openclaw/docs/concepts/context-engine.md

summary	read_when	title
Context engine: pluggable context assembly, compaction, and subagent lifecycle	You want to understand how OpenClaw assembles model context You are switching between the legacy engine and a plugin engine You are building a context engine plugin	Context Engine

Context Engine

A context engine controls how OpenClaw builds model context for each run. It decides 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. Plugins can register alternative engines that replace the entire context pipeline.

Quick start

Check which engine is active:

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

Installing a context engine plugin

Context engine plugins are installed like any other OpenClaw plugin. Install first, then select the engine in the slot:

# Install from npm
openclaw plugins install @martian-engineering/lossless-claw

# Or install from a local path (for development)
openclaw plugins install -l ./my-context-engine

Then enable the plugin and select it as the active engine in your config:

// 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.

To switch back to the built-in engine, set contextEngine to "legacy" (or remove the key entirely — "legacy" is the default).

How it works

Every time OpenClaw runs a model prompt, the context engine participates at four lifecycle points:

1. Ingest — called when a new message is added to the session. The engine can store or index the message in its own data store.
2. Assemble — called before each model run. The engine returns an ordered set of messages (and an optional systemPromptAddition) that fit within the token budget.
3. Compact — called when the context window is full, or when the user runs /compact. The engine summarizes older history to free space.
4. After turn — called after a run completes. The engine can persist state, trigger background compaction, or update indexes.

Subagent lifecycle (optional)

OpenClaw currently calls one subagent lifecycle hook:

• onSubagentEnded — clean up when a subagent session completes or is swept.

The prepareSubagentSpawn hook is part of the interface for future use, but the runtime does not invoke it yet.

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.

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.

Plugin engines

A plugin can register a context engine using the plugin API:

export default function register(api) {
  api.registerContextEngine("my-engine", () => ({
    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 }) {
      // Return messages that fit the budget
      return {
        messages: buildContext(messages, tokenBudget),
        estimatedTokens: countTokens(messages),
        systemPromptAddition: "Use lcm_grep to search history...",
      };
    },

    async compact({ sessionId, force }) {
      // Summarize older context
      return { ok: true, compacted: true };
    },
  }));
}

Then enable it in config:

{
  plugins: {
    slots: {
      contextEngine: "my-engine",
    },
    entries: {
      "my-engine": {
        enabled: true,
      },
    },
  },
}

The ContextEngine interface

Required members:

Member	Kind	Purpose
info	Property	Engine id, name, version, and whether it owns compaction
ingest(params)	Method	Store a single message
assemble(params)	Method	Build context for a model run (returns AssembleResult)
compact(params)	Method	Summarize/reduce context

assemble returns an AssembleResult with:

• messages — the ordered messages to send to the model.
• estimatedTokens (required, number) — the engine's estimate of total tokens in the assembled context. OpenClaw uses this for compaction threshold decisions and diagnostic reporting.
• systemPromptAddition (optional, string) — prepended to the system prompt.

Optional members:

Member	Kind	Purpose
bootstrap(params)	Method	Initialize engine state for a session. Called once when the engine first sees a session (e.g., import history).
ingestBatch(params)	Method	Ingest a completed turn as a batch. Called after a run completes, with all messages from that turn at once.
afterTurn(params)	Method	Post-run lifecycle work (persist state, trigger background compaction).
prepareSubagentSpawn(params)	Method	Set up shared state for a child session.
onSubagentEnded(params)	Method	Clean up after a subagent ends.
dispose()	Method	Release resources. Called during gateway shutdown or plugin reload — not per-session.

ownsCompaction

When info.ownsCompaction is true, the engine manages its own compaction lifecycle. OpenClaw will not trigger the built-in auto-compaction; instead it delegates entirely to the engine's compact() method. The engine may also run compaction proactively in afterTurn().

When false or unset, OpenClaw's built-in auto-compaction logic runs alongside the engine.

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.

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.
• Session pruning (trimming old tool results in-memory) still runs regardless of which context engine is active.

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.

See also: Compaction, Context, Plugins, Plugin manifest.