# Context Compression

Strategies for long-running sessions exceeding context windows.

## Core Insight

Optimize **tokens-per-task** (total to completion), not tokens-per-request.
Aggressive compression causing re-fetching costs more than better retention.

## Compression Methods

| Method                 | Compression | Quality | Best For        |
| ---------------------- | ----------- | ------- | --------------- |
| **Anchored Iterative** | 98.6%       | 3.70/5  | Best balance    |
| **Regenerative Full**  | 98.7%       | 3.44/5  | Readability     |
| **Opaque**             | 99.3%       | 3.35/5  | Max compression |

## Anchored Iterative Summary Template

```markdown
## Session Intent

Original goal: [preserved]

## Files Modified

- file.py: Changes made

## Decisions Made

- Key decisions with rationale

## Current State

Progress summary

## Next Steps

1. Next action items
```

**On compression**: Merge new content into existing sections, don't regenerate.

## Compression Triggers

| Strategy        | Trigger                     | Use Case             |
| --------------- | --------------------------- | -------------------- |
| Fixed threshold | 70-80% utilization          | General purpose      |
| Sliding window  | Keep last N turns + summary | Conversations        |
| Task-boundary   | At logical completion       | Multi-step workflows |

## Artifact Trail Problem

Weakest dimension (2.2-2.5/5.0). Coding agents need explicit tracking of:

- Files created/modified/read
- Function/variable names, error messages

**Solution**: Dedicated artifact section in summary.

## Probe-Based Evaluation

| Probe Type   | Tests             | Example                 |
| ------------ | ----------------- | ----------------------- |
| Recall       | Factual retention | "What was the error?"   |
| Artifact     | File tracking     | "Which files modified?" |
| Continuation | Task planning     | "What next?"            |
| Decision     | Reasoning chains  | "Why chose X?"          |

## Six Evaluation Dimensions

1. **Accuracy** - Technical correctness
2. **Context Awareness** - Conversation state
3. **Artifact Trail** - File tracking (universally weak)
4. **Completeness** - Coverage depth
5. **Continuity** - Work continuation
6. **Instruction Following** - Constraints

## Guidelines

1. Use anchored iterative for best quality/compression
2. Maintain explicit artifact tracking section
3. Trigger compression at 70% utilization
4. Merge into sections, don't regenerate
5. Evaluate with probes, not lexical metrics

## Related

- [Context Optimization](./context-optimization.md)
- [Evaluation](./evaluation.md)