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---
summary: "Trimming old tool results to keep context lean and caching efficient"
title: "Session pruning"
read_when:
- You want to reduce context growth from tool outputs
- You want to understand Anthropic prompt cache optimization
---
Session pruning trims **old tool results** from the context before each LLM
call. It reduces context bloat from accumulated tool outputs (exec results, file
reads, search results) without rewriting normal conversation text.
<Info>
Pruning is in-memory only -- it does not modify the on-disk session transcript.
Your full history is always preserved.
</Info>
## Why it matters
Long sessions accumulate tool output that inflates the context window. This
increases cost and can force [compaction](/concepts/compaction) sooner than
necessary.
Pruning is especially valuable for **Anthropic prompt caching**. After the cache
TTL expires, the next request re-caches the full prompt. Pruning reduces the
cache-write size, directly lowering cost.
## How it works
1. Wait for the cache TTL to expire (default 5 minutes).
2. Find old tool results for normal pruning (conversation text is left alone).
3. **Soft-trim** oversized results -- keep the head and tail, insert `...`.
4. **Hard-clear** the rest -- replace with a placeholder.
5. Reset the TTL so follow-up requests reuse the fresh cache.
## Legacy image cleanup
OpenClaw also runs a separate idempotent cleanup for older legacy sessions that
persisted raw image blocks in history.
- It preserves the **3 most recent completed turns** byte-for-byte so prompt
cache prefixes for recent follow-ups stay stable.
- Older already-processed image blocks in `user` or `toolResult` history can be
replaced with `[image data removed - already processed by model]`.
- This is separate from normal cache-TTL pruning. It exists to stop repeated
image payloads from busting prompt caches on later turns.
## Smart defaults
OpenClaw auto-enables pruning for Anthropic profiles:
| Profile type | Pruning enabled | Heartbeat |
| ------------------------------------------------------- | --------------- | --------- |
| Anthropic OAuth/token auth (including Claude CLI reuse) | Yes | 1 hour |
| API key | Yes | 30 min |
If you set explicit values, OpenClaw does not override them.
## Enable or disable
Pruning is off by default for non-Anthropic providers. To enable:
```json5
{
agents: {
defaults: {
contextPruning: { mode: "cache-ttl", ttl: "5m" },
},
},
}
```
To disable: set `mode: "off"`.
## Pruning vs compaction
| | Pruning | Compaction |
| ---------- | ------------------ | ----------------------- |
| **What** | Trims tool results | Summarizes conversation |
| **Saved?** | No (per-request) | Yes (in transcript) |
| **Scope** | Tool results only | Entire conversation |
They complement each other -- pruning keeps tool output lean between
compaction cycles.
## Further reading
- [Compaction](/concepts/compaction) -- summarization-based context reduction
- [Gateway Configuration](/gateway/configuration) -- all pruning config knobs
(`contextPruning.*`)
## Related
- [Session management](/concepts/session)
- [Session tools](/concepts/session-tool)
- [Context engine](/concepts/context-engine)
[Dauer der Verarbeitung: 0.26 Sekunden, vorverarbeitet 2026-04-27]
