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 }

Each kind's OS, key, presence and locator are one LOCATION_KINDS entry in hermes_platform/resolver/app.py; the parser validates against that table, so a new kind is one entry there. The directories behind each kind live in hermes_platform/resolver/known_dirs.py.

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"

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 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, 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:.