Update an existing contact property in Resend. Only the fallback value can be changed — the key and type cannot be modified after creation.
AI agents use update-contact-property to create or update resources in Email Sending MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Email Sending MCP environment.
This tool creates or modifies data in a reversible manner (updating a contact property's fallback value), which fits the Write category. Severity is medium because modifying contact properties could affect email campaigns or contact management workflows, but the operation is not destructive, financial, or executing arbitrary code.
From the tool's definition Tool description states 'Update an existing contact property in Resend' — this is a modification operation that changes data. The constraint that 'key and type cannot be modified' indicates reversible updates to the fallback value.
Documented attack patterns abuse exactly the kind of access update-contact-property gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Email Sending MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for update-contact-property:
{
"version": "1",
"default": "deny",
"tools": {
"update-contact-property": {
"limits": [
{
"counter": "update-contact-property_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}update-contact-property 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 contact property in Resend. Only the fallback value can be changed — the key and type cannot be modified after creation. It is categorised as a Write tool in the Email Sending MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Email Sending MCP server in PolicyLayer and add a rule for update-contact-property: 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 Email Sending MCP. Nothing to install.
update-contact-property 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-contact-property 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-contact-property. 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-contact-property is provided by the Email Sending MCP server (resend/resend-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 77 Email Sending MCP tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
77 Email Sending MCP tools catalogued and risk-classified — across an index of 42,500+ MCP servers.