AI agents invoke cmux_browser_fill to trigger actions in Cmux Agent. 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.
Filling an input field is a browser interaction/action that triggers external operations in a browser session. It can be used to input data into forms, search fields, authentication prompts, etc. This is an Execute-category action as it manipulates browser state and can have downstream effects depending on what fields are filled (e.g., credentials, form data).
From the tool's definition Fill an input field in the browser
Documented attack patterns abuse exactly the kind of access cmux_browser_fill gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Cmux Agent, and nothing reaches the server without passing your rules. This is the rule we recommend for cmux_browser_fill:
{
"version": "1",
"default": "deny",
"tools": {
"cmux_browser_fill": {
"limits": [
{
"counter": "cmux_browser_fill_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}cmux_browser_fill 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.
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Fill an input field in the browser. It is categorised as a Execute tool in the Cmux Agent MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Cmux Agent MCP server in PolicyLayer and add a rule for cmux_browser_fill: 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 Cmux Agent. Nothing to install.
cmux_browser_fill 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 cmux_browser_fill 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 cmux_browser_fill. 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.
cmux_browser_fill is provided by the Cmux Agent MCP server (multiagentcognition/cmux-agent-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Cmux Agent, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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