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.opencode/skills/context-engineering/references/context-compression.md

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+# 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)