Add changelog for 2025-08-25 (#644)

Author: quantstruct-bot

fern/changelog/2025-08-23.mdx

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+# Voicemail Detection Enhancements
+
+1. **Voicemail Detection Enhancements**: You can now configure voicemail detection across providers with [`Assistant.voicemailDetection`](https://api.vapi.ai/api#:~:text=https://api.vapi.ai/api#:~:text=SessionPaginatedResponse-,Assistant,-AssistantPaginatedResponse), for example with [Vapi](https://api.vapi.ai/api#:~:text=VapiVoicemailDetectionPlan), [Google](https://api.vapi.ai/api#:~:text=GoogleVoicemailDetectionPlan), and [OpenAI](https://api.vapi.ai/api#:~:text=OpenAIVoicemailDetectionPlan).Each plan now supports a `type` property to select between:
+  - `audio`: Native audio model detection (default)
+  - `transcript`: ASR/transcript-based detection
+
+2. **Fine-tuned control**: Under each voicemail detection plan, you can configure backoff plans and beep detection timing with [`Assistant.voicemailDetection["yourVoicemailDetectionPlan"].beepMaxAwaitSeconds`](https://api.vapi.ai/api#:~:text=https://api.vapi.ai/api#:~:text=VapiVoicemailDetectionPlan) for improved voicemail handling in automated calls.

fern/changelog/2025-08-25.mdx

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+1. **New Structured Output Endpoints**: You can now use [new APIs for structured outputs](https://docs.vapi.ai/api-reference/calls/list#:~:text=Delete%20Logs-,Structured%20Outputs,-GET) to define, extract, and manage structured data from conversations.
+
+2. **Configure Structured Output Resources**: You can now define reusable structured data extraction templates, including:
+  - **Custom JSON Schema**: Specify the exact structure and validation rules for extracted data using full JSON Schema support (objects, arrays, enums, validation constraints, and more).
+  - **Model Selection**: Choose the LLM (OpenAI, Anthropic, Google, or custom) for extraction, or provide custom system/user prompts with Liquid templating for advanced scenarios.
+  - **Context Linking**: Link structured outputs to specific workflows or assistants for context-aware extraction.
+  - **Metadata**: Track creation/update timestamps, org linkage, and provide rich descriptions for each structured output.
+
+3. **Assistant Transfer Improvements**: You can now include an optional `name` property to better identify and manage your transfer assistants.