diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index 93860aa..c3498a9 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -1,6 +1,7 @@
 name: Unit Tests
 
 on:
+  pull_request:
   push:
     branches-ignore:
       - '[0-9]*'
diff --git a/docs/cli-usage.rst b/docs/cli-usage.rst
index 77fdf85..cbd2a16 100644
--- a/docs/cli-usage.rst
+++ b/docs/cli-usage.rst
@@ -42,6 +42,16 @@ Below some examples on how to use the command-line program:
 
 If you're using python 3+, you can also use the aliases `ls` in place of `list`, and `rm` in place of `remove`.
 
+Remote paths can start with a provider identifier, such as
+``s3compatsigv4/folder/file.txt``. Providers are discovered from the project's
+storage API. If the first path component names a provider the server offers
+but the project has not connected, the command fails instead of writing into
+``osfstorage``. Addons in the ``other`` category, such as ``binderhub``, do
+not hold files and are not treated as providers. If it matches no provider
+at all, the path refers to the default ``osfstorage``.
+To access a folder whose name matches a provider, explicitly prefix the path
+with ``osfstorage/``.
+
 
 If the project is private you will need to provide authentication
 details. You can set the ``OSF_TOKEN`` environment
diff --git a/osfclient/api.py b/osfclient/api.py
index 529b177..6e72bbc 100644
--- a/osfclient/api.py
+++ b/osfclient/api.py
@@ -1,4 +1,5 @@
 from .exceptions import OSFException
+from .models import Addon
 from .models import OSFCore
 from .models import Project
 
@@ -46,6 +47,16 @@ async def project(self, project_id):
             }
         }, self.session)
 
+    @property
+    async def addons(self):
+        """Iterate over all addons available on the server."""
+        url = self._build_url('addons')
+        while url:
+            response = self._json(await self._get(url), 200)
+            for addon in response['data']:
+                yield Addon(addon, self.session)
+            url = response['links']['next']
+
     @property
     def token(self):
         if 'Authorization' not in self.session.headers:
diff --git a/osfclient/cli.py b/osfclient/cli.py
index 96dedcc..85dc60d 100644
--- a/osfclient/cli.py
+++ b/osfclient/cli.py
@@ -184,8 +184,9 @@ async def fetch(args):
     """Fetch an individual file from a project.
 
     The first part of the remote path is interpreted as the name of the
-    storage provider. If there is no match the default (osfstorage) is
-    used.
+    connected storage provider. A provider that exists on the server but
+    is not connected to the project is an error. If there is no match,
+    the default (osfstorage) is used.
 
     The local path defaults to the name of the remote file.
 
@@ -195,7 +196,9 @@ async def fetch(args):
     If args.force is False but args.update is True, overwrite an existing local
     file only if local and remote files differ.
     """
-    storage, remote_path = split_storage(args.remote)
+    osf = _setup_osf(args)
+    project = await osf.project(args.project)
+    store, remote_path = await split_storage(args.remote, osf, project)
 
     local_path = args.local
     if local_path is None:
@@ -209,10 +212,6 @@ async def fetch(args):
     if directory:
         makedirs(directory, exist_ok=True)
 
-    osf = _setup_osf(args)
-    project = await osf.project(args.project)
-
-    store = await project.storage(storage)
     # only fetching one file so we are done
     file_ = await find_by_path(store, remote_path)
     if file_ is None or is_folder(file_):
@@ -279,8 +278,9 @@ async def upload(args):
     """Upload a new file to an existing project.
 
     The first part of the remote path is interpreted as the name of the
-    storage provider. If there is no match the default (osfstorage) is
-    used.
+    connected storage provider. A provider that exists on the server but
+    is not connected to the project is an error. If there is no match,
+    the default (osfstorage) is used.
 
     If the project is private you need to specify a username or token.
 
@@ -300,9 +300,8 @@ async def upload(args):
         sys.exit('To upload a file you need to provide a token.')
 
     project = await osf.project(args.project)
-    storage, remote_path = split_storage(args.destination)
+    store, remote_path = await split_storage(args.destination, osf, project)
 
-    store = await project.storage(storage)
     if args.recursive:
         if not os.path.isdir(args.source):
             raise RuntimeError("Expected source ({}) to be a directory when "
@@ -333,8 +332,9 @@ async def makefolder(args):
     """Create a new folder in an existing project.
 
     The first part of the remote path is interpreted as the name of the
-    storage provider. If there is no match the default (osfstorage) is
-    used.
+    connected storage provider. A provider that exists on the server but
+    is not connected to the project is an error. If there is no match,
+    the default (osfstorage) is used.
     """
     osf = _setup_osf(args)
     if not osf.has_auth:
@@ -342,9 +342,8 @@ async def makefolder(args):
 
     project = await osf.project(args.project)
 
-    storage, remote_path = split_storage(args.target)
+    store, remote_path = await split_storage(args.target, osf, project)
 
-    store = await project.storage(storage)
     f = await find_ancestral_folder(store, remote_path)
     if f is None:
         parent = store
@@ -362,8 +361,9 @@ async def remove(args):
     """Remove a file from the project's storage.
 
     The first part of the remote path is interpreted as the name of the
-    storage provider. If there is no match the default (osfstorage) is
-    used.
+    connected storage provider. A provider that exists on the server but
+    is not connected to the project is an error. If there is no match,
+    the default (osfstorage) is used.
     """
     osf = _setup_osf(args)
     if not osf.has_auth:
@@ -371,9 +371,8 @@ async def remove(args):
 
     project = await osf.project(args.project)
 
-    storage, remote_path = split_storage(args.target)
+    store, remote_path = await split_storage(args.target, osf, project)
 
-    store = await project.storage(storage)
     f = await find_by_path(store, remote_path)
     if f is None:
         sys.exit('No files found to remove.')
@@ -385,8 +384,9 @@ async def move(args):
     """Move a file to specified location on the project's storage.
 
     The first part of the paths is interpreted as the name of the
-    storage provider. If there is no match the default (osfstorage) is
-    used.
+    connected storage provider. A provider that exists on the server but
+    is not connected to the project is an error. If there is no match,
+    the default (osfstorage) is used.
     """
     osf = _setup_osf(args)
     if not osf.has_auth:
@@ -394,7 +394,9 @@ async def move(args):
 
     project = await osf.project(args.project)
 
-    target_storage, target_path = split_storage(args.target, normalize=False)
+    target_store, target_path = await split_storage(
+        args.target, osf, project, normalize=False)
+    target_storage = target_store.provider
 
     if target_path.endswith('/'):
         target_folder_path = target_path[:-1]
@@ -409,16 +411,14 @@ async def move(args):
     else:
         target_folder_path = None
         target_filename = target_path
-    target_store = await project.storage(target_storage)
     if target_folder_path is None:
         target_folder = target_store
     else:
         target_folder = await _ensure_folder(target_store, target_folder_path)
 
     # Move a file
-    storage, remote_path = split_storage(args.source)
+    store, remote_path = await split_storage(args.source, osf, project)
 
-    store = await project.storage(storage)
     f = await find_by_path(store, remote_path)
     if f is None:
         sys.exit('No files found to move.')
diff --git a/osfclient/models/__init__.py b/osfclient/models/__init__.py
index 35c3271..3bcafe6 100644
--- a/osfclient/models/__init__.py
+++ b/osfclient/models/__init__.py
@@ -3,6 +3,7 @@
 Users should not have to instantiate classes from here, instead they should
 use `osfclient.OSF()` to access the OSF.
 """
+from .addon import Addon
 from .core import OSFCore
 from .file import File
 from .file import Folder
diff --git a/osfclient/models/addon.py b/osfclient/models/addon.py
new file mode 100644
index 0000000..8b754be
--- /dev/null
+++ b/osfclient/models/addon.py
@@ -0,0 +1,11 @@
+from .core import OSFCore
+
+
+class Addon(OSFCore):
+    def _update_attributes(self, addon):
+        self.id = self._get_attribute(addon, 'id')
+        self.name = self._get_attribute(addon, 'attributes', 'name')
+        self.categories = self._get_attribute(addon, 'attributes', 'categories')
+
+    def __str__(self):
+        return '<Addon [{0}]>'.format(self.id)
diff --git a/osfclient/models/project.py b/osfclient/models/project.py
index 6764349..3ab56b7 100644
--- a/osfclient/models/project.py
+++ b/osfclient/models/project.py
@@ -38,12 +38,9 @@ def __str__(self):
 
     async def storage(self, provider='osfstorage'):
         """Return storage `provider`."""
-        stores = self._json(await self._get(self._storages_url), 200)
-        stores = stores['data']
-        for store in stores:
-            provides = self._get_attribute(store, 'attributes', 'provider')
-            if provides == provider:
-                return Storage(store, self.session)
+        async for store in self.storages:
+            if store.provider == provider:
+                return store
 
         raise RuntimeError("Project has no storage "
                            "provider '{}'".format(provider))
@@ -51,7 +48,9 @@ async def storage(self, provider='osfstorage'):
     @property
     async def storages(self):
         """Iterate over all storages for this projects."""
-        stores = self._json(await self._get(self._storages_url), 200)
-        stores = stores['data']
-        for store in stores:
-            yield Storage(store, self.session)
+        url = self._storages_url
+        while url:
+            response = self._json(await self._get(url), 200)
+            for store in response['data']:
+                yield Storage(store, self.session)
+            url = response['links']['next']
diff --git a/osfclient/tests/fake_responses.py b/osfclient/tests/fake_responses.py
index 873d4a1..a6a0c79 100644
--- a/osfclient/tests/fake_responses.py
+++ b/osfclient/tests/fake_responses.py
@@ -53,6 +53,26 @@ def storage_node(project_id, storages=['osfstorage']):
                                'n_storages': len(used_storages)})
 
 
+# Use this to fake a response when asking for the server's addons
+# e.g. osf.addons
+def addons(ids, categories=['storage']):
+    return {
+        'data': [{
+            'id': addon_id,
+            'type': 'addon',
+            'attributes': {'name': addon_id, 'categories': categories},
+            'links': {},
+        } for addon_id in ids],
+        'links': {
+            'first': None,
+            'last': None,
+            'prev': None,
+            'next': None,
+            'meta': {'total': len(ids), 'per_page': 1000},
+        },
+    }
+
+
 def _folder(osf_id, name, storage='osfstorage'):
     template = """{
         "relationships": {
diff --git a/osfclient/tests/mocks.py b/osfclient/tests/mocks.py
index e335188..913c8e5 100644
--- a/osfclient/tests/mocks.py
+++ b/osfclient/tests/mocks.py
@@ -1,4 +1,5 @@
 import asyncio
+from types import SimpleNamespace
 from mock import MagicMock, PropertyMock, AsyncMock
 from ..utils import norm_remote_path
 import copy
@@ -58,7 +59,7 @@ def MockStorage(name):
         MockFolder('/a',folders=a_folders),
         MockFolder('/b',folders=b_folders),
         MockFolder('/c',folders=c_folders)]
-    mock = MagicMock(name='Storage-%s' % name,
+    mock = MagicMock(name='Storage-%s' % name, provider=name,
                      folders=AsyncIterator(folders),
                      children=AsyncIterator(folders))
     mock.create_file = MagicMock(return_value=FutureWrapper())
@@ -99,16 +100,26 @@ def MockStream(path, mode, size=1024):
 
 
 def MockProject(name):
-    mock = MagicMock(name='Project-%s' % name,
-                     storages=AsyncIterator([MockStorage('osfstorage'), MockStorage('gh')]))
+    default_store = MockStorage('osfstorage')
+    mock = MagicMock(name='Project-%s' % name, id=name,
+                     storages=AsyncIterator([default_store, MockStorage('gh')]))
     storage = MagicMock(name='Project-%s-storage' % name,
-                        return_value=FutureMockStorage('osfstorage'))
+                        return_value=FutureWrapper(default_store))
     type(mock).storage = storage
     mock._storage_mock = storage
 
     return mock
 
 
+def MockAddon(addon_id, categories=['storage']):
+    return SimpleNamespace(id=addon_id, name=addon_id, categories=categories)
+
+
+def MockAddons(addons):
+    """Stand-in for the `OSF.addons` property."""
+    return PropertyMock(return_value=AsyncIterator(addons))
+
+
 def MockArgs(output=None, project=None,
              source=None, destination=None, local=None, remote=None,
              target=None, force=False, update=False, recursive=False,
diff --git a/osfclient/tests/test_api.py b/osfclient/tests/test_api.py
index 185f753..9670f5a 100644
--- a/osfclient/tests/test_api.py
+++ b/osfclient/tests/test_api.py
@@ -7,6 +7,7 @@
 from osfclient.models import OSFCore
 from osfclient.models import Project
 
+from osfclient.tests import fake_responses
 from osfclient.tests.mocks import FakeResponse
 
 
@@ -34,3 +35,34 @@ def test_endpoint(session_set_endpoint):
 
     osf = OSF(base_url='https://api.test.osf.io/v2/')
     session_set_endpoint.assert_called_with('https://api.test.osf.io/v2/')
+
+
+@pytest.mark.asyncio
+@patch.object(OSFCore, '_get')
+async def test_addons(OSFCore_get):
+    osf = OSF(base_url='https://api.test.osf.io/v2/')
+    first_url = 'https://api.test.osf.io/v2/addons/'
+    next_url = first_url + '?page=2'
+    first = fake_responses.addons(['s3', 'github'])
+    first['links']['next'] = next_url
+    second = fake_responses.addons(['binderhub'], categories=['other'])
+    OSFCore_get.side_effect = [FakeResponse(200, first),
+                               FakeResponse(200, second)]
+
+    addons = [addon async for addon in osf.addons]
+
+    assert [addon.id for addon in addons] == ['s3', 'github', 'binderhub']
+    assert [addon.categories for addon in addons] == [
+        ['storage'], ['storage'], ['other']]
+    assert all(addon.session is osf.session for addon in addons)
+    assert OSFCore_get.call_args_list == [call(first_url), call(next_url)]
+
+
+@pytest.mark.asyncio
+@patch.object(OSFCore, '_get', return_value=FakeResponse(403, {}))
+async def test_addons_propagates_api_error(OSFCore_get):
+    osf = OSF()
+
+    with pytest.raises(RuntimeError, match='403'):
+        async for _ in osf.addons:
+            pass
diff --git a/osfclient/tests/test_connected_providers.py b/osfclient/tests/test_connected_providers.py
new file mode 100644
index 0000000..e096386
--- /dev/null
+++ b/osfclient/tests/test_connected_providers.py
@@ -0,0 +1,151 @@
+"""Test file transfers using providers discovered from the project."""
+
+import pytest
+from mock import patch
+
+from osfclient import OSF
+from osfclient.cli import fetch, upload, makefolder, remove, move
+from osfclient.tests.mocks import (
+    AsyncIterator, MockAddon, MockAddons, MockArgs, MockProject, MockStorage,
+    MockStream,
+    is_folder_mock, mock_async_open,
+)
+from osfclient.utils import find_by_path
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('provider', [
+    's3compatsigv4', 'dropboxbusiness', 'onedrive', 'future-provider',
+])
+@pytest.mark.parametrize('leading_slash', ['', '/'])
+async def test_upload_connected_provider(provider, leading_slash, monkeypatch):
+    monkeypatch.delenv('KNOWN_PROVIDERS', raising=False)
+    monkeypatch.setenv('OSF_TOKEN', 'secret')
+    project = MockProject('1234')
+    store = MockStorage(provider)
+    project.storages = AsyncIterator([MockStorage('osfstorage'), store])
+    prefix = leading_slash + provider + '/'
+    args = MockArgs(project='1234', source='local.txt',
+                    destination=prefix + 'folder/remote.txt')
+    stream = MockStream('local.txt', 'rb')
+    open_mock = mock_async_open(stream)
+
+    with patch.object(OSF, 'project', return_value=project):
+        with patch('osfclient.cli.aiofiles.open', open_mock):
+            await upload(args)
+
+    open_mock.assert_called_once_with('local.txt', 'rb')
+    store.create_file.assert_called_once_with(
+        'folder/remote.txt', stream, force=False, update=False)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('provider', [
+    's3compatsigv4', 'dropboxbusiness', 'onedrive', 'future-provider',
+])
+@pytest.mark.parametrize('leading_slash', ['', '/'])
+async def test_fetch_connected_provider(provider, leading_slash, monkeypatch,
+                                        tmp_path):
+    monkeypatch.delenv('KNOWN_PROVIDERS', raising=False)
+    monkeypatch.setenv('OSF_TOKEN', 'secret')
+    project = MockProject('1234')
+    store = MockStorage(provider)
+    project.storages = AsyncIterator([MockStorage('osfstorage'), store])
+    prefix = leading_slash + provider + '/'
+    local = str(tmp_path / 'download.txt')
+    args = MockArgs(project='1234', remote=prefix + 'a/a/a', local=local)
+    stream = MockStream(local, 'wb')
+    open_mock = mock_async_open(stream)
+
+    with patch.object(OSF, 'project', return_value=project):
+        with patch('osfclient.cli.aiofiles.open', open_mock):
+            with patch('osfclient.cli.is_folder', side_effect=is_folder_mock):
+                await fetch(args)
+
+    open_mock.assert_called_once_with(local, 'wb')
+    file_ = await find_by_path(store, 'a/a/a')
+    file_.write_to.assert_called_once_with(stream)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('command', [makefolder, remove])
+async def test_modify_connected_provider(command, monkeypatch):
+    monkeypatch.setenv('OSF_TOKEN', 'secret')
+    project = MockProject('1234')
+    default_store = MockStorage('osfstorage')
+    store = MockStorage('future-provider')
+    project.storages = AsyncIterator([default_store, store])
+    path = 'new-folder' if command is makefolder else 'a/a/a'
+    args = MockArgs(project='1234', target='future-provider/' + path)
+
+    with patch.object(OSF, 'project', return_value=project):
+        await command(args)
+
+    if command is makefolder:
+        store.create_folder.assert_called_once_with('new-folder')
+        default_store.create_folder.assert_not_called()
+    else:
+        file_ = await find_by_path(store, 'a/a/a')
+        file_.remove.assert_called_once_with()
+        default_file = await find_by_path(default_store, 'a/a/a')
+        default_file.remove.assert_not_called()
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('target_path, folder_path, filename', [
+    ('future-target/', None, None),
+    ('/future-target/', None, None),
+    ('/future-target/c/', 'c', None),
+    ('future-target/renamed.txt', None, 'renamed.txt'),
+])
+async def test_move_between_connected_providers(target_path, folder_path,
+                                               filename, monkeypatch):
+    monkeypatch.setenv('OSF_TOKEN', 'secret')
+    project = MockProject('1234')
+    source = MockStorage('future-source')
+    target = MockStorage('future-target')
+    project.storages = AsyncIterator([source, target])
+    args = MockArgs(project='1234', source='/future-source/a/a/a',
+                    target=target_path)
+
+    with patch.object(OSF, 'project', return_value=project):
+        with patch('osfclient.cli.is_folder', side_effect=is_folder_mock):
+            await move(args)
+
+    folder = target if folder_path is None else await find_by_path(
+        target, folder_path)
+    file_ = await find_by_path(source, 'a/a/a')
+    file_.move_to.assert_called_once_with(
+        'future-target', folder, to_filename=filename, force=False)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('command, path_args', [
+    (upload, {'source': 'local.txt', 'destination': None}),
+    (fetch, {'remote': None}),
+    (makefolder, {'target': None}),
+    (remove, {'target': None}),
+    (move, {'source': None, 'target': 'osfstorage/c/'}),
+])
+@patch.object(OSF, 'addons', new=MockAddons([
+    MockAddon('binderhub', categories=['other']), MockAddon('s3compatsigv4')]))
+async def test_unconnected_provider_is_rejected(command, path_args,
+                                                monkeypatch):
+    monkeypatch.setenv('OSF_TOKEN', 'secret')
+    project = MockProject('1234')
+    default_store = MockStorage('osfstorage')
+    project.storages = AsyncIterator([default_store])
+    path_args = {key: 's3compatsigv4/folder/file.txt' if value is None else value
+                 for key, value in path_args.items()}
+    args = MockArgs(project='1234', **path_args)
+
+    with patch.object(OSF, 'project', return_value=project):
+        with pytest.raises(RuntimeError, match=(
+                "Storage provider 's3compatsigv4' is not connected to "
+                "project '1234'")):
+            await command(args)
+
+    default_store.create_file.assert_not_called()
+    default_store.create_folder.assert_not_called()
+    file_ = await find_by_path(default_store, 'folder/file.txt')
+    assert file_ is None
diff --git a/osfclient/tests/test_mkdir.py b/osfclient/tests/test_mkdir.py
index 85638be..91ef85d 100644
--- a/osfclient/tests/test_mkdir.py
+++ b/osfclient/tests/test_mkdir.py
@@ -8,6 +8,7 @@
 from osfclient import OSF
 from osfclient.cli import makefolder
 
+from osfclient.tests.mocks import MockAddon, MockAddons
 from osfclient.tests.mocks import MockArgs
 from osfclient.tests.mocks import MockProject
 
@@ -114,6 +115,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_wrong_storage_name(OSF_project):
     args = MockArgs(project='1234', target='DOESNTEXIST/a/a/a')
diff --git a/osfclient/tests/test_moving.py b/osfclient/tests/test_moving.py
index f2f8dbd..cafd689 100644
--- a/osfclient/tests/test_moving.py
+++ b/osfclient/tests/test_moving.py
@@ -8,6 +8,7 @@
 from osfclient import OSF
 from osfclient.cli import move
 
+from osfclient.tests.mocks import MockAddon, MockAddons
 from osfclient.tests.mocks import MockArgs
 from osfclient.tests.mocks import MockProject
 
@@ -328,6 +329,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_wrong_storage_name(OSF_project):
     args = MockArgs(project='1234',
diff --git a/osfclient/tests/test_project.py b/osfclient/tests/test_project.py
index ca8ed24..09c9527 100644
--- a/osfclient/tests/test_project.py
+++ b/osfclient/tests/test_project.py
@@ -1,9 +1,11 @@
 from mock import patch
 import pytest
 
+from osfclient import OSF
 from osfclient.models import OSFCore
 from osfclient.models import Project
 from osfclient.models import Storage
+from osfclient.utils import split_storage
 
 from osfclient.tests import fake_responses
 from osfclient.tests.mocks import FakeResponse
@@ -93,3 +95,56 @@ async def test_pass_down_session_to_storages(OSFCore_get):
 
     async for store in project.storages:
         assert store.session == project.session
+
+
+@pytest.mark.asyncio
+@patch.object(OSFCore, '_get')
+async def test_resolve_storage_on_later_page(OSFCore_get):
+    project = Project({})
+    project._storages_url = 'https://api.osf.io/v2/nodes/f3szh/files/'
+    next_url = project._storages_url + '?page=2'
+    first = fake_responses.storage_node('f3szh', ['osfstorage'])
+    first['links']['next'] = next_url
+    second = fake_responses.storage_node('f3szh', ['new-provider'])
+    second['data'][0]['attributes']['name'] = 'A display name'
+    OSFCore_get.side_effect = [FakeResponse(200, first), FakeResponse(200, second)]
+
+    store, path = await split_storage('/new-provider/folder/file.txt', OSF(), project)
+
+    assert store.provider == 'new-provider'
+    assert store.name == 'A display name'
+    assert store.session is project.session
+    assert path == 'folder/file.txt'
+    assert [c.args[0] for c in OSFCore_get.call_args_list] == [
+        project._storages_url, next_url]
+
+
+@pytest.mark.asyncio
+@patch.object(OSFCore, '_get')
+async def test_resolve_storage_propagates_api_error(OSFCore_get):
+    project = Project({})
+    project._storages_url = 'https://api.osf.io/v2/nodes/f3szh/files/'
+    first = fake_responses.storage_node('f3szh', ['osfstorage'])
+    first['links']['next'] = project._storages_url + '?page=2'
+    OSFCore_get.side_effect = [FakeResponse(200, first), FakeResponse(403, {})]
+
+    with pytest.raises(RuntimeError, match='403'):
+        await split_storage('new-provider/file.txt', OSF(), project)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('missing_field', ['links', 'next'])
+@patch.object(OSFCore, '_get')
+async def test_resolve_storage_rejects_missing_pagination(OSFCore_get,
+                                                        missing_field):
+    project = Project({})
+    project._storages_url = 'https://api.osf.io/v2/nodes/f3szh/files/'
+    response = fake_responses.storage_node('f3szh', ['osfstorage'])
+    if missing_field == 'links':
+        del response['links']
+    else:
+        del response['links']['next']
+    OSFCore_get.return_value = FakeResponse(200, response)
+
+    with pytest.raises(KeyError, match=missing_field):
+        await split_storage('folder/file.txt', OSF(), project)
diff --git a/osfclient/tests/test_removing.py b/osfclient/tests/test_removing.py
index 40e9547..0b181a9 100644
--- a/osfclient/tests/test_removing.py
+++ b/osfclient/tests/test_removing.py
@@ -8,6 +8,7 @@
 from osfclient import OSF
 from osfclient.cli import remove
 
+from osfclient.tests.mocks import MockAddon, MockAddons
 from osfclient.tests.mocks import MockArgs
 from osfclient.tests.mocks import MockProject
 
@@ -52,6 +53,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_wrong_storage_name(OSF_project):
     args = MockArgs(project='1234', target='DOESNTEXIST/a/a/a')
diff --git a/osfclient/tests/test_uploading.py b/osfclient/tests/test_uploading.py
index fa958e3..88be834 100644
--- a/osfclient/tests/test_uploading.py
+++ b/osfclient/tests/test_uploading.py
@@ -10,6 +10,7 @@
 from osfclient import OSF
 from osfclient.cli import upload
 
+from osfclient.tests.mocks import MockAddon, MockAddons
 from osfclient.tests.mocks import MockArgs
 from osfclient.tests.mocks import MockProject
 from osfclient.tests.mocks import mock_async_open, MockStream
@@ -31,6 +32,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_select_project(OSF_project):
     args = MockArgs(project='1234',
@@ -53,9 +55,6 @@ def simple_getenv(key, default=None):
 
     # the mock project created by calling OSF().project()
     fake_project = OSF_project.return_value
-    expected = [call('osfstorage')]
-    assert fake_project._storage_mock.mock_calls == expected
-    # assert fake_project.mock_calls == expected
 
     expected = [call.create_file('bar/bar/foo.txt', fake_stream,
                                  force=False, update=False)]
@@ -66,6 +65,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_recursive_requires_directory(OSF_project):
     # test that we check if source is a directory when using recursive mode
@@ -89,6 +89,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_recursive_upload(OSF_project):
     # test that we check if source is a directory when using recursive mode
@@ -135,6 +136,7 @@ def simple_getenv(key, default=None):
 
 
 @pytest.mark.asyncio
+@patch.object(OSF, 'addons', new=MockAddons([MockAddon('github')]))
 @patch.object(OSF, 'project', return_value=MockProject('1234'))
 async def test_recursive_upload_with_subdir(OSF_project):
     # test that an extra level of subdirectory is created on the remote side
diff --git a/osfclient/tests/test_utils.py b/osfclient/tests/test_utils.py
index 3a0f9bc..07e20eb 100644
--- a/osfclient/tests/test_utils.py
+++ b/osfclient/tests/test_utils.py
@@ -1,3 +1,5 @@
+from types import SimpleNamespace
+
 import pytest
 from mock import call, patch, Mock
 
@@ -5,102 +7,83 @@
 from osfclient.utils import norm_remote_path
 from osfclient.utils import makedirs
 from osfclient.utils import split_storage
-from osfclient.tests.mocks import MockStream
-
-
-def test_default_storage():
-    def simple_getenv(key):
-        return None
-
-    with patch('osfclient.cli.os.getenv',
-               side_effect=simple_getenv) as mock_getenv:
-        store, path = split_storage('foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('/foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz'
-
-
-def test_split_storage():
-    def simple_getenv(key):
-        return None
-
-    with patch('osfclient.cli.os.getenv',
-               side_effect=simple_getenv) as mock_getenv:
-        store, path = split_storage('osfstorage/foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('github/foo/bar/baz')
-        assert store == 'github'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('/github/foo/bar/baz')
-        assert store == 'github'
-        assert path == 'foo/bar/baz'
+from osfclient.tests.mocks import AsyncIterator, MockAddon, MockStream
 
-        store, path = split_storage('figshare/foo/bar/baz')
-        assert store == 'figshare'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('/figshare/foo/bar/baz')
-        assert store == 'figshare'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('googledrive/foo/bar/baz')
-        assert store == 'googledrive'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('/googledrive/foo/bar/baz')
-        assert store == 'googledrive'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('osfstorage/foo/bar/baz/')
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz'
-
-        store, path = split_storage('osfstorage/foo/bar/baz/', normalize=False)
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz/'
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize('remote, normalize, provider, path', [
+    ('folder/file.txt', True, 'osfstorage', 'folder/file.txt'),
+    ('/folder/file.txt', True, 'osfstorage', 'folder/file.txt'),
+    ('custom/files/data.txt', True, 'custom', 'files/data.txt'),
+    ('/custom/files/data.txt', True, 'custom', 'files/data.txt'),
+    ('./custom/files/data.txt', True, 'custom', 'files/data.txt'),
+    ('custom/files/../data.txt', True, 'custom', 'data.txt'),
+    ('/custom//data.txt', True, 'custom', 'data.txt'),
+    ('custom/files/', True, 'custom', 'files'),
+    ('/custom/files/', False, 'custom', 'files/'),
+    ('custom/', True, 'custom', ''),
+    ('custom', True, 'osfstorage', 'custom'),
+    ('/custom/', False, 'custom', ''),
+    ('osfstorage/custom/file.txt', True, 'osfstorage', 'custom/file.txt'),
+    ('github/file.txt', True, 'osfstorage', 'github/file.txt'),
+    ('custom-other/file.txt', True, 'osfstorage', 'custom-other/file.txt'),
+    ('unconnected', True, 'osfstorage', 'unconnected'),
+    ('osfstorage/unconnected/file.txt', True, 'osfstorage',
+     'unconnected/file.txt'),
+    ('binderhub/notebook.ipynb', True, 'osfstorage', 'binderhub/notebook.ipynb'),
+])
+async def test_split_storage(remote, normalize, provider, path, monkeypatch):
+    # Obsolete environment overrides must not hide connected providers.
+    monkeypatch.setenv('KNOWN_PROVIDERS', 'github')
+    stores = [SimpleNamespace(provider=name, name='Display name')
+              for name in ['osfstorage', 'custom']]
+    project = SimpleNamespace(id='1234', storages=AsyncIterator(stores))
+    osf = SimpleNamespace(addons=AsyncIterator([
+        MockAddon('custom'), MockAddon('unconnected'),
+        MockAddon('binderhub', categories=['other'])]))
+    store, actual_path = await split_storage(remote, osf, project,
+                                             normalize=normalize)
+    assert store is stores[['osfstorage', 'custom'].index(provider)]
+    assert actual_path == path
 
-def test_custom_split_storage():
-    def simple_getenv(key):
-        if key == 'KNOWN_PROVIDERS':
-            return 'osfstorage,s3,github'
-        return None
 
-    with patch('osfclient.cli.os.getenv',
-               side_effect=simple_getenv) as mock_getenv:
-        store, path = split_storage('osfstorage/foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'foo/bar/baz'
+@pytest.mark.asyncio
+async def test_split_storage_connected_provider_skips_addons():
+    stores = [SimpleNamespace(provider=name) for name in ['osfstorage', 'custom']]
+    project = SimpleNamespace(id='1234', storages=AsyncIterator(stores))
+    osf = SimpleNamespace(addons=AsyncIterator([MockAddon('custom')]))
 
-        store, path = split_storage('github/foo/bar/baz')
-        assert store == 'github'
-        assert path == 'foo/bar/baz'
+    store, path = await split_storage('custom/file.txt', osf, project)
 
-        store, path = split_storage('/github/foo/bar/baz')
-        assert store == 'github'
-        assert path == 'foo/bar/baz'
+    assert store is stores[1]
+    assert path == 'file.txt'
+    assert not osf.addons.__aiter__.called
 
-        store, path = split_storage('figshare/foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'figshare/foo/bar/baz'
 
-        store, path = split_storage('/figshare/foo/bar/baz')
-        assert store == 'osfstorage'
-        assert path == 'figshare/foo/bar/baz'
+@pytest.mark.asyncio
+@pytest.mark.parametrize('remote, normalize', [
+    ('unconnected/file.txt', True),
+    ('/unconnected/file.txt', True),
+    ('unconnected/', True),
+    ('/unconnected/', False),
+])
+async def test_split_storage_unconnected_provider(remote, normalize):
+    stores = [SimpleNamespace(provider='osfstorage')]
+    project = SimpleNamespace(id='1234', storages=AsyncIterator(stores))
+    osf = SimpleNamespace(addons=AsyncIterator([
+        MockAddon('custom'), MockAddon('unconnected', categories=['citations'])]))
+
+    with pytest.raises(RuntimeError, match=(
+            "Storage provider 'unconnected' is not connected to project '1234'")):
+        await split_storage(remote, osf, project, normalize=normalize)
 
-        store, path = split_storage('s3/foo/bar/baz')
-        assert store == 's3'
-        assert path == 'foo/bar/baz'
 
-        store, path = split_storage('/s3/foo/bar/baz')
-        assert store == 's3'
-        assert path == 'foo/bar/baz'
+@pytest.mark.asyncio
+async def test_split_storage_missing_default():
+    project = SimpleNamespace(id='1234', storages=AsyncIterator([]))
+    osf = SimpleNamespace(addons=AsyncIterator([]))
+    with pytest.raises(RuntimeError, match="no storage provider 'osfstorage'"):
+        await split_storage('folder/file.txt', osf, project)
 
 
 def test_norm_remote_path():
diff --git a/osfclient/utils.py b/osfclient/utils.py
index ba195c7..4d8e5ff 100644
--- a/osfclient/utils.py
+++ b/osfclient/utils.py
@@ -9,15 +9,6 @@
 import aiofiles
 
 
-KNOWN_PROVIDERS = [
-    'osfstorage', 'github', 'figshare', 'googledrive',
-    'azureblobstorage', 'bitbucket', 'box', 'dataverse', 'dropbox',
-    'gitlab', 'iqbrims', 'nextcloud', 'nextcloudinstitutions',
-    'ociinstitutions', 'owncloud', 'onedrivebusiness', 's3', 's3compat',
-    's3compatb3', 's3compatinstitutions', 'swift', 'weko'
-]
-
-
 def norm_remote_path(path: str) -> str:
     """Normalize `path`.
 
@@ -30,29 +21,40 @@ def norm_remote_path(path: str) -> str:
         return path
 
 
-def split_storage(path, default='osfstorage', normalize=True):
-    """Extract storage name from file path.
+async def split_storage(path, osf, project, default='osfstorage',
+                        normalize=True):
+    """Resolve a remote path against the project's connected storages.
 
-    If a path begins with a known storage provider the name is removed
-    from the path. Otherwise the `default` storage provider is returned
-    and the path is not modified.
+    A connected provider at the start of the path selects that storage.
+    A provider the server offers but the project has not connected is an
+    error rather than a folder in the default storage. Addons in the
+    ``other`` category never hold files, so their names stay usable as
+    folder names. Otherwise the entire path belongs to the default storage.
+    Explicitly prefix with ``osfstorage/`` to access a folder named after
+    a provider.
     """
+    is_directory = path.endswith('/')
     if normalize:
         path = norm_remote_path(path)
-    env_known_providers = os.getenv('KNOWN_PROVIDERS')
-    if env_known_providers is not None:
-        known_providers = env_known_providers.split(',')
-    else:
-        known_providers = KNOWN_PROVIDERS
-
-    for provider in known_providers:
-        if path.startswith(provider + '/'):
-            if six.PY3:
-                return path.split('/', maxsplit=1)
-            else:
-                return path.split('/', 1)
-
-    return (default, path)
+    path = path.lstrip('/')
+    provider, separator, remote_path = path.partition('/')
+    has_provider = bool(separator) or is_directory
+    default_store = None
+    async for store in project.storages:
+        if has_provider and store.provider == provider:
+            return store, remote_path
+        if store.provider == default:
+            default_store = store
+
+    if default_store is None:
+        raise RuntimeError("Project has no storage provider '{}'".format(default))
+    if has_provider:
+        async for addon in osf.addons:
+            if addon.id == provider and 'other' not in addon.categories:
+                raise RuntimeError(
+                    "Storage provider '{}' is not connected to project '{}'"
+                    .format(provider, project.id))
+    return default_store, path
 
 
 def makedirs(path, mode=511, exist_ok=False):