diff --git a/.gitignore b/.gitignore
index 2116329..c950518 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 js-sdk/node_modules
 js-sdk/dist
 internal/api/dist
+__pycache__
\ No newline at end of file
diff --git a/internal/deploy/deploy.go b/internal/deploy/deploy.go
index 773ccfc..e62ae9f 100644
--- a/internal/deploy/deploy.go
+++ b/internal/deploy/deploy.go
@@ -8,6 +8,7 @@ import (
 	"net/url"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"sync"
 	"testing"
 	"time"
@@ -88,7 +89,15 @@ func RunNewDeployment(t *testing.T, shouldTCPDump bool) *SlidingSyncDeployment {
 	deployment := complement.Deploy(t, 2)
 	networkName := deployment.Network()
 
-	// make a reverse proxy.
+	workingDir, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("failed to find working directory: %s", err)
+	}
+
+	// Make the mitmproxy and hardcode CONTAINER PORTS for hs1/hs2. HOST PORTS are still dynamically allocated.
+	// By running this container on the same network as the homeservers, we can leverage DNS hence hs1/hs2 URLs.
+	// We also need to preload addons into the proxy, so we bind mount the addons directory. This also allows
+	// test authors to easily add custom addons.
 	hs1ExposedPort := "3000/tcp"
 	hs2ExposedPort := "3001/tcp"
 	mitmproxyContainer, err := testcontainers.GenericContainer(context.Background(), testcontainers.GenericContainerRequest{
@@ -97,13 +106,16 @@ func RunNewDeployment(t *testing.T, shouldTCPDump bool) *SlidingSyncDeployment {
 			ExposedPorts: []string{hs1ExposedPort, hs2ExposedPort},
 			Env:          map[string]string{},
 			Cmd: []string{
-				"mitmdump", "--mode", "reverse:http://hs1:8008@3000", "--mode", "reverse:http://hs2:8008@3001",
+				"mitmdump", "--mode", "reverse:http://hs1:8008@3000", "--mode", "reverse:http://hs2:8008@3001", "-s", "/addons/__init__.py",
 			},
 			// WaitingFor: wait.ForLog("listening"),
 			Networks: []string{networkName},
 			NetworkAliases: map[string][]string{
 				networkName: {"mitmproxy"},
 			},
+			Mounts: testcontainers.Mounts(
+				testcontainers.BindMount(filepath.Join(workingDir, "addons"), "/addons"),
+			),
 		},
 		Started: true,
 	})
diff --git a/tests/addons/__init__.py b/tests/addons/__init__.py
new file mode 100644
index 0000000..feb1d2e
--- /dev/null
+++ b/tests/addons/__init__.py
@@ -0,0 +1,4 @@
+from add_header import AddHeader
+
+
+addons = [AddHeader()]
diff --git a/tests/addons/add_header.py b/tests/addons/add_header.py
new file mode 100644
index 0000000..13c792f
--- /dev/null
+++ b/tests/addons/add_header.py
@@ -0,0 +1,9 @@
+class AddHeader:
+    def __init__(self):
+        self.num = 0
+
+    def response(self, flow):
+        self.num = self.num + 1
+        print("got response num", self.num)
+        flow.response.headers["count"] = str(self.num)
+        
\ No newline at end of file