Summary

I built a context engine plugin slot for Hermes Agent and the first plugin that uses it: hermes-lcm — Lossless Context Management as a standalone installable plugin.

• PR: feat: pluggable context engine slot (enabling LCM and custom compaction) #5700 (the context engine slot — 2 commits, small diff)
• Plugin: stephenschoettler/hermes-lcm

The Problem

Hermes has no way for plugins to replace the context compressor. The ContextCompressor is hardwired in run_agent.py. This means:

1. Anyone wanting LCM (or any custom compaction) must fork and patch core
2. PR feat(lcm): Lossless Context Management — never lose a message again #4033 does exactly this — 12 modules, 278 tests, all jammed into agent/lcm/
3. OpenClaw solved this cleanly with a contextEngine plugin slot and lossless-claw as a standalone plugin

The Solution

Two things:

1. Context engine slot (PR #5700)

A ContextEngine ABC in agent/context_engine.py that makes the compressor swappable:

class ContextEngine(ABC):
    def update_from_response(self, usage) -> None: ...
    def should_compress(self, prompt_tokens) -> bool: ...
    def compress(self, messages, current_tokens) -> list: ...
    # + optional: get_tool_schemas(), handle_tool_call(), on_session_start/end/reset, etc.

ContextCompressor becomes the default implementation. Plugins register a replacement via ctx.register_context_engine(engine). No behavior change for existing users.

2. hermes-lcm plugin

LCM implemented as a plugin against that slot:

• Immutable SQLite message store (every message preserved verbatim)
• Summary DAG with depth-aware nodes (D0 minutes → D1 hours → D2 days)
• 3-level escalation (L1 detailed → L2 bullets → L3 deterministic truncate)
• Agent tools: lcm_grep, lcm_describe, lcm_expand
• Install: cp -r hermes-lcm ~/.hermes/plugins/hermes-lcm

Based on the LCM paper by Ehrlich & Blackman (Voltropy, Feb 2026).

Why This Approach

Status

• PR feat: pluggable context engine slot (enabling LCM and custom compaction) #5700: ready for review
• hermes-lcm: MVP with 35 tests, needs real-world testing with live LLM calls
• Tested against Hermes v0.7.0

Happy to iterate on the ABC design if the team has feedback on the interface.