Manually set a session variable to use in replayed flows.
AI agents use set_session_variable to create or update resources in Mitmproxy — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mitmproxy environment.
This tool modifies session variables that are used in HTTP flow replay, which constitutes a reversible write operation. While not immediately destructive, it has high severity because corrupted or injected session variables could cause replayed requests to modify backend data or trigger unintended side effects.
From the tool's definition Tool description states 'Manually set a session variable to use in replayed flows.' This creates or modifies session state that persists to subsequent HTTP requests.
Documented attack patterns abuse exactly the kind of access set_session_variable gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mitmproxy, and nothing reaches the server without passing your rules. This is the rule we recommend for set_session_variable:
{
"version": "1",
"default": "deny",
"tools": {
"set_session_variable": {
"limits": [
{
"counter": "set_session_variable_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}set_session_variable stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Manually set a session variable to use in replayed flows. It is categorised as a Write tool in the Mitmproxy MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Mitmproxy MCP server in PolicyLayer and add a rule for set_session_variable: 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 Mitmproxy. Nothing to install.
set_session_variable is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the set_session_variable 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 set_session_variable. 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.
set_session_variable is provided by the Mitmproxy MCP server (snapspecter/mitmproxy-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 25 Mitmproxy tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
25 Mitmproxy tools catalogued and risk-classified — across an index of 42,500+ MCP servers.