fix: robust context engine interface — config selection, plugin discovery, ABC completeness

Follow-up fixes for the context engine plugin slot (PR #5700):

- Enhance ContextEngine ABC: add threshold_percent, protect_first_n,
  protect_last_n as class attributes; complete update_model() default
  with threshold recalculation; clarify on_session_end() lifecycle docs
- Add ContextCompressor.update_model() override for model/provider/
  base_url/api_key updates
- Replace all direct compressor internal access in run_agent.py with
  ABC interface: switch_model(), fallback restore, context probing
  all use update_model() now; _context_probed guarded with getattr/
  hasattr for plugin engine compatibility
- Create plugins/context_engine/ directory with discovery module
  (mirrors plugins/memory/ pattern) — discover_context_engines(),
  load_context_engine()
- Add context.engine config key to DEFAULT_CONFIG (default: compressor)
- Config-driven engine selection in run_agent.__init__: checks config,
  then plugins/context_engine/<name>/, then general plugin system,
  falls back to built-in ContextCompressor
- Wire on_session_end() in shutdown_memory_provider() at real session
  boundaries (CLI exit, /reset, gateway expiry)

agent/context_compressor.py

@@ -73,6 +73,22 @@ class ContextCompressor(ContextEngine):
         self._context_probe_persistable = False
         self._previous_summary = None
 
+    def update_model(
+        self,
+        model: str,
+        context_length: int,
+        base_url: str = "",
+        api_key: str = "",
+        provider: str = "",
+    ) -> None:
+        """Update model info after a model switch or fallback activation."""
+        self.model = model
+        self.base_url = base_url
+        self.api_key = api_key
+        self.provider = provider
+        self.context_length = context_length
+        self.threshold_tokens = int(context_length * self.threshold_percent)
+
     def __init__(
         self,
         model: str,

agent/context_engine.py

@@ -3,7 +3,11 @@
 A context engine controls how conversation context is managed when
 approaching the model's token limit. The built-in ContextCompressor
 is the default implementation. Third-party engines (e.g. LCM) can
-replace it by registering via the plugin system.
+replace it via the plugin system or by being placed in the
+``plugins/context_engine/<name>/`` directory.
+
+Selection is config-driven: ``context.engine`` in config.yaml.
+Default is ``"compressor"`` (the built-in). Only one engine is active.
 
 The engine is responsible for:
   - Deciding when compaction should fire
@@ -17,7 +21,8 @@ Lifecycle:
   3. update_from_response() called after each API response with usage data
   4. should_compress() checked after each turn
   5. compress() called when should_compress() returns True
-  6. on_session_end() called when the conversation ends
+  6. on_session_end() called at real session boundaries (CLI exit, /reset,
+     gateway session expiry) — NOT per-turn
 """
 
 from abc import ABC, abstractmethod
@@ -45,6 +50,16 @@ class ContextEngine(ABC):
     context_length: int = 0
     compression_count: int = 0
 
+    # -- Compaction parameters (read by run_agent.py for preflight) --------
+    #
+    # These control the preflight compression check.  Subclasses may
+    # override via __init__ or property; defaults are sensible for most
+    # engines.
+
+    threshold_percent: float = 0.75
+    protect_first_n: int = 3
+    protect_last_n: int = 6
+
     # -- Core interface ----------------------------------------------------
 
     @abstractmethod
@@ -93,9 +108,10 @@ class ContextEngine(ABC):
         """
 
     def on_session_end(self, session_id: str, messages: List[Dict[str, Any]]) -> None:
-        """Called when the conversation ends.
+        """Called at real session boundaries (CLI exit, /reset, gateway expiry).
 
         Use this to flush state, close DB connections, etc.
+        NOT called per-turn — only when the session truly ends.
         """
 
     def on_session_reset(self) -> None:
@@ -158,9 +174,11 @@ class ContextEngine(ABC):
         api_key: str = "",
         provider: str = "",
     ) -> None:
-        """Called when the user switches models mid-session.
+        """Called when the user switches models or on fallback activation.
 
-        Default updates context_length and threshold_tokens. Override if
+        Default updates context_length and recalculates threshold_tokens
-        your engine needs to do more (e.g. recalculate DAG budgets).
+        from threshold_percent. Override if your engine needs more
+        (e.g. recalculate DAG budgets, switch summary models).
         """
         self.context_length = context_length
+        self.threshold_tokens = int(context_length * self.threshold_percent)

hermes_cli/config.py

@@ -504,6 +504,16 @@ DEFAULT_CONFIG = {
         "max_ms": 2500,
     },
     
+    # Context engine -- controls how the context window is managed when
+    # approaching the model's token limit.
+    # "compressor" = built-in lossy summarization (default).
+    # Set to a plugin name to activate an alternative engine (e.g. "lcm"
+    # for Lossless Context Management).  The engine must be installed as
+    # a plugin in plugins/context_engine/<name>/ or ~/.hermes/plugins/.
+    "context": {
+        "engine": "compressor",
+    },
+
     # Persistent memory -- bounded curated memory injected into system prompt
     "memory": {
         "memory_enabled": True,
@@ -1450,7 +1460,7 @@ _KNOWN_ROOT_KEYS = {
     "_config_version", "model", "providers", "fallback_model",
     "fallback_providers", "credential_pool_strategies", "toolsets",
     "agent", "terminal", "display", "compression", "delegation",
-    "auxiliary", "custom_providers", "memory", "gateway",
+    "auxiliary", "custom_providers", "context", "memory", "gateway",
 }
 
 # Valid fields inside a custom_providers list entry

plugins/context_engine/__init__.py

@@ -0,0 +1,219 @@
+"""Context engine plugin discovery.
+
+Scans ``plugins/context_engine/<name>/`` directories for context engine
+plugins.  Each subdirectory must contain ``__init__.py`` with a class
+implementing the ContextEngine ABC.
+
+Context engines are separate from the general plugin system — they live
+in the repo and are always available without user installation.  Only ONE
+can be active at a time, selected via ``context.engine`` in config.yaml.
+The default engine is ``"compressor"`` (the built-in ContextCompressor).
+
+Usage:
+    from plugins.context_engine import discover_context_engines, load_context_engine
+
+    available = discover_context_engines()   # [(name, desc, available), ...]
+    engine = load_context_engine("lcm")      # ContextEngine instance
+"""
+
+from __future__ import annotations
+
+import importlib
+import importlib.util
+import logging
+import sys
+from pathlib import Path
+from typing import List, Optional, Tuple
+
+logger = logging.getLogger(__name__)
+
+_CONTEXT_ENGINE_PLUGINS_DIR = Path(__file__).parent
+
+
+def discover_context_engines() -> List[Tuple[str, str, bool]]:
+    """Scan plugins/context_engine/ for available engines.
+
+    Returns list of (name, description, is_available) tuples.
+    Does NOT import the engines — just reads plugin.yaml for metadata
+    and does a lightweight availability check.
+    """
+    results = []
+    if not _CONTEXT_ENGINE_PLUGINS_DIR.is_dir():
+        return results
+
+    for child in sorted(_CONTEXT_ENGINE_PLUGINS_DIR.iterdir()):
+        if not child.is_dir() or child.name.startswith(("_", ".")):
+            continue
+        init_file = child / "__init__.py"
+        if not init_file.exists():
+            continue
+
+        # Read description from plugin.yaml if available
+        desc = ""
+        yaml_file = child / "plugin.yaml"
+        if yaml_file.exists():
+            try:
+                import yaml
+                with open(yaml_file) as f:
+                    meta = yaml.safe_load(f) or {}
+                desc = meta.get("description", "")
+            except Exception:
+                pass
+
+        # Quick availability check — try loading and calling is_available()
+        available = True
+        try:
+            engine = _load_engine_from_dir(child)
+            if engine is None:
+                available = False
+            elif hasattr(engine, "is_available"):
+                available = engine.is_available()
+        except Exception:
+            available = False
+
+        results.append((child.name, desc, available))
+
+    return results
+
+
+def load_context_engine(name: str) -> Optional["ContextEngine"]:
+    """Load and return a ContextEngine instance by name.
+
+    Returns None if the engine is not found or fails to load.
+    """
+    engine_dir = _CONTEXT_ENGINE_PLUGINS_DIR / name
+    if not engine_dir.is_dir():
+        logger.debug("Context engine '%s' not found in %s", name, _CONTEXT_ENGINE_PLUGINS_DIR)
+        return None
+
+    try:
+        engine = _load_engine_from_dir(engine_dir)
+        if engine:
+            return engine
+        logger.warning("Context engine '%s' loaded but no engine instance found", name)
+        return None
+    except Exception as e:
+        logger.warning("Failed to load context engine '%s': %s", name, e)
+        return None
+
+
+def _load_engine_from_dir(engine_dir: Path) -> Optional["ContextEngine"]:
+    """Import an engine module and extract the ContextEngine instance.
+
+    The module must have either:
+    - A register(ctx) function (plugin-style) — we simulate a ctx
+    - A top-level class that extends ContextEngine — we instantiate it
+    """
+    name = engine_dir.name
+    module_name = f"plugins.context_engine.{name}"
+    init_file = engine_dir / "__init__.py"
+
+    if not init_file.exists():
+        return None
+
+    # Check if already loaded
+    if module_name in sys.modules:
+        mod = sys.modules[module_name]
+    else:
+        # Handle relative imports within the plugin
+        # First ensure the parent packages are registered
+        for parent in ("plugins", "plugins.context_engine"):
+            if parent not in sys.modules:
+                parent_path = Path(__file__).parent
+                if parent == "plugins":
+                    parent_path = parent_path.parent
+                parent_init = parent_path / "__init__.py"
+                if parent_init.exists():
+                    spec = importlib.util.spec_from_file_location(
+                        parent, str(parent_init),
+                        submodule_search_locations=[str(parent_path)]
+                    )
+                    if spec:
+                        parent_mod = importlib.util.module_from_spec(spec)
+                        sys.modules[parent] = parent_mod
+                        try:
+                            spec.loader.exec_module(parent_mod)
+                        except Exception:
+                            pass
+
+        # Now load the engine module
+        spec = importlib.util.spec_from_file_location(
+            module_name, str(init_file),
+            submodule_search_locations=[str(engine_dir)]
+        )
+        if not spec:
+            return None
+
+        mod = importlib.util.module_from_spec(spec)
+        sys.modules[module_name] = mod
+
+        # Register submodules so relative imports work
+        for sub_file in engine_dir.glob("*.py"):
+            if sub_file.name == "__init__.py":
+                continue
+            sub_name = sub_file.stem
+            full_sub_name = f"{module_name}.{sub_name}"
+            if full_sub_name not in sys.modules:
+                sub_spec = importlib.util.spec_from_file_location(
+                    full_sub_name, str(sub_file)
+                )
+                if sub_spec:
+                    sub_mod = importlib.util.module_from_spec(sub_spec)
+                    sys.modules[full_sub_name] = sub_mod
+                    try:
+                        sub_spec.loader.exec_module(sub_mod)
+                    except Exception as e:
+                        logger.debug("Failed to load submodule %s: %s", full_sub_name, e)
+
+        try:
+            spec.loader.exec_module(mod)
+        except Exception as e:
+            logger.debug("Failed to exec_module %s: %s", module_name, e)
+            sys.modules.pop(module_name, None)
+            return None
+
+    # Try register(ctx) pattern first (how plugins are written)
+    if hasattr(mod, "register"):
+        collector = _EngineCollector()
+        try:
+            mod.register(collector)
+            if collector.engine:
+                return collector.engine
+        except Exception as e:
+            logger.debug("register() failed for %s: %s", name, e)
+
+    # Fallback: find a ContextEngine subclass and instantiate it
+    from agent.context_engine import ContextEngine
+    for attr_name in dir(mod):
+        attr = getattr(mod, attr_name, None)
+        if (isinstance(attr, type) and issubclass(attr, ContextEngine)
+                and attr is not ContextEngine):
+            try:
+                return attr()
+            except Exception:
+                pass
+
+    return None
+
+
+class _EngineCollector:
+    """Fake plugin context that captures register_context_engine calls."""
+
+    def __init__(self):
+        self.engine = None
+
+    def register_context_engine(self, engine):
+        self.engine = engine
+
+    # No-op for other registration methods
+    def register_tool(self, *args, **kwargs):
+        pass
+
+    def register_hook(self, *args, **kwargs):
+        pass
+
+    def register_cli_command(self, *args, **kwargs):
+        pass
+
+    def register_memory_provider(self, *args, **kwargs):
+        pass

run_agent.py

@@ -1268,18 +1268,54 @@ class AIAgent:
                                         pass
                         break
         
-        # Check if a plugin registered a custom context engine (e.g. LCM)
+        # Select context engine: config-driven (like memory providers).
-        _plugin_engine = None
+        # 1. Check config.yaml context.engine setting
+        # 2. Check plugins/context_engine/<name>/ directory (repo-shipped)
+        # 3. Check general plugin system (user-installed plugins)
+        # 4. Fall back to built-in ContextCompressor
+        _selected_engine = None
+        _engine_name = "compressor"  # default
         try:
-            from hermes_cli.plugins import get_plugin_context_engine
+            _ctx_cfg = _agent_cfg.get("context", {}) if isinstance(_agent_cfg, dict) else {}
-            _plugin_engine = get_plugin_context_engine()
+            _engine_name = _ctx_cfg.get("engine", "compressor") or "compressor"
         except Exception:
             pass
 
-        if _plugin_engine is not None:
+        if _engine_name != "compressor":
-            self.context_compressor = _plugin_engine
+            # Try loading from plugins/context_engine/<name>/
+            try:
+                from plugins.context_engine import load_context_engine
+                _selected_engine = load_context_engine(_engine_name)
+            except Exception as _ce_load_err:
+                logger.debug("Context engine load from plugins/context_engine/: %s", _ce_load_err)
+
+            # Try general plugin system as fallback
+            if _selected_engine is None:
+                try:
+                    from hermes_cli.plugins import get_plugin_context_engine
+                    _candidate = get_plugin_context_engine()
+                    if _candidate and _candidate.name == _engine_name:
+                        _selected_engine = _candidate
+                except Exception:
+                    pass
+
+            if _selected_engine is None:
+                logger.warning(
+                    "Context engine '%s' not found — falling back to built-in compressor",
+                    _engine_name,
+                )
+        else:
+            # Even with default config, check if a plugin registered one
+            try:
+                from hermes_cli.plugins import get_plugin_context_engine
+                _selected_engine = get_plugin_context_engine()
+            except Exception:
+                pass
+
+        if _selected_engine is not None:
+            self.context_compressor = _selected_engine
             if not self.quiet_mode:
-                logger.info("Using plugin context engine: %s", _plugin_engine.name)
+                logger.info("Using context engine: %s", _selected_engine.name)
         else:
             self.context_compressor = ContextCompressor(
                 model=self.model,
@@ -1385,11 +1421,13 @@ class AIAgent:
             "api_key": getattr(self, "api_key", ""),
             "client_kwargs": dict(self._client_kwargs),
             "use_prompt_caching": self._use_prompt_caching,
-            # Compressor state that _try_activate_fallback() overwrites
+            # Context engine state that _try_activate_fallback() overwrites.
-            "compressor_model": _cc.model,
+            # Use getattr for model/base_url/api_key/provider since plugin
-            "compressor_base_url": _cc.base_url,
+            # engines may not have these (they're ContextCompressor-specific).
+            "compressor_model": getattr(_cc, "model", self.model),
+            "compressor_base_url": getattr(_cc, "base_url", self.base_url),
             "compressor_api_key": getattr(_cc, "api_key", ""),
-            "compressor_provider": _cc.provider,
+            "compressor_provider": getattr(_cc, "provider", self.provider),
             "compressor_context_length": _cc.context_length,
             "compressor_threshold_tokens": _cc.threshold_tokens,
         }
@@ -1518,13 +1556,12 @@ class AIAgent:
                 provider=self.provider,
                 config_context_length=getattr(self, "_config_context_length", None),
             )
-            self.context_compressor.model = self.model
+            self.context_compressor.update_model(
-            self.context_compressor.base_url = self.base_url
+                model=self.model,
-            self.context_compressor.api_key = self.api_key
+                context_length=new_context_length,
-            self.context_compressor.provider = self.provider
+                base_url=self.base_url,
-            self.context_compressor.context_length = new_context_length
+                api_key=getattr(self, "api_key", ""),
-            self.context_compressor.threshold_tokens = int(
+                provider=self.provider,
-                new_context_length * self.context_compressor.threshold_percent
             )
 
         # ── Invalidate cached system prompt so it rebuilds next turn ──
@@ -1540,10 +1577,10 @@ class AIAgent:
             "api_key": getattr(self, "api_key", ""),
             "client_kwargs": dict(self._client_kwargs),
             "use_prompt_caching": self._use_prompt_caching,
-            "compressor_model": _cc.model if _cc else self.model,
+            "compressor_model": getattr(_cc, "model", self.model) if _cc else self.model,
-            "compressor_base_url": _cc.base_url if _cc else self.base_url,
+            "compressor_base_url": getattr(_cc, "base_url", self.base_url) if _cc else self.base_url,
             "compressor_api_key": getattr(_cc, "api_key", "") if _cc else "",
-            "compressor_provider": _cc.provider if _cc else self.provider,
+            "compressor_provider": getattr(_cc, "provider", self.provider) if _cc else self.provider,
             "compressor_context_length": _cc.context_length if _cc else 0,
             "compressor_threshold_tokens": _cc.threshold_tokens if _cc else 0,
         }
@@ -2740,10 +2777,11 @@ class AIAgent:
         }
 
     def shutdown_memory_provider(self, messages: list = None) -> None:
-        """Shut down the memory provider — call at actual session boundaries.
+        """Shut down the memory provider and context engine — call at actual session boundaries.
 
         This calls on_session_end() then shutdown_all() on the memory
-        manager. NOT called per-turn — only at CLI exit, /reset, gateway
+        manager, and on_session_end() on the context engine.
+        NOT called per-turn — only at CLI exit, /reset, gateway
         session expiry, etc.
         """
         if self._memory_manager:
@@ -2755,6 +2793,15 @@ class AIAgent:
                 self._memory_manager.shutdown_all()
             except Exception:
                 pass
+        # Notify context engine of session end (flush DAG, close DBs, etc.)
+        if hasattr(self, "context_compressor") and self.context_compressor:
+            try:
+                self.context_compressor.on_session_end(
+                    self.session_id or "",
+                    messages or [],
+                )
+            except Exception:
+                pass
     
     def close(self) -> None:
         """Release all resources held by this agent instance.
@@ -5272,13 +5319,12 @@ class AIAgent:
                     self.model, base_url=self.base_url,
                     api_key=self.api_key, provider=self.provider,
                 )
-                self.context_compressor.model = self.model
+                self.context_compressor.update_model(
-                self.context_compressor.base_url = self.base_url
+                    model=self.model,
-                self.context_compressor.api_key = self.api_key
+                    context_length=fb_context_length,
-                self.context_compressor.provider = self.provider
+                    base_url=self.base_url,
-                self.context_compressor.context_length = fb_context_length
+                    api_key=getattr(self, "api_key", ""),
-                self.context_compressor.threshold_tokens = int(
+                    provider=self.provider,
-                    fb_context_length * self.context_compressor.threshold_percent
                 )
 
             self._emit_status(
@@ -5338,14 +5384,15 @@ class AIAgent:
                     shared=True,
                 )
 
-            # ── Restore context compressor state ──
+            # ── Restore context engine state ──
             cc = self.context_compressor
-            cc.model = rt["compressor_model"]
+            cc.update_model(
-            cc.base_url = rt["compressor_base_url"]
+                model=rt["compressor_model"],
-            cc.api_key = rt["compressor_api_key"]
+                context_length=rt["compressor_context_length"],
-            cc.provider = rt["compressor_provider"]
+                base_url=rt["compressor_base_url"],
-            cc.context_length = rt["compressor_context_length"]
+                api_key=rt["compressor_api_key"],
-            cc.threshold_tokens = rt["compressor_threshold_tokens"]
+                provider=rt["compressor_provider"],
+            )
 
             # ── Reset fallback chain for the new turn ──
             self._fallback_activated = False
@@ -8247,7 +8294,7 @@ class AIAgent:
                         # Cache discovered context length after successful call.
                         # Only persist limits confirmed by the provider (parsed
                         # from the error message), not guessed probe tiers.
-                        if self.context_compressor._context_probed:
+                        if getattr(self.context_compressor, "_context_probed", False):
                             ctx = self.context_compressor.context_length
                             if getattr(self.context_compressor, "_context_probe_persistable", False):
                                 save_context_length(self.model, self.base_url, ctx)
@@ -8586,15 +8633,21 @@ class AIAgent:
                         compressor = self.context_compressor
                         old_ctx = compressor.context_length
                         if old_ctx > _reduced_ctx:
-                            compressor.context_length = _reduced_ctx
+                            compressor.update_model(
-                            compressor.threshold_tokens = int(
+                                model=self.model,
-                                _reduced_ctx * compressor.threshold_percent
+                                context_length=_reduced_ctx,
+                                base_url=self.base_url,
+                                api_key=getattr(self, "api_key", ""),
+                                provider=self.provider,
                             )
+                            # Context probing flags — only set on built-in
+                            # compressor (plugin engines manage their own).
+                            if hasattr(compressor, "_context_probed"):
                                 compressor._context_probed = True
                                 # Don't persist — this is a subscription-tier
-                            # limitation, not a model capability.  If the user
+                                # limitation, not a model capability.  If the
-                            # later enables extra usage the 1M limit should
+                                # user later enables extra usage the 1M limit
-                            # come back automatically.
+                                # should come back automatically.
                                 compressor._context_probe_persistable = False
                             self._vprint(
                                 f"{self.log_prefix}⚠️  Anthropic long-context tier "
@@ -8759,8 +8812,16 @@ class AIAgent:
                             new_ctx = get_next_probe_tier(old_ctx)
 
                         if new_ctx and new_ctx < old_ctx:
-                            compressor.context_length = new_ctx
+                            compressor.update_model(
-                            compressor.threshold_tokens = int(new_ctx * compressor.threshold_percent)
+                                model=self.model,
+                                context_length=new_ctx,
+                                base_url=self.base_url,
+                                api_key=getattr(self, "api_key", ""),
+                                provider=self.provider,
+                            )
+                            # Context probing flags — only set on built-in
+                            # compressor (plugin engines manage their own).
+                            if hasattr(compressor, "_context_probed"):
                                 compressor._context_probed = True
                                 # Only persist limits parsed from the provider's
                                 # error message (a real number).  Guessed fallback