AI agents call signal_list_chats to retrieve information from Signal without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool retrieves or queries chat data (listing conversations) with no side effects. It falls under Read category. Severity is high rather than medium because access to Signal messages—a private communication platform—carries significant privacy implications if exposed to unauthorized agents, though the blast radius is limited to information disclosure rather than modification or financial impact.
From the tool's definition Tool name 'signal_list_chats' and server description 'MCP Server for retrieving Signal messages' indicate data retrieval without modification.
Documented attack patterns abuse exactly the kind of access signal_list_chats gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Signal, and nothing reaches the server without passing your rules. This is the rule we recommend for signal_list_chats:
{
"version": "1",
"default": "deny",
"tools": {
"signal_list_chats": {}
}
}signal_list_chats is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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signal_list_chats. It is categorised as a Read tool in the Signal MCP Server, which means it retrieves data without modifying state.
Register the Signal MCP server in PolicyLayer and add a rule for signal_list_chats: 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 Signal. Nothing to install.
signal_list_chats is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the signal_list_chats 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 signal_list_chats. 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.
signal_list_chats is provided by the Signal MCP server (stefanstranger/signal-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Signal, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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3 Signal tools catalogued and risk-classified — across an index of 43,000+ MCP servers.