diff --git a/swh/lister/phabricator/lister.py b/swh/lister/phabricator/lister.py
--- a/swh/lister/phabricator/lister.py
+++ b/swh/lister/phabricator/lister.py
@@ -5,6 +5,7 @@
 import logging
 import random
 from typing import Any, Dict, Iterator, List, Optional
+from urllib.parse import urljoin
 
 import requests
 
@@ -21,6 +22,7 @@
 
 class PhabricatorLister(StatelessLister[PageType]):
     LISTER_NAME = "phabricator"
+    API_REPOSITORIES_PATH = "/api/diffusion.repository.search"
 
     def __init__(
         self,
@@ -30,7 +32,9 @@
         api_token: Optional[str] = None,
         credentials: CredentialsType = None,
     ):
-        super().__init__(scheduler, url, instance, credentials)
+        super().__init__(
+            scheduler, urljoin(url, self.API_REPOSITORIES_PATH), instance, credentials
+        )
 
         self.session = requests.Session()
         self.session.headers.update(
diff --git a/swh/lister/phabricator/tests/test_lister.py b/swh/lister/phabricator/tests/test_lister.py
--- a/swh/lister/phabricator/tests/test_lister.py
+++ b/swh/lister/phabricator/tests/test_lister.py
@@ -6,7 +6,7 @@
 import importlib.resources
 import json
 
-from swh.lister.phabricator.lister import get_repo_url
+from swh.lister.phabricator.lister import PhabricatorLister, get_repo_url
 
 
 def test_get_repo_url():
@@ -27,3 +27,23 @@
         repo = json.load(f)
     expected_name = "https://svn.blender.org/svnroot/bf-blender/"
     assert get_repo_url(repo["attachments"]["uris"]["uris"]) == expected_name
+
+
+def test_lister_url_param(swh_scheduler):
+    FORGE_BASE_URL = "https://forge.softwareheritage.org"
+    API_REPOSITORIES_PATH = "/api/diffusion.repository.search"
+
+    for url in (
+        FORGE_BASE_URL,
+        f"{FORGE_BASE_URL}/",
+        f"{FORGE_BASE_URL}/{API_REPOSITORIES_PATH}",
+        f"{FORGE_BASE_URL}/{API_REPOSITORIES_PATH}/",
+    ):
+
+        lister = PhabricatorLister(
+            scheduler=swh_scheduler, url=FORGE_BASE_URL, instance="swh", api_token="foo"
+        )
+
+        expected_url = f"{FORGE_BASE_URL}{API_REPOSITORIES_PATH}"
+
+        assert lister.url == expected_url
diff --git a/swh/lister/phabricator/tests/test_tasks.py b/swh/lister/phabricator/tests/test_tasks.py
--- a/swh/lister/phabricator/tests/test_tasks.py
+++ b/swh/lister/phabricator/tests/test_tasks.py
@@ -25,7 +25,7 @@
     lister.run.return_value = lister_stats
 
     task_params = {
-        "url": "https://forge.softwareheritage.org/api/diffusion.repository.search",
+        "url": "https://forge.softwareheritage.org",
         "instance": "swh",
         "api_token": None,
     }