Update an existing Custom App with new Docker Compose configuration
AI agents use update_custom_app to create or update resources in TrueNAS Scale MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your TrueNAS Scale MCP Server environment.
This tool modifies application state reversibly (update operation) rather than permanently deleting it. While the changes are significant and could affect running services, they can be reverted by updating again with previous configuration. The blast radius is high because misconfigured Docker Compose could break application availability, but the operation itself is not destructive or irreversible.
From the tool's definition Tool description states 'Update an existing Custom App with new Docker Compose configuration' — directly modifies application configuration.
Documented attack patterns abuse exactly the kind of access update_custom_app gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and TrueNAS Scale MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for update_custom_app:
{
"version": "1",
"default": "deny",
"tools": {
"update_custom_app": {
"limits": [
{
"counter": "update_custom_app_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}update_custom_app 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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Update an existing Custom App with new Docker Compose configuration. It is categorised as a Write tool in the TrueNAS Scale MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the TrueNAS Scale MCP Server MCP server in PolicyLayer and add a rule for update_custom_app: 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 TrueNAS Scale MCP Server. Nothing to install.
update_custom_app 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_custom_app 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_custom_app. 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_custom_app is provided by the TrueNAS Scale MCP Server MCP server (svnstfns/truenas-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from TrueNAS Scale MCP Server, 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.
13 TrueNAS Scale MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.