# Keeping Tool Output Under Control — Prompt & Context Engineering

Source: https://www.skillbyai.com/en/prompt-context-engineering/m-tools

> Large tool results can flood the context of an agent.

## Truncate, summarise, or fetch on demand

Agents call tools that can return huge results: logs, web pages, database dumps. Put the raw result into the context and a single call can consume the whole window. Options: **truncate** with head and tail (errors are often at the end), **summarise** or extract the fields needed, **paginate** (let the agent request more), or store the full result and give the model a **handle** it can query. Always mark tool output as untrusted data, and tell the model when content was cut so it does not assume it saw everything.

## Head-and-tail truncation of a long log, run

I ran this with plain Python 3 (standard library only); the data is made-up example data. A 501-line log (6,417 characters) is cut to the first three and last three lines plus an omission marker (112 characters); the final ERROR line survives.

```python
log = "\n".join(f"line {i}: ok" for i in range(1, 501)) + "\nline 501: ERROR disk full"
def head_tail(text, keep=6):
    lines = text.splitlines()
    if len(lines) <= 2 * keep:
        return text
    cut = len(lines) - 2 * keep
    return "\n".join(lines[:keep] + [f"... [{cut} lines omitted] ..."] + lines[-keep:])
short = head_tail(log, keep=3)
print(short)
print("chars:", len(log), "->", len(short))
```

Output:

```
line 1: ok
line 2: ok
line 3: ok
... [495 lines omitted] ...
line 499: ok
line 500: ok
line 501: ERROR disk full
chars: 6417 -> 112
```

## Design tools to return less

Give tools parameters like limit, fields and filters so they return only what the task needs.

**Quiz:** Why keep the tail when truncating a long log?

- [ ] Models only read the tail
- [ ] The tail is always shorter
- [x] Errors and final status often appear at the end
- [ ] The head is never useful

*Answer:* Errors and final status often appear at the end. Head-and-tail keeps context and outcome.