diff --git a/molecule-shared/homeserver-stub.py b/molecule-shared/homeserver-stub.py
new file mode 100644
index 000000000..b0a334f58
--- /dev/null
+++ b/molecule-shared/homeserver-stub.py
@@ -0,0 +1,137 @@
+# SPDX-FileCopyrightText: 2026 Slavi Pantaleev
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+"""A stand-in homeserver for Molecule scenarios.
+
+Most components in this playbook talk to a homeserver while starting up and
+exit if it is unreachable, so a scenario cannot get them running without one.
+Standing up a real Synapse for every role would dominate the run time and drag
+in Postgres, and the scenarios are not testing Synapse - they are testing that
+the role's configuration reaches the component and that it starts.
+
+So this answers the handful of endpoints components touch during startup, with
+the blandest plausible response in each case. It is deliberately permissive: an
+unknown path returns `{}` with a 200 rather than a 404, because the goal is to
+get the component past its startup checks, not to model the Matrix spec.
+
+What it is NOT: an authentication check, a room state machine, or anything a
+scenario should assert *about*. Assert on what the role rendered and on what the
+component reports about itself. If a scenario starts needing this stub to behave
+like a real homeserver, that scenario has outgrown what these tests are for.
+"""
+
+import json
+import os
+import re
+import sys
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+
+SERVER_NAME = os.environ.get("STUB_SERVER_NAME", "molecule.local")
+PORT = int(os.environ.get("STUB_PORT", "8008"))
+
+# Rooms reported as already joined. Components that resolve a room mapping at
+# startup (matrix-alertmanager-receiver, for one) fail if the rooms they were
+# configured with are missing, so a scenario passes its own room IDs in.
+JOINED_ROOMS = [r for r in os.environ.get("STUB_JOINED_ROOMS", "").split(",") if r]
+
+USER_ID = os.environ.get("STUB_USER_ID", f"@stub:{SERVER_NAME}")
+
+
+class Handler(BaseHTTPRequestHandler):
+    protocol_version = "HTTP/1.1"
+
+    def _send(self, payload, status=200):
+        body = json.dumps(payload).encode()
+        self.send_response(status)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(body)))
+        self.end_headers()
+        self.wfile.write(body)
+
+    def _route(self):
+        path = self.path.split("?", 1)[0]
+
+        if path.endswith("/joined_rooms"):
+            return {"joined_rooms": JOINED_ROOMS}
+
+        if path.endswith("/whoami"):
+            return {"user_id": USER_ID, "device_id": "STUBDEVICE"}
+
+        if path.endswith("/versions"):
+            return {
+                "versions": ["v1.1", "v1.2", "v1.3", "v1.4", "v1.5", "v1.6"],
+                "unstable_features": {},
+            }
+
+        if path.endswith("/capabilities"):
+            return {"capabilities": {}}
+
+        if path.endswith("/_matrix/client/v3/login"):
+            return {
+                "user_id": USER_ID,
+                "access_token": "stub_access_token",
+                "device_id": "STUBDEVICE",
+                "home_server": SERVER_NAME,
+            }
+
+        if path.endswith("/createRoom"):
+            return {"room_id": f"!stub-room:{SERVER_NAME}"}
+
+        if re.search(r"/rooms/[^/]+/join$", path) or path.endswith("/join"):
+            return {"room_id": f"!stub-room:{SERVER_NAME}"}
+
+        if "/send/" in path or "/state/" in path:
+            return {"event_id": f"$stub-event:{SERVER_NAME}"}
+
+        if path.endswith("/register"):
+            return {
+                "user_id": USER_ID,
+                "access_token": "stub_access_token",
+                "device_id": "STUBDEVICE",
+                "home_server": SERVER_NAME,
+            }
+
+        if path.endswith("/profile") or "/profile/" in path:
+            return {"displayname": "stub"}
+
+        if path.startswith("/_matrix/key/"):
+            return {"server_name": SERVER_NAME, "verify_keys": {}, "old_verify_keys": {}}
+
+        if path.startswith("/.well-known/matrix/client"):
+            return {"m.homeserver": {"base_url": f"http://{SERVER_NAME}:{PORT}"}}
+
+        if path.startswith("/.well-known/matrix/server"):
+            return {"m.server": f"{SERVER_NAME}:{PORT}"}
+
+        if path.endswith("/health") or path.endswith("/_matrix/federation/v1/version"):
+            return {"server": {"name": "molecule-stub", "version": "0"}}
+
+        # Anything unrecognised: an empty object, so a component doing a startup
+        # probe of an endpoint not listed here still gets past it.
+        return {}
+
+    def do_GET(self):
+        self._send(self._route())
+
+    def do_POST(self):
+        length = int(self.headers.get("Content-Length") or 0)
+        if length:
+            self.rfile.read(length)
+        self._send(self._route())
+
+    def do_PUT(self):
+        self.do_POST()
+
+    def do_DELETE(self):
+        self._send({})
+
+    def log_message(self, fmt, *args):
+        # Quiet by default; STUB_VERBOSE=1 when a scenario will not start and you
+        # need to see what the component is actually asking for.
+        if os.environ.get("STUB_VERBOSE"):
+            sys.stderr.write("stub: " + (fmt % args) + "\n")
+
+
+if __name__ == "__main__":
+    ThreadingHTTPServer(("0.0.0.0", PORT), Handler).serve_forever()
diff --git a/molecule-shared/homeserver-stub.py.license b/molecule-shared/homeserver-stub.py.license
new file mode 100644
index 000000000..dbb307901
--- /dev/null
+++ b/molecule-shared/homeserver-stub.py.license
@@ -0,0 +1,3 @@
+SPDX-FileCopyrightText: 2026 Slavi Pantaleev
+
+SPDX-License-Identifier: AGPL-3.0-or-later
diff --git a/molecule-shared/tasks/homeserver-stub.yml b/molecule-shared/tasks/homeserver-stub.yml
new file mode 100644
index 000000000..5fbe375dd
--- /dev/null
+++ b/molecule-shared/tasks/homeserver-stub.yml
@@ -0,0 +1,77 @@
+# SPDX-FileCopyrightText: 2026 Slavi Pantaleev
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+---
+# Stands up a stand-in homeserver on a container network, for scenarios whose
+# component contacts a homeserver while starting up.
+#
+# Include from a scenario's prepare.yml:
+#
+#   - name: Ensure the homeserver stub is running
+#     ansible.builtin.include_tasks:
+#       file: "{{ lookup('env', 'MOLECULE_PROJECT_DIRECTORY') }}/../../../molecule-shared/tasks/homeserver-stub.yml"
+#     vars:
+#       molecule_shared_stub_network: "{{ <role>_container_network }}"
+#       molecule_shared_stub_joined_rooms: ["!some-room:molecule.local"]
+#
+# The component should then be pointed at http://matrix.molecule.local:8008.
+#
+# See molecule-shared/homeserver-stub.py for what it answers and, more
+# importantly, for what it is not.
+
+- name: Ensure the homeserver stub script is present
+  ansible.builtin.copy:
+    src: "{{ lookup('env', 'MOLECULE_PROJECT_DIRECTORY') }}/../../../molecule-shared/homeserver-stub.py"
+    dest: /root/molecule-homeserver-stub.py
+    mode: "0755"
+
+- name: Ensure a previous homeserver stub is gone
+  ansible.builtin.command:
+    argv:
+      - docker
+      - rm
+      - --force
+      - "{{ molecule_shared_stub_name | default('matrix-homeserver-stub') }}"
+  register: molecule_shared_stub_removal
+  changed_when: molecule_shared_stub_removal.rc == 0
+  failed_when: false
+
+# The alias is what the component resolves, so its configuration can name a
+# hostname rather than a container name.
+- name: Ensure the homeserver stub is running
+  ansible.builtin.command:
+    argv:
+      - docker
+      - run
+      - --detach
+      - --name={{ molecule_shared_stub_name | default('matrix-homeserver-stub') }}
+      - --network={{ molecule_shared_stub_network }}
+      - --network-alias={{ molecule_shared_stub_hostname | default('matrix.molecule.local') }}
+      - --env=STUB_SERVER_NAME={{ molecule_shared_stub_server_name | default('molecule.local') }}
+      - --env=STUB_JOINED_ROOMS={{ (molecule_shared_stub_joined_rooms | default([])) | join(',') }}
+      - --volume=/root/molecule-homeserver-stub.py:/stub.py:ro
+      - "{{ molecule_shared_image_python }}"
+      - python3
+      - /stub.py
+  register: molecule_shared_stub_start
+  changed_when: molecule_shared_stub_start.rc == 0
+
+- name: Wait for the homeserver stub to answer
+  ansible.builtin.command:
+    argv:
+      - docker
+      - run
+      - --rm
+      - --network={{ molecule_shared_stub_network }}
+      - "{{ molecule_shared_image_curl }}"
+      - --silent
+      - --fail
+      - --max-time
+      - "5"
+      - "http://{{ molecule_shared_stub_hostname | default('matrix.molecule.local') }}:8008/_matrix/client/versions"
+  register: molecule_shared_stub_ready
+  changed_when: false
+  until: molecule_shared_stub_ready.rc == 0
+  retries: 12
+  delay: 5
diff --git a/roles/custom/matrix-alertmanager-receiver/molecule/default/prepare.yml b/roles/custom/matrix-alertmanager-receiver/molecule/default/prepare.yml
index 739a172ca..0066bb740 100644
--- a/roles/custom/matrix-alertmanager-receiver/molecule/default/prepare.yml
+++ b/roles/custom/matrix-alertmanager-receiver/molecule/default/prepare.yml
@@ -70,59 +70,12 @@
         - "'already exists' not in matrix_alertmanager_receiver_molecule_network.stderr"
 
     # matrix-alertmanager-receiver contacts the homeserver while starting up -
-    # it fetches /_matrix/client/v3/joined_rooms to resolve its room mapping -
-    # and exits 1 if that fails. So a homeserver has to exist for the service
-    # to come up at all. A stub is enough: the scenario is testing this role,
-    # not Synapse, and it keeps the run offline and fast.
-    - name: Ensure the Matrix homeserver stub script exists
-      ansible.builtin.copy:
-        dest: /root/matrix-homeserver-stub.py
-        mode: "0755"
-        content: |
-          import json
-          from http.server import BaseHTTPRequestHandler, HTTPServer
-
-          ROOMS = {"joined_rooms": ["{{ matrix_alertmanager_receiver_config_matrix_room_mapping['molecule-room'] }}"]}
-
-          class Handler(BaseHTTPRequestHandler):
-              def _send(self, payload):
-                  body = json.dumps(payload).encode()
-                  self.send_response(200)
-                  self.send_header("Content-Type", "application/json")
-                  self.send_header("Content-Length", str(len(body)))
-                  self.end_headers()
-                  self.wfile.write(body)
-
-              def do_GET(self):
-                  if self.path.endswith("/joined_rooms"):
-                      self._send(ROOMS)
-                  else:
-                      self._send({})
-
-              def do_POST(self):
-                  self._send({"event_id": "$molecule-event-id"})
-
-              def log_message(self, *args):
-                  pass
-
-          HTTPServer(("0.0.0.0", 8008), Handler).serve_forever()
-
-    - name: Ensure the Matrix homeserver stub is running on the role's network
-      ansible.builtin.command:
-        argv:
-          - docker
-          - run
-          - --detach
-          - --rm
-          - --name=matrix-homeserver-stub
-          - --network={{ matrix_alertmanager_receiver_container_network }}
-          - --network-alias=matrix.molecule.local
-          - --volume=/root/matrix-homeserver-stub.py:/stub.py:ro
-          - "{{ molecule_shared_image_python }}"
-          - python3
-          - /stub.py
-      register: matrix_alertmanager_receiver_molecule_stub
-      changed_when: matrix_alertmanager_receiver_molecule_stub.rc == 0
-      failed_when:
-        - matrix_alertmanager_receiver_molecule_stub.rc != 0
-        - "'already in use' not in matrix_alertmanager_receiver_molecule_stub.stderr"
+    # it fetches /_matrix/client/v3/joined_rooms to resolve its room mapping and
+    # exits 1 if that fails - so a homeserver has to exist for it to come up at
+    # all. The shared stub is enough; see molecule-shared/homeserver-stub.py.
+    - name: Ensure the homeserver stub is running
+      ansible.builtin.include_tasks:
+        file: "{{ lookup('env', 'MOLECULE_PROJECT_DIRECTORY') }}/../../../molecule-shared/tasks/homeserver-stub.yml"
+      vars:
+        molecule_shared_stub_network: "{{ matrix_alertmanager_receiver_container_network }}"
+        molecule_shared_stub_joined_rooms: "{{ matrix_alertmanager_receiver_config_matrix_room_mapping.values() | list }}"