Post a message to a channel. Optionally reply to a parent message.
AI agents use post_channel_message to create or update resources in Open WebUI MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Open WebUI MCP Server environment.
This tool creates new messages in channels, which is a reversible write operation. It does not delete data (Destructive), execute arbitrary code (Execute), involve financial transactions (Financial), or merely read information (Read).
From the tool's definition Tool name 'post_channel_message' and description 'Post a message to a channel' indicate creation of new message content. The capability to 'reply to a parent message' further confirms this modifies channel state by adding new data.
Documented attack patterns abuse exactly the kind of access post_channel_message gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Open WebUI MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for post_channel_message:
{
"version": "1",
"default": "deny",
"tools": {
"post_channel_message": {
"limits": [
{
"counter": "post_channel_message_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}post_channel_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. Optionally reply to a parent message. It is categorised as a Write tool in the Open WebUI MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Open WebUI MCP Server MCP server in PolicyLayer and add a rule for post_channel_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 Open WebUI MCP Server. Nothing to install.
post_channel_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 post_channel_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 post_channel_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.
post_channel_message is provided by the Open WebUI MCP Server MCP server (troylar/open-webui-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Open WebUI MCP Server, 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.
82 Open WebUI MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.