Upload an image to Substack's CDN and return its public URL.
AI agents use upload_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.
Uploading an image creates a new asset in Substack's CDN but is reversible (the image can be deleted or replaced). It does not execute arbitrary code, delete data, move money, or cause irreversible harm. The blast radius is minimal—worst case, an attacker uploads unwanted images to the user's CDN, which can be cleaned up. This is clearly a Write operation with low severity.
From the tool's definition Tool description states 'Upload an image to Substack's CDN and return its public URL.' The verb 'upload' indicates creation of new media assets.
Documented attack patterns abuse exactly the kind of access upload_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 upload_image:
{
"version": "1",
"default": "deny",
"tools": {
"upload_image": {
"limits": [
{
"counter": "upload_image_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}upload_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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Upload an image to Substack's CDN and return its public URL. 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 upload_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.
upload_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 upload_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 upload_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.
upload_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.