---
sidebar_position: 9
title: "Context Engine Plugins"
description: "How to build a context engine plugin that replaces the built-in ContextCompressor"
---

# Building a Context Engine Plugin

Context engine plugins replace the built-in `ContextCompressor` with an alternative strategy for managing conversation context. For example, a Lossless Context Management (LCM) engine that builds a knowledge DAG instead of lossy summarization.

## How it works

The agent's context management is built on the `ContextEngine` ABC (`agent/context_engine.py`). The built-in `ContextCompressor` is the default implementation. Plugin engines must implement the same interface.

Only **one** context engine can be active at a time. Selection is config-driven:

```yaml
# config.yaml
context:
  engine: "compressor"    # default built-in
  engine: "lcm"           # activates a plugin engine named "lcm"
```

Plugin engines are **never auto-activated** — the user must explicitly set `context.engine` to the plugin's name.

## Directory structure

Each context engine lives in `plugins/context_engine/<name>/`:

```
plugins/context_engine/lcm/
├── __init__.py      # exports the ContextEngine subclass
├── plugin.yaml      # metadata (name, description, version)
└── ...              # any other modules your engine needs
```

## The ContextEngine ABC

Your engine must implement these **required** methods:

```python
from agent.context_engine import ContextEngine

class LCMEngine(ContextEngine):

    @property
    def name(self) -> str:
        """Short identifier, e.g. 'lcm'. Must match config.yaml value."""
        return "lcm"

    def update_from_response(self, usage: dict) -> None:
        """Called after every LLM call with the usage dict.

        Update self.last_prompt_tokens, self.last_completion_tokens,
        self.last_total_tokens from the response.
        """

    def should_compress(self, prompt_tokens: int = None) -> bool:
        """Return True if compaction should fire this turn."""

    def compress(self, messages: list, current_tokens: int = None,
                 focus_topic: str = None) -> list:
        """Compact the message list and return a new (possibly shorter) list.

        The returned list must be a valid OpenAI-format message sequence.

        ``focus_topic`` is an optional topic string from manual
        ``/compress <focus>``; engines that support guided compression should
        prioritise preserving information related to it, others may ignore it.
        """
```

### Class attributes your engine must maintain

The agent reads these directly for display and logging:

```python
last_prompt_tokens: int = 0
last_completion_tokens: int = 0
last_total_tokens: int = 0
threshold_tokens: int = 0        # when compression triggers
context_length: int = 0          # model's full context window
compression_count: int = 0       # how many times compress() has run
```

### Optional methods

These have sensible defaults in the ABC. Override as needed:

| Method | Default | Override when |
|--------|---------|--------------|
| `on_session_start(session_id, **kwargs)` | No-op | You need to load persisted state (DAG, DB) |
| `on_session_end(session_id, messages)` | No-op | You need to flush state, close connections |
| `on_session_reset()` | Resets token counters | You have per-session state to clear |
| `update_model(model, context_length, ...)` | Updates context_length + threshold | You need to recalculate budgets on model switch |
| `get_tool_schemas()` | Returns `[]` | Your engine provides agent-callable tools (e.g., `lcm_grep`) |
| `handle_tool_call(name, args, **kwargs)` | Returns error JSON | You implement tool handlers |
| `should_compress_preflight(messages)` | Returns `False` | You can do a cheap pre-API-call estimate |
| `get_status()` | Standard token/threshold dict | You have custom metrics to expose |
| `select_context(request_messages, *, conversation_messages, incoming_message, budget_tokens)` | Returns `None` (no-op) | You select/route which context enters **this** request (retrieval, topic routing) — see below |
| `on_turn_complete(messages, usage=None, **kwargs)` | No-op | You ingest/index/observe the finished turn — see below |

## Per-turn context selection and observation

`compress()` answers "context is too long → make it shorter". Two optional,
no-op-default hooks cover the orthogonal *selection / observation* axis, so an
engine no longer has to force `should_compress()` to `True` and abuse
`compress()` as a per-turn callback:

```python
def select_context(self, request_messages, *, conversation_messages=None,
                   incoming_message=None, budget_tokens=0):
    """Choose/replace the context for THIS request, before dispatch.

    Return a new message list to use for this one provider call (retrieval,
    topic routing, role/branch switching), or None to leave it unchanged.
    Request-only: the persisted conversation history is never mutated.
    """

def on_turn_complete(self, messages, usage=None, **kwargs):
    """Observe a finished turn after the assistant/tool loop completes.

    Receives a shallow copy of the finalized transcript plus the turn's
    canonical usage dict (or None if no provider response was reached), so the
    engine can ingest/index/summarize for the next select_context(). The return
    value is ignored.
    """
```

Contract:

- **No-op by default, fail-open.** Both default to `return None`. A missing hook, an exception, or an invalid return value leaves the request untouched — so a failing engine is never worse than not installing one. The host also identity-checks for the inherited ABC default and skips it entirely, so non-implementing engines (including the built-in compressor) pay no per-request work at all.
- **`select_context()` is request-only.** The returned list replaces the messages for a single provider call; persisted history is never written. Returning `None`, `[]`, a non-list, or a list containing non-dicts all fall open to the unmodified request.
- **Ordering / cache stability.** The hook runs **before** prompt cache-control and every request sanitizer, so (a) a replacement still passes the same validation as any request, and (b) the no-op default leaves the request byte-identical — prompt-cache behaviour is unchanged for non-implementing engines. An engine that replaces the list changes only its own cache prefix. Evaluated per provider request (re-runs on retries).
- **`on_turn_complete()`** is post-turn observation only; treat `messages` as read-only. **Coverage is best-effort:** it fires from the standard turn-finalization seam. Some abnormal early-return paths in the loop (e.g. a content-policy block or a provider terminal failure) persist and return without routing through finalization, so they do not currently emit this hook — treat it as a best-effort observation for completed turns, not a guaranteed callback for every early exit. Unifying all terminal paths behind one finalization seam is a separate follow-up.

### When to use these hooks — and when NOT to

- **Implement `select_context()` only when your engine must *replace* the
  per-request context** — retrieval-augmented selection, topic/branch routing,
  role switching. It is the only verb that can swap which messages enter a
  request: the `pre_llm_call` plugin hook is inject-only by documented design
  (it appends to the user message and never rewrites the list, to preserve the
  prompt-cache prefix). If you don't need replacement, don't implement it.
- **If your plugin only needs post-turn observation / ingestion** (indexing,
  memory sync, analytics), implement a **memory provider** (`sync_turn()` —
  see [Memory Provider Plugins](./memory-provider-plugin.md)) instead of a
  context engine. A context engine takes ownership of the session's compaction
  policy; a memory provider observes turns without owning anything.
  `on_turn_complete()` exists as the observation mirror for engines that
  *already* need `select_context()` — so the same component can learn from the
  turn it just routed — not as a general-purpose turn callback.
- **Prompt-cache impact of a real `select_context()`.** A non-no-op selection
  naturally changes the prompt-cache prefix for the turns where it changes the
  selection — that request's prefix no longer matches the provider's cached
  prefix, so those turns re-write cache instead of reading it. Engines should
  return **stable selections when nothing has changed** (same object or an
  equal list), and only reshape the context when the routing decision actually
  differs; a selection that shuffles per turn silently forfeits cache reuse
  every turn.

## Engine tools

Context engines can expose tools the agent calls directly. Return schemas from `get_tool_schemas()` and handle calls in `handle_tool_call()`:

```python
def get_tool_schemas(self):
    return [{
        "name": "lcm_grep",
        "description": "Search the context knowledge graph",
        "parameters": {
            "type": "object",
            "properties": {
                "query": {"type": "string", "description": "Search query"}
            },
            "required": ["query"],
        },
    }]

def handle_tool_call(self, name, args, **kwargs):
    if name == "lcm_grep":
        results = self._search_dag(args["query"])
        return json.dumps({"results": results})
    return json.dumps({"error": f"Unknown tool: {name}"})
```

Engine tools are injected into the agent's tool list at startup and dispatched automatically — no registry registration needed.

## Registration

### Via directory (recommended)

Place your engine in `plugins/context_engine/<name>/`. The `__init__.py` must export a `ContextEngine` subclass. The discovery system finds and instantiates it automatically.

### Via general plugin system

A general plugin can also register a context engine:

```python
def register(ctx):
    engine = LCMEngine(context_length=200000)
    ctx.register_context_engine(engine)
```

Only one engine can be registered. A second plugin attempting to register is rejected with a warning.

## Lifecycle

```
1. Engine instantiated (plugin load or directory discovery)
2. on_session_start() — conversation begins
3. update_from_response() — after each API call
4. should_compress() — checked each turn
5. compress() — called when should_compress() returns True
6. on_session_end() — session boundary (CLI exit, /reset, gateway expiry)
```

`on_session_reset()` is called on `/new` or `/reset` to clear per-session state without a full shutdown.

## Configuration

Users select your engine via `hermes plugins` → Provider Plugins → Context Engine, or by editing `config.yaml`:

```yaml
context:
  engine: "lcm"   # must match your engine's name property
```

The `compression` config block (`compression.threshold`, `compression.protect_last_n`, etc.) is specific to the built-in `ContextCompressor`, with one explicit exception: `compression.model_thresholds` (per-model threshold overrides) is part of the context-engine contract. The host assigns the resolved map to `engine.model_thresholds` *before* the initial `update_model()` call, and the base-class `update_model()` applies it (longest substring match, falling back to the engine's configured threshold). Engines that override `update_model()` own their own compaction policy and may honor or ignore the map — `from agent.context_compressor import resolve_model_threshold` to reuse the same resolution logic. For everything else, your engine should define its own config format if needed, reading from `config.yaml` during initialization.

## Testing

```python
from agent.context_engine import ContextEngine

def test_engine_satisfies_abc():
    engine = YourEngine(context_length=200000)
    assert isinstance(engine, ContextEngine)
    assert engine.name == "your-name"

def test_compress_returns_valid_messages():
    engine = YourEngine(context_length=200000)
    msgs = [{"role": "user", "content": "hello"}]
    result = engine.compress(msgs)
    assert isinstance(result, list)
    assert all("role" in m for m in result)
```

See `tests/agent/test_context_engine.py` for the full ABC contract test suite.

## Thread safety

When `compression.context_timeout_seconds > 0` (the default), Hermes runs the
whole compression pass — including your engine's `compress()` and boundary
callbacks, and any memory provider's `on_pre_compress` /
`on_session_switch` — on a pooled daemon thread with a host-side timeout.
Your engine must therefore assume:

- Calls may arrive on an arbitrary pooled thread. Do not rely on thread
  affinity or `threading.local` state shared with the conversation thread.
- The message list you receive is a private deep snapshot; mutating it in
  place is allowed (legacy contract), but the mutation only becomes visible
  if the pass commits. After a host timeout your still-running work is
  discarded — never publish to external/durable state outside the commit.
- Passes for *different* sessions can run concurrently on pool siblings; a
  single engine/provider instance shared across sessions must be thread-safe.

## See also

- [Context Compression and Caching](/developer-guide/context-compression-and-caching) — how the built-in compressor works
- [Memory Provider Plugins](/developer-guide/memory-provider-plugin) — analogous single-select plugin system for memory
- [Plugins](/user-guide/features/plugins) — general plugin system overview
