AI agents use slack_add_reaction to create or update resources in MCP-Brave-Search — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCP-Brave-Search environment.
This tool creates new data (a reaction) but is fully reversible (reactions can be removed). It has minimal blast radius—the worst case is spam reactions on Slack messages, causing minor disruption. It falls under Write (creates/modifies data reversibly) rather than Execute or higher categories, as it only affects Slack message metadata without triggering external operations or side effects beyond the platform.
From the tool's definition Tool adds a reaction emoji to a message, which creates/modifies data reversibly. The description explicitly states 'Add a reaction emoji to a message', indicating a create operation on message metadata.
Documented attack patterns abuse exactly the kind of access slack_add_reaction gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and MCP-Brave-Search, and nothing reaches the server without passing your rules. This is the rule we recommend for slack_add_reaction:
{
"version": "1",
"default": "deny",
"tools": {
"slack_add_reaction": {
"limits": [
{
"counter": "slack_add_reaction_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}slack_add_reaction 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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Add a reaction emoji to a message. It is categorised as a Write tool in the MCP-Brave-Search MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MCP-Brave-Search MCP server in PolicyLayer and add a rule for slack_add_reaction: 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 MCP-Brave-Search. Nothing to install.
slack_add_reaction 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 slack_add_reaction 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 slack_add_reaction. 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.
slack_add_reaction is provided by the MCP-Brave-Search MCP server (modelcontextprotocol/servers-archived). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from MCP-Brave-Search, 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.
59 MCP-Brave-Search tools catalogued and risk-classified — across an index of 43,000+ MCP servers.