hermes-webui/docs/EXTENSIONS.md

WebUI Extensions

Hermes WebUI supports a small, opt-in extension surface for self-hosted installs. It lets an administrator serve local static assets and inject same-origin CSS or JavaScript into the app shell without editing the WebUI source tree.

Trust model — read this first. Extensions execute with full WebUI session authority. An extension JS file can call any API the logged-in user can call, including reading conversation history, sending messages, modifying settings, and triggering tool actions. Only enable extensions you wrote yourself or from sources you trust as much as the WebUI source itself. If your WebUI is shared with users you do not fully trust, do not enable extensions. If you set HERMES_WEBUI_EXTENSION_DIR yourself, do not point it at a user-writable directory on a shared host.

This is intentionally not a plugin marketplace or dependency system. It is a safe escape hatch for local dashboards, internal tooling, and workflow-specific panels that should not live in core Hermes WebUI.

What extensions can do

Extensions can:

• serve files from one configured local directory at /extensions/...
• inject configured same-origin stylesheets into <head>
• inject configured same-origin scripts before </body>
• read a local JSON manifest that lists bundled scripts/styles to inject
• call the normal WebUI APIs available to the browser session
• call trusted local loopback sidecars directly from extension JavaScript when the browser Content Security Policy allows that origin

Extensions cannot, by themselves:

• bypass WebUI authentication
• serve files outside the configured extension directory
• load third-party scripts/styles through the built-in injection config
• register new WebUI backend routes or proxy arbitrary sidecar/backend traffic
• change Hermes Agent permissions, models, memory, or tools unless they call existing authenticated APIs that already allow those changes

Configuration

One-click install (no configuration required)

For a single-user self-hosted instance you do not need to configure anything. Open Settings → Extensions, pick an extension from the gallery, and click Install — it just works. The first install creates a WebUI-managed extension directory under your state dir (STATE_DIR/extensions, e.g. ~/.hermes/webui/extensions/) and installs into it; gallery-installed extensions load automatically on the next app-shell render with no environment variables and no restart of your shell.

The managed directory lives alongside your sessions and settings in the WebUI-owned state dir. That is a different trust domain from "a world-writable directory on a shared box": only the WebUI process (and whoever can already write your ~/.hermes state) can place code there. The trust model below still applies — installed extension code runs with full session authority — so only install extensions from the vetted gallery or sources you trust as much as the WebUI source itself.

Some gallery entries need more than WebUI assets. If an extension declares post-install guidance or lifecycle requirements such as a loopback sidecar or a native host, Settings -> Extensions shows a Next step note on the card after install. For example, Desktop Companion can install the WebUI bridge from the gallery, but the desktop pet is only visible after the local Desktop Companion app is started.

Manual / advanced configuration (optional)

HERMES_WEBUI_EXTENSION_DIR is optional and overrides the managed default. Set it when you want extensions to live in a specific directory you control (e.g. a checked-out bundle, or a path mounted into a container). When set it must point to an existing directory before any script or stylesheet URLs are injected; WebUI never auto-creates an admin-specified path:

export HERMES_WEBUI_EXTENSION_DIR=/path/to/my-extension/static
export HERMES_WEBUI_EXTENSION_SCRIPT_URLS=/extensions/app.js
export HERMES_WEBUI_EXTENSION_STYLESHEET_URLS=/extensions/app.css
./start.sh

Multiple URLs may be comma-separated:

export HERMES_WEBUI_EXTENSION_SCRIPT_URLS=/extensions/runtime.js,/extensions/app.js
export HERMES_WEBUI_EXTENSION_STYLESHEET_URLS=/extensions/base.css,/extensions/theme.css

For bundled or multi-extension installs, you may list assets in a manifest file inside HERMES_WEBUI_EXTENSION_DIR instead of maintaining long comma-separated environment variables:

cat > ~/.hermes/webui-extension-bundle/extensions.json <<'JSON'
{
  "extensions": [
    {
      "id": "templates",
      "scripts": ["templates/templates.js"],
      "stylesheets": ["templates/templates.css"]
    },
    {
      "id": "sidebar-tools",
      "scripts": ["sidebar-tools/sidebar-tools.js"],
      "stylesheets": ["sidebar-tools/sidebar-tools.css"]
    }
  ]
}
JSON

HERMES_WEBUI_EXTENSION_DIR=~/.hermes/webui-extension-bundle \
HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json \
./start.sh

Manifest entries use the same URL safety rules as the environment variables. Bare relative entries such as templates/templates.js resolve to /extensions/templates/templates.js; absolute same-origin entries such as /extensions/shared.js or /static/theme.css are also accepted. A manifest may be an object with top-level scripts / stylesheets, an object with an extensions array, or a top-level array of extension objects. Disabled entries may be kept in the manifest with the JSON boolean "enabled": false. Explicit HERMES_WEBUI_EXTENSION_SCRIPT_URLS and HERMES_WEBUI_EXTENSION_STYLESHEET_URLS still work and are appended after manifest assets, with duplicates ignored.

When an extension is installed from Settings -> Extensions, WebUI records the installed package and loads that package's manifest.json automatically on the next app-shell render. In this gallery-installed mode, a manifest located at HERMES_WEBUI_EXTENSION_DIR/<extension-id>/manifest.json resolves bare relative assets relative to that package directory. For example, "scripts": ["assets/companion-adapter.js"] in desktop-companion/manifest.json injects /extensions/desktop-companion/assets/companion-adapter.js.

Manual manifests configured with HERMES_WEBUI_EXTENSION_MANIFEST follow the same rule: relative assets resolve from the manifest file's directory. A root manifest such as extensions.json keeps the existing /extensions/<asset-path> behavior, while a subdirectory manifest such as desktop-companion/manifest.json resolves relative assets under /extensions/desktop-companion/.

Extension entries may also declare a read-only loopback sidecar for diagnostics:

{
  "extensions": [
    {
      "id": "desktop-companion",
      "name": "Desktop Companion",
      "scripts": ["companion-adapter.js"],
      "stylesheets": ["companion-adapter.css"],
      "sidecar": {
        "type": "loopback",
        "origin": "http://127.0.0.1:17787",
        "health_path": "/health"
      }
    }
  ]
}

Loopback sidecars do not change asset injection behavior. They are only reported by diagnostics so an operator can see that a local companion service was declared and optionally check its health from the browser.

URL rules

Injected asset URLs are deliberately restricted:

• must be same-origin paths
• must start with /extensions/ or /static/ after manifest normalization
• must not include a URL scheme, host, fragment, quote, angle bracket, newline, NUL byte, or backslash
• must not contain dot-segments or dotfiles after percent-decoding

Allowed examples:

/extensions/app.js
/extensions/app.css
/extensions/app.js?v=1
/static/theme.css

Rejected examples:

https://example.com/app.js
//example.com/app.js
javascript:alert(1)
/api/session
/extensions/app.js#fragment

These restrictions keep the existing Content Security Policy intact and avoid turning the extension hook into a third-party script loader. Invalid configured URLs are ignored rather than injected.

Trusted local sidecars

Manifest-bundled extensions may integrate with a trusted local sidecar process, such as a desktop companion listening on http://127.0.0.1:17787. The injected extension JavaScript talks to that sidecar directly from the browser; Hermes WebUI does not proxy those requests and does not create extension-owned backend routes.

Loopback sidecar origins are already included in WebUI's enforced CSP connect-src directive:

http://127.0.0.1:*
http://localhost:*
http://ipc.localhost
ws://127.0.0.1:*
ws://localhost:*

The wildcard ports above cover any loopback port, including http://127.0.0.1:17787. For a trusted non-loopback origin that you explicitly control, append the exact origin with HERMES_WEBUI_CSP_CONNECT_EXTRA before starting WebUI:

HERMES_WEBUI_CSP_CONNECT_EXTRA=https://companion.example.internal HERMES_WEBUI_EXTENSION_DIR=/path/to/my-extension/static HERMES_WEBUI_EXTENSION_MANIFEST=extensions.json ./start.sh

HERMES_WEBUI_CSP_CONNECT_EXTRA accepts space-separated http(s):// or ws(s):// origins only. It rejects paths, directive injection, and invalid port numbers. Avoid wildcard or remote origins unless you fully control the target; extension JavaScript runs with the logged-in WebUI session's authority.

Loopback sidecar declarations

Sidecar declarations are sanitized before they appear in diagnostics:

• only "type": "loopback" is supported
• origin must be an http or https origin on 127.0.0.1, localhost, or [::1]
• origin must not include a username, password, path, query string, or fragment
• health_path is optional and defaults to /health
• when present, health_path must start with / and must not contain a scheme, host, query string, fragment, quotes, control characters, backslashes, empty segments, whitespace, or path traversal

Invalid sidecars are skipped with a stable warning code such as sidecar_origin_rejected, sidecar_type_unsupported, sidecar_health_path_rejected, or sidecar_invalid. Raw rejected origins and paths are never returned by the status endpoint. If health_path is omitted, diagnostics use /health; if health_path is present but invalid, the sidecar is skipped rather than probed.

Static file serving

When HERMES_WEBUI_EXTENSION_DIR points at an existing directory, files under that directory are available below /extensions/:

/path/to/my-extension/static/app.js  ->  /extensions/app.js
/path/to/my-extension/static/ui.css  ->  /extensions/ui.css

The static handler is sandboxed:

• path traversal is rejected, including encoded traversal
• dotfiles and dot-directories are not served
• symlinks that resolve outside the extension directory are rejected
• missing or invalid extension directories behave as disabled
• manifest paths must be relative files inside the configured extension directory
• malformed, missing, or oversized manifests are ignored without enabling unsafe URLs
• failures return a generic 404 without exposing local filesystem paths

Security notes

Only enable extensions from directories you control. Extension JavaScript runs in the WebUI origin and can call the same authenticated WebUI APIs as the logged-in browser session.

For shared or remotely exposed installations:

• keep HERMES_WEBUI_PASSWORD enabled
• bind to loopback unless you intentionally expose the service
• review extension code before enabling it
• prefer small, auditable extension files
• avoid serving generated or user-writable directories as extension roots

Extension authoring guidance

Extensions share the page with the WebUI app, so they should be additive and reversible. Prefer small, well-scoped DOM changes that can be removed or hidden without breaking the built-in Chat, Tasks, Settings, or session views.

Recommended patterns:

• create extension-specific containers with unique IDs or class prefixes
• add UI next to existing views instead of replacing large app containers
• keep event listeners scoped to extension-owned elements where possible
• preserve built-in navigation behavior and restore any view state you change
• use hidden, aria-*, and extension-scoped CSS for panels or overlays
• guard initialization so reloading or re-injecting the script does not create duplicate buttons, panels, timers, or event listeners

Avoid destructive mutations such as replacing document.body.innerHTML, main.innerHTML, or other broad WebUI containers. Those patterns can remove or mask the app's existing panels and leave normal navigation unable to recover after an extension view is opened.

For custom pages, prefer adding a dedicated panel and toggling it alongside the built-in views:

(() => {
  if (document.getElementById('my-extension-panel')) return;

  const panel = document.createElement('section');
  panel.id = 'my-extension-panel';
  panel.className = 'main-view my-extension-panel';
  panel.hidden = true;
  panel.textContent = 'My extension page';

  document.querySelector('main')?.appendChild(panel);

  function showPanel() {
    document.querySelectorAll('main > .main-view').forEach((view) => {
      view.hidden = view !== panel;
    });
  }

  // Wire showPanel() to an extension-owned button or menu item.
})();

If host CSS overrides [hidden], add an extension-scoped rule such as:

.my-extension-panel[hidden] {
  display: none !important;
}

Minimal example

Create a local extension directory:

mkdir -p ~/.hermes/webui-extension
cat > ~/.hermes/webui-extension/app.css <<'CSS'
.my-extension-badge {
  position: fixed;
  right: 12px;
  bottom: 12px;
  padding: 6px 10px;
  border-radius: 999px;
  background: #202236;
  color: #fff;
  font: 12px system-ui, sans-serif;
  z-index: 9999;
}
CSS
cat > ~/.hermes/webui-extension/app.js <<'JS'
(() => {
  const badge = document.createElement('div');
  badge.className = 'my-extension-badge';
  badge.textContent = 'Extension loaded';
  document.body.appendChild(badge);
})();
JS

Start WebUI with the extension enabled:

HERMES_WEBUI_EXTENSION_DIR=~/.hermes/webui-extension \
HERMES_WEBUI_EXTENSION_STYLESHEET_URLS=/extensions/app.css \
HERMES_WEBUI_EXTENSION_SCRIPT_URLS=/extensions/app.js \
./start.sh

Open the WebUI and confirm the badge appears.

Diagnostics

Authenticated administrators can inspect sanitized extension configuration at:

GET /api/extensions/status

The status endpoint is read-only and follows the normal WebUI authentication rules. The same sanitized diagnostics are also shown in Settings → Extensions for operators who prefer to inspect extension state from the browser. Installed manifest entries can be enabled or disabled from that panel through the authenticated POST /api/extensions/toggle endpoint. The toggle writes only a WebUI-managed override in the WebUI state directory; it does not edit extension manifests, fetch new extension assets, uninstall files, proxy sidecars, or add extension-owned backend routes. Manifest entries with "enabled": false remain manifest-disabled and cannot be re-enabled from WebUI.

The diagnostics return the same public asset URLs that can already be injected into the HTML, plus coarse manifest status, per-extension effective state, asset counts, sanitized declared loopback sidecars, and warning codes for rejected or unavailable configuration. manifest.script_count and manifest.stylesheet_count count accepted assets from the effectively enabled manifest entries only; manifest.sidecar_count counts accepted enabled loopback sidecars from the manifest. counts.script_urls and counts.stylesheet_urls count the final post-env-merge URLs, while counts.sidecars counts the sanitized sidecar list returned in sidecars. counts.manifest_extensions counts sanitized manifest extension entries with valid IDs, and counts.user_disabled counts installed manifest entries currently suppressed by the WebUI-managed override. manifest.entry_count counts the loaded top-level manifest object and effectively enabled extension entries that were inspected for injection, not every extension object in the file. The endpoint and Settings panel do not return HERMES_WEBUI_EXTENSION_DIR, resolved manifest paths, raw environment values, rejected URL strings, rejected sidecar origins, rejected health paths, or the override state-file path.

When sanitized loopback sidecars are present, Settings → Extensions renders a read-only sidecar monitor card. The browser checks each declared health_url directly with fetch(..., { credentials: 'omit', cache: 'no-store' }) and a short timeout. WebUI does not proxy sidecar requests and does not send WebUI cookies to sidecars. A successful HTTP response is shown as healthy, a non-OK HTTP response as unhealthy, and CORS/network/timeouts as unreachable or blocked; raw health response bodies are never rendered. If a healthy response includes an optional top-level runtime object, the panel may parse it and render only allowlisted scalar fields such as sidecar, native_host, bridge, last_seen_at, and webui_origin. This keeps sidecar-specific diagnostics machine-readable without making WebUI depend on any one extension's private payload shape.