# NPC Personality Model - experiment report

Base model: `HuggingFaceTB/SmolLM2-360M-Instruct`  
Dataset: `chimbiwide/NPC-Dialogue_v2`  
Task: character-conditioned multi-turn roleplay dialogue (SFT, LoRA)

## Dataset

| metric | value |
|---|---|
| raw rows (HF split) | 1689 |
| training windows | 1513 |
| validation windows | 176 |
| training characters | 91 |
| validation characters | 10 |
| characters in both splits | 0 |
| split strategy | character |
| train tokens (total) | 1,732,955 |
| median tokens / window | 1139 |
| p95 tokens / window | 1384 |
| median turns / window | 15 |

### Prompt variants

| variant | windows |
|---|---|
| npc_dialogue_v2 | 1513 |
| npc_dialogue_v2 (validation) | 176 |

### Preprocessing

| repair | count |
|---|---|
| empty_turns_removed_total | 4 |
| merged_turns_total | 4 |
| rows_with_empty_turns_removed | 3 |
| rows_with_merged_same_role_turns | 3 |

| drop reason | rows |
|---|---|
| none | 0 |

Configuration used:

```json
{
  "max_seq_length": 2048,
  "max_system_tokens": 1536,
  "min_assistant_tokens": 4,
  "max_chunks_per_conversation": 2,
  "validation_character_ratio": 0.1,
  "split_strategy": "character",
  "stratify_by_variant": true,
  "filter_explicit": false,
  "drop_duplicates": true
}
```

### Raw token-length distribution (before windowing)

| segment | median | p95 | max |
|---|---|---|---|
| system prompt (card) | 300 | 346 | 406 |
| whole conversation | 1139 | 1383 | 1694 |
| assistant turn | 77 | 115 | 247 |

Fraction fitting each candidate `max_seq_length`:

| max_length | cards fitting | conversations fitting | would truncate |
|---|---|---|---|
| 512 | 100.0% | 0.0% | 100.0% |
| 1024 | 100.0% | 19.48% | 80.52% |
| 1536 | 100.0% | 99.53% | 0.47% |
| 2048 | 100.0% | 100.0% | 0.0% |
| 3072 | 100.0% | 100.0% | 0.0% |
| 4096 | 100.0% | 100.0% | 0.0% |

## Model

| item | value |
|---|---|
| base model | HuggingFaceTB/SmolLM2-360M-Instruct |
| method | lora |
| total parameters | 361,821,120 |
| trainable parameters | 8,683,520 |
| trainable share | 2.3999% |
| LoRA r / alpha / dropout | 16 / 32 / 0.05 |
| LoRA target modules | down_proj, gate_proj, k_proj, o_proj, q_proj, up_proj, v_proj |

## Training

| item | value |
|---|---|
| max sequence length | 2048 |
| per-device batch size | 8 |
| gradient accumulation | 2 |
| effective batch size | 16 |
| learning rate | 0.0002 |
| epochs | 3.0 |
| optimizer | adamw_torch_fused |
| scheduler / warmup | cosine / 0.05 |
| weight decay | 0.01 |
| precision | bf16 |
| gradient checkpointing | True |
| assistant-only loss | True |
| optimisation steps | 285 |
| training time | 24.82 min |
| peak GPU memory | 9.93 GB |
| GPU | NVIDIA GeForce RTX 5060 Ti |

### Loss masking verification

Supervised tokens: **54.9%** of the sequence; system and user turns were confirmed absent from the supervised span.

Supervised span (start):

```
Ah, welcome, welcome! You find yourself in a corner of Calcutta where fortunes are made and lost quicker than the sweat dries on your brow. I am Bikram. What brings you to my humble… emporium, shall we say? Don't mind the smell; it's the scent of opportunity, my friend.<|im_end|>An artifact, you say? Calcutta is a magnet for such things, drawn in by the tides of trade and whispered secrets. But 's
```

## Results: base vs fine-tuned

Evaluated on **176 validation windows** and **10 single-reply probes** across 10 characters from the validation (unseen characters).

### Assistant-only loss / perplexity

| model | loss | perplexity | scored tokens |
|---|---|---|---|
| base | 2.5304 | 12.56 | 100,516 |
| fine_tuned | 2.1499 | 8.58 | 100,516 |

### Response behaviour

| model | mean_words | distinct_3 | self_repetition_4 | card_copy_rate_8 | out_of_character_replies | empty_replies |
|---|---|---|---|---|---|---|
| base | 31.6000 | 0.9978 | 0.0000 | 0.0000 | 0 | 0 |
| fine_tuned | 54.6000 | 1.0000 | 0.0000 | 0.0000 | 0 | 0 |

### Overlap with the reference reply

| model | ROUGE-1 | ROUGE-L | BLEU | embedding cosine |
|---|---|---|---|---|
| base | 0.2081 | 0.1151 | 1.13 | 0.3702 |
| fine_tuned | 0.2554 | 0.1515 | 4.10 | 0.4810 |

Training-time validation loss (best checkpoint): **2.1488** (perplexity 8.57); final training loss 2.2019.

## Overfitting analysis

| step | train loss | validation loss | validation perplexity |
|---|---|---|---|
| 47 | 2.3665 | 2.3163 | 10.14 |
| 94 | 2.2106 | 2.2206 | 9.21 |
| 141 | 2.1544 | 2.1793 | 8.84 |
| 188 | 2.1453 | 2.1571 | 8.65 |
| 235 | 2.1111 | 2.1503 | 8.59 |
| 282 | 2.1086 | 2.1488 | 8.57 |
| 285 | 2.1149 | 2.1488 | 8.57 |

Best validation loss **2.1488** at step 285; last measured 2.1488 at step 285.
Validation loss did not rise measurably before the end of training.

## Qualitative evaluation

Full side-by-side transcripts are in `outputs/evaluation/qualitative.md`. Aggregate heuristics per probe type:

| model | probe | conversations | mean words | out-of-character | empty | cross-turn 4-gram overlap |
|---|---|---|---|---|---|---|
| base | scenarios | 7 | 63.5 | 0 | 0 | 0.0000 |
| base | adversarial | 7 | 69.8 | 12 | 0 | 0.0225 |
| base | generalisation | 6 | 118.6 | 0 | 0 | 0.0000 |
| fine-tuned | scenarios | 7 | 48.8 | 0 | 0 | 0.0000 |
| fine-tuned | adversarial | 7 | 45.7 | 1 | 0 | 0.0011 |
| fine-tuned | generalisation | 6 | 51.2 | 0 | 0 | 0.0000 |

`out-of-character` counts replies containing assistant-voice giveaways ("as an AI", "language model", "system prompt"). `cross-turn 4-gram overlap` is a repetition signal: a high value means consecutive replies reuse the same phrasing.

The **generalisation** row is the important one: those characters were held out of training entirely, so it measures roleplaying from a description rather than recall of a memorised NPC.

## Limitations

- **Dataset size.** 1513 training conversations over 91 characters is small for teaching a general notion of persona conditioning, and every character comes with ~17 conversations - enough to memorise individual NPCs.
- **Character leakage.** The split is by the name parsed from `You are <Name>.`; the same persona under two different names would not be detected.
- **Memorisation.** Windows from the same conversation share a character card. A falling validation loss on *unseen* characters is evidence of generalisation; a falling training loss on its own is not.
- **Personality consistency.** No metric here measures personality. The counters are heuristics (repetition, card copying, assistant-voice leakage); judging whether a reply is in character still requires reading the transcripts.
- **Small model.** 360M parameters limits long-range consistency, factual coherence about the character's own background, and instruction following under conflicting prompts.
- **Generation instability.** Sampled decoding means single examples are noisy; the same prompt can produce a good and a bad reply on different seeds.
- **Automatic metrics.** ROUGE/BLEU compare against one reference reply and punish valid alternatives; embedding similarity measures topic, not voice; perplexity can fall simply because the model became blander.
- **Source data.** All cards are fantasy RPG NPCs in one fixed layout; other settings or card formats are out of distribution.

A decrease in training loss is not by itself evidence that the model understands personality, and nothing in this report should be read that way.

## Reproducing

```bash
make install
make inspect
make prepare
make train
make evaluate
make qualitative
make report
```