Update an existing Substack draft post. WARNING: This tool COMPLETELY REPLACES the specified fields - it does NOT make partial edits. If you provide content, it will REPLACE ALL existing content. To make small edits, first use get_post_content to read the current content, make your changes, then ...
AI agents use update_post to create or update resources in Substack Mcp Plus — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Substack Mcp Plus environment.
The tool modifies existing draft posts by replacing their content, which is a Write operation (reversible modification of data). Severity is high because misuse could replace important draft content with unintended text, though the operation targets drafts (not published posts) and is theoretically reversible via undo or re-editing.
From the tool's definition Tool description states it 'Update an existing Substack draft post' and 'COMPLETELY REPLACES the specified fields' including all content. This is a create/modify operation on existing data.
Risk signalsBulk/mass operation — affects multiple targets
Documented attack patterns abuse exactly the kind of access update_post gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Substack Mcp Plus, and nothing reaches the server without passing your rules. This is the rule we recommend for update_post:
{
"version": "1",
"default": "deny",
"tools": {
"update_post": {
"limits": [
{
"counter": "update_post_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}update_post 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.
Free to start. No card required.
Update an existing Substack draft post. WARNING: This tool COMPLETELY REPLACES the specified fields - it does NOT make partial edits. If you provide content, it will REPLACE ALL existing content. To make small edits, first use get_post_content to read the current content, make your changes, then provide the ENTIRE updated content. IMPORTANT: You MUST ALWAYS ask the user to confirm updates in a follow-up message BEFORE calling this tool with confirm_update=true. Never set confirm_update=true on the first request. It is categorised as a Write tool in the Substack Mcp Plus MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Substack Mcp Plus MCP server in PolicyLayer and add a rule for update_post: 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 Mcp Plus. Nothing to install.
update_post 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 update_post 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 update_post. 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.
update_post is provided by the Substack Mcp Plus MCP server (ty13r/substack-mcp-plus). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Substack Mcp Plus, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
12 Substack Mcp Plus tools catalogued and risk-classified — across an index of 43,000+ MCP servers.