# Host-Agent MCP Server Integration

The host-agent process exposes a Streamable HTTP MCP server on the same
port as the local console (default `127.0.0.1:8765`), at path `/mcp`. This
lets any MCP-compatible client — Hermes Agent, Claude Desktop, custom
scripts using the `mcp` Python SDK — drive devices directly through the
same `DeviceManager` the cloud worker uses.

## Prerequisites

- Host-agent built from this repo (see `docs/MACOS_IPHONE_SETUP.md`).
- An MCP client that supports the Streamable HTTP transport (mcp SDK
  1.20+ on the client side).

## Get the bearer token

The first time host-agent starts after this feature ships, it generates
a random bearer token and writes it to:

    <identity_path.parent>/host_mcp_token.json

(Default: `tasks/host_mcp_token.json` next to `host_identity.json`.)

To print it for copy/paste:

    device-host-agent mcp-token

To rotate: delete the file and restart host-agent. Old tokens stop
working immediately.

## Hermes Agent configuration

Add to `~/.hermes/config.yaml`:

```yaml
mcp_servers:
  apex_device:
    url: "http://127.0.0.1:8765/mcp"
    headers:
      Authorization: "Bearer <paste-token-here>"
```

Start (or restart) Hermes. Verify by asking Hermes to list devices:

> Use the apex_device MCP to list connected devices.

## Tools exposed

All 11 device tools from `api/mcp.py`:

- `take_screenshot(device_id?)`
- `tap(x, y, device_id?)`
- `swipe(start_x, start_y, end_x, end_y, duration_ms?, device_id?)`
- `input_text(text, device_id?)`
- `launch_app(app_id, device_id?)`
- `find_text(query, device_id?)`
- `find_icon(name, device_id?)`
- `get_ui_tree(device_id?, include_app_info?)`
- `describe_screen(device_id?)`
- `list_devices()`
- `device_status(device_id)`

## Concurrency model

- The cloud worker and MCP clients share the same `DeviceManager`.
- Per-device, session-level locking: the first caller (cloud or MCP) to
  touch a device holds it; the other side sees a busy error.
- MCP sessions hold their lock until **20 seconds of inactivity**
  (the `McpBusyTracker` default TTL). The mcp SDK 1.28.1 does not expose
  a per-session shutdown callback, so a clean Hermes disconnect is also
  recovered via the 20s TTL sweep — see the implementation note in
  spec §6.5. Cloud assignments hold theirs until the assignment
  terminates.
- The cloud scheduler is told about MCP-held devices via the heartbeat
  `mcp_busy_device_ids` field, so it normally won't even try to dispatch
  to them. A 30-second window exists between an MCP acquire and the next
  heartbeat; during that window cloud may dispatch, and the host-agent
  will fail-fast the assignment with `failure_reason="device held by an
  active MCP session"`.

## Network binding

The MCP endpoint is bound to the same address as the local console. By
default this is `127.0.0.1` (loopback only). To expose on a different
interface, set `HOST_AGENT_CONSOLE_BIND_HOST` AND
`HOST_AGENT_CONSOLE_ALLOW_NON_LOOPBACK=true` — both are required. This
is the same escape hatch the local console uses; there is no MCP-only
override.

## Error responses

The mcp SDK 1.28.1 forces tool errors into `CallToolResult(isError=true,
content=[TextContent(message)])` — there is no public path that surfaces
JSON-RPC `-32000` with a structured `data.busy_owner` field from a tool
call site. The busy-owner value lives inside the text content (full
string for `cloud_assignment`, truncated session_id prefix for
`mcp_session:` collisions).

| Condition | JSON-RPC envelope | `result.content[0].text` |
|---|---|---|
| Missing/wrong bearer token | HTTP 401 (transport-level) | `{"error": "invalid token"}` + `WWW-Authenticate: Bearer` |
| Device busy (cloud) | `result.isError = true` | `"device <X> is busy (held by cloud assignment)"` |
| Device busy (other MCP) | `result.isError = true` | `"device <X> is busy (held by mcp_session:<8-char-prefix>)"` |
| Unknown device | `result.isError = false` | JSON `{"ok": false, "error": "device not found: <X>"}` |
| Tool error | `result.isError = true` | `"Error executing tool <name>: <original-message>"` |

## Troubleshooting

- **`list_devices` returns `[]`**: no devices registered. Use the local
  console at `http://127.0.0.1:8765/` to add one (Login → Devices).
- **`device X is busy` even when cloud console says device is idle**:
  check whether another MCP session is holding it. The local console
  dashboard shows active MCP sessions and held device_ids.
- **Token verification fails after restart**: confirm you copied the
  token from the current `host_mcp_token.json`, not an older one.
  Rotation = delete file + restart.

## Out of scope (current version)

- `wait_until_usable` MCP tool: implemented internally but not exposed.
  MVP callers must handle busy errors themselves.
- MCP call history in the local console: only current state is surfaced,
  not a call log.
- Token rotation CLI: use delete-and-restart for now.
- Non-loopback binding without explicit opt-in.