API-based endpoint discovery of resources in cloud edge locations embedded in telecommunications networks

Techniques for API-based endpoint discovery involving provider substrate extension resources are described. A discovery coordinator service located within the provider network can identify one or more endpoints from a set of potentially distributed endpoints for a client to utilize, where endpoints may be located within provider substrate extensions of the provider network. The discovery coordinator service can utilize location values of the client provided via an API request, such as its network address or geographic coordinates, to identify a nearby resource that may be most optimal for the client to use via providing minimal latency of access.