Add changelog for 2025-06-20 (#527)

Author: quantstruct-bot

fern/changelog/2025-06-20.mdx

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+# New Campaigns APIs and Assistant Improvements
+
+<Card title="Campaign API Endpoints" icon="bullhorn">
+  **Create, retrieve, and manage campaigns** using the new [`/campaign` endpoints](https://docs.vapi.ai/api-reference/calls/list#:~:text=Campaign). Build automated call campaigns with specified customers and schedules.
+</Card>
+
+<Check>
+  **General Availability**: [`Assistant.modelOutputInMessagesEnabled`](https://api.vapi.ai/api#:~:text=SessionPaginatedResponse-,Assistant,-AssistantPaginatedResponse) is now generally available without beta limitations. You can decide whether to use the model's output in conversation history instead of the assistant's speech transcription.
+</Check>
+
+1. **Simplified Assistant Property Structure**: Properties like `serverMessages`, `clientMessages`, and `serverUrl` have been moved under [`Assistant.monitorPlan`](https://api.vapi.ai/api#:~:text=SessionPaginatedResponse-,Assistant,-AssistantPaginatedResponse). This reorganization simplifies how you configure monitoring for your assistants.
+
+2. **Node-Level Overrides for Model and Voice**: In [`Conversation Node`](https://api.vapi.ai/api#:~:text=ConversationNode), properties like `model`, `voice`, and `transcriber` now explicitly override the workflow's settings. This allows you to customize these settings for individual nodes within a workflow for greater control.
+
+3. **Enhanced Credential Configuration in Assistants**: Assistants now support `credentials` and `credentialIds`, similar to workflows. This allows you to specify dynamic credentials specifically for assistant calls, enhancing security and flexibility.
+
+4. **New Models Available in `ConversationNode`**: You can now use [`Google Models`](https://api.vapi.ai/api#:~:text=WorkflowGoogleModel) and [`Custom Models`](https://api.vapi.ai/api#:~:text=WorkflowCustomModel) in conversation nodes. This expands the range of language models that can be integrated into conversation nodes.
+
+<Frame caption="New Workflow Models">
+    <img src="../static/images/changelog/new-workflow-models.png" />
+</Frame>