Cancel a downstream MCP task by opaque task ID.
AI agents invoke gateway.tasks_cancel to trigger actions in PMCP. 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.
Cancelling a task is an action that triggers an external operation (stopping a running task). It is not purely a read, nor does it delete data or move money. It falls under Execute because it causes a side effect (task termination) whose impact depends on which task is cancelled.
From the tool's definition 'Cancel a downstream MCP task by opaque task ID' — actively terminates/stops a running downstream operation
Documented attack patterns abuse exactly the kind of access gateway.tasks_cancel gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and PMCP, and nothing reaches the server without passing your rules. This is the rule we recommend for gateway.tasks_cancel:
{
"version": "1",
"default": "deny",
"tools": {
"gateway.tasks_cancel": {
"limits": [
{
"counter": "gateway.tasks_cancel_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}gateway.tasks_cancel 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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Cancel a downstream MCP task by opaque task ID. It is categorised as a Execute tool in the PMCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the P MCP server in PolicyLayer and add a rule for gateway.tasks_cancel: 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 PMCP. Nothing to install.
gateway.tasks_cancel 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 gateway.tasks_cancel 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 gateway.tasks_cancel. 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.
gateway.tasks_cancel is provided by the P MCP server (viperjuice/pmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from PMCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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