AI agents use modify_message to create or update resources in Gmail MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Gmail MCP environment.
This tool modifies email metadata (labels) rather than content, and the operation is reversible—labels can be added or removed. It does not delete or send emails, placing it in the Write category rather than Destructive or Execute. Medium severity because mislabeling bulk messages could cause organizational disruption or hide critical emails, but the effect is recoverable.
From the tool's definition Tool name is 'modify_message' and description states 'Modify the labels on a message'. Modifying labels (marking as read, spam, important, etc.) changes the state of email metadata reversibly.
Documented attack patterns abuse exactly the kind of access modify_message gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Gmail MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for modify_message:
{
"version": "1",
"default": "deny",
"tools": {
"modify_message": {
"limits": [
{
"counter": "modify_message_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}modify_message 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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Modify the labels on a message. It is categorised as a Write tool in the Gmail MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Gmail MCP server in PolicyLayer and add a rule for modify_message: 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 Gmail MCP. Nothing to install.
modify_message 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 modify_message 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 modify_message. 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.
modify_message is provided by the Gmail MCP server (shinzo-labs/gmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 65 Gmail MCP tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
65 Gmail MCP tools catalogued and risk-classified — across an index of 42,500+ MCP servers.