Crate tool_output_truncate 

tool-output-truncate

Truncate tool output before adding it to LLM message history.

When an agent runs cat large_file.log, ripgrep, or a database query, the result can be megabytes. Naively appending it to the conversation blows the context window. The standard fix is to keep the head and the tail and replace the middle with an elision marker. This crate gives you that, char-aware (UTF-8 safe) and line-aware, with zero deps.

Quick example

use tool_output_truncate::truncate_middle;

let big = (0..1000).map(|i| format!("row {i}\n")).collect::<String>();
let trimmed = truncate_middle(&big, 200);
assert!(trimmed.len() <= 200 + 64); // text + elision marker overhead
assert!(trimmed.starts_with("row 0"));
assert!(trimmed.contains("truncated"));

Strategies

• truncate_middle - keep half at the start, half at the end. Best default for arbitrary text where head and tail both carry signal (logs, stack traces, file contents).
• truncate_head - keep the first N chars only. Use when the tail is noise (e.g. very long lists where order matters).
• truncate_tail - keep the last N chars only. Use when the head is preamble (e.g. command output with banner lines).
• truncate_middle_lines - line-aware version of truncate_middle. Splits at line boundaries so you don’t show half a line of JSON.

All functions are no-op when the input already fits.

What it does NOT do

• No tokenization. Pass a char cap; if you need token caps use chars * 4 as a rough Anthropic/OpenAI proxy.
• No structured truncation (JSON, YAML, XML). For JSON specifically, parse first and decide which fields to keep.
• No summarization. This is byte/char arithmetic only.

Functions

truncate_head

Keep up to max_chars characters from the start. Everything after is replaced with an elision marker indicating the count of omitted chars.

truncate_middle

Keep up to max_chars characters by taking half from the start and half from the end, replacing the middle with an elision marker.

truncate_middle_lines

Keep up to max_lines lines by taking half from the start and half from the end, replacing the middle with an elision marker. The marker reports the number of omitted lines and the number of omitted chars.

truncate_tail

Keep up to max_chars characters from the end. Everything before is replaced with an elision marker indicating the count of omitted chars.