diff --git a/docs/tools-custom/authentication.md b/docs/tools-custom/authentication.md
index 232d8178b3..25cac2d514 100644
--- a/docs/tools-custom/authentication.md
+++ b/docs/tools-custom/authentication.md
@@ -127,7 +127,9 @@ specific interactive process with your ***Agent Client*** application.
     not require an authentication exchange.
 *   **OAUTH2:** Provides standard OAuth 2.0 authentication flows, and requires
     configuration with client ID, secret, and scopes. This method often
-    triggers an interactive flow for user consent.
+    triggers an interactive flow for user consent. For enhanced security in OIDC
+    flows, the `OAuth2Auth` model now includes a `nonce` field to help prevent
+    replay attacks.
 *   **OPEN\_ID\_CONNECT:** Provides authentication based on OpenID Connect.
     Similar to OAuth2, this type often requires configuration and user
     interaction.
@@ -135,6 +137,20 @@ specific interactive process with your ***Agent Client*** application.
     JSON key or Application Default Credentials. This type typically exchanges a
     Bearer token.
 
+### Custom authentication schemes
+
+In addition to the standard OpenAPI authentication schemes, you can define your own
+custom authentication schemes by subclassing `CustomAuthScheme`. This is useful for
+integrating with proprietary or legacy enterprise authentication systems that do not
+follow OpenAPI standards.
+
+When creating a custom authentication scheme, you must define a default value for
+the `type_` field.
+
+For easier registration of your custom authentication provider, you can also define
+a `supported_auth_schemes` property on your `BaseAuthProvider` implementation. This
+property should return a tuple of the scheme types your provider supports.
+
 ## Tools and integrations quick guide
 
 Here is a quick guide to authentication for key ADK toolsets:
@@ -806,4 +822,4 @@ except Exception as e:
                required:
                   - code
                   - message
-         ```
+         ```
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