AI agents use post_note to create or update resources in Substack — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Substack environment.
Based on the server's purpose (content creation and publishing) and naming convention alongside sibling write operations, post_note most likely creates or modifies notes reversibly. It does not delete (Destructive), execute arbitrary code (Execute), or move money (Financial).
From the tool's definition The tool is named 'post_note' and resides on a Substack MCP server whose sibling tools include create_draft, publish_draft, update_draft, and upload_image—all write operations. The pattern suggests post_note creates or modifies content.
Documented attack patterns abuse exactly the kind of access post_note gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Substack, and nothing reaches the server without passing your rules. This is the rule we recommend for post_note:
{
"version": "1",
"default": "deny",
"tools": {
"post_note": {
"limits": [
{
"counter": "post_note_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}post_note 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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post_note. It is categorised as a Write tool in the Substack MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Substack MCP server in PolicyLayer and add a rule for post_note: 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 Substack. Nothing to install.
post_note 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 post_note 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 post_note. 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.
post_note is provided by the Substack MCP server (nanameru/substack-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Substack, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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11 Substack tools catalogued and risk-classified — across an index of 43,000+ MCP servers.