Post a message to a channel, private channel, or direct message
AI agents use slack_conversations_add_message to create or update resources in Sage MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Sage MCP environment.
This tool creates new data (messages) in Slack, which is a reversible write operation. While posts are permanent in Slack's logs, they can be edited or deleted by authorized users. The severity is medium because posting messages could be misused to spam channels, spread misinformation, or impersonate users, but it lacks the irreversibility of destructive operations or financial impact.
From the tool's definition Tool description explicitly states 'Post a message to a channel, private channel, or direct message', indicating creation of new message data that modifies Slack conversation state.
Documented attack patterns abuse exactly the kind of access slack_conversations_add_message gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Sage MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for slack_conversations_add_message:
{
"version": "1",
"default": "deny",
"tools": {
"slack_conversations_add_message": {
"limits": [
{
"counter": "slack_conversations_add_message_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}slack_conversations_add_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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Post a message to a channel, private channel, or direct message. It is categorised as a Write tool in the Sage MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Sage MCP server in PolicyLayer and add a rule for slack_conversations_add_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 Sage MCP. Nothing to install.
slack_conversations_add_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 slack_conversations_add_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 slack_conversations_add_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.
slack_conversations_add_message is provided by the Sage MCP server (sagemcp/sagemcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 359 Sage MCP tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
359 Sage MCP tools catalogued and risk-classified — across an index of 42,500+ MCP servers.