diff --git a/CHANGELOG.md b/CHANGELOG.md
index e12012aa..16db4ae8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,23 @@
 <!-- This file is generated, please add to it using `knope document-change` in the client-library-templates repo -->
 # Changelog
 
+## 3.18.1 (2026-10-06)
+
+### Fixes
+
+#### Stop the generated idempotency key persisting in caller-owned state
+
+The generated `Idempotency-Key` was written into state owned by the caller: the
+headers dictionary in Python, and the request object in .NET. An application that
+reused either across independent create calls — a module-level custom-header
+template, or one request object in a loop — sent the first call's key every time.
+The API answers each repeat with `409 idempotent_creation_conflict`, which these
+clients resolve by default by returning the resource the first call created, so
+later creates returned the wrong resource with no error.
+
+The key is now generated per call without touching the caller's own objects, and
+remains stable across network retries of the same call.
+
 ## 3.18.0 (2026-10-06)
 
 ### Features
diff --git a/gocardless_pro/__init__.py b/gocardless_pro/__init__.py
index 14883fab..b6e69b3e 100644
--- a/gocardless_pro/__init__.py
+++ b/gocardless_pro/__init__.py
@@ -2,5 +2,5 @@
 
 from .client import Client
 
-__version__ = '3.18.0'
+__version__ = '3.18.1'
 
diff --git a/gocardless_pro/api_client.py b/gocardless_pro/api_client.py
index 3227757f..db27f1ae 100644
--- a/gocardless_pro/api_client.py
+++ b/gocardless_pro/api_client.py
@@ -192,7 +192,7 @@ def _default_headers(self):
             'Authorization': 'Bearer {0}'.format(self.access_token),
             'Content-Type': 'application/json',
             'GoCardless-Client-Library': 'gocardless-pro-python',
-            'GoCardless-Client-Version': '3.18.0',
+            'GoCardless-Client-Version': '3.18.1',
             'User-Agent': self._user_agent(),
             'GoCardless-Version': '2015-07-06',
         }
@@ -201,7 +201,7 @@ def _user_agent(self):
         python_version = '.'.join(platform.python_version_tuple()[0:2])
         vm_version = '{}.{}.{}-{}{}'.format(*sys.version_info)
         return ' '.join([
-            'gocardless-pro-python/3.18.0',
+            'gocardless-pro-python/3.18.1',
             'python/{0}'.format(python_version),
             '{0}/{1}'.format(platform.python_implementation(), vm_version),
             '{0}/{1}'.format(platform.system(), platform.release()),
diff --git a/setup.py b/setup.py
index 269bee93..8756ec96 100644
--- a/setup.py
+++ b/setup.py
@@ -6,7 +6,7 @@
 
 setup(
     name = 'gocardless_pro',
-    version = '3.18.0',
+    version = '3.18.1',
     packages = find_packages(exclude=['tests']),
     install_requires = ['requests>=2.34.2'],
     python_requires = '>=3.6',