diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2813,6 +2813,7 @@
     'PhabricatorJiraIssueHasObjectEdgeType' => 'applications/doorkeeper/edge/PhabricatorJiraIssueHasObjectEdgeType.php',
     'PhabricatorJumpNavHandler' => 'applications/search/engine/PhabricatorJumpNavHandler.php',
     'PhabricatorKeyValueDatabaseCache' => 'applications/cache/PhabricatorKeyValueDatabaseCache.php',
+    'PhabricatorKeyValueSerializingCacheProxy' => 'applications/cache/PhabricatorKeyValueSerializingCacheProxy.php',
     'PhabricatorKeyboardRemarkupRule' => 'infrastructure/markup/rule/PhabricatorKeyboardRemarkupRule.php',
     'PhabricatorKeyring' => 'applications/files/keyring/PhabricatorKeyring.php',
     'PhabricatorKeyringConfigOptionType' => 'applications/files/keyring/PhabricatorKeyringConfigOptionType.php',
@@ -7798,6 +7799,7 @@
     'PhabricatorJiraIssueHasObjectEdgeType' => 'PhabricatorEdgeType',
     'PhabricatorJumpNavHandler' => 'Phobject',
     'PhabricatorKeyValueDatabaseCache' => 'PhutilKeyValueCache',
+    'PhabricatorKeyValueSerializingCacheProxy' => 'PhutilKeyValueCacheProxy',
     'PhabricatorKeyboardRemarkupRule' => 'PhutilRemarkupRule',
     'PhabricatorKeyring' => 'Phobject',
     'PhabricatorKeyringConfigOptionType' => 'PhabricatorConfigJSONOptionType',
diff --git a/src/applications/cache/PhabricatorCaches.php b/src/applications/cache/PhabricatorCaches.php
--- a/src/applications/cache/PhabricatorCaches.php
+++ b/src/applications/cache/PhabricatorCaches.php
@@ -116,6 +116,24 @@
     return $caches;
   }
 
+  public static function getMutableStructureCache() {
+    static $cache;
+    if (!$cache) {
+      $caches = self::buildMutableStructureCaches();
+      $cache = self::newStackFromCaches($caches);
+    }
+    return $cache;
+  }
+
+  private static function buildMutableStructureCaches() {
+    $caches = array();
+
+    $cache = new PhabricatorKeyValueDatabaseCache();
+    $cache = new PhabricatorKeyValueSerializingCacheProxy($cache);
+    $caches[] = $cache;
+
+    return $caches;
+  }
 
 /* -(  Runtime Cache  )------------------------------------------------------ */
 
diff --git a/src/applications/cache/PhabricatorKeyValueSerializingCacheProxy.php b/src/applications/cache/PhabricatorKeyValueSerializingCacheProxy.php
new file mode 100644
--- /dev/null
+++ b/src/applications/cache/PhabricatorKeyValueSerializingCacheProxy.php
@@ -0,0 +1,55 @@
+<?php
+
+/**
+ * Proxies another cache and serializes values.
+ *
+ * This allows more complex data to be stored in a cache which can only store
+ * strings.
+ */
+final class PhabricatorKeyValueSerializingCacheProxy
+  extends PhutilKeyValueCacheProxy {
+
+  public function getKeys(array $keys) {
+    $results = parent::getKeys($keys);
+
+    $reads = array();
+    foreach ($results as $key => $result) {
+      $structure = @unserialize($result);
+
+      // The unserialize() function returns false when unserializing a
+      // literal `false`, and also when it fails. If we get a literal
+      // `false`, test if the serialized form is the same as the
+      // serialization of `false` and miss the cache otherwise.
+      if ($structure === false) {
+        static $serialized_false;
+        if ($serialized_false === null) {
+          $serialized_false = serialize(false);
+        }
+        if ($result !== $serialized_false) {
+          continue;
+        }
+      }
+
+      $reads[$key] = $structure;
+    }
+
+    return $reads;
+  }
+
+  public function setKeys(array $keys, $ttl = null) {
+    $writes = array();
+    foreach ($keys as $key => $value) {
+      if (is_object($value)) {
+        throw new Exception(
+          pht(
+            'Serializing cache can not write objects (for key "%s")!',
+            $key));
+      }
+      $writes[$key] = serialize($value);
+    }
+
+    return parent::setKeys($writes, $ttl);
+  }
+
+
+}