// MCP TOOL REFERENCE

start_session

ServerNova Act
CategoryExecute

WHAT IT DOES

What start_session does on Nova Act

AI agents invoke start_session to trigger actions in Nova Act. 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.

THE RISK

High Risk

Why start_session needs a policy

start_session triggers browser automation with external side effects (launching browser, navigating to URLs, establishing sessions). While it doesn't directly modify data or delete anything, it executes operations that interact with external systems and can access arbitrary web content based on provided URLs.

From the tool's definition "Starts and initializes a new browser session controlled by Nova Act, navigating to the specified URL" - initiates external browser automation and navigation operations whose effects depend on the URL argument provided.

Documented attack patterns abuse exactly the kind of access start_session gives an agent:

POLICY

How to control start_session

PolicyLayer is an MCP gateway — it sits between your AI agents and Nova Act, and nothing reaches the server without passing your rules. This is the rule we recommend for start_session:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "start_session": {
      "limits": [
        {
          "counter": "start_session_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

start_session 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.

Create a free account and register Nova Act — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Nova Act tools

Destructive end_session Closes the specified browser session and cleans up resources. Call this when done with a s Execute execute_instruction Executes a natural language instruction in an active browser session. Use task for the mai Execute compress_logs Compresses Nova Act log files to reduce size while preserving important information. Optio Read fetch_file Retrieves the contents of a file from the server, with optional base64 encoding. Primarily Read inspect_browser Retrieves current state (URL, title) of an active browser session. Screenshot is omitted b Read list_browser_sessions Lists all active Nova Act browser sessions managed by this MCP server. Returns details abo Read view_compressed_log Retrieves and displays the contents of a compressed log file (.gz) previously created by t Read view_html_log Retrieves and displays HTML log files generated by Nova Act browser sessions. These logs c

All 9 Nova Act tools →

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Go deeper

High-risk tools in Nova Act → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about start_session

What does the start_session tool do? +

Starts and initializes a new browser session controlled by Nova Act, navigating to the specified URL. Returns session details including a unique session_id required for other browser tools. This is a long-running operation (20-60s) and will send progress notifications. It is categorised as a Execute tool in the Nova Act MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on start_session? +

Register the Nova Act MCP server in PolicyLayer and add a rule for start_session: 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 Nova Act. Nothing to install.

What risk level is start_session? +

start_session is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit start_session? +

Yes. Add a rate_limit block to the start_session 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.

How do I block start_session completely? +

Set action: deny in the PolicyLayer policy for start_session. 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.

What MCP server provides start_session? +

start_session is provided by the Nova Act MCP server (madtank/nova-act-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Nova Act tool call.

Start from Nova Act, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →

Free to start. No card required.

9 Nova Act tools catalogued and risk-classified — across an index of 43,000+ MCP servers.