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I'm still relatively new to colcon myself, but as I understand it, it does not know what is inside a package other than what build system it uses, and so has no way to know if a package is modified or not. That is up to each package's own build scripts. If each package can skip its own build if there is nothing to do (as most build systems do), then colcon will merely be calling a build script that does nearly nothing and returns. It's not as fast as colcon doing the skipping, but you shouldn't be seeing packages being actually rebuilt when there is nothing changed.