Running a massive codebase migration with Claude Code

i gave Claude Code + Opus 5.5 a 680,000 line migration and expected it to choke somewhere in the middle then it kept going long enough that the interesting part stopped being the code it was the lack of babysitting Opus 5.5 kept tracing dependencies, changing files, checking its own work and pushing the migration forward without me constantly rebuilding the context that’s the shift i care about with Claude Code now not “can it write this function” but how much ugly, long running engineering work can i hand over before i need to come back Anthropic says one early tester finished a 680,000 line migration in less than a day with Opus 5.5 that number is obviously insane but the more important part is what it implies the unit of work for coding agents is moving from a prompt to a project