In a distributed system that includes a debug server and debug clients coupled for data communications through a data communications network, where the debug server includes a debug administrator, a message router, a back-end debugger, and a debuggee, collaborative software debugging includes receiving, from the debug clients, application-level messages, including receiving, from a symbol locking debug client, a request to lock one or more symbols from value modification; routing, by the message router, the application-level messages among the debug clients, the debug administrator, and the back-end debugger, including: establishing a lock for each of the one or more symbols; and returning, to the debug clients, client-specific debug results including, returning, responsive to a request to modify a value of a locked symbol, an indication that the symbol is locked without modifying the value of the locked symbol as requested.