diff --git a/Strand/BLE/BLEManager.swift b/Strand/BLE/BLEManager.swift
index 0feffde48b..02d9042979 100644
--- a/Strand/BLE/BLEManager.swift
+++ b/Strand/BLE/BLEManager.swift
@@ -6904,7 +6904,8 @@ extension BLEManager: @preconcurrency CBPeripheralDelegate {
             return
         }
 
-        if !didBond {
+        switch Whoop4BondAck.classify(didBond: didBond, encryptedBond: state.encryptedBond) {
+        case .firstBond:
             didBond = true
             state.bonded = true
             state.encryptedBond = true   // WHOOP 4 confirmed-write bond is always genuine — #69
@@ -6912,6 +6913,15 @@ extension BLEManager: @preconcurrency CBPeripheralDelegate {
             noteGenuineBond(of: peripheral)   // #52: this strap bonds fine; clears any pin-refusal streak
             emitConnectionBondState("encryptedBond family=whoop4 (confirmed write acked)")
             log("BONDED (confirmed write acknowledged) — custom channels should now flow")
+        case .reprove:
+            // didConnect re-entered with no disconnect before it (a Bluetooth power cycle) and cleared the
+            // flag; this ack is the same proof the first bond recorded. See `Whoop4BondAck`.
+            state.encryptedBond = true
+            noteGenuineBond(of: peripheral)
+            emitConnectionBondState("encryptedBond family=whoop4 (confirmed write acked, re-entered link)")
+            log("BONDED again (confirmed write acknowledged on a re-entered link)")
+        case .alreadyProven:
+            break
         }
         // Run the connect handshake EXACTLY ONCE per connection. didWriteValueFor re-fires on EVERY
         // .withResponse write — the bond write, every SEND_HISTORICAL, every HISTORY_END ack. Without
diff --git a/Strand/BLE/Whoop4BondAck.swift b/Strand/BLE/Whoop4BondAck.swift
new file mode 100644
index 0000000000..49593bb9d8
--- /dev/null
+++ b/Strand/BLE/Whoop4BondAck.swift
@@ -0,0 +1,26 @@
+import Foundation
+
+/// What a WHOOP 4.0 confirmed-write acknowledgement records on the current link.
+///
+/// The ack is the per-connection proof of the encrypted bond (#69): `didConnect` clears `encryptedBond` so
+/// each link earns it again at this site. `didBond` is cleared only by the disconnect callback, and
+/// CoreBluetooth sends no disconnect when the Mac's Bluetooth radio powers off. On the next `poweredOn` the
+/// app re-attaches to the strap macOS still holds, so `didConnect` arrives with `didBond` still true.
+/// Recording the bond only when `didBond` was false then never re-proved it: the strap read "Live HR (not
+/// fully paired)" for the rest of the link while history sync, which `didBond` gates, kept working, and
+/// buzz, which `encryptedBond` gates, was refused.
+///
+/// Pure so the rule is unit-tested without a radio.
+enum Whoop4BondAck: Equatable {
+    /// The first bond on this link: record it and run the post-bond bookkeeping.
+    case firstBond
+    /// Already bonded, but a re-entered `didConnect` cleared the encrypted flag: this ack restores it.
+    case reprove
+    /// Already bonded and already proven on this link: nothing to record.
+    case alreadyProven
+
+    static func classify(didBond: Bool, encryptedBond: Bool) -> Whoop4BondAck {
+        if !didBond { return .firstBond }
+        return encryptedBond ? .alreadyProven : .reprove
+    }
+}
diff --git a/StrandTests/Whoop4BondAckTests.swift b/StrandTests/Whoop4BondAckTests.swift
new file mode 100644
index 0000000000..d2f21572d2
--- /dev/null
+++ b/StrandTests/Whoop4BondAckTests.swift
@@ -0,0 +1,27 @@
+import XCTest
+@testable import Strand
+
+/// Pins what a WHOOP 4.0 confirmed-write ack records. The case that motivated it: the Mac's Bluetooth radio
+/// powers off and back on, CoreBluetooth sends no disconnect, the app re-attaches, and `didConnect` clears
+/// `encryptedBond` while `didBond` survives. The next ack must restore the encrypted bond rather than skip it,
+/// or the strap reads "Live HR (not fully paired)" and buzz is refused for the rest of the link.
+final class Whoop4BondAckTests: XCTestCase {
+
+    func testFirstAckOnALinkIsTheFirstBond() {
+        XCTAssertEqual(Whoop4BondAck.classify(didBond: false, encryptedBond: false), .firstBond)
+    }
+
+    func testAckAfterAReenteredConnectReprovesTheEncryptedBond() {
+        XCTAssertEqual(Whoop4BondAck.classify(didBond: true, encryptedBond: false), .reprove)
+    }
+
+    func testLaterAcksOnAProvenLinkRecordNothing() {
+        // Every SEND_HISTORICAL / HISTORY_END ack lands here during an offload; none may re-run bond bookkeeping.
+        XCTAssertEqual(Whoop4BondAck.classify(didBond: true, encryptedBond: true), .alreadyProven)
+    }
+
+    func testAStaleEncryptedFlagWithoutABondIsStillTheFirstBond() {
+        // didBond is the link-level fact; a first ack always runs the full first-bond path.
+        XCTAssertEqual(Whoop4BondAck.classify(didBond: false, encryptedBond: true), .firstBond)
+    }
+}