diff --git a/changelog.d/17031.feature b/changelog.d/17031.feature
new file mode 100644
index 000000000..0f28cbbcd
--- /dev/null
+++ b/changelog.d/17031.feature
@@ -0,0 +1 @@
+OIDC: try to JWT decode userinfo response if JSON parsing failed.
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 08a8634da..4718517c9 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -390,6 +390,13 @@ class BaseHttpClient:
                         cooperator=self._cooperator,
                     )
 
+                # Always make sure we add a user agent to the request
+                if headers is None:
+                    headers = Headers()
+
+                if not headers.hasHeader("User-Agent"):
+                    headers.addRawHeader("User-Agent", self.user_agent)
+
                 request_deferred: defer.Deferred = treq.request(
                     method,
                     uri,