# M3.8.2 — CacheAligner: HTTP Headers + Metrics

| Field | Value |
|---|---|
| Phase | M3.8 — Context optimization |
| Size | M — 1–2 days |
| Status | ✅ COMPLETE |
| Depends | M3.8.1 (CacheAligner complete) |
| Blocks | M3.8.3 |

## Goal

Integrate CacheAligner output into HTTP response headers and observability metrics
so that:
1. LLM provider can use cache hints
2. Monitoring can track cache effectiveness
3. Debugging can identify cache misses

## Deliverables

### 1. PromptBuilder::cache_metrics()

New method returning cache metadata:

```rust
pub struct CacheMetrics {
    pub stable_prefix_bytes: usize,
    pub dynamic_tail_bytes: usize,
    pub drift_metric: f32,  // 0.0-1.0 ratio
    pub cache_eligible: bool,  // true if drift < 0.3
}

impl PromptBuilder {
    pub fn cache_metrics(query: &Query, chunk: &Chunk) -> Result<CacheMetrics>
}
```

Tests (3):
- `test_cache_metrics_stable_query`
- `test_cache_metrics_high_drift`
- `test_cache_metrics_zero_drift`

### 2. HTTP Response Headers

Add to PromptBuilder output:
- `X-Cache-Stable-Bytes`: size of cacheable prefix
- `X-Cache-Drift`: 0.0-1.0 ratio
- `X-Cache-Eligible`: "true"/"false"
- `X-Compression-Ratio`: original vs. compressed

Tests (4):
- `test_headers_present_in_response`
- `test_headers_accurate_values`
- `test_headers_skipped_when_disabled`
- `test_headers_format_valid`

### 3. Observability Hooks

Integrate with logging:
```rust
pub fn log_cache_metrics(metrics: &CacheMetrics) {
    tracing::info!(
        stable_bytes = metrics.stable_prefix_bytes,
        drift = metrics.drift_metric,
        eligible = metrics.cache_eligible,
        "cache_alignment"
    );
}
```

Tests (2):
- `test_metrics_logged_on_alignment`
- `test_drift_high_triggers_warning`

## Acceptance

- All 9 new tests passing
- Existing 114 mem-core tests still pass
- Cache metrics accurately reflect alignment
- HTTP headers present and valid
- Zero performance overhead (< 1ms additional)