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Documentation Index

Fetch the complete documentation index at: https://docs.openclaw.ai/llms.txt

Use this file to discover all available pages before exploring further.

A context engine controls how OpenClaw builds model context for each run: which messages to include, how to summarize older history, and how to manage context across subagent boundaries. OpenClaw ships with a built-in legacy engine and uses it by default — most users never need to change this. Install and select a plugin engine only when you want different assembly, compaction, or cross-session recall behavior.

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Quick start

1

Check which engine is active

openclaw doctor
# or inspect config directly:
cat ~/.openclaw/openclaw.json | jq '.plugins.slots.contextEngine'
2

Install a plugin engine

Context engine plugins are installed like any other OpenClaw plugin.
openclaw plugins install @martian-engineering/lossless-claw
3

Enable and select the engine

// openclaw.json
{
  plugins: {
    slots: {
      contextEngine: "lossless-claw", // must match the plugin's registered engine id
    },
    entries: {
      "lossless-claw": {
        enabled: true,
        // Plugin-specific config goes here (see the plugin's docs)
      },
    },
  },
}
Restart the gateway after installing and configuring.
4

Switch back to legacy (optional)

Set contextEngine to "legacy" (or remove the key entirely — "legacy" is the default).

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How it works

Every time OpenClaw runs a model prompt, the context engine participates at four lifecycle points:
Called when a new message is added to the session. The engine can store or index the message in its own data store.
Called before each model run. The engine returns an ordered set of messages (and an optional systemPromptAddition) that fit within the token budget.
Called when the context window is full, or when the user runs /compact. The engine summarizes older history to free space.
Called after a run completes. The engine can persist state, trigger background compaction, or update indexes.
For the bundled non-ACP Codex harness, OpenClaw applies the same lifecycle by projecting assembled context into Codex developer instructions and the current turn prompt. Codex still owns its native thread history and native compactor.

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Subagent lifecycle (optional)

OpenClaw calls two optional subagent lifecycle hooks:
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prepareSubagentSpawn
method
Prepare shared context state before a child run starts. The hook receives parent/child session keys, contextMode (isolated or fork), available transcript ids/files, and optional TTL. If it returns a rollback handle, OpenClaw calls it when spawn fails after preparation succeeds.
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onSubagentEnded
method
Clean up when a subagent session completes or is swept.

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System prompt addition

The assemble method can return a systemPromptAddition string. OpenClaw prepends this to the system prompt for the run. This lets engines inject dynamic recall guidance, retrieval instructions, or context-aware hints without requiring static workspace files.

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The legacy engine

The built-in legacy engine preserves OpenClaw’s original behavior:
  • Ingest: no-op (the session manager handles message persistence directly).
  • Assemble: pass-through (the existing sanitize → validate → limit pipeline in the runtime handles context assembly).
  • Compact: delegates to the built-in summarization compaction, which creates a single summary of older messages and keeps recent messages intact.
  • After turn: no-op.
The legacy engine does not register tools or provide a systemPromptAddition. When no plugins.slots.contextEngine is set (or it’s set to "legacy"), this engine is used automatically.

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Plugin engines

A plugin can register a context engine using the plugin API: 
import { buildMemorySystemPromptAddition } from "openclaw/plugin-sdk/core";

export default function register(api) {
  api.registerContextEngine("my-engine", (ctx) => ({
    info: {
      id: "my-engine",
      name: "My Context Engine",
      ownsCompaction: true,
    },

    async ingest({ sessionId, message, isHeartbeat }) {
      // Store the message in your data store
      return { ingested: true };
    },

    async assemble({ sessionId, messages, tokenBudget, availableTools, citationsMode }) {
      // Return messages that fit the budget
      return {
        messages: buildContext(messages, tokenBudget),
        estimatedTokens: countTokens(messages),
        systemPromptAddition: buildMemorySystemPromptAddition({
          availableTools: availableTools ?? new Set(),
          citationsMode,
        }),
      };
    },

    async compact({ sessionId, force }) {
      // Summarize older context
      return { ok: true, compacted: true };
    },
  }));
}
The factory ctx includes optional config, agentDir, and workspaceDir values so plugins can initialize per-agent or per-workspace state before the first lifecycle hook runs. Then enable it in config: 
{
  plugins: {
    slots: {
      contextEngine: "my-engine",
    },
    entries: {
      "my-engine": {
        enabled: true,
      },
    },
  },
}

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The ContextEngine interface

Required members:
MemberKindPurpose
infoPropertyEngine id, name, version, and whether it owns compaction
ingest(params)MethodStore a single message
assemble(params)MethodBuild context for a model run (returns AssembleResult)
compact(params)MethodSummarize/reduce context
assemble returns an AssembleResult with:
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messages
Message[]
required
The ordered messages to send to the model.
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estimatedTokens
number
required
The engine’s estimate of total tokens in the assembled context. OpenClaw uses this for compaction threshold decisions and diagnostic reporting.
​
systemPromptAddition
string
Prepended to the system prompt.
compact returns a CompactResult. When compaction rotates the active transcript, result.sessionId and result.sessionFile identify the successor session that the next retry or turn must use. Optional members:
MemberKindPurpose
bootstrap(params)MethodInitialize engine state for a session. Called once when the engine first sees a session (e.g., import history).
ingestBatch(params)MethodIngest a completed turn as a batch. Called after a run completes, with all messages from that turn at once.
afterTurn(params)MethodPost-run lifecycle work (persist state, trigger background compaction).
prepareSubagentSpawn(params)MethodSet up shared state for a child session before it starts.
onSubagentEnded(params)MethodClean up after a subagent ends.
dispose()MethodRelease resources. Called during gateway shutdown or plugin reload — not per-session.

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ownsCompaction

ownsCompaction controls whether Pi’s built-in in-attempt auto-compaction stays enabled for the run:
The engine owns compaction behavior. OpenClaw disables Pi’s built-in auto-compaction for that run, and the engine’s compact() implementation is responsible for /compact, overflow recovery compaction, and any proactive compaction it wants to do in afterTurn(). OpenClaw may still run the pre-prompt overflow safeguard; when it predicts the full transcript will overflow, the recovery path calls the active engine’s compact() before submitting another prompt.
Pi’s built-in auto-compaction may still run during prompt execution, but the active engine’s compact() method is still called for /compact and overflow recovery.
ownsCompaction: false does not mean OpenClaw automatically falls back to the legacy engine’s compaction path.
That means there are two valid plugin patterns:
Implement your own compaction algorithm and set ownsCompaction: true.
A no-op compact() is unsafe for an active non-owning engine because it disables the normal /compact and overflow-recovery compaction path for that engine slot.

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Configuration reference

{
  plugins: {
    slots: {
      // Select the active context engine. Default: "legacy".
      // Set to a plugin id to use a plugin engine.
      contextEngine: "legacy",
    },
  },
}
The slot is exclusive at run time — only one registered context engine is resolved for a given run or compaction operation. Other enabled kind: "context-engine" plugins can still load and run their registration code; plugins.slots.contextEngine only selects which registered engine id OpenClaw resolves when it needs a context engine.
Plugin uninstall: when you uninstall the plugin currently selected as plugins.slots.contextEngine, OpenClaw resets the slot back to the default (legacy). The same reset behavior applies to plugins.slots.memory. No manual config edit is required.

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Relationship to compaction and memory

Compaction is one responsibility of the context engine. The legacy engine delegates to OpenClaw’s built-in summarization. Plugin engines can implement any compaction strategy (DAG summaries, vector retrieval, etc.).
Memory plugins (plugins.slots.memory) are separate from context engines. Memory plugins provide search/retrieval; context engines control what the model sees. They can work together — a context engine might use memory plugin data during assembly. Plugin engines that want the active memory prompt path should prefer buildMemorySystemPromptAddition(...) from openclaw/plugin-sdk/core, which converts the active memory prompt sections into a ready-to-prepend systemPromptAddition. If an engine needs lower-level control, it can still pull raw lines from openclaw/plugin-sdk/memory-host-core via buildActiveMemoryPromptSection(...).
Trimming old tool results in-memory still runs regardless of which context engine is active.

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Tips

  • Use openclaw doctor to verify your engine is loading correctly.
  • If switching engines, existing sessions continue with their current history. The new engine takes over for future runs.
  • Engine errors are logged and surfaced in diagnostics. If a plugin engine fails to register or the selected engine id cannot be resolved, OpenClaw does not fall back automatically; runs fail until you fix the plugin or switch plugins.slots.contextEngine back to "legacy".
  • For development, use openclaw plugins install -l ./my-engine to link a local plugin directory without copying.