diff --git a/internal/chrome/exec.go b/internal/api/js/chrome/chrome.go
similarity index 55%
rename from internal/chrome/exec.go
rename to internal/api/js/chrome/chrome.go
index e0ecc57..f580860 100644
--- a/internal/chrome/exec.go
+++ b/internal/api/js/chrome/chrome.go
@@ -1,3 +1,7 @@
+// package chrome provides helper functions to execute JS in a Chrome browser
+//
+// This would ordinarily be done via a Chrome struct but Go does not allow
+// generic methods, only generic static functions, producing "method must have no type parameters".
 package chrome
 
 import (
@@ -6,9 +10,42 @@ import (
 
 	"github.com/chromedp/cdproto/runtime"
 	"github.com/chromedp/chromedp"
+	"github.com/matrix-org/complement-crypto/internal/api"
 	"github.com/matrix-org/complement/must"
 )
 
+// Void is a type which can be used when you want to run an async function without returning anything.
+// It can stop large responses causing errors "Object reference chain is too long (-32000)"
+// when we don't care about the response.
+type Void *runtime.RemoteObject
+
+// Run an anonymous async iffe in the browser. Set the type parameter to a basic data type
+// which can be returned as JSON e.g string, map[string]any, []string. If you do not want
+// to return anything, use chrome.Void
+func RunAsyncFn[T any](t *testing.T, ctx context.Context, js string) (*T, error) {
+	t.Helper()
+	out := new(T)
+	err := chromedp.Run(ctx,
+		chromedp.Evaluate(`(async () => {`+js+`})()`, &out, func(p *runtime.EvaluateParams) *runtime.EvaluateParams {
+			return p.WithAwaitPromise(true)
+		}),
+	)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// MustRunAsyncFn is RunAsyncFn but fails the test if an error is returned when executing.
+func MustRunAsyncFn[T any](t *testing.T, ctx context.Context, js string) *T {
+	t.Helper()
+	result, err := RunAsyncFn[T](t, ctx, js)
+	if err != nil {
+		api.Fatalf(t, "MustRunAsyncFn: %s", err)
+	}
+	return result
+}
+
 func MustExecuteInto[T any](t *testing.T, ctx context.Context, js string) T {
 	t.Helper()
 	out, err := ExecuteInto[T](t, ctx, js)
diff --git a/internal/api/js/js.go b/internal/api/js/js.go
index 91c2cd7..f717569 100644
--- a/internal/api/js/js.go
+++ b/internal/api/js/js.go
@@ -19,7 +19,7 @@ import (
 	"github.com/chromedp/cdproto/runtime"
 	"github.com/chromedp/chromedp"
 	"github.com/matrix-org/complement-crypto/internal/api"
-	"github.com/matrix-org/complement-crypto/internal/chrome"
+	"github.com/matrix-org/complement-crypto/internal/api/js/chrome"
 	"github.com/matrix-org/complement/must"
 	"github.com/tidwall/gjson"
 )
@@ -336,7 +336,8 @@ func (c *JSClient) MustBackpaginate(t *testing.T, roomID string, count int) {
 }
 
 func (c *JSClient) MustBackupKeys(t *testing.T) (recoveryKey string) {
-	key, err := chrome.AwaitExecuteInto[string](t, c.ctx, `(async () => {
+	t.Helper()
+	key := chrome.MustRunAsyncFn[string](t, c.ctx, `
 		// we need to ensure that we have a recovery key first, though we don't actually care about it..?
 		const recoveryKey = await window.__client.getCrypto().createRecoveryKeyFromPassphrase();
 		// now use said key to make backups
@@ -347,11 +348,7 @@ func (c *JSClient) MustBackupKeys(t *testing.T) (recoveryKey string) {
 		});
 		// now we can enable key backups
 		await window.__client.getCrypto().checkKeyBackupAndEnable();
-		return recoveryKey.encodedPrivateKey;
-	})()`)
-	if err != nil {
-		api.Fatalf(t, "MustBackupKeys: %s", err)
-	}
+		return recoveryKey.encodedPrivateKey;`)
 	// the backup loop which sends keys will wait between 0-10s before uploading keys...
 	// See https://github.com/matrix-org/matrix-js-sdk/blob/49624d5d7308e772ebee84322886a39d2e866869/src/rust-crypto/backup.ts#L319
 	// Ideally this would be configurable..
@@ -360,7 +357,7 @@ func (c *JSClient) MustBackupKeys(t *testing.T) (recoveryKey string) {
 }
 
 func (c *JSClient) MustLoadBackup(t *testing.T, recoveryKey string) {
-	chrome.MustAwaitExecute(t, c.ctx, fmt.Sprintf(`(async () => {
+	chrome.MustRunAsyncFn[chrome.Void](t, c.ctx, fmt.Sprintf(`
 		// we assume the recovery key is the private key for the default key id so
 		// figure out what that key id is.
 		const keyId = await window.__client.secretStorage.getDefaultKeyId();
@@ -374,8 +371,8 @@ func (c *JSClient) MustLoadBackup(t *testing.T, recoveryKey string) {
 		console.log("key backup: ", JSON.stringify(keyBackupCheck));
 		// FIXME: this just doesn't seem to work, causing 'Error: getSecretStorageKey callback returned invalid data' because the key ID
 		// cannot be found...
-		await window.__client.restoreKeyBackupWithSecretStorage(keyBackupCheck ? keyBackupCheck.backupInfo : null, undefined, undefined);
-	})()`, recoveryKey))
+		await window.__client.restoreKeyBackupWithSecretStorage(keyBackupCheck ? keyBackupCheck.backupInfo : null, undefined, undefined);`,
+		recoveryKey))
 }
 
 func (c *JSClient) WaitUntilEventInRoom(t *testing.T, roomID string, checker func(e api.Event) bool) api.Waiter {
diff --git a/internal/chrome/callbacks.go b/internal/chrome/callbacks.go
deleted file mode 100644
index 7b44280..0000000
--- a/internal/chrome/callbacks.go
+++ /dev/null
@@ -1 +0,0 @@
-package chrome
diff --git a/tests/addons/README.md b/tests/addons/README.md
new file mode 100644
index 0000000..0550e01
--- /dev/null
+++ b/tests/addons/README.md
@@ -0,0 +1,37 @@
+### mitmproxy
+
+This directory contains code that will be used as a [mitmproxy addon](https://docs.mitmproxy.org/stable/addons-overview/).
+
+How this works:
+ - A vanilla `mitmproxy` is run in the same network as the homeservers.
+ - It is told to proxy both hs1 and hs2 i.e `mitmdump --mode reverse:http://hs1:8008@3000`
+ - It is also told to run a normal proxy, to which a Flask HTTP server is attached.
+ - The Flask HTTP server can be used to control mitmproxy at test runtime. This is done via the Controller HTTP API.
+
+
+### Controller HTTP API
+
+**This is highly experimental and will change without warning.**
+
+`mitmproxy` is run once for all tests. To avoid test pollution, the controller is "locked" for the duration
+of a test and must be "unlocked" afterwards. When acquiring the lock, options can be set on `mitmproxy`.
+
+```
+POST /options/lock
+ {
+   "options": {
+     "body_size_limit": "3m",
+   }
+ }
+ HTTP/1.1 200 OK
+ {
+   "reset_id": "some_opaque_string"
+ }
+```
+
+```
+POST /options/unlock
+{
+   "reset_id": "some_opaque_string"
+}
+```
\ No newline at end of file