PRISMA.md

PRISMA: 16-Bit Temporal Introspection Mechanism - Implementation Specification

Architecture Overview: PRISMA adds a cross-step feedback loop where model uncertainty from the previous forward pass modulates input embeddings for the current step. This enables introspective behavior without modifying internal transformer layers.

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Core Components to Add

# 1. In your model class (e.g., LlamaModel, MistralModel)
self.uncertainty_embeddings = nn.Embedding(65536, hidden_dim)  # 16-bit codes
self.register_buffer('prev_uncertainty_code', None)  # [batch, prev_seq_len]

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Initialization Details

• Embedding Table: Initialize weights from N(0, σ²) where σ = config.initializer_range (typically 0.02)
• Buffer: prev_uncertainty_code starts as None; will be lazily initialized on first forward pass
• Device/Dtype: Buffer automatically inherits model's device; ensure uncertainty_embeddings runs in same dtype as model (typically bfloat16)

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Forward Pass Modifications (Input Side)

Location: Immediately after input embedding lookup, before transformer layers

# Pseudocode for model forward()
def forward(self, input_ids, inputs_embeds=None, ...):
    if inputs_embeds is None:
        inputs_embeds = self.embed_tokens(input_ids)
    
    # === PRISMA INJECTION POINT ===
    batch_size, seq_len = inputs_embeds.shape[:2]
    
    # Handle uncertainty state initialization
    if self.prev_uncertainty_code is None or self.prev_uncertainty_code.shape[0] != batch_size:
        # First pass or batch size changed: use neutral uncertainty
        uncertainty_code = torch.full(
            (batch_size, seq_len), 32768,  # N/2 = neutral
            dtype=torch.long, device=inputs_embeds.device
        )
    else:
        # Pad or truncate to match current sequence length
        prev_len = self.prev_uncertainty_code.shape[1]
        if prev_len < seq_len:
            padding = torch.full(
                (batch_size, seq_len - prev_len), 32768,
                dtype=torch.long, device=inputs_embeds.device
            )
            uncertainty_code = torch.cat([self.prev_uncertainty_code, padding], dim=1)
        else:
            uncertainty_code = self.prev_uncertainty_code[:, :seq_len]
    
    # Lookup and shift embeddings (position t gets uncertainty from t-1)
    uncertainty_embeds = self.uncertainty_embeddings(uncertainty_code)  # [B, S, D]
    uncertainty_shifted = F.pad(
        uncertainty_embeds[:, :-1, :], (0, 0, 1, 0), value=0.0
    )  # First position gets zero
    
    # Inject into main embeddings
    inputs_embeds = inputs_embeds + uncertainty_shifted
    # === END PRISMA INJECTION ===
    
    # Proceed to transformer layers as normal
    hidden_states = self.layers(inputs_embeds, ...)
    return hidden_states

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Forward Pass Modifications (Output Side)

Location: In your CausalLM class (e.g., LlamaForCausalLM) after computing logits

# Pseudocode for CausalLM forward()
def forward(self, ..., labels=None, return_dict=True):
    outputs = self.model(...)
    hidden_states = outputs.last_hidden_state
    logits = self.lm_head(hidden_states)
    
    # === PRISMA UNCERTAINTY COMPUTATION ===
    if self.training or logits is not None:  # Compute during both train and inference
        with torch.no_grad():
            # Detach to avoid gradient flow into uncertainty mechanism
            probs = logits.detach().softmax(dim=-1)  # [B, S, V]
            
            # Compute normalized entropy
            log_probs = torch.log(probs.clamp(min=1e-9))
            entropy = -(probs * log_probs).sum(dim=-1)  # [B, S]
            
            # Normalize by uniform distribution entropy
            max_entropy = math.log(probs.size(-1))
            entropy_norm = (entropy / max_entropy).clamp(0.0, 1.0)
            
            # Quantize to 16-bit integer codes [0, 65535]
            self.model.prev_uncertainty_code = (
                entropy_norm * 65535
            ).long().clamp(0, 65535)
    # === END PRISMA COMPUTATION ===
    
    # Compute loss, return outputs as normal
    loss = None
    if labels is not None:
        loss = self.loss_function(logits, labels)
    
    return CausalLMOutputWithPast(
        loss=loss,
        logits=logits,
        past_key_values=outputs.past_key_values,
    )

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Generation Loop Integration

Required: Reset uncertainty state between generation runs

# Add this method to your CausalLM class
def reset_uncertainty(self):
    """Call this before each new generation to clear uncertainty state"""
    self.model.prev_uncertainty_code = None

# In your generation code:
model.reset_uncertainty()  # Essential!
outputs = model.generate(**inputs)

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Key Implementation Notes for Arbitrary Models

Model Type	Integration Points
Standard Decoder (Llama, Mistral)	Inject in forward() after self.embed_tokens(); compute uncertainty in ForCausalLM.forward()
Encoder-Decoder (T5)	Inject in decoder embedding; compute uncertainty from decoder output logits
Vision-Language (LLaVA, DeepSeek-VL)	Inject after multimodal projections; ensure prev_uncertainty_code tracks text token positions only
MoE Models (Mixtral)	Inject before expert routing; uncertainty overhead is negligible compared to MoE computation

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Edge Cases & State Management

1. Dynamic Sequence Lengths: The padding/truncation logic ensures prev_uncertainty_code always matches current seq_len
2. Batch Size Changes: When batch size changes mid-generation, reinitialize with neutral codes
3. KV Cache: prev_uncertainty_code does not participate in KV cache; it's purely a side-channel
4. Gradient Checkpointing: The mechanism is checkpointing-safe; embeddings are recomputed during backward
5. Multi-GPU: uncertainty_embeddings are part of model parameters and get sharded automatically; prev_uncertainty_code stays on same device as model

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Performance Characteristics

Component	Parameters	FLOPs	Memory	Latency
Uncertainty Embeddings	65,536 × hidden_dim	0	~134MB (if d=2048)	Negligible
Entropy Computation	0	O(B×S×V)	O(1)	<0.1ms
Embedding Addition	0	O(B×S×D)	O(1)	<0.01ms

Total Overhead: <1% additional compute, ~0.1% additional memory

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Theoretical Intuition

PRISMA transforms autoregressive generation from a memoryless process P(y_t | x, y_<t) into a stateful process P(y_t | x, y_<t, H(P(y_<t))), where H is the uncertainty quantizer. The model learns to use this "confidence memory" to:

• Be more cautious after uncertain predictions (high c_t → modified embeddings)
• Maintain momentum after confident predictions (low c_t → near-zero injection)
• Develop meta-cognitive strategies without architectural depth changes

The 16-bit quantization provides sufficient resolution (65,536 levels) to capture subtle confidence gradations while maintaining a computationally efficient lookup table.

Recipe: Add 16-bit Uncertainty Feedback to Any Model

Goal

Feed model confidence from the previous step back into the next token's embedding using entropy-based uncertainty. The signal rides along through the network, gaining strength until the model can act on it explicitly.

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1. Add persistent uncertainty state

Where: model __init__

self.n_bits = 16
self.n_levels = 2 ** self.n_bits
self.uncertainty_embed = nn.Embedding(self.n_levels, hidden_size)
self.register_buffer("prev_uncertainty_code", None)

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2. Inject uncertainty into input embeddings

Where: model forward, immediately after inputs_embeds is created

B, T, D = inputs_embeds.shape

if self.prev_uncertainty_code is None or self.prev_uncertainty_code.shape[0] != B:
    code = torch.full((B, T), self.n_levels // 2, dtype=torch.long, device=inputs_embeds.device)
else:
    prev = self.prev_uncertainty_code
    if prev.shape[1] >= T:
        code = prev[:, :T]
    else:
        code = F.pad(prev, (0, T - prev.shape[1]), value=self.n_levels // 2)

u = self.uncertainty_embed(code)
u = F.pad(u[:, :-1], (0, 0, 1, 0))  # shift right: position i gets uncertainty from i-1
inputs_embeds = inputs_embeds + u

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3. Compute uncertainty from logits

Where: LM head forward, after logits are computed

Note: If your buffer lives on an inner model (e.g., self.model), update self.model.prev_uncertainty_code instead.

with torch.no_grad():
    probs = logits.softmax(dim=-1)
    entropy = -(probs * torch.log(probs.clamp_min(1e-9))).sum(dim=-1)
    entropy = entropy / math.log(probs.size(-1))  # normalize to [0, 1]
    self.prev_uncertainty_code = (
        entropy * (self.n_levels - 1)
    ).long().clamp(0, self.n_levels - 1)

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4. Reset hook

def reset_uncertainty(self):
    self.prev_uncertainty_code = None

Call before each new generation or when switching between unrelated sequences.

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5. Generation rules

• Do NOT clear prev_uncertainty_code between decoding steps within a sequence
• DO clear it between unrelated sequences or batches

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Why this works

The uncertainty signal rides along in the residual stream from the first layer. Early on, it competes with stronger signals—token semantics, position, attention patterns. But because it correlates with prediction difficulty, the model learns to preserve it.

By approximately two-thirds through the network, the signal has accumulated enough relative strength to influence decisions explicitly. The model doesn't just feel uncertain—it can act on uncertainty: hedge, qualify, or change course.

You don't inject at a specific layer because you don't know where introspection should live. The model discovers that for itself. You just ensure the information is present from the start.

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What to watch for

• Ablation test: Zero out the uncertainty injection and measure perplexity change. If it hurts, the signal is being used.
• Attention probe: Check whether high-uncertainty positions receive more attention in later layers.
• Behavioral test: Does the model hedge more after high-entropy predictions? Does it recover better from mistakes?

If uncertainty is truly integrated, the model's behavior will reflect its confidence—not because you trained it to say "I'm not sure," but because knowing its own uncertainty became useful for prediction.