diff --git a/actions/apitimelineuser.php b/actions/apitimelineuser.php
index 0057693068..9696ea3615 100644
--- a/actions/apitimelineuser.php
+++ b/actions/apitimelineuser.php
@@ -420,9 +420,9 @@ class ApiTimelineUserAction extends ApiBareAuthAction
 
         // Check for optional attributes...
 
-        if (!empty($activity->context)) {
+        if ($activity->context instanceof ActivityContext) {
 
-            foreach ($activity->context->attention as $uri) {
+            foreach ($activity->context->attention as $uri=>$type) {
 
                 $profile = Profile::fromURI($uri);
 
diff --git a/classes/Message.php b/classes/Message.php
index c5aefcc4e8..211284ecec 100644
--- a/classes/Message.php
+++ b/classes/Message.php
@@ -198,8 +198,7 @@ class Message extends Managed_DataObject
                 throw new Exception(sprintf("Receiver profile not found: %d", $this->to_profile));
             }
 
-            $ctx->attention[] = $rprofile->getUri();
-            $ctx->attentionType[$rprofile->getUri()] = ActivityObject::PERSON;
+            $ctx->attention[$rprofile->getUri()] = ActivityObject::PERSON;
 
             $act->context = $ctx;
 
diff --git a/classes/Notice.php b/classes/Notice.php
index d3c56bac54..df05ce4650 100644
--- a/classes/Notice.php
+++ b/classes/Notice.php
@@ -1577,27 +1577,23 @@ class Notice extends Managed_DataObject
             foreach ($reply_ids as $id) {
                 $rprofile = Profile::getKV('id', $id);
                 if ($rprofile instanceof Profile) {
-                    $ctx->attention[] = $rprofile->getUri();
-                    $ctx->attentionType[$rprofile->getUri()] = ActivityObject::PERSON;
+                    $ctx->attention[$rprofile->getUri()] = ActivityObject::PERSON;
                 }
             }
 
             $groups = $this->getGroups();
 
             foreach ($groups as $group) {
-                $ctx->attention[] = $group->getUri();
-                $ctx->attentionType[$group->getUri()] = ActivityObject::GROUP;
+                $ctx->attention[$group->getUri()] = ActivityObject::GROUP;
             }
 
             switch ($this->scope) {
             case Notice::PUBLIC_SCOPE:
-                $ctx->attention[] = "http://activityschema.org/collection/public";
-                $ctx->attentionType["http://activityschema.org/collection/public"] = ActivityObject::COLLECTION;
+                $ctx->attention[ActivityContext::ATTN_PUBLIC] = ActivityObject::COLLECTION;
                 break;
             case Notice::FOLLOWER_SCOPE:
                 $surl = common_local_url("subscribers", array('nickname' => $profile->nickname));
-                $ctx->attention[] = $surl;
-                $ctx->attentionType[$surl] = ActivityObject::COLLECTION;
+                $ctx->attention[$surl] = ActivityObject::COLLECTION;
                 break;
             }
 
diff --git a/lib/activity.php b/lib/activity.php
index eaa7cf1c2d..8be37c5681 100644
--- a/lib/activity.php
+++ b/lib/activity.php
@@ -626,8 +626,9 @@ class Activity
                                            'href' => $this->context->conversation));
             }
 
-            foreach ($this->context->attention as $attnURI) {
+            foreach ($this->context->attention as $attnURI=>$type) {
                 $xs->element('link', array('rel' => ActivityContext::MENTIONED,
+                                           ActivityContext::OBJECTTYPE => $type,  // FIXME: undocumented 
                                            'href' => $attnURI));
             }
 
diff --git a/lib/activitycontext.php b/lib/activitycontext.php
index 371eab3e64..cebd72ea90 100644
--- a/lib/activitycontext.php
+++ b/lib/activitycontext.php
@@ -37,8 +37,7 @@ class ActivityContext
     public $replyToID;
     public $replyToUrl;
     public $location;
-    public $attention = array();
-    public $attentionType = array();
+    public $attention = array();    // 'uri' => 'type'
     public $conversation;
     public $scope;
 
@@ -46,15 +45,18 @@ class ActivityContext
     const GEORSS  = 'http://www.georss.org/georss';
     const OSTATUS = 'http://ostatus.org/schema/1.0';
 
-    const INREPLYTO = 'in-reply-to';
-    const REF       = 'ref';
-    const HREF      = 'href';
+    const INREPLYTO  = 'in-reply-to';
+    const REF        = 'ref';
+    const HREF       = 'href';
+    const OBJECTTYPE = 'ostatus:object-type';   // FIXME: Undocumented!
 
     const POINT     = 'point';
 
     const MENTIONED    = 'mentioned';
     const CONVERSATION = 'ostatus:conversation';
 
+    const ATTN_PUBLIC  = 'http://activityschema.org/collection/public';
+
     function __construct($element = null)
     {
         if (empty($element)) {
@@ -76,17 +78,15 @@ class ActivityContext
 
         $links = $element->getElementsByTagNameNS(ActivityUtils::ATOM, ActivityUtils::LINK);
 
-        $attention = array();
         for ($i = 0; $i < $links->length; $i++) {
             $link = $links->item($i);
 
-            $linkRel = $link->getAttribute(ActivityUtils::REL);
-
-            if ($linkRel == self::MENTIONED) {
-                $attention[] = $link->getAttribute(self::HREF);
+            $linkRel  = $link->getAttribute(ActivityUtils::REL);
+            $linkHref = $link->getAttribute(self::HREF);
+            if ($linkRel == self::MENTIONED && $linkHref !== '') {
+                $this->attention[$linkHref] = $link->getAttribute(ActivityContext::OBJECTTYPE);
             }
         }
-        $this->attention = array_unique($attention);
     }
 
     /**
@@ -159,15 +159,10 @@ class ActivityContext
     {
         $tos = array();
 
-        foreach ($this->attention as $attnUrl) {
-            if (array_key_exists($attnUrl, $this->attentionType)) {
-                $type = ActivityObject::canonicalType($this->attentionType[$attnUrl]);
-            } else {
-                $type = ActivityObject::canonicalType(ActivityObject::PERSON);
-            }
+        foreach ($this->attention as $attnUrl => $attnType) {
             $to = array(
-                'objectType' => $type,
-                'id'         => $attnUrl
+                'objectType' => $attnType,  // can be empty
+                'id'         => $attnUrl,
             );
             $tos[] = $to;
         }
diff --git a/lib/activityimporter.php b/lib/activityimporter.php
index c2bdc447b5..1c34b64d35 100644
--- a/lib/activityimporter.php
+++ b/lib/activityimporter.php
@@ -290,19 +290,19 @@ class ActivityImporter extends QueueHandler
         return $saved;
     }
 
-    function filterAttention($attn)
+    protected function filterAttention(array $attn)
     {
-        $groups = array();
-        $replies = array();
+        $groups = array();  // TODO: context->attention
+        $replies = array(); // TODO: context->attention
 
-        foreach (array_unique($attn) as $recipient) {
+        foreach ($attn as $recipient=>$type) {
 
             // Is the recipient a local user?
 
             $user = User::getKV('uri', $recipient);
 
-            if ($user) {
-                // @fixme sender verification, spam etc?
+            if ($user instanceof User) {
+                // TODO: @fixme sender verification, spam etc?
                 $replies[] = $recipient;
                 continue;
             }
@@ -319,7 +319,7 @@ class ActivityImporter extends QueueHandler
             }
 
             // Is the recipient a local group?
-            // @fixme uri on user_group isn't reliable yet
+            // TODO: @fixme uri on user_group isn't reliable yet
             // $group = User_group::getKV('uri', $recipient);
             $id = OStatusPlugin::localGroupFromUrl($recipient);
 
diff --git a/lib/microappplugin.php b/lib/microappplugin.php
index 30542ffa73..3ed9a76e95 100644
--- a/lib/microappplugin.php
+++ b/lib/microappplugin.php
@@ -390,10 +390,10 @@ abstract class MicroAppPlugin extends Plugin
 
             if ($target instanceof User_group) {
                 $uri = $target->getUri();
-                if (!in_array($uri, $activity->context->attention)) {
+                if (!array_key_exists($uri, $activity->context->attention)) {
                     // @todo FIXME: please document (i18n).
                     // TRANS: Client exception thrown when ...
-                    throw new ClientException(_('Bookmark not posted to this group.'));
+                    throw new ClientException(_('Object not posted to this group.'));
                 }
             } else if ($target instanceof User) {
                 $uri      = $target->uri;
@@ -402,7 +402,7 @@ abstract class MicroAppPlugin extends Plugin
                     $original = Notice::getKV('uri',
                                                   $activity->context->replyToID);
                 }
-                if (!in_array($uri, $activity->context->attention) &&
+                if (!array_key_exists($uri, $activity->context->attention) &&
                     (empty($original) ||
                      $original->profile_id != $target->id)) {
                     // @todo FIXME: Please document (i18n).
diff --git a/plugins/Bookmark/BookmarkPlugin.php b/plugins/Bookmark/BookmarkPlugin.php
index 6dacdb085e..6461227f98 100644
--- a/plugins/Bookmark/BookmarkPlugin.php
+++ b/plugins/Bookmark/BookmarkPlugin.php
@@ -450,19 +450,19 @@ class BookmarkPlugin extends MicroAppPlugin
             }
         }
 
-        $replies = $activity->context->attention;
-
         $options['groups']  = array();
-        $options['replies'] = array();
+        $options['replies'] = array();  // TODO: context->attention
 
-        foreach ($replies as $replyURI) {
-            $other = Profile::fromURI($replyURI);
-            if (!empty($other)) {
-                $options['replies'][] = $replyURI;
+        foreach ($activity->context->attention as $attnUrl=>$type) {
+            $other = Profile::fromURI($attnUrl);
+            if ($other instanceof Profile) {
+                $options['replies'][] = $attnUrl;
             } else {
-                $group = User_group::getKV('uri', $replyURI);
-                if (!empty($group)) {
-                    $options['groups'][] = $replyURI;
+                // Maybe we can get rid of this since every User_group got a Profile?
+                // TODO: Make sure the above replies get sorted properly for groups (or handled afterwards)
+                $group = User_group::getKV('uri', $attnUrl);
+                if ($group instanceof User_group) {
+                    $options['groups'][] = $attnUrl;
                 }
             }
         }
diff --git a/plugins/OStatus/actions/groupsalmon.php b/plugins/OStatus/actions/groupsalmon.php
index 14e66f5ca0..c8b2a73b68 100644
--- a/plugins/OStatus/actions/groupsalmon.php
+++ b/plugins/OStatus/actions/groupsalmon.php
@@ -78,14 +78,13 @@ class GroupsalmonAction extends SalmonAction
         }
 
         // Notice must be to the attention of this group
-        $context = $this->activity->context;
-
-        if (empty($context->attention)) {
+        if (empty($this->activity->context->attention)) {
             // TRANS: Client exception.
             throw new ClientException("Not to the attention of anyone.");
         } else {
             $uri = common_local_url('groupbyid', array('id' => $this->group->id));
-            if (!in_array($uri, $context->attention)) {
+
+            if (!array_key_exists($uri, $this->activity->context->attention)) {
                 // TRANS: Client exception.
                 throw new ClientException("Not to the attention of this group.");
             }
diff --git a/plugins/OStatus/actions/usersalmon.php b/plugins/OStatus/actions/usersalmon.php
index ed2c7b3d5f..296fd6c6c7 100644
--- a/plugins/OStatus/actions/usersalmon.php
+++ b/plugins/OStatus/actions/usersalmon.php
@@ -91,9 +91,9 @@ class UsersalmonAction extends SalmonAction
                 throw new ClientException(_m('In reply to a notice not by this user and not mentioning this user.'));
             }
         } else if (!empty($context->attention)) {
-            if (!in_array($this->user->uri, $context->attention) &&
-                !in_array(common_profile_url($this->user->nickname), $context->attention)) {
-                common_log(LOG_ERR, "{$this->user->uri} not in attention list (".implode(',', $context->attention).")");
+            if (!array_key_exists($this->user->uri, $context->attention) &&
+                !array_key_exists(common_profile_url($this->user->nickname), $context->attention)) {
+                common_log(LOG_ERR, "{$this->user->uri} not in attention list (".implode(',', array_keys($context->attention)).')');
                 // TRANS: Client exception.
                 throw new ClientException(_m('To the attention of user(s), not including this one.'));
             }
diff --git a/plugins/OStatus/classes/Ostatus_profile.php b/plugins/OStatus/classes/Ostatus_profile.php
index 0edb45f825..1e49323653 100644
--- a/plugins/OStatus/classes/Ostatus_profile.php
+++ b/plugins/OStatus/classes/Ostatus_profile.php
@@ -656,10 +656,9 @@ class Ostatus_profile extends Managed_DataObject
         }
 
         if ($activity->context) {
-            // Any individual or group attn: targets?
-            $replies = $activity->context->attention;
-            $options['groups'] = $this->filterReplies($oprofile, $replies);
-            $options['replies'] = $replies;
+            // TODO: context->attention
+            list($options['groups'], $options['replies'])
+                = $this->filterReplies($oprofile, $activity->context->attention);
 
             // Maintain direct reply associations
             // @todo FIXME: What about conversation ID?
@@ -826,10 +825,9 @@ class Ostatus_profile extends Managed_DataObject
         }
 
         if ($activity->context) {
-            // Any individual or group attn: targets?
-            $replies = $activity->context->attention;
-            $options['groups'] = $this->filterReplies($oprofile, $replies);
-            $options['replies'] = $replies;
+            // TODO: context->attention
+            list($options['groups'], $options['replies'])
+                = $this->filterReplies($oprofile, $activity->context->attention);
 
             // Maintain direct reply associations
             // @todo FIXME: What about conversation ID?
@@ -908,26 +906,26 @@ class Ostatus_profile extends Managed_DataObject
      * @param array in/out &$attention_uris set of URIs, will be pruned on output
      * @return array of group IDs
      */
-    protected function filterReplies($sender, &$attention_uris)
+    protected function filterReplies($sender, array $attention)
     {
-        common_log(LOG_DEBUG, "Original reply recipients: " . implode(', ', $attention_uris));
+        common_log(LOG_DEBUG, "Original reply recipients: " . implode(', ', $attention));
         $groups = array();
         $replies = array();
-        foreach (array_unique($attention_uris) as $recipient) {
+        foreach ($attention as $recipient=>$type) {
             // Is the recipient a local user?
             $user = User::getKV('uri', $recipient);
-            if ($user) {
+            if ($user instanceof User) {
                 // @todo FIXME: Sender verification, spam etc?
                 $replies[] = $recipient;
                 continue;
             }
 
             // Is the recipient a local group?
-            // $group = User_group::getKV('uri', $recipient);
+            // TODO: $group = User_group::getKV('uri', $recipient);
             $id = OStatusPlugin::localGroupFromUrl($recipient);
             if ($id) {
                 $group = User_group::getKV('id', $id);
-                if ($group) {
+                if ($group instanceof User_group) {
                     // Deliver to all members of this local group if allowed.
                     $profile = $sender->localProfile();
                     if ($profile->isMember($group)) {
@@ -959,10 +957,9 @@ class Ostatus_profile extends Managed_DataObject
             }
 
         }
-        $attention_uris = $replies;
         common_log(LOG_DEBUG, "Local reply recipients: " . implode(', ', $replies));
         common_log(LOG_DEBUG, "Local group recipients: " . implode(', ', $groups));
-        return $groups;
+        return array($groups, $replies);
     }
 
     /**
diff --git a/tests/ActivityParseTests.php b/tests/ActivityParseTests.php
index 71b90aadb6..27b751b75e 100644
--- a/tests/ActivityParseTests.php
+++ b/tests/ActivityParseTests.php
@@ -101,7 +101,7 @@ class ActivityParseTests extends PHPUnit_Framework_TestCase
         $this->assertEquals('http://example.net/notice/12', $act->context->replyToID);
         $this->assertEquals('http://example.net/notice/12', $act->context->replyToUrl);
         $this->assertEquals('http://example.net/conversation/11', $act->context->conversation);
-        $this->assertEquals(array('http://example.net/user/1'), $act->context->attention);
+        $this->assertEquals(array('http://example.net/user/1'), array_keys($act->context->attention));
 
         $this->assertFalse(empty($act->objects[0]));
         $this->assertEquals($act->objects[0]->content,