diff --git a/mocks/mock-google-oidc/.gitignore b/mocks/mock-google-oidc/.gitignore
new file mode 100644
index 000000000..504afef81
--- /dev/null
+++ b/mocks/mock-google-oidc/.gitignore
@@ -0,0 +1,2 @@
+node_modules/
+package-lock.json
diff --git a/mocks/mock-google-oidc/README.md b/mocks/mock-google-oidc/README.md
new file mode 100644
index 000000000..ad00cc2a6
--- /dev/null
+++ b/mocks/mock-google-oidc/README.md
@@ -0,0 +1,73 @@
+# Mock Google OIDC Provider
+
+A lightweight mock OpenID Connect provider that simulates Google login for local MAS development. It lets you test the upstream OAuth2 / "Sign in with Google" flow without needing real Google credentials.
+
+## Prerequisites
+
+- Node.js 18+
+- MAS configured with an upstream OAuth2 provider pointing to this mock
+
+## Quick start
+
+```sh
+cd mock-google-oidc
+npm install
+npm start
+```
+
+The server starts at `http://localhost:5556`.
+
+## How it works
+
+The mock uses [`oidc-provider`](https://github.com/panva/node-oidc-provider) to run a standards-compliant OIDC server with dev interactions enabled. When MAS redirects to the mock for login, you'll see a simple form where you can enter any email-like value (e.g. `taylor@gmail.com`). The mock will return that as the authenticated user with synthetic profile claims.
+
+## MAS configuration
+
+Add an upstream OAuth2 provider entry in your MAS `config.yaml`:
+
+```yaml
+upstream_oauth2:
+  providers:
+    - id: "01JQ0FAKEG00G1E0D1CPR0V1D3"
+      human_name: "Google"
+      issuer: "http://localhost:5556"
+      client_id: "mas-dev"
+      client_secret: "mas-dev-secret"
+      token_endpoint_auth_method: "client_secret_post"
+      scope: "openid email profile"
+      claims_imports:
+        localpart:
+          action: suggest
+          template: "{{ user.preferred_username }}"
+        displayname:
+          action: suggest
+          template: "{{ user.name }}"
+        email:
+          action: suggest
+          template: "{{ user.email }}"
+          set_email_verification: always
+```
+
+## Environment variables
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `MOCK_OIDC_ISSUER` | `http://localhost:5556` | The issuer URL |
+| `MAS_PROVIDER_ID` | `01JQ0FAKEG00G1E0D1CPR0V1D3` | The upstream provider ID configured in MAS |
+| `MOCK_OIDC_CLIENT_ID` | `mas-dev` | OAuth2 client ID |
+| `MOCK_OIDC_CLIENT_SECRET` | `mas-dev-secret` | OAuth2 client secret |
+
+## Running with the full dev stack
+
+```sh
+# Terminal 1: Mock Synapse (handles homeserver API calls)
+python3 mock_synapse.py
+
+# Terminal 2: Mock Google OIDC
+cd mock-google-oidc && npm start
+
+# Terminal 3: MAS
+cargo run -- server -c config.yaml
+```
+
+Then visit `http://[::]:8080/login` and click "Sign in with Google".
diff --git a/mocks/mock-google-oidc/package.json b/mocks/mock-google-oidc/package.json
new file mode 100644
index 000000000..a17d78baa
--- /dev/null
+++ b/mocks/mock-google-oidc/package.json
@@ -0,0 +1,12 @@
+{
+  "name": "mock-google-oidc",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "start": "node server.mjs"
+  },
+  "dependencies": {
+    "jose": "^6.0.11",
+    "oidc-provider": "^9.5.1"
+  }
+}
diff --git a/mocks/mock-google-oidc/server.mjs b/mocks/mock-google-oidc/server.mjs
new file mode 100644
index 000000000..a8b8efeca
--- /dev/null
+++ b/mocks/mock-google-oidc/server.mjs
@@ -0,0 +1,77 @@
+import { exportJWK, generateKeyPair } from 'jose';
+import { Provider } from 'oidc-provider';
+
+const ISSUER = process.env.MOCK_OIDC_ISSUER ?? 'http://localhost:5556';
+const PROVIDER_ID =
+  process.env.MAS_PROVIDER_ID ?? '01JQ0FAKEG00G1E0D1CPR0V1D3';
+const CLIENT_ID = process.env.MOCK_OIDC_CLIENT_ID ?? 'mas-dev';
+const CLIENT_SECRET =
+  process.env.MOCK_OIDC_CLIENT_SECRET ?? 'mas-dev-secret';
+
+const redirectUris = [
+  `http://localhost:8080/upstream/callback/${PROVIDER_ID}`,
+  `http://127.0.0.1:8080/upstream/callback/${PROVIDER_ID}`,
+  `http://[::]:8080/upstream/callback/${PROVIDER_ID}`,
+];
+
+const { privateKey } = await generateKeyPair('RS256', { extractable: true });
+const jwk = await exportJWK(privateKey);
+jwk.use = 'sig';
+jwk.alg = 'RS256';
+jwk.kid = 'mock-google-rs256';
+
+const configuration = {
+  clients: [
+    {
+      client_id: CLIENT_ID,
+      client_secret: CLIENT_SECRET,
+      redirect_uris: redirectUris,
+      response_types: ['code'],
+      grant_types: ['authorization_code'],
+      token_endpoint_auth_method: 'client_secret_post',
+    },
+  ],
+  jwks: { keys: [jwk] },
+  claims: {
+    openid: ['sub'],
+    email: ['email', 'email_verified'],
+    profile: ['name', 'preferred_username', 'picture'],
+  },
+  cookies: {
+    keys: ['mock-google-oidc-dev-key-1', 'mock-google-oidc-dev-key-2'],
+  },
+  features: {
+    devInteractions: { enabled: true },
+    rpInitiatedLogout: { enabled: false },
+  },
+  async findAccount(_ctx, sub) {
+    const email = sub.includes('@') ? sub : `${sub}@gmail.com`;
+    const preferredUsername = email.split('@')[0];
+
+    return {
+      accountId: sub,
+      async claims(_use, _scope) {
+        return {
+          sub,
+          email,
+          email_verified: true,
+          name: 'Taylor Google User',
+          preferred_username: preferredUsername,
+          picture:
+            'https://www.gstatic.com/images/branding/product/1x/avatar_circle_blue_512dp.png',
+        };
+      },
+    };
+  },
+};
+
+const provider = new Provider(ISSUER, configuration);
+
+provider.listen(5556, () => {
+  console.log(`Mock Google OIDC running at ${ISSUER}`);
+  console.log('Configured redirect URIs:');
+  for (const uri of redirectUris) {
+    console.log(`  - ${uri}`);
+  }
+  console.log('Use any email-like value in the dev interaction login form.');
+});
diff --git a/mocks/mock_synapse.py b/mocks/mock_synapse.py
new file mode 100644
index 000000000..d0bc9c518
--- /dev/null
+++ b/mocks/mock_synapse.py
@@ -0,0 +1,266 @@
+#!/usr/bin/env python3
+"""
+Mock Synapse server for testing MAS UI without a real homeserver.
+
+Handles both Modern (_synapse/mas/...) and Legacy (_synapse/admin/...)
+API endpoints that MAS calls on Synapse.
+
+Usage:
+    python3 mock_synapse.py
+
+Then start MAS normally:
+    cargo run -- server -c config.yaml
+"""
+
+from http.server import HTTPServer, BaseHTTPRequestHandler
+from urllib.parse import urlparse, parse_qs
+import json
+import sys
+
+
+# In-memory store of provisioned users: localpart -> user data
+provisioned_users = {}
+
+
+class MockSynapseHandler(BaseHTTPRequestHandler):
+
+    def _send_json(self, data, status=200):
+        self.send_response(status)
+        self.send_header("Content-Type", "application/json")
+        self.end_headers()
+        self.wfile.write(json.dumps(data).encode())
+
+    def _read_body(self):
+        length = int(self.headers.get("Content-Length", 0))
+        if length:
+            return json.loads(self.rfile.read(length))
+        return {}
+
+    # ── GET endpoints ──────────────────────────────────────────────
+
+    def do_GET(self):
+        parsed = urlparse(self.path)
+        path = parsed.path.rstrip("/")
+        qs = parse_qs(parsed.query)
+
+        # Modern: check if localpart is available
+        if path == "/_synapse/mas/is_localpart_available":
+            localpart = qs.get("localpart", [""])[0]
+            if localpart in provisioned_users:
+                print(f"  [mock] is_localpart_available: {localpart} → in use")
+                self._send_json({"errcode": "M_USER_IN_USE", "error": "User ID already taken"}, 409)
+            else:
+                print(f"  [mock] is_localpart_available: {localpart} → available")
+                self._send_json({})  # 200 = available
+            return
+
+        # Modern: query user info
+        if path == "/_synapse/mas/query_user":
+            localpart = qs.get("localpart", [""])[0]
+            if localpart in provisioned_users:
+                user = provisioned_users[localpart]
+                print(f"  [mock] query_user: {localpart} → found")
+                self._send_json({
+                    "user_id": f"@{localpart}:localhost:8008",
+                    "display_name": user.get("display_name"),
+                    "avatar_url": user.get("avatar_url"),
+                    "is_suspended": False,
+                    "is_deactivated": False,
+                })
+            else:
+                print(f"  [mock] query_user: {localpart} → not found (404)")
+                self._send_json({"errcode": "M_NOT_FOUND", "error": "User not found"}, 404)
+            return
+
+        # Legacy: username available
+        if path == "/_synapse/admin/v1/username_available":
+            username = qs.get("username", [""])[0]
+            print(f"  [mock] username_available: {username} → available=true")
+            self._send_json({"available": True})
+            return
+
+        # Legacy: get user info
+        if "/_synapse/admin/v2/users/" in path and "/devices" not in path:
+            print(f"  [mock] get user → not found (404)")
+            self._send_json({"errcode": "M_NOT_FOUND", "error": "User not found"}, 404)
+            return
+
+        # Legacy: list devices
+        if "/_synapse/admin/v2/users/" in path and path.endswith("/devices"):
+            print(f"  [mock] list devices → empty")
+            self._send_json({"devices": []})
+            return
+
+        # Matrix client versions
+        if path == "/_matrix/client/versions":
+            self._send_json({"versions": ["v1.1", "v1.2", "v1.3"]})
+            return
+
+        # Catch-all
+        print(f"  [mock] Unhandled GET: {self.path}")
+        self._send_json({})
+
+    # ── POST endpoints ─────────────────────────────────────────────
+
+    def do_POST(self):
+        parsed = urlparse(self.path)
+        path = parsed.path.rstrip("/")
+        body = self._read_body()
+
+        # Modern: provision user
+        if path == "/_synapse/mas/provision_user":
+            localpart = body.get("localpart", "?")
+            is_new = localpart not in provisioned_users
+            provisioned_users[localpart] = {
+                "display_name": body.get("set_displayname"),
+                "avatar_url": body.get("set_avatar_url"),
+                "emails": body.get("set_emails", []),
+            }
+            status = 201 if is_new else 200
+            print(f"  [mock] provision_user: {localpart} → {status} {'created' if is_new else 'updated'}")
+            self._send_json({}, status)
+            return
+
+        # Modern: upsert device
+        if path == "/_synapse/mas/upsert_device":
+            print(f"  [mock] upsert_device: {body.get('device_id', '?')} → ok")
+            self._send_json({})
+            return
+
+        # Modern: update device display name
+        if path == "/_synapse/mas/update_device_display_name":
+            print(f"  [mock] update_device_display_name → ok")
+            self._send_json({})
+            return
+
+        # Modern: delete device
+        if path == "/_synapse/mas/delete_device":
+            print(f"  [mock] delete_device → ok")
+            self._send_json({})
+            return
+
+        # Modern: sync devices
+        if path == "/_synapse/mas/sync_devices":
+            print(f"  [mock] sync_devices → ok")
+            self._send_json({})
+            return
+
+        # Modern: delete user
+        if path == "/_synapse/mas/delete_user":
+            print(f"  [mock] delete_user → ok")
+            self._send_json({})
+            return
+
+        # Modern: reactivate user
+        if path == "/_synapse/mas/reactivate_user":
+            print(f"  [mock] reactivate_user → ok")
+            self._send_json({})
+            return
+
+        # Modern: set displayname
+        if path == "/_synapse/mas/set_displayname":
+            localpart = body.get("localpart", "?")
+            if localpart in provisioned_users:
+                provisioned_users[localpart]["display_name"] = body.get("displayname")
+            print(f"  [mock] set_displayname: {localpart} → ok")
+            self._send_json({})
+            return
+
+        # Modern: unset displayname
+        if path == "/_synapse/mas/unset_displayname":
+            localpart = body.get("localpart", "?")
+            if localpart in provisioned_users:
+                provisioned_users[localpart]["display_name"] = None
+            print(f"  [mock] unset_displayname: {localpart} → ok")
+            self._send_json({})
+            return
+
+        # Modern: allow cross-signing reset
+        if path == "/_synapse/mas/allow_cross_signing_reset":
+            print(f"  [mock] allow_cross_signing_reset → ok")
+            self._send_json({})
+            return
+
+        # Legacy: create device
+        if "/_synapse/admin/v2/users/" in path and path.endswith("/devices"):
+            print(f"  [mock] legacy create device → 201")
+            self._send_json({}, 201)
+            return
+
+        # Legacy: bulk delete devices
+        if "/_synapse/admin/v2/users/" in path and "delete_devices" in path:
+            print(f"  [mock] legacy bulk delete devices → ok")
+            self._send_json({})
+            return
+
+        # Legacy: deactivate user
+        if "/_synapse/admin/v1/deactivate/" in path:
+            print(f"  [mock] legacy deactivate → ok")
+            self._send_json({})
+            return
+
+        # Legacy: allow cross-signing reset
+        if "_allow_cross_signing_replacement_without_uia" in path:
+            print(f"  [mock] legacy allow cross-signing reset → ok")
+            self._send_json({})
+            return
+
+        # Catch-all
+        print(f"  [mock] Unhandled POST: {path} body={body}")
+        self._send_json({})
+
+    # ── PUT endpoints ──────────────────────────────────────────────
+
+    def do_PUT(self):
+        parsed = urlparse(self.path)
+        path = parsed.path.rstrip("/")
+        body = self._read_body()
+
+        # Legacy: create/update user
+        if "/_synapse/admin/v2/users/" in path and "/devices/" not in path:
+            print(f"  [mock] legacy put user → 201 created")
+            self._send_json({}, 201)
+            return
+
+        # Legacy: update device display name
+        if "/_synapse/admin/v2/users/" in path and "/devices/" in path:
+            print(f"  [mock] legacy update device display name → ok")
+            self._send_json({})
+            return
+
+        # Client API: set displayname
+        if "/_matrix/client/v3/profile/" in path and "/displayname" in path:
+            print(f"  [mock] set profile displayname → ok")
+            self._send_json({})
+            return
+
+        # Catch-all
+        print(f"  [mock] Unhandled PUT: {path} body={body}")
+        self._send_json({})
+
+    # ── DELETE endpoints ───────────────────────────────────────────
+
+    def do_DELETE(self):
+        parsed = urlparse(self.path)
+        path = parsed.path.rstrip("/")
+
+        # Legacy: delete device
+        if "/_synapse/admin/v2/users/" in path and "/devices/" in path:
+            print(f"  [mock] legacy delete device → ok")
+            self._send_json({})
+            return
+
+        # Catch-all
+        print(f"  [mock] Unhandled DELETE: {path}")
+        self._send_json({})
+
+
+if __name__ == "__main__":
+    port = int(sys.argv[1]) if len(sys.argv) > 1 else 8008
+    server = HTTPServer(("localhost", port), MockSynapseHandler)
+    print(f"Mock Synapse running on http://localhost:{port}")
+    print("Press Ctrl+C to stop\n")
+    try:
+        server.serve_forever()
+    except KeyboardInterrupt:
+        print("\nStopped.")