Updated troubleshooting for dropped message handoffs

fern/squads/handoff/troubleshooting.mdx

@@ -45,3 +45,43 @@ attention to these handoffs in your conversation with the user.
 - Test variable extraction schemas with sample data
 - Validate that assistant names exist in the same squad
 - Verify rejection plan conditions use correct regex syntax (remember to double-escape `\\` in JSON)
+
+### Destination assistant ignores part of its system prompt
+
+If a destination assistant has more than one system message in `model.messages`, only the first system message is sent to the model after a handoff. Any additional system messages are dropped. The same assistant follows all of its instructions when you call it directly.
+
+To fix this, combine all system instructions into a single system message. Use the [`model`](/api-reference/assistants/update#request.body.model) field of the [Update Assistant](/api-reference/assistants/update) endpoint to replace `model.messages` with one system message. Include the full `model` object in your request, since the update replaces the existing `model` configuration.
+
+For example, this configuration loses the second system message after a handoff:
+
+```json
+{
+  "model": {
+    "messages": [
+      {
+        "role": "system",
+        "content": "You are a support agent for Acme."
+      },
+      {
+        "role": "system",
+        "content": "Only discuss billing questions."
+      }
+    ]
+  }
+}
+```
+
+Combine the instructions into one system message instead:
+
+```json
+{
+  "model": {
+    "messages": [
+      {
+        "role": "system",
+        "content": "You are a support agent for Acme.\n\nOnly discuss billing questions."
+      }
+    ]
+  }
+}
+```