diff --git a/RELEASE.md b/RELEASE.md
new file mode 100644
index 0000000000..de8f153a6b
--- /dev/null
+++ b/RELEASE.md
@@ -0,0 +1,13 @@
+Release type: patch
+
+This release fixes schema codegen to support custom directive definitions. Previously, schemas containing custom directives like `@authz` would fail with `NotImplementedError: Unknown definition None`. Now directive definitions are gracefully skipped since they don't require code generation.
+
+```graphql
+directive @authz(resource: String!, action: String!) on FIELD_DEFINITION
+
+type Query {
+    hello: String! @authz(resource: "greeting", action: "read")
+}
+```
+
+This also fixes the error message for truly unknown definition types to show the actual type name instead of `None`.
diff --git a/strawberry/schema_codegen/__init__.py b/strawberry/schema_codegen/__init__.py
index 3c8ceb2018..5222f21324 100644
--- a/strawberry/schema_codegen/__init__.py
+++ b/strawberry/schema_codegen/__init__.py
@@ -9,6 +9,7 @@
 
 import libcst as cst
 from graphql import (
+    DirectiveDefinitionNode,
     EnumTypeDefinitionNode,
     EnumValueDefinitionNode,
     FieldDefinitionNode,
@@ -748,8 +749,14 @@ def codegen(schema: str) -> str:
                 _is_federation_link_directive(directive)
                 for directive in graphql_definition.directives
             )
+        elif isinstance(graphql_definition, DirectiveDefinitionNode):
+            # Custom directive definitions don't need code generation,
+            # they're only used by the GraphQL schema itself
+            pass
         else:
-            raise NotImplementedError(f"Unknown definition {definition}")
+            raise NotImplementedError(
+                f"Unknown definition {type(graphql_definition).__name__}"
+            )
 
         if definition is not None:
             definitions[definition.name] = definition
diff --git a/tests/cli/test_schema_codegen.py b/tests/cli/test_schema_codegen.py
index 9d3e3286a5..d5bb8e9c7b 100644
--- a/tests/cli/test_schema_codegen.py
+++ b/tests/cli/test_schema_codegen.py
@@ -5,8 +5,10 @@
 from typer.testing import CliRunner
 
 schema = """
+directive @foo on FIELD_DEFINITION
+
 type Query {
-    hello: String!
+    hello: String! @foo
 }
 """