105 lines
4.2 KiB
Markdown
105 lines
4.2 KiB
Markdown
---
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summary: "Session pruning: tool-result trimming to reduce context bloat"
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read_when:
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- You want to reduce LLM context growth from tool outputs
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- You are tuning agents.defaults.contextPruning
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---
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# Session Pruning
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Session pruning trims **old tool results** from the in-memory context right before each LLM call. It does **not** rewrite the on-disk session history (`*.jsonl`).
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## When it runs
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- When `mode: "cache-ttl"` is enabled and the last Anthropic call for the session is older than `ttl`.
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- Only affects the messages sent to the model for that request.
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- Only active for Anthropic API calls (and OpenRouter Anthropic models).
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- For best results, match `ttl` to your model `cacheControlTtl`.
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- After a prune, the TTL window resets so subsequent requests keep cache until `ttl` expires again.
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## Smart defaults (Anthropic)
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- **OAuth or setup-token** profiles: enable `cache-ttl` pruning and set heartbeat to `1h`.
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- **API key** profiles: enable `cache-ttl` pruning, set heartbeat to `30m`, and default `cacheControlTtl` to `1h` on Anthropic models.
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- If you set any of these values explicitly, Moltbot does **not** override them.
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## What this improves (cost + cache behavior)
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- **Why prune:** Anthropic prompt caching only applies within the TTL. If a session goes idle past the TTL, the next request re-caches the full prompt unless you trim it first.
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- **What gets cheaper:** pruning reduces the **cacheWrite** size for that first request after the TTL expires.
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- **Why the TTL reset matters:** once pruning runs, the cache window resets, so follow‑up requests can reuse the freshly cached prompt instead of re-caching the full history again.
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- **What it does not do:** pruning doesn’t add tokens or “double” costs; it only changes what gets cached on that first post‑TTL request.
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## What can be pruned
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- Only `toolResult` messages.
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- User + assistant messages are **never** modified.
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- The last `keepLastAssistants` assistant messages are protected; tool results after that cutoff are not pruned.
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- If there aren’t enough assistant messages to establish the cutoff, pruning is skipped.
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- Tool results containing **image blocks** are skipped (never trimmed/cleared).
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## Context window estimation
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Pruning uses an estimated context window (chars ≈ tokens × 4). The window size is resolved in this order:
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1) Model definition `contextWindow` (from the model registry).
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2) `models.providers.*.models[].contextWindow` override.
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3) `agents.defaults.contextTokens`.
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4) Default `200000` tokens.
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## Mode
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### cache-ttl
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- Pruning only runs if the last Anthropic call is older than `ttl` (default `5m`).
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- When it runs: same soft-trim + hard-clear behavior as before.
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## Soft vs hard pruning
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- **Soft-trim**: only for oversized tool results.
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- Keeps head + tail, inserts `...`, and appends a note with the original size.
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- Skips results with image blocks.
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- **Hard-clear**: replaces the entire tool result with `hardClear.placeholder`.
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## Tool selection
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- `tools.allow` / `tools.deny` support `*` wildcards.
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- Deny wins.
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- Matching is case-insensitive.
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- Empty allow list => all tools allowed.
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## Interaction with other limits
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- Built-in tools already truncate their own output; session pruning is an extra layer that prevents long-running chats from accumulating too much tool output in the model context.
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- Compaction is separate: compaction summarizes and persists, pruning is transient per request. See [/concepts/compaction](/concepts/compaction).
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## Defaults (when enabled)
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- `ttl`: `"5m"`
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- `keepLastAssistants`: `3`
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- `softTrimRatio`: `0.3`
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- `hardClearRatio`: `0.5`
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- `minPrunableToolChars`: `50000`
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- `softTrim`: `{ maxChars: 4000, headChars: 1500, tailChars: 1500 }`
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- `hardClear`: `{ enabled: true, placeholder: "[Old tool result content cleared]" }`
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## Examples
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Default (off):
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```json5
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{
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agent: {
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contextPruning: { mode: "off" }
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}
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}
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```
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Enable TTL-aware pruning:
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```json5
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{
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agent: {
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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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Restrict pruning to specific tools:
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```json5
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{
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agent: {
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contextPruning: {
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mode: "cache-ttl",
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tools: { allow: ["exec", "read"], deny: ["*image*"] }
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}
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}
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}
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```
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See config reference: [Gateway Configuration](/gateway/configuration)
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