expect(Buffer.byteLength(output, "utf8")).toBe(16);

});

it("does not exceed the byte cap when clipping multibyte text", () => {

const output = appendBoundedTextTail("ignored", "é", 1);

expect(output).toBe("");

expect(Buffer.byteLength(output, "utf8")).toBeLessThanOrEqual(1);

});

it("keeps complete multibyte characters at the bounded tail", () => {

const output = appendBoundedTextTail("prefix", "aé", 2);

expect(output).toBe("é");

expect(Buffer.byteLength(output, "utf8")).toBe(2);

});

it("uses the session stderr tail limit by default", () => {

const output = appendBoundedTextTail("", "x".repeat(SESSION_TOOL_STDERR_TAIL_BYTES + 1));

export const SESSION_TOOL_STDERR_TAIL_BYTES = 64 * 1024;

function decodeUtf8TextTail(buffer: Buffer, maxBytes: number): string {

const chars = Array.from(buffer.toString("utf8"));

const kept: string[] = [];

let bytes = 0;

for (let i = chars.length - 1; i >= 0; i--) {

const char = chars[i] ?? "";

const charBytes = Buffer.byteLength(char, "utf8");

if (bytes + charBytes > maxBytes) {

break;

}

kept.push(char);

bytes += charBytes;

}

return kept.toReversed().join("");

}

export function appendBoundedTextTail(

current: string,

chunk: Buffer | string,

const effectiveMaxBytes = normalizePositiveLimit(maxBytes, SESSION_TOOL_STDERR_TAIL_BYTES);

const chunkBuffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);

if (chunkBuffer.byteLength >= effectiveMaxBytes) {

return chunkBuffer.subarray(chunkBuffer.byteLength - effectiveMaxBytes).toString("utf8");

return decodeUtf8TextTail(chunkBuffer, effectiveMaxBytes);

}

const currentBuffer = Buffer.from(current);

const currentTailBytes = Math.max(0, effectiveMaxBytes - chunkBuffer.byteLength);

const currentTail = currentBuffer.subarray(currentBuffer.byteLength - currentTailBytes);

return Buffer.concat([currentTail, chunkBuffer], effectiveMaxBytes).toString("utf8");

return decodeUtf8TextTail(Buffer.concat([currentTail, chunkBuffer]), effectiveMaxBytes);

}