Start the observer daemon to continuously watch an app window. Captures frames via CGWindowListCreateImage, runs OCR only when pixels change, detects popups. Zero overhead on engine — reads a JSON file.
AI agents invoke observer_start to trigger actions in ScreenHand. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
This tool initiates a daemon process that monitors and captures application window states, runs OCR analysis, and detects UI changes. Daemon startup and persistent monitoring of application windows constitute execution of system-level operations whose effects depend on which app is targeted and how the captured data is subsequently used by an agent.
From the tool's definition Start the observer daemon to continuously watch an app window. Captures frames via CGWindowListCreateImage, runs OCR only when pixels change, detects popups.
Documented attack patterns abuse exactly the kind of access observer_start gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and ScreenHand, and nothing reaches the server without passing your rules. This is the rule we recommend for observer_start:
{
"version": "1",
"default": "deny",
"tools": {
"observer_start": {
"limits": [
{
"counter": "observer_start_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}observer_start stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
Free to start. No card required.
Start the observer daemon to continuously watch an app window. Captures frames via CGWindowListCreateImage, runs OCR only when pixels change, detects popups. Zero overhead on engine — reads a JSON file. It is categorised as a Execute tool in the ScreenHand MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the ScreenHand MCP server in PolicyLayer and add a rule for observer_start: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches ScreenHand. Nothing to install.
observer_start is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the observer_start rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for observer_start. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
observer_start is provided by the ScreenHand MCP server (manushi4/screenhand). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from ScreenHand, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
89 ScreenHand tools catalogued and risk-classified — across an index of 43,000+ MCP servers.