType != Record.r25 &&
         kKnownRecordVersions.contains(recType)) {
       // Every gen4 layout version protocol has a field map for — not just
diff --git a/lib/data/models.dart b/lib/data/models.dart
index 95f7f1a46..6b127b382 100644
--- a/lib/data/models.dart
+++ b/lib/data/models.dart
@@ -152,6 +152,22 @@ class Sample {
   /// (analytics `band_offset.dart`) — never to create, extend or stage one.
   final int? bandSleepState;
 
+  /// The gen5 v18 SpO2 candidate byte (protocol `spo2CandidateRaw`,
+  /// inner[74]) — GEN5/MG ONLY, raw and unvalidated.
+  ///
+  /// Zero in ~99% of records, nonzero only during band-declared sleep
+  /// (community-observed bursts of 30 s every 1,200 s), encoding not fully
+  /// pinned (values above 128 decompose as 128 + flag). Carried as the raw
+  /// byte for capture correlation; NEVER displayed or stored as a
+  /// percentage — see `Spo2Data` in `models/payloads.dart` for the refusal.
+  /// Not persisted (no column); in-memory diagnostic only, never logged
+  /// (FileLog is external storage — CWE-532). After the validation gate in
+  /// docs/proposals/whoop-spo2.md passes, this byte is the raw input that
+  /// feeds `Spo2Data` — the single publication path. The validation gate this
+  /// refers to lives in docs/proposals/whoop-spo2.md, added by PR #17
+  /// (upstream OpenStrap/edge#675) — not part of this branch.
+  final int? spo2CandidateRaw;
+
   Sample({
     required this.tsEpoch,
     required this.counter,
@@ -177,6 +193,7 @@ class Sample {
     this.dynAccelG,
     this.tsSubsec,
     this.bandSleepState,
+    this.spo2CandidateRaw,
   });
 
   /// Copy with an overridden [tsEpoch] — used by the clock-offset salvage path
@@ -208,6 +225,7 @@ class Sample {
     dynAccelG: dynAccelG,
     tsSubsec: tsSubsec,
     bandSleepState: bandSleepState,
+    spo2CandidateRaw: spo2CandidateRaw,
   );
 
   bool get hasDecodedOneHz =>
diff --git a/test/gen5_spo2_candidate_test.dart b/test/gen5_spo2_candidate_test.dart
new file mode 100644
index 000000000..4f7311017
--- /dev/null
+++ b/test/gen5_spo2_candidate_test.dart
@@ -0,0 +1,68 @@
+// The gen5 v18 SpO2 candidate byte (protocol `spo2CandidateRaw`, inner[74])
+// must be carried through `decodeGen5HistoricalSample` as a RAW byte — never
+// gated, never converted to a percentage. The byte is unvalidated community
+// knowledge (nonzero only in band-declared sleep bursts), so the contract
+// under test is passthrough + null on gen4-shaped input, not a value claim.
+
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:openstrap_edge/ble/ble_engine.dart';
+import 'package:openstrap_edge/data/models.dart';
+import 'package:openstrap_edge/models/payloads.dart';
+import 'package:openstrap_protocol/openstrap_protocol.dart';
+
+/// A minimal decodable v18 inner: length 112, version byte 18 at inner[1].
+Uint8List v18Inner({int spo2Candidate = 0}) {
+  final b = Uint8List(kGen5V18InnerLen);
+  b[1] = 18;
+  final v = ByteData.sublistView(b);
+  v.setUint32(7, 1780916150, Endian.little); // unix
+  b[73] = 2; // sleepStateByte: sleep
+  b[74] = spo2Candidate;
+  return b;
+}
+
+void main() {
+  test('gen5 v18 spo2CandidateRaw is carried through as the raw byte', () {
+    final s = decodeGen5HistoricalSample(v18Inner(spo2Candidate: 97));
+    expect(s, isNotNull);
+    expect(s!.spo2CandidateRaw, 97);
+    expect(s.bandSleepState, isNotNull);
+  });
+
+  test('zero candidate stays zero — absent measurement, not a gated null', () {
+    final s = decodeGen5HistoricalSample(v18Inner(spo2Candidate: 0));
+    expect(s!.spo2CandidateRaw, 0);
+  });
+
+  test('flag-style values above 128 are passed through unmasked', () {
+    final s = decodeGen5HistoricalSample(v18Inner(spo2Candidate: 128 + 97));
+    expect(s!.spo2CandidateRaw, 128 + 97);
+  });
+
+  // copyWith must carry the candidate along — a regression that drops
+  // spo2CandidateRaw from copyWith would silently lose it on every copy.
+  // (copyWith only overrides tsEpoch; the field must pass through unchanged.)
+  test('copyWith preserves a nonzero spo2CandidateRaw', () {
+    final s = decodeGen5HistoricalSample(v18Inner(spo2Candidate: 97))!;
+    expect(s.copyWith(tsEpoch: s.tsEpoch + 1).spo2CandidateRaw, 97);
+  });
+
+  test('copyWith preserves an absent (null) spo2CandidateRaw', () {
+    // A gen4-shaped sample carries no candidate byte at all.
+    final s = Sample(tsEpoch: 1000, counter: 0, hr: 60);
+    expect(s.spo2CandidateRaw, isNull);
+    expect(s.copyWith(tsEpoch: 1001).spo2CandidateRaw, isNull);
+  });
+
+  // The seam to PR #472: a captured candidate byte must NOT unlock the
+  // publication path. `Spo2Data` (payloads.dart) stays unread until the
+  // validation gate passes — capturing is not displaying.
+  test('captured candidate stays unread: Spo2Data refusal is intact', () {
+    final s = decodeGen5HistoricalSample(v18Inner(spo2Candidate: 97))!;
+    expect(s.spo2CandidateRaw, 97);
+    final payload = TodayData.fromJson({'spo2': const <String, dynamic>{}});
+    expect(payload.spo2, isNull, reason: 'no odi_per_hour -> Spo2Data unread');
+  });
+}