Add changelog for 2025-07-18

[quantstruct-bot]

Description

1. Custom headers for custom LLM models: You can now add custom headers to both Custom LLM Models and Workflow Custom Models (including in assistants, squads, and workflows). This lets you send custom HTTP headers—such as for authentication or extra metadata—when Vapi calls your custom LLM API. The headers property overrides default headers (except Authorization, which should be set via your custom-llm credential).

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

[github-actions]

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