httpAdditionalHeaders

httpAdditionalHeaders is a property of URLSessionConfiguration in the Foundation framework. It holds a dictionary of additional HTTP header fields that URLSession should attach to every request made with a session configured with this object. If no value is set, default headers are used.

You set it before creating a URLSession. For example: let config = URLSessionConfiguration.default; config.httpAdditionalHeaders = ["Authorization": "Bearer token", "Accept": "application/json"]; let session = URLSession(configuration: config). This configuration ensures the specified headers are included in all requests made by that session unless overridden.

If a request provides its own header with the same field name, the request’s header value takes precedence over the value in httpAdditionalHeaders. The HTTP header field names are case-insensitive, but using canonical names is recommended for consistency.

This feature is useful for applying headers consistently across requests, such as authorization tokens, custom user-agent strings, or preferred response formats. Security considerations include avoiding embedding sensitive data in code or logs, since header values can be exposed. Header values are strings, and per-request overrides can be performed by setting headers directly on a URLRequest; for different header sets, separate URLSessionConfigurations or sessions can be used. Related concepts include URLSession, URLRequest, and standard HTTP header semantics.