Your build log is 40,000 lines. Your agent's context window is not.

Your build log is 40,000 lines. Your agent's context window is not.

Every time my build failed, I did the same thing: copy the whole log,
paste it to my agent, and watch it burn half its context on progress
spam before it ever reached the actual error. head -100 cuts the
error at the end. tail -100 cuts the setup at the start. grep -i
error cuts everything that explains why the error happened.

So I built a tiny tool that sits in a pipe and keeps all three:
logtrim.

What it does

One command shrinks verbose logs and tool output down to a token
budget — keeping the beginning, the end, and every error line, while
collapsing the repetitive middle into one-line markers that say what
was dropped.

npm run build 2>&1 | logtrim --budget 1500

How it trims:

1. Collapse blank runs — 5 blank lines become 1.
2. Collapse identical runs — 8 copies of the same warning become 2 copies plus ... [6 identical lines omitted] ....
3. Collapse similar runs — 12 Processed N/1000 records lines become 2 plus ... [10 similar lines omitted] .... "Similar" means the same shape with only numbers differing. Error lines are never collapsed this way.
4. Truncate absurdly long lines — a 3 KB single-line JSON blob gets cut to --max-line chars with a note.
5. If still over budget — keep head + error lines + tail, drop the middle, and mark each gap. Middle lines go first so the start and end survive.
6. Last resort — if error lines alone exceed the budget, they are truncated to share it. Output never exceeds the budget.

Token counts are estimates (characters / 4). Accurate enough to decide
what to keep, and it keeps the tool dependency-free.

Demo

The repo ships a 62-line build log full of progress spam, repeated
warnings, and one real failure:

node bin/logtrim.js examples/sample.log --stats --budget 300

Result: 62 → 23 lines, 715 → 251 tokens — and the two errors, the
stack trace, and the start of the log all survive. With the default
budget (2000 tokens) the same file goes to 48 lines / 559 tokens,
dropping only the collapsed repeats.

Try it

git clone https://github.com/techdevfun/logtrim.git
cd logtrim
npm link   # makes `logtrim` available everywhere

# Trim a failing build before pasting it to your agent
npm run build 2>&1 | logtrim --budget 1500

# Trim a saved log file, see what was cut
logtrim --budget 800 --stats deploy.log

Zero dependencies, plain Node.js 18+. Tests + CI included.

Repo: https://github.com/techdevfun/logtrim

If you try it on a real log and it drops something it shouldn't have,
open an issue — that's the feedback that shapes what gets built next.