pty_spawn · Pty MCP: Risk & Policy

WHAT IT DOES

What pty_spawn does on Pty

AI agents invoke pty_spawn to trigger actions in Pty. 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.

WHY IT NEEDS A POLICY

Why pty_spawn needs a policy

pty_spawn executes arbitrary commands in a pseudo-terminal environment. This gives an AI agent full shell-level code execution capability, allowing it to run any program, script, or shell command. The blast radius is critical because a misused or hijacked agent could execute destructive, financial, or exfiltration commands.

From the tool's definition 'Spawn a command in a pseudo-terminal' and server description 'enables agents to autonomously spawn processes' — arbitrary command execution in a PTY shell

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

pty_spawn Rate-limited

pty_spawn 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.

View as policy code

policy.json

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

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Pty, so you can run agents against it safely.

FAQ

Questions about pty_spawn

What does the pty_spawn tool do? +

Spawn a command in a pseudo-terminal. Returns session ID and initial output. Use pty_write to send input to interactive prompts. It is categorised as a Execute tool in the Pty MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on pty_spawn? +

Register the Pty MCP server in PolicyLayer and add a rule for pty_spawn: 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 Pty. Nothing to install.

What risk level is pty_spawn? +

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

Can I rate-limit pty_spawn? +

Yes. Add a rate_limit block to the pty_spawn 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 pty_spawn completely? +

Set action: deny in the PolicyLayer policy for pty_spawn. 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 pty_spawn? +

pty_spawn is provided by the Pty MCP server (reedm121/pty-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.