Methods and systems for advertisement insertion into streaming media content are described. In one method, an advertisement insertion tool receives an advertisement insertion input. The advertisement insertion input identifies a presentation time in media content to insert an advertisement. In response to receiving the advertisement insertion input, the advertisement insertion tool inspects an original slice of the media content that contains the media content at the presentation time to locate a key frame at or prior to the presentation time. The advertisement insertion tool creates a first new slice comprising the media content of the original slice up to the presentation time and creates a second new slice comprising the media content of the original slice after the presentation time. The advertisement insertion tool updates a descriptor file to include the first new slice, the advertisement, and the second new slice for streaming.