diff --git a/lib/REST/TinyTinyRSS/API.php b/lib/REST/TinyTinyRSS/API.php
index f0648de..e34081d 100644
--- a/lib/REST/TinyTinyRSS/API.php
+++ b/lib/REST/TinyTinyRSS/API.php
@@ -130,4 +130,33 @@ class API extends \JKingWeb\Arsse\REST\AbstractHandler {
         // session validity is already checked by the dispatcher, so we need only return true
         return ['status' => true];
     }
+
+    public function opAddCategory(array $data) {
+        $in = [
+            'name'   => isset($data['caption']) ? $data['caption'] : "",
+            'parent' => isset($data['parent_id']) ? $data['parent_id'] : null,
+        ];
+        if (!$in['parent']) {
+            $in['parent'] = null;
+        }
+        try {
+            return Arsse::$db->folderAdd(Arsse::$user->id, $in);
+        } catch (ExceptionInput $e) {
+            switch ($e->getCode()) {
+                // folder already exists
+                case 10236: 
+                    // retrieve the ID of the existing folder; duplicating a category silently returns the existing one
+                    $folders = Arsse::$db->folderList(Arsse::$user->id, $in['parent'], false);
+                    foreach ($folders as $folder) {
+                        if ($folder['name']==$in['name']) {
+                            return (int) $folder['id'];
+                        }
+                    }
+                // parent folder does not exist; this returns false as an ID
+                case 10235: return false;
+                // other errors related to input
+                default: throw new Exception("INCORRECT_USAGE");
+            }
+        }
+    }
 }
\ No newline at end of file
diff --git a/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php b/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
index f18adea..ebb42b8 100644
--- a/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
+++ b/tests/REST/TinyTinyRSS/TestTinyTinyAPI.php
@@ -143,7 +143,7 @@ class TestTinyTinyAPI extends Test\AbstractTest {
         ]
     ];
 
-    protected function respGood(array $content, $seq = 0): Response {
+    protected function respGood($content = null, $seq = 0): Response {
         return new Response(200, [
             'seq' => $seq,
             'status' => 0,
@@ -151,7 +151,7 @@ class TestTinyTinyAPI extends Test\AbstractTest {
         ]);
     }
 
-    protected function respErr(string $msg, array $content = [], $seq = 0): Response {
+    protected function respErr(string $msg, $content = [], $seq = 0): Response {
         $err = ['error' => $msg];
         return new Response(200, [
             'seq' => $seq,
@@ -248,4 +248,49 @@ class TestTinyTinyAPI extends Test\AbstractTest {
         $exp = $this->respGood(['level' => \JKingWeb\Arsse\REST\TinyTinyRSS\API::LEVEL]);
         $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($data))));
     }
+
+    public function testAddACategory() {
+        $in = [
+            ['op' => "addCategory", 'sid' => "PriestsOfSyrinx", 'caption' => "Software"],
+            ['op' => "addCategory", 'sid' => "PriestsOfSyrinx", 'caption' => "Hardware", 'parent_id' => 1],
+            ['op' => "addCategory", 'sid' => "PriestsOfSyrinx"],
+            ['op' => "addCategory", 'sid' => "PriestsOfSyrinx", 'caption' => ""],
+            ['op' => "addCategory", 'sid' => "PriestsOfSyrinx", 'caption' => "   "],
+        ];
+        $db = [
+            ['name' => "Software", 'parent' => null],
+            ['name' => "Hardware", 'parent' => 1],
+        ];
+        $out = [
+            ['id' => 2, 'name' => "Software", 'parent' => null],
+            ['id' => 3, 'name' => "Hardware", 'parent' => 1],
+            ['id' => 1, 'name' => "Politics", 'parent' => null],
+        ];
+        // set of various mocks for testing
+        Phake::when(Arsse::$db)->folderAdd(Arsse::$user->id, $db[0])->thenReturn(2)->thenThrow(new ExceptionInput("constraintViolation")); // error on the second call
+        Phake::when(Arsse::$db)->folderAdd(Arsse::$user->id, $db[1])->thenReturn(3)->thenThrow(new ExceptionInput("constraintViolation")); // error on the second call
+        Phake::when(Arsse::$db)->folderList(Arsse::$user->id, null, false)->thenReturn(new Result([$out[0], $out[2]]));
+        Phake::when(Arsse::$db)->folderList(Arsse::$user->id,    1, false)->thenReturn(new Result([$out[1]]));
+        // set up mocks that produce errors
+        Phake::when(Arsse::$db)->folderAdd(Arsse::$user->id, [])->thenThrow(new ExceptionInput("missing"));
+        Phake::when(Arsse::$db)->folderAdd(Arsse::$user->id, ['name' => "",    'parent' => null])->thenThrow(new ExceptionInput("missing"));
+        Phake::when(Arsse::$db)->folderAdd(Arsse::$user->id, ['name' => "   ", 'parent' => null])->thenThrow(new ExceptionInput("whitespace"));
+        // correctly add two folders
+        $exp = $this->respGood(2);
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[0]))));
+        $exp = $this->respGood(3);
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[1]))));
+        // attempt to add the two folders again
+        $exp = $this->respGood(2);
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[0]))));
+        $exp = $this->respGood(3);
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[1]))));
+        Phake::verify(Arsse::$db)->folderList(Arsse::$user->id, null, false);
+        Phake::verify(Arsse::$db)->folderList(Arsse::$user->id,    1, false);
+        // add some invalid folders
+        $exp = $this->respErr("INCORRECT_USAGE");
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[2]))));
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[3]))));
+        $this->assertEquals($exp, $this->h->dispatch(new Request("POST", "", json_encode($in[4]))));
+    }
 }
\ No newline at end of file