journal_entry · Nexus Core MCP: Risk & Policy

WHAT IT DOES

What journal_entry does on Nexus Core

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

WHY IT NEEDS A POLICY

Why journal_entry needs a policy

This tool writes/appends a new journal entry with mood and tags. It creates new data in a journal, which is a reversible write operation. The blast radius is low as it only affects personal journal data.

From the tool's definition Append a journal entry. Optional mood + tags.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

journal_entry Rate-limited

journal_entry 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": {
    "journal_entry": {
      "limits": [
        {
          "counter": "journal_entry_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 Nexus Core, so you can run agents against it safely.

FAQ

Questions about journal_entry

What does the journal_entry tool do? +

Append a journal entry. Optional mood + tags. It is categorised as a Write tool in the Nexus Core MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on journal_entry? +

Register the Nexus Core MCP server in PolicyLayer and add a rule for journal_entry: 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 Nexus Core. Nothing to install.

What risk level is journal_entry? +

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

Can I rate-limit journal_entry? +

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

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

journal_entry is provided by the Nexus Core MCP server (noumenon-ai/nexus-core). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.