AI agents use mattermost_add_reaction to create or update resources in Mattermost MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mattermost MCP Server environment.
Adding a reaction is a reversible, non-destructive modification operation. It creates metadata associated with a message but does not delete, overwrite, execute code, or transfer funds. The blast radius is minimal—worst case, an agent adds unwanted emoji reactions to messages, which can be easily undone.
From the tool's definition Tool adds a reaction emoji to a message, which creates a new artifact (the reaction) in the Mattermost system. The description explicitly states 'Add a reaction emoji', indicating a write operation that modifies message state by appending a reaction.
Documented attack patterns abuse exactly the kind of access mattermost_add_reaction gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mattermost MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for mattermost_add_reaction:
{
"version": "1",
"default": "deny",
"tools": {
"mattermost_add_reaction": {
"limits": [
{
"counter": "mattermost_add_reaction_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}mattermost_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.
Free to start. No card required.
Add a reaction emoji to a message. It is categorised as a Write tool in the Mattermost MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Mattermost MCP Server MCP server in PolicyLayer and add a rule for mattermost_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 Mattermost MCP Server. Nothing to install.
mattermost_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 mattermost_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 mattermost_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.
mattermost_add_reaction is provided by the Mattermost MCP Server MCP server (pvev/mattermost-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 9 Mattermost MCP Server tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
9 Mattermost MCP Server tools catalogued and risk-classified — across an index of 42,500+ MCP servers.