memory_invalidate · Mementos MCP: Risk & Policy

WHAT IT DOES

What memory_invalidate does on Mementos

AI agents use memory_invalidate to create or update resources in Mementos — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mementos environment.

WHY IT NEEDS A POLICY

Why memory_invalidate needs a policy

The tool modifies an existing memory record by setting its valid_until timestamp to the current time, effectively expiring it. This is a reversible write operation (the data is not deleted, just marked as invalid/expired), so it falls under Write rather than Destructive. Misuse could corrupt an agent's memory state, giving it medium severity.

From the tool's definition Invalidate an existing fact by setting valid_until to now... old fact should be superseded

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

memory_invalidate Rate-limited

memory_invalidate 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "memory_invalidate": {
      "limits": [
        {
          "counter": "memory_invalidate_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about memory_invalidate

What does the memory_invalidate tool do? +

Invalidate an existing fact by setting valid_until to now. Use when a contradiction is confirmed and the old fact should be superseded. Optionally link to the new superseding memory. It is categorised as a Write tool in the Mementos MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on memory_invalidate? +

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

What risk level is memory_invalidate? +

memory_invalidate is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit memory_invalidate? +

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

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

memory_invalidate is provided by the Mementos MCP server (@hasna/mementos). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.