diff --git a/test/unit/test_builder.py b/test/unit/test_builder.py
index 3e10a24..54457e6 100644
--- a/test/unit/test_builder.py
+++ b/test/unit/test_builder.py
@@ -2,16 +2,152 @@
 # koji hub plugin unit tests
 #
 
+import json
+import os
+import uuid
+from flexmock import flexmock
 
 import koji
-from flexmock import flexmock
+import httpretty
 
 from plugintest import PluginTest
 
 
+class MockComposer:
+    def __init__(self, *, architectures=["x86_64"]):
+        self.architectures = architectures[:]
+        self.composes = {}
+        self.errors = []
+        self.build_id = 1
+
+    def httpretty_regsiter(self):
+        httpretty.register_uri(
+            httpretty.POST,
+            "http://localhost:8701/compose",
+            body=self.compose_create
+        )
+
+    def next_build_id(self):
+        build_id = self.build_id
+        self.build_id += 1
+        return build_id
+
+    def compose_create(self, request, _uri, response_headers):
+        content_type = request.headers.get("Content-Type")
+        if content_type != "application/json":
+            return [400, response_headers, "Bad Request"]
+
+        js = json.loads(request.body)
+        ireqs = js.get("image_requests")
+        if not ireqs:
+            return [400, response_headers, "Bad Request"]
+
+        for it in ireqs:
+            arch = it.get("architecture")
+            if arch not in self.architectures:
+                return [400, response_headers, "Unsupported Architrecture"]
+
+        compose_id = str(uuid.uuid4())
+        build_id = self.next_build_id()
+        compose = {
+            "id": compose_id,
+            "koji_build_id": build_id,
+        }
+
+        self.composes[compose_id] = {
+            "request": js,
+            "result": compose,
+            "status": "success",
+        }
+
+        httpretty.register_uri(
+            httpretty.GET,
+            "http://localhost:8701/compose/" + compose_id,
+            body=self.compose_status
+        )
+
+        return [201, response_headers, json.dumps(compose)]
+
+    def compose_status(self, _request, uri, response_headers):
+        target = os.path.basename(uri)
+        compose = self.composes.get(target)
+        if not compose:
+            return [400, response_headers, f"Unknown compose: {target}"]
+
+        ireqs = compose["request"]["image_requests"]
+        result = {
+            "status": compose["status"],
+            "koji_task_id": compose["request"]["koji"]["task_id"],
+            "image_statuses": [
+                {"status": compose["status"] for _ in ireqs}
+            ]
+        }
+        return [200, response_headers, json.dumps(result)]
+
+
 @PluginTest.load_plugin("builder")
 class TestBuilderPlugin(PluginTest):
 
+    @staticmethod
+    def mock_session():
+        session = flexmock()
+
+        build_target = {
+            "build_tag": 23,
+            "build_tag_name": "fedora-build",
+            "dest_tag": 42,
+            "dest_tag_name": "fedora-dest"
+        }
+
+        tag_info = {
+            "id": build_target["dest_tag"],
+            "name": build_target["dest_tag_name"],
+            "locked": False
+        }
+
+        build_config = {
+            "arches": "x86_64"
+        }
+
+        repo_info = {
+            "id": 20201015,
+            "tag": build_target["build_tag_name"],
+            "tag_id": build_target["build_tag"],
+            "event_id": 2121,
+        }
+
+        session.should_receive("getBuildTarget") \
+               .with_args("fedora-candidate", strict=True) \
+               .and_return(build_target)
+
+        session.should_receive('getBuildConfig') \
+               .with_args(build_target["build_tag"]) \
+               .and_return(build_config)
+
+        session.should_receive("getTag") \
+               .with_args(build_target["build_tag"], strict=True) \
+               .and_return(tag_info)
+
+        session.should_receive('getRepo') \
+               .with_args(build_target["build_tag"]) \
+               .and_return(repo_info)
+
+        session.should_receive('getNextRelease') \
+               .with_args(dict) \
+               .and_return("20201015")
+
+        return session
+
+    @staticmethod
+    def mock_options():
+        options = flexmock(
+            allowed_scms='pkg.osbuild.org:/*:no',
+            workdir="/tmp",
+            topurl="http://localhost/kojifiles"
+        )
+        return options
+
+
     def test_unknown_build_target(self):
         session = flexmock()
 
@@ -72,3 +208,29 @@ class TestBuilderPlugin(PluginTest):
         with self.assertRaises(koji.BuildError) as err:
             handler.handler(*args)
             self.assertTrue(str(err).startswith("Unsupported"))
+
+
+    @httpretty.activate
+    def test_bad_request(self):
+        # Simulate a bad request by asking for an unsupported architecture
+        session = self.mock_session()
+        options = self.mock_options()
+
+        handler = self.plugin.OSBuildImage(1,
+                                           "osbuildImage",
+                                           "params",
+                                           session,
+                                           options)
+
+        args = ["name", "version", "distro",
+                ["image_type"],
+                "fedora-candidate",
+                ["x86_64"],
+                {}]
+
+        composer = MockComposer(architectures=["s390x"])
+        composer.httpretty_regsiter()
+
+        with self.assertRaises(koji.GenericError):
+            handler.handler(*args)
+