diff --git a/.changeset/pydantic-cycle-fix.md b/.changeset/pydantic-cycle-fix.md
new file mode 100644
index 000000000..f18c53749
--- /dev/null
+++ b/.changeset/pydantic-cycle-fix.md
@@ -0,0 +1,7 @@
+---
+default: patch
+---
+
+# Fix for rare Pydantic error
+
+Fixed a subtle issue with forward type references in Pydantic classes that could, in very rare cases, cause the parser to fail with the error "`Parameter` is not fully defined".
diff --git a/openapi_python_client/schema/openapi_schema_pydantic/header.py b/openapi_python_client/schema/openapi_schema_pydantic/header.py
index 3223c199b..2d2471ea9 100644
--- a/openapi_python_client/schema/openapi_schema_pydantic/header.py
+++ b/openapi_python_client/schema/openapi_schema_pydantic/header.py
@@ -29,3 +29,10 @@ class Header(Parameter):
             ]
         },
     )
+
+
+# Calling model_rebuild() here helps Pydantic to resolve the forward references that were used
+# in defining Parameter and Encoding. Without this call, any subtle change to the loading order
+# of schema submodules could result in an error like "Parameter is not fully defined".
+# See: https://docs.pydantic.dev/latest/concepts/models/#rebuilding-model-schema
+Parameter.model_rebuild()