hermes-harness-plugin

A Hermes Agent plugin that optimizes Hermes for use with harness.

Scope. This plugin is purpose-built for the boldblackai harness container images. It assumes mise lives at /usr/local/bin/mise, auto-trusts the nearest mise config on session start, and injects harness-specific environment facts into every LLM turn. It is not intended for — and will misbehave outside of — those images. If you pip-install it elsewhere, expect the hardcoded paths and bundled context to be wrong for your environment.

It bundles a mise skill and three lifecycle hooks:

1. a pre_tool_call hook that transparently activates mise for every terminal() command when a mise config file is present;
2. an on_session_start hook that trusts the nearest mise config on startup; and
3. a pre_llm_call hook that injects a bundled context.md as additional context into every LLM turn.

The mise activation is modeled on pi-mise (the same idea, for the pi coding agent).

What it does

When Hermes starts and this plugin is enabled, every terminal() command is transparently run inside an activated mise shell — but only when it matters:

1. a mise config file (mise.toml / .mise.toml / .tool-versions) exists in the working directory or any parent.

Harness images always ship mise at /usr/local/bin/mise, so the hook uses that path directly — it never tries to detect or resolve mise from PATH. If no config is found, the hook no-ops — zero overhead. It is also idempotent: it never double-wraps a command that's already activating mise.

Concretely, a terminal(command="bundle install") issued in a repo with a mise.toml becomes:

eval "$(/usr/local/bin/mise activate bash)" && bundle install

An on_session_start hook also runs mise trust on the nearest config so activation is frictionless on fresh clones. (This auto-trusts whatever config is in the working tree — appropriate inside a trusted harness image, but another reason not to run this plugin elsewhere.)

It also contributes a mise skill (skill_view("hermes-harness-plugin:mise")) covering manual mise exec, tasks, installs, trust, and the common pitfalls.

Context injection (pre_llm_call)

A pre_llm_call hook injects the contents of the bundled context.md (shipped inside the package) as additional context into every LLM turn. Edit src/hermes_harness_plugin/context.md in the repo to control what the model sees on every turn. By design the injection is per-turn: the hook returns the text for the host to splice into the current turn only. How the host handles persistence, the system prompt, and prompt caching is a Hermes decision outside this plugin's control — see the Hermes docs for those semantics.

Prerequisites

• Python >= 3.13.
• Hermes Agent — this is a plugin, not a standalone app; it does nothing without Hermes loaded as the host.
• mise at /usr/local/bin/mise (preinstalled on harness images).

Install

# via pip (or uv pip)
pip install hermes-harness-plugin

# or from source with uv
uv pip install .

Hermes auto-discovers the plugin via the hermes_agent.plugins entry point on next startup. Enable it (plugins are opt-in):

hermes plugins enable hermes-harness-plugin

Confirm with /plugins in a session.

How it works

Hermes' pre_tool_call hook receives the same args dict that is later dispatched to the terminal handler, so mutating args["command"] in place inside the hook changes what actually executes. The plugin prepends the activation prefix (using the hardcoded mise path), checks for a config file, and rewrites the command. All failures are caught and logged — a broken hook never breaks the agent loop.

Repo layout

hermes-harness-plugin/
├── pyproject.toml                      # uv/pip packaging + entry point
├── README.md
├── LICENSE
└── src/hermes_harness_plugin/
    ├── __init__.py                     # register(ctx): skill + hooks
    ├── plugin.yaml                     # manifest (also enables directory-plugin use)
    ├── mise.py                         # mise activation hook logic
    ├── context.py                      # pre_llm_call context-injection hook
    ├── context.md                      # bundled context (injected every turn)
    └── skills/
        └── mise/
            └── SKILL.md                # bundled skill

Develop

uv sync                                 # create venv + install dev deps
uv run pytest                           # run tests
uv build                                # build sdist + wheel into dist/

License

MIT