Opt-in Langfuse tracing for Hermes conversations — LLM calls, tool
usage, usage/cost breakdown per span. Hooks into pre/post_api_request,
pre/post_llm_call, pre/post_tool_call. SDK is optional; missing SDK or
credentials renders the plugin inert.
Salvaged from PR #16845 by @kshitijk4poor, who wrote the plugin
(~875 LOC, 6 hooks, Langfuse usage-details/cost-details normalization,
read_file payload summarization).
Salvage scope (why this isn't PR #16845 as-authored):
- Lives at plugins/observability/langfuse/ (standalone kind, opt-in via
plugins.enabled) instead of a new parallel optional-plugins/
directory. Standalone bundled plugins are already opt-in — only their
plugin.yaml is scanned at startup; the Python module is not imported
unless the user enables it. The premise of optional-plugins/ (avoid
import cost for users who don't want it) is already solved by the
existing plugin system.
- Dropped the triple activation gate (plugins.enabled +
plugins.langfuse.enabled + HERMES_LANGFUSE_ENABLED). The Hermes plugin
system's own enable/disable is authoritative; runtime credentials
gate whether the hook actually traces.
- Rewrote _is_enabled() → cached _get_langfuse() with an _INIT_FAILED
sentinel. The original called hermes_cli.config.load_config() from
every hook invocation (full yaml parse + deep merge + env expansion
on every pre/post_tool_call, potentially 100+ times per turn). The
cached version reads env once and returns the cached client or None
on every subsequent call with zero further work.
- hermes tools → Langfuse Observability post-setup adds
observability/langfuse to plugins.enabled directly (via
_save_enabled_set) instead of going through an install-copy flow.
Enable:
hermes tools # interactive
hermes plugins enable observability/langfuse # manual
Required env (set by `hermes tools` or in ~/.hermes/.env):
HERMES_LANGFUSE_PUBLIC_KEY
HERMES_LANGFUSE_SECRET_KEY
HERMES_LANGFUSE_BASE_URL # optional
Co-authored-by: kshitijk4poor <kshitijk4poor@gmail.com>
