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 }

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	required; drive-rooted (C:\\...) on Windows, or starting with ~ / %VAR% / $VAR; UNC paths are rejected so a presence check never touches the network; no .. segment or URL scheme; expansion at lookup	.location
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 applicable app.<os> must declare a real version.kind; compared numerically per segment, non-numeric characters in a segment are dropped (2.3.0.12594 ≥ 2.3.0; prerelease suffixes are not ordered)
gpu	str	nvidia; the server is offered only on a host where hermes_platform.host.facts.gpu_class() reports that vendor. Independent of app: a server with no app: block can require a GPU.

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

gpu is for an application that has no install location to check, or that needs the hardware whatever is installed. gpu_class() reads the registry on Windows, sysfs on Linux and the CPU architecture on macOS, never a subprocess or a driver library. When it cannot read the GPU (unknown), the requirement passes: a failed read must not lock out a machine that has the GPU, and the connection check still applies. Only nvidia is accepted: gpu_class() reports the highest-priority vendor present, which answers "is an NVIDIA GPU here" exactly and would not answer the same question for AMD or Intel on a machine that also has an NVIDIA GPU.

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 | unsupported_gpu | 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, and requires.gpu (if any) is met; the application gate passes, but the connection check still applies.
• unsupported_os: requires.app and no app.<this os> block. Zero I/O.
• unsupported_gpu: requires.gpu names a vendor this host's GPU is not. The install is refused (… is unavailable: unsupported_gpu, needs an NVIDIA GPU.), and a registered server's status sentence is "<app> needs an NVIDIA GPU; none was found on this machine. Use <app> on a machine with an NVIDIA GPU."
• missing_app: locate found nothing at 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, optional version inspection and the cached GPU fact. 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:.