Tee

tee

When the agent runs a command, piping straight to tail -n N keeps the chat readable — and silently drops every line above the tail, including the first error of a cascading failure, the warning that produced the summary, or any earlier hint.

The fix is a one-token splice: add tee "$log" before the tail/head stage so the full output lands in a file inside a mktemp -d directory. The visible slice stays exactly as it was. The file is there for follow-up Read, grep, tail, head.

Rule

log=$(mktemp -d)/out.log
cmd 2>&1 | tee "$log" | tail -n 100
echo "full log: $log"

• Allocate the directory with mktemp -d, not a hardcoded /tmp/... path — mktemp -d returns a unique private directory on stdout each call; hardcoded paths collide with concurrent runs and leftover state from prior ones.
• Redirect stderr to stdout (2>&1) before tee, so errors and warnings land in the file too.
• Pipe through tail -n N (or head -n N) for the visible slice; pick tail for failure tails and summaries, head when the first lines carry the signal.
• Print the full log path on the last line so the file can be re-opened with Read, grep, tail, head.

When NOT to use

• Short, bounded output. The | tail -n N reflex was never needed — drop the pipe entirely (git status, git rev-parse, command -v foo, a single-line jq query).
• Output expected to be very long — e.g. find /, grep -r over a huge tree, verbose package installs that dump tens of MB. Capturing the full stream into a temp file fills disk for no real gain. Leave the command as the reflex would write it (| tail -n N and accept the loss), or narrow the output at the source first (--quiet, --summary, -q, per-tool filters).
• Interactive commands (vim, less, REPL) — pipes break their UI.
• Output the human is following live — e.g. a tail -f on a server log the user is reading.

Examples

Bad — straight pipe to tail discards earlier output:

npm test 2>&1 | tail -n 100

Good — splice in tee to a temp file:

log=$(mktemp -d)/out.log
npm test 2>&1 | tee "$log" | tail -n 100
echo "full log: $log"

Same shape with head when the first lines carry the signal:

log=$(mktemp -d)/out.log
make build 2>&1 | tee "$log" | head -n 50
echo "full log: $log"

After the command, the printed path can be read directly — e.g. widen the same slice (tail/head with a bigger N), search by keyword (grep), or read a known line range (Read):

tail -n 500 "$log"
head -n 200 "$log"
grep -nE 'error|fail|warning' "$log"

Scope

Applies to commands the agent runs — primarily Bash tool invocations and shell snippets under scripts/ that the agent itself executes. Snippets shown to a human reader (install instructions, copy-paste examples) stay free of this pattern unless the human asked for output capture too.