registerWithId
Registers a user for an application. If you provide the User and the UserRegistration object on this request, it will create the user as well as register them for the application. This is called a Full Registration. However, if you only provide the UserRegistration object, then the user must alre...
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What registerWithId does on Fusionauth
AI agents use registerWithId to create or update resources in Fusionauth, usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Fusionauth environment.
Why registerWithId is rated Medium
This tool creates new user accounts and/or application registrations. While these are reversible (users can be deleted or unregistered), the action modifies system state by adding user data and application associations. This is a Write operation.
From the tool's definition Description states: 'Registers a user for an application. If you provide the User and the UserRegistration object on this request, it will create the user as well as register them for the application.' The verbs 'create' and 'register' indicate data…
Attacks that exploit this kind of access
The rule that runs registerWithId safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Fusionauth, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For registerWithId, this is the rule to start with:
registerWithId 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.
The button opens the PolicyLayer dashboard: create your workspace, connect Fusionauth, apply this rule, and every registerWithId call is checked against it from then on.
Questions about registerWithId
Registers a user for an application. If you provide the User and the UserRegistration object on this request, it will create the user as well as register them for the application. This is called a Full Registration. However, if you only provide the UserRegistration object, then the user must already exist and they will be registered for the application. The user Id can also be provided and it will either be used to look up an existing user or it will be used for the newly created User. It is categorised as a Write tool in the Fusionauth MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Fusionauth MCP server in PolicyLayer and add a rule for registerWithId: 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 Fusionauth. Nothing to install.
registerWithId 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 registerWithId 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 registerWithId. 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.
registerWithId is provided by the Fusionauth MCP server (@fusionauth/mcp-api). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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