When the Fine-tune Misbehaves

A first check on every output

// A first check on every model output: exactly one JSON object, nothing else.
function problem(raw: string): string {
  const text = raw.trim();
  if (/<\|[a-z_]+\|>/.test(text)) return "template tokens";
  if (text.startsWith("```")) return "code fence";
  if (!text.startsWith("{")) return "prose";
  const end = text.lastIndexOf("}");
  try { JSON.parse(text.slice(0, text.indexOf("}") + 1)); } catch { return "invalid or cut off"; }
  return end === text.length - 1 && text.indexOf("}") === end ? "" : "text after the JSON";
}

const outputs = [
  '{"intent":"cancel","order":"2518"}',
  '{"intent":"cancel","order":"2518"}<|im_end|>',
  'Sure! I have cancelled it.',
  '{"intent":"cancel","ord',
  '{"intent":"cancel","order":"2518"}\nAnything else?',
  '```json\n{"intent":"cancel"}\n```',
];
for (const o of outputs) console.log(`${(problem(o) || "ok").padEnd(20)} ${JSON.stringify(o).slice(0, 44)}`);

A first check on every output

using System.Text.Json;
using System.Text.RegularExpressions;

// A first check on every model output: exactly one JSON object, nothing else.
string Problem(string raw)
{
    var text = raw.Trim();
    if (Regex.IsMatch(text, @"<\|[a-z_]+\|>")) return "template tokens";
    if (text.StartsWith("```")) return "code fence";
    if (!text.StartsWith("{")) return "prose";
    var end = text.LastIndexOf('}');
    try { JsonDocument.Parse(text.Substring(0, text.IndexOf('}') + 1)); } catch { return "invalid or cut off"; }
    return end == text.Length - 1 && text.IndexOf('}') == end ? "" : "text after the JSON";
}

var jsonOptions = new JsonSerializerOptions { Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping };
string[] outputs = [
    "{\"intent\":\"cancel\",\"order\":\"2518\"}",
    "{\"intent\":\"cancel\",\"order\":\"2518\"}<|im_end|>",
    "Sure! I have cancelled it.",
    "{\"intent\":\"cancel\",\"ord",
    "{\"intent\":\"cancel\",\"order\":\"2518\"}\nAnything else?",
    "```json\n{\"intent\":\"cancel\"}\n```",
];
foreach (var o in outputs)
{
    var p = Problem(o);
    var json = JsonSerializer.Serialize(o, jsonOptions);
    Console.WriteLine($"{(p == "" ? "ok" : p).PadRight(20)} {json.Substring(0, Math.Min(44, json.Length))}");
}

Detect repetition

// Spot a run-on answer: does any 8-character piece repeat 3+ times?
function repeats(text: string, size = 8): number {
  const seen = new Map<string, number>();
  for (let i = 0; i + size <= text.length; i++) {
    const piece = text.slice(i, i + size);
    seen.set(piece, (seen.get(piece) ?? 0) + 1);
  }
  return Math.max(0, ...seen.values());
}

const answers = [
  '{"intent":"cancel","order":"2518","priority":"high"}',
  '{"intent":"refund"}"}"}"}"}"}"}"}"}"}"}"}"}',
  '{"intent":"where"}{"intent":"where"}{"intent":"where"}',
];
for (const a of answers) console.log(`max repeats ${repeats(a)}  ${repeats(a) >= 3 ? "RUN-ON" : "ok"}`);

Detect repetition

// Spot a run-on answer: does any 8-character piece repeat 3+ times?
int Repeats(string text, int size = 8)
{
    var seen = new Dictionary<string, int>();
    for (var i = 0; i + size <= text.Length; i++)
    {
        var piece = text.Substring(i, size);
        seen[piece] = seen.GetValueOrDefault(piece) + 1;
    }
    return seen.Values.Prepend(0).Max();
}

string[] answers = [
    "{\"intent\":\"cancel\",\"order\":\"2518\",\"priority\":\"high\"}",
    "{\"intent\":\"refund\"}\"}\"}\"}\"}\"}\"}\"}\"}\"}\"}\"}\"}",
    "{\"intent\":\"where\"}{\"intent\":\"where\"}{\"intent\":\"where\"}",
];
foreach (var a in answers) Console.WriteLine($"max repeats {Repeats(a)}  {(Repeats(a) >= 3 ? "RUN-ON" : "ok")}");