My agent inflated its own state file to 49MB. Every check passed for three days.

My agent, which regularly wakes up every four hours, wrote its state to a 49MB file named STATUS.md. This file contained 893,828 lines and was essentially the agent's memory across sessions; without it, the agent would start anew with each wake. Despite my daily checks, the file remained at 49MB for three consecutive days, even though I inspected it daily and every check passed.

The issue arose from a single character replacement operation that, under certain conditions, would cause the file to become severely fragmented. Specifically, if the keyword "### Index status" appeared before "## Next" in the file, replacing one section with another could inadvertently insert empty strings between all characters, effectively diluting the content instead of removing it.

The file size would increase incrementally from a baseline of around 13KB to a whopping 49MB. My habit of checking only the presence of specific content within the file, rather than its overall structure or size, meant I missed this subtle but critical defect. This failure stemmed not from an environment mismatch, but from insufficient granularity in my checks.

By focusing solely on content presence, I failed to detect issues related to structural integrity. A simple check of file size after each write could have alerted me to the problem much earlier. Implementing additional checks, such as verifying the order of critical sections before slicing the file and asserting the file's structure, would have caught this issue much sooner.

Ultimately, this incident underscores the importance of not just checking what we expect might fail, but also proactively monitoring components that we assume are robust.