diff --git a/lib/REST/TinyTinyRSS/API.php b/lib/REST/TinyTinyRSS/API.php
index f2e6058..fe54ba2 100644
--- a/lib/REST/TinyTinyRSS/API.php
+++ b/lib/REST/TinyTinyRSS/API.php
@@ -27,6 +27,7 @@ Protocol difference so far:
     - The "Published" virtual feed is non-functional (this will not be implemented in the near term)
     - setArticleLabel responds with errors for invalid labels where TT-RSS simply returns a zero result
     - The result of setArticleLabel counts only records which actually changed rather than all entries attempted
+    - Top-level categories in getFeedTree have a 'parent_id' property (set to null); in TT-RSS the property is absent
 */
 
 
@@ -439,6 +440,7 @@ class API extends \JKingWeb\Arsse\REST\AbstractHandler {
                 'unread'     => 0,
                 'auxcounter' => 0,
                 'checkbox'   => false,
+                // NOTE: feeds don't have a type property (even though both labels and special feeds do); don't ask me why
             ];
         }
         return $out;
@@ -459,7 +461,7 @@ class API extends \JKingWeb\Arsse\REST\AbstractHandler {
                 'name'         => $c['name'],
                 'id'           => "CAT:".$c['id'],
                 'bare_id'      => $c['id'],
-                'parent_id'    => $c['parent'],
+                'parent_id'    => $c['parent'], // top-level categories are not supposed to have this property; we deviated and have the property set to null because it's simpler that way
                 'type'         => "category",
                 'auxcounter'   => 0,
                 'unread'       => 0,
@@ -754,9 +756,13 @@ class API extends \JKingWeb\Arsse\REST\AbstractHandler {
         return ['status' => "OK"];
     }
 
-    protected function labelIn($id): int {
+    protected function labelIn($id, bool $throw = true): int {
         if (!(ValueInfo::int($id) & ValueInfo::NEG) || $id > (-1 - self::LABEL_OFFSET)) {
-            throw new Exception("INCORRECT_USAGE");
+            if ($throw) {
+                throw new Exception("INCORRECT_USAGE");
+            } else {
+                return 0;
+            }
         }
         return (abs($id) - self::LABEL_OFFSET);
     }
@@ -848,4 +854,96 @@ class API extends \JKingWeb\Arsse\REST\AbstractHandler {
         }
         return ['status' => "OK", 'updated' => $out];
     }
+
+    public function opCatchUpFeed(array $data): array {
+        $id = $data['feed_id'] ?? self::FEED_ARCHIVED;
+        $cat = $data['is_cat'] ?? false;
+        $out = ['status' => "OK"];
+        // first prepare the context; unsupported contexts simply return early, whereas some valid contexts are special cases
+        $c = new Context;
+        if ($cat) { // categories
+            switch ($id) {
+                case self::CAT_SPECIAL:
+                case self::CAT_NOT_SPECIAL:
+                case self::CAT_ALL:
+                    // not valid
+                    return $out;
+                case self::CAT_UNCATEGORIZED:
+                    // this is a special case
+                    try {
+                        $tr = Arsse::$db->begin();
+                        // filter the subscription list to return only uncategorized, and get their IDs
+                        $list = array_column(array_filter(Arsse::$db->subscriptionList(Arsse::$user->id)->getAll(), function($value) {return is_null($value['folder']);}), "id");
+                        // perform marking for each applicable subscription
+                        foreach ($list as $id) {
+                            Arsse::$db->articleMark(Arsse::$user->id, ['read' => true], (new Context)->subscription($id));
+                        }
+                        $tr->commit();
+                    } catch (ExceptionInput $e) {
+                        // ignore errors
+                    }
+                    return $out;
+                case self::CAT_LABELS:
+                    // this is also a special case
+                    try {
+                        $tr = Arsse::$db->begin();
+                        // list all non-empty labels
+                        $list = array_column(Arsse::$db->labelList(Arsse::$user->id, false)->getAll(), "id");
+                        // perform marking for each label
+                        foreach ($list as $id) {
+                            try {
+                                Arsse::$db->articleMark(Arsse::$user->id, ['read' => true], (new Context)->label($id));
+                            } catch (ExceptionInput $e) {
+                                // ignore errors
+                            }                            
+                        }
+                        $tr->commit();
+                    } catch (ExceptionInput $e) {
+                        // ignore errors
+                    }
+                    return $out;
+                default:
+                    // any actual category
+                    $c->folder($id);
+                    break;
+            }
+        } else { // feeds
+            if ($this->labelIn($id, false)) { // labels
+                $c->label($this->labelIn($id));
+            } else {
+                switch ($id) {
+                    case self::FEED_ARCHIVED:
+                        // not implemented (also, evidently, not implemented in TTRSS)
+                        return $out;
+                    case self::FEED_STARRED:
+                        $c->starred(true);
+                        break;
+                    case self::FEED_PUBLISHED:
+                        // not implemented
+                        // TODO: if the Published feed is implemented, the catchup function needs to be modified accordingly
+                        return $out;
+                    case self::FEED_FRESH:
+                        $c->modifiedSince(Date::sub("PT24H"));
+                        break;
+                    case self::FEED_ALL:
+                        // no context needed here
+                        break;
+                    case self::FEED_READ:
+                        // everything in the Recently read feed is, by definition, already read
+                        return $out;
+                    default:
+                        // any actual feed
+                        $c->subscription($id);
+                }
+            }
+        }
+        // perform the marking
+        try {
+            Arsse::$db->articleMark(Arsse::$user->id, ['read' => true], $c);
+        } catch (ExceptionInput $e) {
+            // ignore all errors
+        }
+        // return boilerplate output
+        return $out;
+    }
 }
diff --git a/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php b/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
index 211a71a..dcae6b0 100644
--- a/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
+++ b/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
@@ -879,4 +879,70 @@ class TestTinyTinyAPI extends Test\AbstractTest {
         $exp = ['categories'=>['identifier'=>'id','label'=>'name','items'=>[['id'=>'CAT:-1','items'=>[['id'=>'FEED:-4','name'=>'All articles','unread'=>35,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/folder.png','bare_id'=>-4,'auxcounter'=>0,],['id'=>'FEED:-3','name'=>'Fresh articles','unread'=>7,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/fresh.png','bare_id'=>-3,'auxcounter'=>0,],['id'=>'FEED:-1','name'=>'Starred articles','unread'=>4,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/star.png','bare_id'=>-1,'auxcounter'=>0,],['id'=>'FEED:-2','name'=>'Published articles','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/feed.png','bare_id'=>-2,'auxcounter'=>0,],['id'=>'FEED:0','name'=>'Archived articles','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/archive.png','bare_id'=>0,'auxcounter'=>0,],['id'=>'FEED:-6','name'=>'Recently read','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/time.png','bare_id'=>-6,'auxcounter'=>0,],],'name'=>'Special','type'=>'category','unread'=>0,'bare_id'=>-1,],['id'=>'CAT:-2','items'=>[['id'=>'FEED:-1027','name'=>'Fascinating','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/label.png','bare_id'=>-1027,'auxcounter'=>0,'fg_color'=>'','bg_color'=>'',],['id'=>'FEED:-1029','name'=>'Interesting','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/label.png','bare_id'=>-1029,'auxcounter'=>0,'fg_color'=>'','bg_color'=>'',],['id'=>'FEED:-1025','name'=>'Logical','unread'=>0,'type'=>'feed','error'=>'','updated'=>'','icon'=>'images/label.png','bare_id'=>-1025,'auxcounter'=>0,'fg_color'=>'','bg_color'=>'',],],'name'=>'Labels','type'=>'category','unread'=>8,'bare_id'=>-2,],['id'=>'CAT:3','bare_id'=>3,'auxcounter'=>0,'name'=>'Politics','items'=>[['id'=>'CAT:5','bare_id'=>5,'name'=>'Local','items'=>[['id'=>'FEED:2','bare_id'=>2,'auxcounter'=>0,'name'=>'Toronto Star','checkbox'=>false,'unread'=>0,'error'=>'oops','icon'=>'feed-icons/2.ico','param'=>'2011-11-11T11:11:11',],],'checkbox'=>false,'type'=>'category','unread'=>0,'child_unread'=>0,'auxcounter'=>0,'parent_id'=>3,'param'=>'(1 feed)',],['id'=>'CAT:6','bare_id'=>6,'name'=>'National','items'=>[['id'=>'FEED:4','bare_id'=>4,'auxcounter'=>0,'name'=>'CBC News','checkbox'=>false,'unread'=>0,'error'=>'','icon'=>'feed-icons/4.ico','param'=>'2017-10-09T15:58:34',],['id'=>'FEED:5','bare_id'=>5,'auxcounter'=>0,'name'=>'Ottawa Citizen','checkbox'=>false,'unread'=>0,'error'=>'','icon'=>false,'param'=>'2017-07-07T17:07:17',],],'checkbox'=>false,'type'=>'category','unread'=>0,'child_unread'=>0,'auxcounter'=>0,'parent_id'=>3,'param'=>'(2 feeds)',],],'checkbox'=>false,'type'=>'category','unread'=>0,'child_unread'=>0,'parent_id'=>null,'param'=>'(3 feeds)',],['id'=>'CAT:1','bare_id'=>1,'auxcounter'=>0,'name'=>'Science','items'=>[['id'=>'CAT:2','bare_id'=>2,'name'=>'Rocketry','items'=>[['id'=>'FEED:1','bare_id'=>1,'auxcounter'=>0,'name'=>'NASA JPL','checkbox'=>false,'unread'=>0,'error'=>'','icon'=>false,'param'=>'2017-09-15T22:54:16',],],'checkbox'=>false,'type'=>'category','unread'=>0,'child_unread'=>0,'auxcounter'=>0,'parent_id'=>1,'param'=>'(1 feed)',],['id'=>'FEED:3','bare_id'=>3,'auxcounter'=>0,'name'=>'Ars Technica','checkbox'=>false,'unread'=>0,'error'=>'argh','icon'=>'feed-icons/3.ico','param'=>'2016-05-23T06:40:02',],],'checkbox'=>false,'type'=>'category','unread'=>0,'child_unread'=>0,'parent_id'=>null,'param'=>'(2 feeds)',],['id'=>'CAT:0','bare_id'=>0,'auxcounter'=>0,'name'=>'Uncategorized','items'=>[['id'=>'FEED:6','bare_id'=>6,'auxcounter'=>0,'name'=>'Eurogamer','checkbox'=>false,'error'=>'','icon'=>'feed-icons/6.ico','param'=>'2010-02-12T20:08:47','unread'=>0,],],'type'=>'category','checkbox'=>false,'unread'=>0,'child_unread'=>0,'parent_id'=>null,'param'=>'(1 feed)',],],],];
         $this->assertEquals($this->respGood($exp), $this->h->dispatch(new Request("POST", "", json_encode($in[1]))));
     }
+
+    public function testMarkFeedsAsRead() {
+        $in1 = [
+            // no-ops
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx"],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => 0],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -2],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -6],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -1, 'is_cat' => true],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -3, 'is_cat' => true],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -4, 'is_cat' => true],
+        ];
+        $in2 = [
+            // simple contexts
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -1],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -4],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -2112],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => 2112],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => 42, 'is_cat' => true],
+        ];
+        $in3 = [
+            // complex context
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => 0, 'is_cat' => true],
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -2, 'is_cat' => true],
+        ];
+        $in4 = [
+            // this one has a tricky time-based context and will be handled last
+            ['op' => "catchupFeed", 'sid' => "PriestsOfSyrinx", 'feed_id' => -3],
+        ];
+        Phake::when(Arsse::$db)->articleMark->thenThrow(new ExceptionInput("typeViolation"));
+        $exp = $this->respGood(['status' => "OK"]);
+        // verify the above are in fact no-ops
+        for ($a = 0; $a < sizeof($in1); $a++) {
+            $this->assertResponse($exp, $this->h->dispatch(new Request("POST", "", json_encode($in1[$a]))), "Test $a failed");
+        }
+        Phake::verify(Arsse::$db, Phake::times(0))->articleMark;
+        // verify the simple contexts
+        for ($a = 0; $a < sizeof($in2); $a++) {
+            $this->assertResponse($exp, $this->h->dispatch(new Request("POST", "", json_encode($in2[$a]))), "Test $a failed");
+        }
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], new Context);
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->starred(true));
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->label(1088));
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->subscription(2112));
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->folder(42));
+        // reset the database mock
+        $this->setUp();
+        Phake::when(Arsse::$db)->articleMark->thenThrow(new ExceptionInput("typeViolation"));
+        Phake::when(Arsse::$db)->subscriptionList($this->anything())->thenReturn(new Result($this->subscriptions));
+        Phake::when(Arsse::$db)->labelList->thenReturn(new Result($this->labels));
+        Phake::when(Arsse::$db)->labelList($this->anything(), false)->thenReturn(new Result($this->usedLabels));
+        // verify the complex contexts
+        for ($a = 0; $a < sizeof($in3); $a++) {
+            $this->assertResponse($exp, $this->h->dispatch(new Request("POST", "", json_encode($in3[$a]))), "Test $a failed");
+        }
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->subscription(6));
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->label(3));
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->label(1));
+        Phake::verify(Arsse::$db, Phake::times(3))->articleMark;
+        // verify the time-based mock
+        $t = Date::sub("PT24H");
+        for ($a = 0; $a < sizeof($in4); $a++) {
+            $this->assertResponse($exp, $this->h->dispatch(new Request("POST", "", json_encode($in4[$a]))), "Test $a failed");
+        }
+        Phake::verify(Arsse::$db)->articleMark($this->anything(), ['read' => true], (new Context)->modifiedSince($t));
+    }
 }