truncateTail returns empty content when the last line exceeds the byte budget, so long single-line output reaches the model as (no output)

[Jiaaqiliu]

Environment. prime-agent 0.7.3, main at f8f0036, Node 22.17.0, macOS 15 (darwin 25.5.0).

Summary

truncateTail returns empty content whenever the output ends with a newline and its last line is longer than the byte budget. Because almost every command's stdout ends with a newline, a single long line means the model receives (no output).

Reproduction

import { truncateTail } from "./src/core/tools/truncate.js";

truncateTail("A".repeat(100 * 1024)).content.length;        // 51200  (correct)
truncateTail("A".repeat(100 * 1024) + "\n").content.length; // 0
truncateTail(JSON.stringify({ d: "x".repeat(60 * 1024) }) + "\n").content.length; // 0
truncateTail("short1\nshort2\n" + "B".repeat(80 * 1024) + "\n").content.length;   // 0

In practice this is cat on a single-line JSON file, jq -c, base64 file, a minified bundle, or one long log line. What the model sees is:

(no output)

[Showing lines 2-2 of 2 (50.0KB limit). Full output: /tmp/pi-bash-....log]

The footer says content was shown, and lastLinePartial is false, so nothing downstream can tell that the output was lost rather than empty.

Root cause

core/tools/truncate.ts:182. The backwards scan is supposed to fall back to keeping the end of an oversized last line, which the function's own doc comment describes. The fallback is gated on outputLinesArr.length === 0:

if (outputBytesCount + lineBytes > maxBytes) {
    truncatedBy = "bytes";
    if (outputLinesArr.length === 0) {
        const truncatedLine = truncateStringToBytesFromEnd(line, maxBytes);
        ...
    }
    break;
}

content.split("\n") on content ending in a newline yields a trailing "" element. The scan starts at that element, so outputLinesArr.length is already 1 when it reaches the oversized line. The guard is false, the rescue is skipped, and the loop breaks with only [""] collected.

Scope

truncateTail is on the model-facing path through OutputAccumulator.snapshot (core/tools/output-accumulator.ts:90) and executeBashWithOperations (core/bash-executor.ts:96,115), and on the display path through interactive-mode.ts:4464 and components/bash-execution.ts:117.

Suggested fix

Gate the fallback on whether any line with content has been collected, and subtract the newline the already-collected empty element contributes so the result still fits maxBytes. No currently-correct case changes: with no trailing newline the collected array is empty and the arithmetic is identical, and a tail that already holds real content still prefers whole lines.

There is currently no test file for this module; I added one.

---

I have a fix with a regression test on a branch: fix/truncate-tail-empty-output. npm run check passes and the surrounding suites still pass. I opened it as a PR first and the contribution gate closed it, which is what CONTRIBUTING.md says should happen, so I am bringing it here instead. Happy to leave it as is, adjust it, or drop it entirely if you would rather fix this differently.