ucp_checkout_set_fulfillment
AI agents use ucp_checkout_set_fulfillment to create or update resources in Ucp — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Ucp environment.
This tool modifies fulfillment settings within an active checkout session. While it does not complete the payment (that is ucp_checkout_complete) nor charge the user (Financial category requires actual money movement), it alters the terms of a transaction in progress—specifically delivery/fulfillment instructions—which is reversible via ucp_checkout_update but represents a significant Write action.
From the tool's definition Tool name contains 'checkout_set_fulfillment' which modifies fulfillment details during a checkout workflow. Sibling tools include 'ucp_checkout_complete' and 'ucp_checkout_create', indicating this tool updates checkout state.
Documented attack patterns abuse exactly the kind of access ucp_checkout_set_fulfillment gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Ucp, and nothing reaches the server without passing your rules. This is the rule we recommend for ucp_checkout_set_fulfillment:
{
"version": "1",
"default": "deny",
"tools": {
"ucp_checkout_set_fulfillment": {
"limits": [
{
"counter": "ucp_checkout_set_fulfillment_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}ucp_checkout_set_fulfillment 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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ucp_checkout_set_fulfillment. It is categorised as a Write tool in the Ucp MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Ucp MCP server in PolicyLayer and add a rule for ucp_checkout_set_fulfillment: 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 Ucp. Nothing to install.
ucp_checkout_set_fulfillment 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 ucp_checkout_set_fulfillment 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 ucp_checkout_set_fulfillment. 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.
ucp_checkout_set_fulfillment is provided by the Ucp MCP server (nguthrie/ucp-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Ucp, 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.
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