Systems and methods are described to enable request-driven encoding of content portions to be played back as part of a content stream. A content stream can represent multiple types of content, each initially encoded according to different formatting parameters. Thus, playback of the multiple types of content as initially encoded would result in an inconsistently formatted content stream. To address this, a streaming content delivery service may respond to requests by determining whether consistently formatted content portions are available, and if such portions are not available, select fallback content portions for inclusion in the stream. The streaming content service can further initiate generation of the consistently formatted content portions in response to the request, such that subsequent requests may be serviced using the consistently formatted content portions.