# ADR-016: Cost Efficiency Ranking Algorithm

**Status**: Accepted | Implemented
**Date**: 2024-11-01
**Deciders**: Cost Architecture Team
**Technical Story**: Ranking LLM providers by quality-to-cost ratio to prevent cost overfitting

---

## Decision

Implementar **Cost Efficiency Ranking** con fórmula `efficiency = (quality_score * 100) / (cost_cents + 1)`.

---

## Rationale

1. **Prevents Cost Overfitting**: No preferir siempre provider más barato (quality importa)
2. **Balances Quality and Cost**: Fórmula explícita que combina ambas dimensiones
3. **Handles Zero-Cost**: `+ 1` evita division-by-zero para Ollama ($0)
4. **Normalized Scale**: Scores comparables entre providers

---

## Alternatives Considered

### ❌ Quality Only (Ignore Cost)
- **Pros**: Highest quality
- **Cons**: Unbounded costs

### ❌ Cost Only (Ignore Quality)
- **Pros**: Lowest cost
- **Cons**: Poor quality results

### ✅ Quality/Cost Ratio (CHOSEN)
- Balances both dimensions mathematically

---

## Trade-offs

**Pros**:
- ✅ Single metric for comparison
- ✅ Prevents cost overfitting
- ✅ Prevents quality overfitting
- ✅ Handles zero-cost providers
- ✅ Easy to understand and explain

**Cons**:
- ⚠️ Formula is simplified (assumes linear quality/cost)
- ⚠️ Quality scores must be comparable across providers
- ⚠️ May not capture all cost factors (latency, tokens)

---

## Implementation

**Quality Scores (Baseline)**:
```rust
// crates/vapora-llm-router/src/cost_ranker.rs

pub struct ProviderQuality {
    provider: String,
    model: String,
    quality_score: f32,  // 0.0 - 1.0
}

pub const QUALITY_SCORES: &[ProviderQuality] = &[
    ProviderQuality {
        provider: "claude",
        model: "claude-opus",
        quality_score: 0.95,  // Best reasoning
    },
    ProviderQuality {
        provider: "openai",
        model: "gpt-4",
        quality_score: 0.92,  // Excellent code generation
    },
    ProviderQuality {
        provider: "gemini",
        model: "gemini-2.0-flash",
        quality_score: 0.88,  // Good balance
    },
    ProviderQuality {
        provider: "ollama",
        model: "llama2",
        quality_score: 0.75,  // Lower quality (local)
    },
];
```

**Cost Efficiency Calculation**:
```rust
pub struct CostEfficiency {
    provider: String,
    quality_score: f32,
    cost_cents: u32,
    efficiency_score: f32,
}

impl CostEfficiency {
    pub fn calculate(
        provider: &str,
        quality: f32,
        cost_cents: u32,
    ) -> f32 {
        (quality * 100.0) / ((cost_cents as f32) + 1.0)
    }

    pub fn from_provider(
        provider: &str,
        quality: f32,
        cost_cents: u32,
    ) -> Self {
        let efficiency = Self::calculate(provider, quality, cost_cents);

        Self {
            provider: provider.to_string(),
            quality_score: quality,
            cost_cents,
            efficiency_score: efficiency,
        }
    }
}

// Examples:
// Claude Opus: quality=0.95, cost=50¢ → efficiency = (0.95*100)/(50+1) = 1.86
// GPT-4:       quality=0.92, cost=30¢ → efficiency = (0.92*100)/(30+1) = 2.97
// Gemini:      quality=0.88, cost=5¢  → efficiency = (0.88*100)/(5+1) = 14.67
// Ollama:      quality=0.75, cost=0¢  → efficiency = (0.75*100)/(0+1) = 75.0
```

**Ranking by Efficiency**:
```rust
pub async fn rank_providers_by_efficiency(
    providers: &[LLMClient],
    task_type: &str,
) -> Result<Vec<(String, f32)>> {
    let mut efficiencies = Vec::new();

    for provider in providers {
        let quality = get_quality_for_task(&provider.id, task_type)?;
        let cost_per_token = provider.cost_per_token();
        let estimated_tokens = estimate_tokens_for_task(task_type);
        let total_cost_cents = (cost_per_token * estimated_tokens as f64) as u32;

        let efficiency = CostEfficiency::calculate(
            &provider.id,
            quality,
            total_cost_cents,
        );

        efficiencies.push((provider.id.clone(), efficiency));
    }

    // Sort by efficiency descending
    efficiencies.sort_by(|a, b| {
        b.1.partial_cmp(&a.1).unwrap_or(std::cmp::Ordering::Equal)
    });

    Ok(efficiencies)
}
```

**Provider Selection with Efficiency**:
```rust
pub async fn select_best_provider_by_efficiency(
    task: &Task,
    available_providers: &[LLMClient],
) -> Result<&'_ LLMClient> {
    let ranked = rank_providers_by_efficiency(available_providers, &task.task_type).await?;

    // Return highest efficiency
    ranked
        .first()
        .and_then(|(provider_id, _)| {
            available_providers.iter().find(|p| p.id == *provider_id)
        })
        .ok_or(Error::NoProvidersAvailable)
}
```

**Efficiency Metrics**:
```rust
pub async fn report_efficiency(
    db: &Surreal<Ws>,
) -> Result<String> {
    // Query: execution history with cost and quality
    let query = r#"
        SELECT
            provider,
            avg(quality_score) as avg_quality,
            avg(cost_cents) as avg_cost,
            (avg(quality_score) * 100) / (avg(cost_cents) + 1) as avg_efficiency
        FROM executions
        WHERE timestamp > now() - 1d  -- Last 24 hours
        GROUP BY provider
        ORDER BY avg_efficiency DESC
    "#;

    let results = db.query(query).await?;
    Ok(format_efficiency_report(results))
}
```

**Key Files**:
- `/crates/vapora-llm-router/src/cost_ranker.rs` (efficiency calculations)
- `/crates/vapora-llm-router/src/router.rs` (provider selection)
- `/crates/vapora-backend/src/services/` (cost analysis)

---

## Verification

```bash
# Test efficiency calculation with various costs
cargo test -p vapora-llm-router test_cost_efficiency_calculation

# Test zero-cost handling (Ollama)
cargo test -p vapora-llm-router test_zero_cost_efficiency

# Test provider ranking by efficiency
cargo test -p vapora-llm-router test_provider_ranking_efficiency

# Test efficiency comparison across providers
cargo test -p vapora-llm-router test_efficiency_comparison

# Integration: select best provider by efficiency
cargo test -p vapora-llm-router test_select_by_efficiency
```

**Expected Output**:
- Claude Opus ranked well despite higher cost (quality offset)
- Ollama ranked very high (zero cost, decent quality)
- Gemini ranked between (good efficiency)
- GPT-4 ranked based on balanced cost/quality
- Rankings consistent across multiple runs

---

## Consequences

### Cost Optimization
- Prevents pure cost minimization (quality matters)
- Prevents pure quality maximization (cost matters)
- Balanced strategy emerges

### Provider Selection
- No single provider always selected (depends on task)
- Ollama used frequently (high efficiency)
- Premium providers used for high-quality tasks only

### Reporting
- Efficiency metrics tracked over time
- Identify providers underperforming cost-wise
- Guide budget allocation

### Monitoring
- Alert if efficiency drops for any provider
- Track efficiency trends
- Recommend provider switches if efficiency improves

---

## References

- `/crates/vapora-llm-router/src/cost_ranker.rs` (implementation)
- `/crates/vapora-llm-router/src/router.rs` (usage)
- ADR-007 (Multi-Provider LLM)
- ADR-015 (Budget Enforcement)

---

**Related ADRs**: ADR-007 (Multi-Provider), ADR-015 (Budget), ADR-012 (Routing Tiers)