hermes-agent/website/docs/developer-guide/plugins/application-declarations.md

Application declarations

A plugin whose MCP server fronts a desktop application declares which application that is and what the server needs of it. The core evaluates the declaration on the host and gates the server's tools, and any skill that names the application, on the answer. The parser imports only the standard library and hermes_platform.

The vocabulary lives in hermes_platform/declaration.py. A declaration is data plus policy, parsed from plain mappings (already-decoded YAML, JSON, a dict literal — the parser never touches a file):

from hermes_platform import declaration

decl = declaration.parse_declaration(
    "my-server",
    raw_app={"linux": {"presence": "executable", "location": "/opt/my-app/server"}},
    raw_requires={"app": True},
    where="my-plugin/plugin.yaml",   # human label used in error messages
)
declaration.register("my-server", decl)

register(server_name, decl) stores one declaration under the configured server name in a process-local registry. Loader integration is separate work; core does not read plugin YAML automatically.

An unregistered MCP server keeps its connection-only check. A skill that explicitly names an unregistered server is hidden. clear() removes every registration and is not a per-plugin unload operation. Registrations are process-wide, not profile-scoped.

app — how to find the application on each OS

app:
  win32:
    presence: executable
    location: "%ProgramFiles%/Vendor/Vendor App/McpServer/Server.exe"
    version: { kind: uninstall_registry, display_name_prefix: "Vendor App" }
    liveness:
      kind: server_json
      path: "%LOCALAPPDATA%/Vendor/Vendor App/McpServer/server.json"
      pid_key: pid
      url_key: http
      token_key: token
      endpoint_path: /mcp
  darwin:
    presence: bundle
    location: /Applications/Vendor.app
    version: { kind: plist }

location is one path or an ordered list of places to look; the first present one wins, except that with requires.min_version every present copy's version is read and the first copy that qualifies wins. A list item is a path or a mapping that names a location kind:

app:
  win32:
    presence: executable
    location:
      - { kind: uninstall_registry, display_name_prefix: "Vendor App", file: vendor.exe }
      - "%ProgramFiles%/Vendor/Vendor App */vendor.exe"
    version: { kind: pe_resource }
  darwin:
    presence: bundle
    location: [{ kind: app_bundle, name: Vendor.app }]
    version: { kind: plist }
  linux:
    presence: executable
    location:
      - { kind: command, name: vendor }
      - { kind: flatpak, app_id: com.vendor.App }
      - { kind: snap, name: vendor }

location kind	OS	key	where it looks
(a path string)	any	—	that path; a * stands for a versioned folder, highest version first
command	any	name	PATH (executable only)
uninstall_registry	win32	display_name_prefix, file	InstallLocation of each matching uninstall entry, joined with file
app_bundle	darwin	name	/Applications, then ~/Applications (bundle only)
flatpak	linux	app_id	the system, then the per-user flatpak exports/bin
snap	linux	name	/snap/bin

The directories behind each kind live in hermes_platform/resolver/known_dirs.py.

field	type	rule	maps to AppDef
<os>	win32 | darwin | linux	at least one; unknown key is an error	AppDef.os_family
presence	executable | bundle	required per OS	.presence
location	str | list	required; a path is drive-rooted (C:\\...) on Windows, or starts with ~ / %VAR% / $VAR; UNC paths are rejected so a presence check never touches the network; no .. segment, ** or URL scheme; expansion at lookup. A list holds paths and location-kind mappings (above)	.locations
version.kind	pe_resource | plist | uninstall_registry | none	default none; pe_resource/uninstall_registry only under win32, plist only under darwin	.version_kind
version.display_name_prefix	str	required when uninstall_registry	.version_arg
liveness.kind	server_json | none	default none	.liveness_kind
liveness.path	str	required when server_json	.liveness_path
liveness.pid_key / url_key / token_key	str	defaults pid / http / token	.liveness_*_key
liveness.endpoint_path	str	default /mcp; the path used for initialize, never the one in the file	.endpoint_path

When requires.app is true, an OS missing from app: gives unsupported_os.

requires — what the server needs before it is offered

requires:
  app: true
  min_version: "2.3.0"

field	type	rule
app	bool	when true, app: must exist and the server is gated on presence
min_version	str	requires app: true; dotted numeric; every app.win32 and app.darwin must declare a real version.kind; app.linux may omit it (Linux has no version source) and is then gated on presence only, but a Linux-only declaration cannot set a minimum; compared numerically per segment, non-numeric characters in a segment are dropped (2.3.0.12594 ≥ 2.3.0; prerelease suffixes are not ordered)

requires.app: true with no app: block is a DeclarationError.

Availability: the one evaluation every reader uses

hermes_platform/resolver/availability.py::availability(decl) -> Availability

Availability(
  state:   available | installed_not_running | missing_app | version_too_old
         | unsupported_os | no_requirements,
  version: str | None,       # inspected, when present
  path:    str | None,       # where the app was found or looked for
  min_version: str | None,   # from requires
)

• no_requirements: no requires.app; the application gate passes, but the connection check still applies.
• unsupported_os: requires.app and no app.<this os> block. Zero I/O.
• missing_app: locate found nothing at any location.
• version_too_old: the version is below the minimum or cannot be read.
• available: present, version acceptable or not required.
• installed_not_running: reserved vocabulary; this evaluator never produces it.

Evaluation uses locate and optional version inspection. It never probes a server, launches an application, or connects. The tool registry retains its existing availability cache.

The two gates

• MCP check_fn (tools/mcp_tool_handlers.py::_make_check_fn): connection alive AND, when a declaration with requires.app is registered for the server, availability(decl).offerable. Returns a plain bool because the registry caches bool(fn()).
• Skill requires_apps: frontmatter (agent/skill_utils.py::skill_matches_apps): each name resolves through declaration.lookup; an unknown name hides the skill (fail closed). Offer-time filter, like environments:.