A system for providing streaming content, the system comprising: a data store including a first set of content portions corresponding to a main content, wherein the first set of content portions are encoded according to one or more first parameters; and one or more computing devices configured with computer-executable instructions that, when executed, cause the computing devices to: obtain a request from a client computing device to provide a content stream including the main content; determine that an additional content is to be included in the content stream; determine that the data store does not include a second set of content portions representing the additional content encoded according to the one or more first parameters; select a fallback content for inclusion within the content stream, wherein the data store further includes a third set of content portions representing the fallback content encoded according to the one or more first parameters; generate a manifest for the content stream, wherein the manifest includes identifiers for respective content portions within the first set of content portions and identifiers for respective content portions within the third set of content portions; transmit the manifest to the client computing device; and responsive to the request to provide the content stream, begin encoding the additional content according to the one or more first parameters to result in the second set of content portions representing the additional content encoded according to the one or more first parameters. a data store including a first set of content portions corresponding to a main content, wherein the first set of content portions are encoded according to one or more first parameters; and one or more computing devices configured with computer-executable instructions that, when executed, cause the computing devices to: obtain a request from a client computing device to provide a content stream including the main content; determine that an additional content is to be included in the content stream; determine that the data store does not include a second set of content portions representing the additional content encoded according to the one or more first parameters; select a fallback content for inclusion within the content stream, wherein the data store further includes a third set of content portions representing the fallback content encoded according to the one or more first parameters; generate a manifest for the content stream, wherein the manifest includes identifiers for respective content portions within the first set of content portions and identifiers for respective content portions within the third set of content portions; transmit the manifest to the client computing device; and responsive to the request to provide the content stream, begin encoding the additional content according to the one or more first parameters to result in the second set of content portions representing the additional content encoded according to the one or more first parameters. obtain a request from a client computing device to provide a content stream including the main content; determine that an additional content is to be included in the content stream; determine that the data store does not include a second set of content portions representing the additional content encoded according to the one or more first parameters; select a fallback content for inclusion within the content stream, wherein the data store further includes a third set of content portions representing the fallback content encoded according to the one or more first parameters; generate a manifest for the content stream, wherein the manifest includes identifiers for respective content portions within the first set of content portions and identifiers for respective content portions within the third set of content portions; transmit the manifest to the client computing device; and responsive to the request to provide the content stream, begin encoding the additional content according to the one or more first parameters to result in the second set of content portions representing the additional content encoded according to the one or more first parameters.