# Fidelity Metrics Framework

> Sprint: SPRINT_3403_0001_0001_fidelity_metrics

This document describes the three-tier fidelity metrics framework for measuring deterministic reproducibility in StellaOps scanner outputs.

## Overview

Fidelity metrics quantify how consistently the scanner produces outputs across replay runs. The framework provides three tiers of measurement, each capturing different aspects of reproducibility:

| Metric | Abbrev. | Description | Target |
|--------|---------|-------------|--------|
| Bitwise Fidelity | BF | Byte-for-byte identical outputs | ≥ 0.98 |
| Semantic Fidelity | SF | Normalized object equivalence | ≥ 0.99 |
| Policy Fidelity | PF | Policy decision consistency | ≈ 1.0 |

## Metric Definitions

### Bitwise Fidelity (BF)

Measures the proportion of replay runs that produce byte-for-byte identical outputs.

```
BF = identical_outputs / total_replays
```

**What it captures:**
- SHA-256 hash equivalence of all output artifacts
- Timestamp consistency
- JSON formatting consistency
- Field ordering consistency

**When BF < 1.0:**
- Timestamps embedded in outputs
- Non-deterministic field ordering
- Floating-point rounding differences
- Random identifiers (UUIDs)

### Semantic Fidelity (SF)

Measures the proportion of replay runs that produce semantically equivalent outputs, ignoring formatting differences.

```
SF = semantic_matches / total_replays
```

**What it compares:**
- Package PURLs and versions
- CVE identifiers
- Severity levels (normalized to uppercase)
- VEX verdicts
- Reason codes

**When SF < 1.0 but BF = SF:**
- No actual content differences
- Only formatting differences

**When SF < 1.0:**
- Different packages detected
- Different CVEs matched
- Different severity assignments

### Policy Fidelity (PF)

Measures the proportion of replay runs that produce matching policy decisions.

```
PF = policy_matches / total_replays
```

**What it compares:**
- Final pass/fail decision
- Reason codes (sorted for comparison)
- Policy rule triggering

**When PF < 1.0:**
- Policy outcome differs between runs
- Indicates a non-determinism bug that affects user-visible decisions

## Prometheus Metrics

The fidelity framework exports the following metrics:

| Metric Name | Type | Labels | Description |
|-------------|------|--------|-------------|
| `fidelity_bitwise_ratio` | Gauge | tenant_id, surface_id | Bitwise fidelity ratio |
| `fidelity_semantic_ratio` | Gauge | tenant_id, surface_id | Semantic fidelity ratio |
| `fidelity_policy_ratio` | Gauge | tenant_id, surface_id | Policy fidelity ratio |
| `fidelity_total_replays` | Gauge | tenant_id, surface_id | Number of replays |
| `fidelity_slo_breach_total` | Counter | breach_type, tenant_id | SLO breach count |

## SLO Thresholds

Default SLO thresholds (configurable):

| Metric | Warning | Critical |
|--------|---------|----------|
| Bitwise Fidelity | < 0.98 | < 0.90 |
| Semantic Fidelity | < 0.99 | < 0.95 |
| Policy Fidelity | < 1.0 | < 0.99 |

## Integration with DeterminismReport

Fidelity metrics are integrated into the `DeterminismReport` record:

```csharp
public sealed record DeterminismReport(
    // ... existing fields ...
    FidelityMetrics? Fidelity = null);

public sealed record DeterminismImageReport(
    // ... existing fields ...
    FidelityMetrics? Fidelity = null);
```

## Usage Example

```csharp
// Create fidelity metrics service
var service = new FidelityMetricsService(
    new BitwiseFidelityCalculator(),
    new SemanticFidelityCalculator(),
    new PolicyFidelityCalculator());

// Compute fidelity from baseline and replays
var baseline = LoadScanResult("scan-baseline.json");
var replays = LoadReplayScanResults();
var fidelity = service.Compute(baseline, replays);

// Check thresholds
if (fidelity.BitwiseFidelity < 0.98)
{
    logger.LogWarning("BF below threshold: {BF}", fidelity.BitwiseFidelity);
}

// Include in determinism report
var report = new DeterminismReport(
    // ... other fields ...
    Fidelity: fidelity);
```

## Mismatch Diagnostics

When fidelity is below threshold, the framework provides diagnostic information:

```csharp
public sealed record FidelityMismatch
{
    public required int RunIndex { get; init; }
    public required FidelityMismatchType Type { get; init; }
    public required string Description { get; init; }
    public IReadOnlyList<string>? AffectedArtifacts { get; init; }
}

public enum FidelityMismatchType
{
    BitwiseOnly,    // Hash differs but content equivalent
    SemanticOnly,   // Content differs but policy matches
    PolicyDrift     // Policy decision differs
}
```

## Configuration

Configure fidelity options via `FidelityThresholds`:

```json
{
  "Fidelity": {
    "BitwiseThreshold": 0.98,
    "SemanticThreshold": 0.99,
    "PolicyThreshold": 1.0,
    "EnableDiagnostics": true,
    "MaxMismatchesRecorded": 100
  }
}
```

## Related Documentation

- [Determinism and Reproducibility Technical Reference](../product-advisories/14-Dec-2025%20-%20Determinism%20and%20Reproducibility%20Technical%20Reference.md)
- [Determinism Scoring Foundations Sprint](../implplan/SPRINT_3401_0001_0001_determinism_scoring_foundations.md)
- [Scanner Architecture](../modules/scanner/architecture.md)

## Source Files

- `src/Scanner/StellaOps.Scanner.Worker/Determinism/FidelityMetrics.cs`
- `src/Scanner/StellaOps.Scanner.Worker/Determinism/FidelityMetricsService.cs`
- `src/Scanner/StellaOps.Scanner.Worker/Determinism/Calculators/`
- `src/Telemetry/StellaOps.Telemetry.Core/FidelityMetricsTelemetry.cs`
- `src/Telemetry/StellaOps.Telemetry.Core/FidelitySloAlertingService.cs`