For people: A test suite documents the intended behaviour and makes changes safe. Counted is whether there is one of reasonable size, not its coverage.

For AI agents: Tests let an agent check its own work; without them its mistakes surface later, in review or in production.

For people: Long functions do many things at once: they are hard to name, read, test and reuse.

For AI agents: To change one line, an agent reads and often rewrites the whole function: bigger edits, more tokens, more breakage.

For people: A fix in one copy must be repeated in all the others; a missed copy becomes a bug.

For AI agents: Agents copy the patterns they find, so duplicates multiply; a change to one copy misses the others, and every copy costs context.

For people: Complex files concentrate risk: understanding one means keeping many paths in mind.

For AI agents: Much to read and much logic to get right in one place: the costliest combination for an agent.

For people: Large files are hard to navigate, mix responsibilities and attract merge conflicts.

For AI agents: Agents read whole files into their context: every change to a large file costs many tokens and crowds out other relevant code.

For people: Every branch is another path to understand and test; complex logic is where mistakes are made and missed in review.

For AI agents: Nested conditions are where generated changes break edge cases; complex units need more tests and review before an agent's change can be trusted.

For people: More code is more to learn, search and keep in mind; onboarding and judging a change's impact slow down as it grows.

For AI agents: An agent sees only a slice at a time: the bigger the code base, the more it searches, and the likelier it misses relevant code or rewrites an existing helper.

For AI agents: Tests let an agent check its own work; without them its mistakes surface later, in review or in production.

For people: A test suite documents the intended behaviour and makes changes safe. Counted is whether there is one of reasonable size, not its coverage.

For AI agents: Agents copy the patterns they find, so duplicates multiply; a change to one copy misses the others, and every copy costs context.

For people: A fix in one copy must be repeated in all the others; a missed copy becomes a bug.

For AI agents: Agents read whole files into their context: every change to a large file costs many tokens and crowds out other relevant code.

For people: Large files are hard to navigate, mix responsibilities and attract merge conflicts.

For AI agents: Much to read and much logic to get right in one place: the costliest combination for an agent.

For people: Complex files concentrate risk: understanding one means keeping many paths in mind.

For AI agents: To change one line, an agent reads and often rewrites the whole function: bigger edits, more tokens, more breakage.

For people: Long functions do many things at once: they are hard to name, read, test and reuse.

For AI agents: Nested conditions are where generated changes break edge cases; complex units need more tests and review before an agent's change can be trusted.

For people: Every branch is another path to understand and test; complex logic is where mistakes are made and missed in review.

For AI agents: An agent sees only a slice at a time: the bigger the code base, the more it searches, and the likelier it misses relevant code or rewrites an existing helper.

For people: More code is more to learn, search and keep in mind; onboarding and judging a change's impact slow down as it grows.