Keep the failed attempts
Why a useful build log has room for the version that did not work.
Sample text for the reading-page design. This is not a published article by Anh.
A finished demo makes the path to it look tidy. The feature works, the interface makes sense, and the explanation follows a clear sequence. The discarded attempts rarely fit into that story.
But a build log has a different job. It can preserve the decisions that would otherwise disappear, including the ideas that looked reasonable and turned out to be wrong.
Keep enough to reproduce the surprise
A note that says an approach failed is hard to use later. Keep the example that exposed the problem and describe what happened. If a change fixed it, record that change alongside the result.
There is no need to save every intermediate thought. Keep the moments that changed the direction of the work. A screenshot, a small input, or a short observation can be enough.
Separate observation from explanation
What happened and why it happened are different things. The observation may be clear while the explanation remains a guess. Writing them separately keeps a tentative interpretation from becoming an unquestioned fact.
Later attempts can revisit that guess. A build log becomes useful when it helps someone ask a better question, including the person who wrote it.