PrismaV2_ToDo.md

# Prisma V2 — ToDo.md

## Goal

Evolve PRISMA from a **Python-stateful uncertainty feedback mechanism** (V1) into a **fully stateless, runtime-portable architecture** compatible with:

* Hugging Face Transformers
* vLLM
* llama.cpp
* MLX

while preserving the core idea:

> **Condition future predictions on the model’s own uncertainty to reduce confident errors and hallucinations.**

---

## Background (Why V2 Is Needed)

Prisma V1 relies on mutable per-step Python state:

```python
self.prev_uncertainty_code
```

This works in Hugging Face `generate()`, but fails in modern inference engines:

* **vLLM** — batched, reordered, stateless execution
* **llama.cpp** — compiled C/C++ loop with fixed KV cache semantics
* **MLX** — pure functional graph execution

All target runtimes require decoding to be **purely functional**:

> All information needed for the next step must be carried explicitly in tensors, tokens, or KV cache — not Python object state.

---

## Core Design Change (Prisma V2)

### Replace measured uncertainty with **predicted uncertainty**

Instead of:

* computing entropy post-hoc from logits
* storing uncertainty in a mutable buffer

**The model learns to predict uncertainty directly.**

At each position, the model outputs:

1. **Next-token logits** (existing)
2. **Uncertainty logits for the next position** (new)

Uncertainty becomes a **learned latent variable**, not a side effect.

---

## Architecture Changes

### 1. Add an uncertainty prediction head

```python
self.n_uncertainty_levels = 256  # V2: smaller, sufficient
self.uncertainty_head = nn.Linear(hidden_size, n_uncertainty_levels, bias=False)
```

### 2. Uncertainty head initialization (important)

The uncertainty head is added to a pretrained model. To avoid destabilizing early training, use **zero initialization**:

```python
self.uncertainty_head.weight.data.zero_()
```

**Rationale:**

* Model initially predicts neutral uncertainty everywhere
* Early training behaves identically to the base model
* Uncertainty signal is learned gradually, only when useful

Alternative initializations (small random, copying from `lm_head`) are left for experimentation.

---

### 3. Keep uncertainty embeddings (input side)

```python
self.uncertainty_embeddings = nn.Embedding(
    n_uncertainty_levels,
    hidden_size
)
```

---

### 4. Modify forward signature

```python
def forward(
    input_ids: torch.Tensor,
    uncertainty_codes: Optional[torch.Tensor] = None,  # [B, S]
    **kwargs
):
```

* `uncertainty_codes[t]` conditions token `t`
* No hidden buffers
* No mutable Python state

---

## Forward Pass Logic (V2)

1. Embed tokens
2. If `uncertainty_codes` provided:

   * lookup `uncertainty_embeddings`
   * add to `inputs_embeds`
3. Run transformer
4. Produce:

   * `logits` (next token)
   * `uncertainty_logits` (next uncertainty)

```python
return {
    "logits": logits,
    "uncertainty_logits": uncertainty_logits,
}
```

---

## Temporal Semantics

| Position | Input                 | Predicts                 |
| -------- | --------------------- | ------------------------ |
| t        | tokenₜ + uncertaintyₜ | tokenₜ₊₁, uncertaintyₜ₊₁ |

This preserves the original PRISMA temporal feedback loop without mutable state.

---

## Training Plan

### Uncertainty supervision (teacher signal)

During training, entropy is used **only as a teacher signal**, not as the definition of uncertainty.

```python
entropy = -∑ p log p
normalized = entropy / log(vocab_size)
uncertainty_label = quantize(normalized)
```

---

### Single-pass training (preferred)

A second forward pass is **not required**.

```python
outputs = model(
    input_ids,
    uncertainty_codes=uncertainty_input
)

with torch.no_grad():
    uncertainty_labels = quantize_entropy(outputs.logits)

loss = (
    loss_lm(outputs.logits, labels)
    + λ * loss_uncertainty(outputs.uncertainty_logits, uncertainty_labels)
)
```

**Key point:**

* Entropy is a **bootstrap target**
* The model is free to learn uncertainty representations that diverge from entropy over time
* This allows uncertainty to correlate better with *error* than raw entropy does

---

### Loss definition

```python
loss = loss_lm + λ * loss_uncertainty
```

* `loss_lm`: standard next-token cross-entropy
* `loss_uncertainty`: cross-entropy over uncertainty codes
* λ ≈ 0.1 (to tune)

---

## Inference (All Runtimes)

### Decode loop (conceptual)

```text
(tokenₜ, uncertaintyₜ) → model → (tokenₜ₊₁, uncertaintyₜ₊₁)
```

### Runtime responsibilities

* **Transformers**: custom `generate()` tracks uncertainty tensor
* **vLLM**: sampler tracks `uncertainty_code` per request
* **llama.cpp**: store one small uncertainty code in `llama_context`
* **MLX**: works naturally (pure tensor graph)

No runtime needs to preserve Python object state.

---

## Compatibility Matrix

| Runtime         | Prisma V1 | Prisma V2 |
| --------------- | --------- | --------- |
| Transformers    | ✅         | ✅         |
| vLLM            | ❌         | ✅         |
| llama.cpp       | ❌         | ✅         |
| MLX             | ❌         | ✅         |
| Tensor Parallel | ⚠️        | ✅         |

---

## Design Decisions

### Classification vs regression

**Chosen:** classification (quantized uncertainty)

Reasons:

* Stable training
* Matches embedding lookup
* Discrete semantics
* Easier runtime handling

Regression remains an experimental alternative.

---

### Uncertainty resolution

* V1: 65,536 levels (overkill)
* V2: **256 levels** (sufficient, efficient, portable)

---

## Known Limitations

* Uncertainty reflects **model confidence**, not correctness
* Learned uncertainty may differ from Shannon entropy
* Does not guarantee abstention or correctness
* Behavior depends on training data and loss weighting

---

## Open Questions / Future Work

* Tune `n_uncertainty_levels`
* Tune λ (uncertainty loss weight)
* Explore uncertainty-aware decoding strategies
* Compare uncertainty prediction vs uncertainty tokens
* Investigate bootstrapping without entropy supervision

---

## Definition of Done (Prisma V2)

* [ ] No mutable per-step Python state
* [ ] Uncertainty passed explicitly as tensor
* [ ] Works in Transformers, vLLM, llama.cpp, MLX
* [ ] Zero-init uncertainty head
* [ ] Single-pass training loop
* [ ] Updated model card + documentation
* [ ] Reference implementation + example

---

### Guiding Principle (V2)

> **Uncertainty must be data, not memory.**