diff --git a/packages/SwingSet/src/kernel/state/kernelKeeper.js b/packages/SwingSet/src/kernel/state/kernelKeeper.js
index c21a7a56de32..2405429c0078 100644
--- a/packages/SwingSet/src/kernel/state/kernelKeeper.js
+++ b/packages/SwingSet/src/kernel/state/kernelKeeper.js
@@ -2,6 +2,7 @@ import { Nat } from '@agoric/nat';
 import { assert, details as X } from '@agoric/assert';
 import { initializeVatState, makeVatKeeper } from './vatKeeper';
 import { initializeDeviceState, makeDeviceKeeper } from './deviceKeeper';
+import { parseReachableAndVatSlot } from './reachable';
 import { insistEnhancedStorageAPI } from '../../storageAPI';
 import {
   insistKernelType,
@@ -248,6 +249,11 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
     setGCActions(actions);
   }
 
+  function getReachableAndVatSlot(vatID, kernelSlot) {
+    const kernelKey = `${vatID}.c.${kernelSlot}`;
+    return parseReachableAndVatSlot(kvStore.get(kernelKey));
+  }
+
   function addKernelObject(ownerID) {
     insistVatID(ownerID);
     const id = Nat(BigInt(getRequired('ko.nextID')));
@@ -707,6 +713,7 @@ export default function makeKernelKeeper(kvStore, streamStore, kernelSlog) {
       addKernelPromiseForVat,
       incrementRefCount,
       decrementRefCount,
+      getReachableAndVatSlot,
       incStat,
       decStat,
       getCrankNumber,
diff --git a/packages/SwingSet/src/kernel/state/reachable.js b/packages/SwingSet/src/kernel/state/reachable.js
new file mode 100644
index 000000000000..cc36c28173c8
--- /dev/null
+++ b/packages/SwingSet/src/kernel/state/reachable.js
@@ -0,0 +1,23 @@
+import { assert, details as X } from '@agoric/assert';
+
+export function parseReachableAndVatSlot(value) {
+  assert.typeof(value, 'string', X`non-string value: ${value}`);
+  const flag = value.slice(0, 1);
+  assert.equal(value.slice(1, 2), ' ');
+  const vatSlot = value.slice(2);
+  let isReachable;
+  if (flag === 'R') {
+    isReachable = true;
+  } else if (flag === '_') {
+    isReachable = false;
+  } else {
+    assert(`flag (${flag}) must be 'R' or '_'`);
+  }
+  return { isReachable, vatSlot };
+}
+harden(parseReachableAndVatSlot);
+
+export function buildReachableAndVatSlot(isReachable, vatSlot) {
+  return `${isReachable ? 'R' : '_'} ${vatSlot}`;
+}
+harden(buildReachableAndVatSlot);
diff --git a/packages/SwingSet/src/kernel/state/vatKeeper.js b/packages/SwingSet/src/kernel/state/vatKeeper.js
index 20a9c160240c..a585d105fe06 100644
--- a/packages/SwingSet/src/kernel/state/vatKeeper.js
+++ b/packages/SwingSet/src/kernel/state/vatKeeper.js
@@ -8,6 +8,10 @@ import { parseKernelSlot } from '../parseKernelSlots';
 import { makeVatSlot, parseVatSlot } from '../../parseVatSlots';
 import { insistVatID } from '../id';
 import { kdebug } from '../kdebug';
+import {
+  parseReachableAndVatSlot,
+  buildReachableAndVatSlot,
+} from './reachable';
 
 // makeVatKeeper is a pure function: all state is kept in the argument object
 
@@ -47,6 +51,7 @@ export function initializeVatState(kvStore, streamStore, vatID) {
  * kernel's mapping tables.
  * @param {*} incrementRefCount
  * @param {*} decrementRefCount
+ * @param {(vatID: string, kernelSlot: string) => {reachable: boolean, vatSlot: string}} getReachableAndVatSlot
  * @param {*} incStat
  * @param {*} decStat
  * @param {*} getCrankNumber
@@ -61,6 +66,7 @@ export function makeVatKeeper(
   addKernelPromiseForVat,
   incrementRefCount,
   decrementRefCount,
+  getReachableAndVatSlot,
   incStat,
   decStat,
   getCrankNumber,
@@ -84,40 +90,35 @@ export function makeVatKeeper(
     return harden({ source, options });
   }
 
-  function parseReachableAndVatSlot(value) {
-    assert.typeof(value, 'string', X`non-string value: ${value}`);
-    const flag = value.slice(0, 1);
-    assert.equal(value.slice(1, 2), ' ');
-    const vatSlot = value.slice(2);
-    let reachable;
-    if (flag === 'R') {
-      reachable = true;
-    } else if (flag === '_') {
-      reachable = false;
-    } else {
-      assert(`flag (${flag}) must be 'R' or '_'`);
-    }
-    return { reachable, vatSlot };
-  }
-
-  function buildReachableAndVatSlot(reachable, vatSlot) {
-    return `${reachable ? 'R' : '_'} ${vatSlot}`;
+  function getReachableFlag(kernelSlot) {
+    const kernelKey = `${vatID}.c.${kernelSlot}`;
+    const data = kvStore.get(kernelKey);
+    const { isReachable } = parseReachableAndVatSlot(data);
+    return isReachable;
   }
 
-  function getReachableAndVatSlot(kernelSlot) {
-    const kernelKey = `${vatID}.c.${kernelSlot}`;
-    return parseReachableAndVatSlot(kvStore.get(kernelKey));
+  function insistNotReachable(kernelSlot) {
+    const isReachable = getReachableFlag(kernelSlot);
+    assert.equal(isReachable, false, X`${kernelSlot} was reachable, oops`);
   }
 
   function setReachableFlag(kernelSlot) {
+    const { type } = parseKernelSlot(kernelSlot);
     const kernelKey = `${vatID}.c.${kernelSlot}`;
-    const { vatSlot } = parseReachableAndVatSlot(kvStore.get(kernelKey));
+    const { isReachable, vatSlot } = parseReachableAndVatSlot(
+      kvStore.get(kernelKey),
+    );
+    const { allocatedByVat } = parseVatSlot(vatSlot);
     kvStore.set(kernelKey, buildReachableAndVatSlot(true, vatSlot));
   }
 
   function clearReachableFlag(kernelSlot) {
+    const { type } = parseKernelSlot(kernelSlot);
     const kernelKey = `${vatID}.c.${kernelSlot}`;
-    const { vatSlot } = parseReachableAndVatSlot(kvStore.get(kernelKey));
+    const { isReachable, vatSlot } = parseReachableAndVatSlot(
+      kvStore.get(kernelKey),
+    );
+    const { allocatedByVat } = parseVatSlot(vatSlot);
     kvStore.set(kernelKey, buildReachableAndVatSlot(false, vatSlot));
   }
 
@@ -181,8 +182,8 @@ export function makeVatKeeper(
         setReachableFlag(kernelSlot);
       } else {
         // imports must be reachable
-        const { reachable } = getReachableAndVatSlot(kernelSlot);
-        assert(reachable, X`vat tried to access unreachable import`);
+        const { isReachable } = getReachableAndVatSlot(vatID, kernelSlot);
+        assert(isReachable, X`vat tried to access unreachable import`);
       }
     }
     return kernelSlot;
@@ -235,7 +236,7 @@ export function makeVatKeeper(
       kdebug(`Add mapping k->v ${kernelKey}<=>${vatKey}`);
     }
 
-    const { reachable, vatSlot } = getReachableAndVatSlot(kernelSlot);
+    const { isReachable, vatSlot } = getReachableAndVatSlot(vatID, kernelSlot);
     const { allocatedByVat } = parseVatSlot(vatSlot);
     if (setReachable) {
       if (!allocatedByVat) {
@@ -244,7 +245,7 @@ export function makeVatKeeper(
       } else {
         // if the kernel is sending non-reachable exports back into
         // exporting vat, that's a kernel bug
-        assert(reachable, X`kernel sent unreachable export`);
+        assert(isReachable, X`kernel sent unreachable export ${kernelSlot}`);
       }
     }
     return vatSlot;
@@ -383,6 +384,8 @@ export function makeVatKeeper(
     getSourceAndOptions,
     mapVatSlotToKernelSlot,
     mapKernelSlotToVatSlot,
+    getReachableFlag,
+    insistNotReachable,
     setReachableFlag,
     clearReachableFlag,
     hasCListEntry,