diff --git a/talisker/requests.py b/talisker/requests.py
index fb133915..b104c35b 100644
--- a/talisker/requests.py
+++ b/talisker/requests.py
@@ -169,6 +169,8 @@ def send(request, **kwargs):
             deadline = datetime.utcfromtimestamp(Context.current.deadline)
             formatted = deadline.isoformat() + 'Z'
             request.headers[config.deadline_header] = formatted
+        if Context.debug:
+            request.headers['X-Debug'] = '1'
         try:
             return func(request, **kwargs)
         except Exception as e:
diff --git a/tests/test_requests.py b/tests/test_requests.py
index ed92b9ca..66e2ce56 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -223,6 +223,7 @@ def test_metric_hook_registered_endpoint(
 def test_configured_session(context, get_breadcrumbs):
     deadline = time.time() + 10
     expected_deadline = datetime.utcfromtimestamp(deadline).isoformat() + 'Z'
+    Context.set_debug()
     Context.current.deadline = deadline
     session = requests.Session()
     talisker.requests.configure(session)
@@ -242,6 +243,7 @@ def test_configured_session(context, get_breadcrumbs):
     headers = responses.calls[0].request.headers
     assert headers['X-Request-Id'] == 'XXX'
     assert headers['X-Request-Deadline'] == expected_deadline
+    assert headers['X-Debug'] == '1'
     assert context.statsd[0] == 'requests.count.localhost.view:1|c'
     assert context.statsd[1].startswith(
         'requests.latency.localhost.view.200:')