diff --git a/fern/server-url/server-authentication.mdx b/fern/server-url/server-authentication.mdx
index 1b21996aa..0309460f7 100644
--- a/fern/server-url/server-authentication.mdx
+++ b/fern/server-url/server-authentication.mdx
@@ -405,6 +405,25 @@ If you supply the server URL in your API request, your endpoint must authenticat
 
 For webhooks sent to a server URL supplied in the API request, Vapi also strips credential fields from the payload. Any `apiKey`, `secret`, `password`, `authToken`, or similar field on the embedded `assistant`, `session`, or `chat` object arrives as the string `"[REDACTED]"` rather than its real value. The rest of the payload is unchanged.
 
+#### Subsystem server URLs
+
+Some webhooks go to a URL owned by a subsystem rather than to your assistant's webhook URL. Tools, knowledge bases, custom voices, dynamic handoff and transfer destinations, and custom endpointing providers each carry their own `server` block:
+
+| Webhook | Server it is sent to |
+| --- | --- |
+| `tool-calls`, `function-call` | the tool's |
+| `knowledge-base-request` | the knowledge base's |
+| `voice-request` | the custom voice's |
+| `handoff-destination-request` | the handoff destination's |
+| `transfer-destination-request` | the transfer destination's |
+| `call.endpointing.request` | the endpointing provider's |
+
+**These follow the assistant that carried them.** A tool on a saved assistant is org-configured and keeps your credentials. The same tool on a transient assistant or in `assistantOverrides` is caller-authored, and Vapi sends to its URL without them.
+
+<Warning>
+A saved assistant can carry a caller-authored subsystem server: if `assistantOverrides` replaces the voice, the tools, or the knowledge base, those replacements are caller-authored even though the assistant itself is saved. Give any subsystem server you supply in a request its own authentication in that block's `headers`.
+</Warning>
+
 #### Confirming a withheld send in your logs
 
 When Vapi withholds credentials, it also writes an entry to the call's webhook logs:
diff --git a/fern/squads/handoff/destinations.mdx b/fern/squads/handoff/destinations.mdx
index 056b93522..bd8ba7711 100644
--- a/fern/squads/handoff/destinations.mdx
+++ b/fern/squads/handoff/destinations.mdx
@@ -149,6 +149,10 @@ Best for Anthropic models -- single tool with multiple destination options:
 
 The destination is determined at runtime via the `handoff-destination-request` webhook:
 
+<Warning>
+A destination's `server` block follows the assistant carrying it. On a saved assistant it keeps your org's webhook credentials; supplied through a transient assistant or `assistantOverrides` it is caller-authored, and Vapi sends to it without them. Give it its own authentication in `server.headers`, as the examples below do. See [Server authentication](/server-url/server-authentication#subsystem-server-urls).
+</Warning>
+
 ```json
 {
   "tools": [
@@ -217,7 +221,10 @@ Pass additional context to your webhook for intelligent routing:
         {
           "type": "dynamic",
           "server": {
-            "url": "https://api.example.com/intelligent-routing"
+            "url": "https://api.example.com/intelligent-routing",
+            "headers": {
+              "Authorization": "Bearer YOUR_API_KEY"
+            }
           }
         }
       ],