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81 lines
2.5 KiB
Markdown
81 lines
2.5 KiB
Markdown
---
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title: "Session Pruning"
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summary: "Trimming old tool results to keep context lean and caching efficient"
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read_when:
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- You want to reduce context growth from tool outputs
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- You want to understand Anthropic prompt cache optimization
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---
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# Session Pruning
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Session pruning trims **old tool results** from the context before each LLM
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call. It reduces context bloat from accumulated tool outputs (exec results, file
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reads, search results) without touching your conversation messages.
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<Info>
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Pruning is in-memory only -- it does not modify the on-disk session transcript.
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Your full history is always preserved.
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</Info>
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## Why it matters
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Long sessions accumulate tool output that inflates the context window. This
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increases cost and can force [compaction](/concepts/compaction) sooner than
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necessary.
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Pruning is especially valuable for **Anthropic prompt caching**. After the cache
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TTL expires, the next request re-caches the full prompt. Pruning reduces the
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cache-write size, directly lowering cost.
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## How it works
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1. Wait for the cache TTL to expire (default 5 minutes).
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2. Find old tool results (user and assistant messages are never touched).
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3. **Soft-trim** oversized results -- keep the head and tail, insert `...`.
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4. **Hard-clear** the rest -- replace with a placeholder.
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5. Reset the TTL so follow-up requests reuse the fresh cache.
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## Smart defaults
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OpenClaw auto-enables pruning for Anthropic profiles:
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| Profile type | Pruning enabled | Heartbeat |
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| -------------------- | --------------- | --------- |
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| OAuth or setup-token | Yes | 1 hour |
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| API key | Yes | 30 min |
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If you set explicit values, OpenClaw does not override them.
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## Enable or disable
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Pruning is off by default for non-Anthropic providers. To enable:
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```json5
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{
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agents: {
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defaults: {
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contextPruning: { mode: "cache-ttl", ttl: "5m" },
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},
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},
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}
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```
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To disable: set `mode: "off"`.
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## Pruning vs compaction
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| | Pruning | Compaction |
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| ---------- | ------------------ | ----------------------- |
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| **What** | Trims tool results | Summarizes conversation |
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| **Saved?** | No (per-request) | Yes (in transcript) |
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| **Scope** | Tool results only | Entire conversation |
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They complement each other -- pruning keeps tool output lean between
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compaction cycles.
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## Further reading
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- [Compaction](/concepts/compaction) -- summarization-based context reduction
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- [Gateway Configuration](/gateway/configuration) -- all pruning config knobs
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(`contextPruning.*`)
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