AI agents use set_cover_image 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.
Setting a cover image creates or modifies post metadata reversibly—it can be changed or removed later. This is a Write operation rather than Read (it modifies state), Execute (it doesn't run arbitrary code), or Destructive (it's undoable). Medium severity reflects that misuse could alter post appearance and audience perception, but the change is easily reversible.
From the tool's definition Tool is part of a publishing server alongside create_draft, update_draft, and publish_draft. The name 'set_cover_image' indicates it modifies metadata of a post (the cover image), which is a reversible write operation.
Documented attack patterns abuse exactly the kind of access set_cover_image 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 set_cover_image:
{
"version": "1",
"default": "deny",
"tools": {
"set_cover_image": {
"limits": [
{
"counter": "set_cover_image_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}set_cover_image 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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set_cover_image. 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 set_cover_image: 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.
set_cover_image 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 set_cover_image 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 set_cover_image. 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.
set_cover_image 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.