I found a bug that made my LLM look 14x better than it was — here's what I learned

I've been building GhostLM for the past few months — a decoder-only transformer trained entirely from scratch in PyTorch on cybersecurity data. No pretrained weights, no HuggingFace wrappers. Every component hand-written.
Phase 1 looked promising. After 10,000 steps on ghost-tiny (14.7M params), I had a validation loss of 2.74 and a cybersecurity perplexity of 2,183.94 against my benchmark set.
Then I found the bug.

The leaky split
My train/validation split was random. Sounds fine. But random splits on text data have a subtle failure mode — if you're not careful, near-duplicate or semantically similar records end up on both sides of the split. Your model "validates" on text it essentially already saw during training.
The fix was switching to a deterministic hash-based split. Every record gets assigned to train or validation based on a hash of its content. Identical texts always land in the same bucket. No leakage, no matter how you shuffle.
I rebuilt the corpus from scratch with this fix, added a data audit script that checks for leakage explicitly, and re-ran training.

Phase 2 results
The new validation loss was 3.78 — higher than Phase 1's 2.74. On the surface that looks like regression.
It isn't. The 2.74 was a lie. The 3.78 is the first honest number.
The real comparison is perplexity on a hardcoded external benchmark set — the same samples both phases ran against:
ModelPerplexityghost-tiny Phase 12,183.94ghost-tiny Phase 2152.71GPT-2 124M baseline26.76
Phase 2 is 14.3x better than Phase 1. Entirely from corpus quality — same architecture, same steps, same hardware.
ghost-tiny is still 5.7x behind GPT-2, which is expected. It's a 14.7M parameter model trained on 2.7M tokens vs a 124M model trained on 40B tokens of WebText. The gap is compute and data, not architecture.

What the model actually generates
Here's a real sample at temperature 0.8:
Prompt: A SQL injection attack works by

...the login page. The login page is used to the login page's name of the login page does not properly sanitization of the password, which allows attackers to cause a denial of service via a long GET request...

It has absorbed surface-level cybersecurity vocabulary — CTF terminology, exploit techniques, CVE string format. But it has no semantic grounding. Broken grammar, hallucinated version chains, topic drift. This is exactly what you'd predict for this scale.
The fix isn't more steps. It's scale. ghost-small at 55M params is next.

What I actually learned
Data quality compounds more than training time. Fixing a leaky split gave me a 14.3x perplexity improvement with zero extra compute. If I had just kept training Phase 1, I would have been optimizing a benchmark that was secretly measuring memorization.
Honest evaluation is harder than training. Designing a benchmark that doesn't leak, doesn't overfit, and actually measures what you care about is genuinely difficult. I got it wrong the first time and only caught it by auditing.
From scratch means you own every mistake. There's no from_pretrained to blame. When something breaks, it's yours. That's painful but it's also the fastest way to actually understand what's happening.

What's next
Corpus expansion — targeting 10-100x current size. CTFtime archives, Project Zero blog posts, PortSwigger research, MITRE ATT&CK, tool documentation.
Then ghost-small at 55M params on the Mac Mini M4 with MPS acceleration. That's the first rung where domain-coherent generation might start to emerge.
Realistic timeline to something genuinely useful: 2-3 years. No shortcuts for from-scratch at scale.
GitHub: https://github.com/joemunene-by/GhostLM