diff --git a/src/Fedi/Activity.hs b/src/Fedi/Activity.hs
index 6b4ee9c..e36f633 100644
--- a/src/Fedi/Activity.hs
+++ b/src/Fedi/Activity.hs
@@ -58,7 +58,8 @@ data OrderedCollectionPage a
 
 data Collection order a
   = Collection
-    { summary :: String
+    { id :: Url
+    , summary :: String
     , items :: [a]
     , first :: Maybe Url
     , last :: Maybe Url
@@ -101,6 +102,7 @@ instance A.ToJSON a => A.ToJSON (Collection Ordered a) where
       [ "@context" A..=
         [ "https://www.w3.org/ns/activitystreams" :: String
         ]
+      , "id" A..= collection.id
       , "type" A..= ("OrderedCollection" :: String)
       , "summary" A..= collection.summary
       , "totalItems" A..= length collection.items
@@ -115,6 +117,7 @@ instance A.ToJSON a => A.ToJSON (Collection Unordered a) where
       [ "@context" A..=
         [ "https://www.w3.org/ns/activitystreams" :: String
         ]
+      , "id" A..= collection.id
       , "type" A..= ("Collection" :: String)
       , "summary" A..= collection.summary
       , "totalItems" A..= length collection.items
diff --git a/src/Fedi/Routes.hs b/src/Fedi/Routes.hs
index ed8c546..c791b41 100644
--- a/src/Fedi/Routes.hs
+++ b/src/Fedi/Routes.hs
@@ -139,7 +139,8 @@ handleOutbox details items = do
             content :: Outbox
             content =
               Collection
-                { summary = details.username <> "'s notes"
+                { id = outboxUrl
+                , summary = details.username <> "'s notes"
                 , items = items
                 , first = Just $ outboxUrl <> "?page=true"
                 , last = Just $ outboxUrl <> "?page=true"