Architecture Decision Records

2026-07-13: Scope plugin manager state by Hermes home/profile (keyed cache)

Status: Accepted

Context: Hermes supports multiple profiles via different Hermes home directories. Homes are switched two ways in a running process: the HERMES_HOME environment variable (single-profile CLI/gateway processes), and the context-local set_hermes_home_override() (hermes_constants.py), which the multiplexed gateway worker (gateway/run.py's _profile_scope) and subagent/embedded callers use to serve several profiles from one long-lived process. The override is a ContextVar and deliberately does not mutate os.environ, since that would leak one profile's home into every other concurrent task in the same process.

The plugin manager was a process-global single-slot singleton (_plugin_manager). User-installed plugins are discovered from get_hermes_home() / "plugins", and context-engine plugins (e.g. hermes-lcm) capture profile-scoped state — such as the LCM database path — at registration time. A single-slot cache meant:

1. Switching homes via set_hermes_home_override() was invisible to a naive "did HERMES_HOME change" check, so the singleton silently kept serving the first profile's manager to every other profile in the process.
2. Even when a fresh PluginManager was created for a new home, plugin modules are imported into sys.modules as hermes_plugins.<slug> by _load_directory_module, and only that top-level module was ever replaced. A same-slug plugin's relative imports (from . import state) are cached separately under hermes_plugins.<slug>.<submodule>, and Python's import machinery resolves those from sys.modules first — so a profile switch could silently keep serving a previous profile's already-imported submodule code/state instead of re-executing the new profile's plugin.

Decision:

• Replace the single-slot singleton with a cache keyed on the resolved Hermes home path (_plugin_managers_by_home: Dict[Path, PluginManager]). get_plugin_manager() resolves the current home via get_hermes_home() (which itself already consults get_hermes_home_override() before os.environ), so both the env-var and context-local override paths are covered uniformly.
• _plugin_manager (the old single-slot name) is kept as a thin "last manager returned" pointer purely for backward compatibility with existing test code that does monkeypatch.setattr(plugins_mod, "_plugin_manager", some_manager). When that name is monkeypatched to a manager the keyed cache doesn't know about, get_plugin_manager() treats it as an explicit injection and adopts it into the cache under the current resolved home, rather than discarding it.
• Both PluginManager._load_directory_module (initial/force=True reload within the same home) and the shared _clear_plugin_submodules helper (profile switch / test teardown) evict sys.modules[module_name] and every name prefixed with module_name + "." before a plugin slug is (re-)imported, so relative-import submodules can never survive a reload or a home switch.
• Test isolation (tests/conftest.py's _hermetic_environment fixture) calls a new _reset_plugin_managers_for_tests() helper that drops the entire keyed cache and purges every plugin submodule from sys.modules between tests, instead of only resetting the single-slot pointer.

Consequences:

• Per-profile LCM instances (and any other context-engine plugin) use their own {home}/lcm.db regardless of whether the profile switch went through HERMES_HOME or set_hermes_home_override().
• Plugin discovery remains cached within a profile for normal performance, and re-entering a previously-seen profile reuses its cached manager instead of rebuilding from scratch.
• Sequential and interleaved profile switching — in tests, the gateway multiplexer worker, or embedded callers using the context-local override — no longer leaks context-engine state, plugin module state, or stale relative-import submodules across profiles.
• Regression coverage exercises the real production path (set_hermes_home_override()) rather than only the env-var path, and includes a dedicated relative-import leak test.