From 3e90012509300c4a0fd8083370f50d11a0cdb705 Mon Sep 17 00:00:00 2001
From: Brian Pandola <bpandola@gmail.com>
Date: Tue, 19 Mar 2024 16:51:34 -0700
Subject: [PATCH] Add StateTransitionReason to ec2:DescribeInstances response
 (#7495)

---
 moto/ec2/responses/instances.py  | 2 +-
 tests/test_ec2/test_instances.py | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/moto/ec2/responses/instances.py b/moto/ec2/responses/instances.py
index 479cc3edb..78b1ccfc9 100644
--- a/moto/ec2/responses/instances.py
+++ b/moto/ec2/responses/instances.py
@@ -463,7 +463,7 @@ INSTANCE_TEMPLATE = """<item>
           <privateDnsName>{{ instance.private_dns }}</privateDnsName>
           <publicDnsName>{{ instance.public_dns }}</publicDnsName>
           <dnsName>{{ instance.public_dns }}</dnsName>
-          <reason/>
+          <reason>{{ instance._reason }}</reason>
           {% if instance.key_name is not none %}
              <keyName>{{ instance.key_name }}</keyName>
           {% endif %}
diff --git a/tests/test_ec2/test_instances.py b/tests/test_ec2/test_instances.py
index 161938d7b..7cd80dcbb 100644
--- a/tests/test_ec2/test_instances.py
+++ b/tests/test_ec2/test_instances.py
@@ -973,6 +973,7 @@ def test_instance_start_and_stop():
 
     instance1.reload()
     assert instance1.state == {"Code": 80, "Name": "stopped"}
+    assert instance1.state_transition_reason.startswith("User initiated")
 
     with pytest.raises(ClientError) as ex:
         client.start_instances(InstanceIds=[instance1.id], DryRun=True)