This record as markdown: /tools/karbassi-slack-mcp/chat-append-stream.md
What chat_append_stream does on Slack
AI agents use chat_append_stream to create or update resources in Slack, usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Slack environment.
Why chat_append_stream is rated Medium
The tool appends to chat content, which is a write operation that modifies data reversibly. While the description is empty (lowering confidence slightly), the name and server context strongly suggest message modification. Severity is high because uncontrolled message appending could manipulate conversations, inject false information, or spam channels.
From the tool's definition Tool name 'chat_append_stream' indicates appending to chat messages; server description states it provides access to 'messages' among other Slack data; 'append' suggests modifying/adding content to existing messages or streams.
Attacks that exploit this kind of access
The rule that runs chat_append_stream safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Slack, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For chat_append_stream, this is the rule to start with:
chat_append_stream 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.
The button opens the PolicyLayer dashboard: create your workspace, connect Slack, apply this rule, and every chat_append_stream call is checked against it from then on.
Questions about chat_append_stream
chat_append_stream is a write tool on the Slack MCP server. It is categorised as a Write tool in the Slack MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Slack MCP server in PolicyLayer and add a rule for chat_append_stream: 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 Slack. Nothing to install.
chat_append_stream 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 chat_append_stream 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 chat_append_stream. 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.
chat_append_stream is provided by the Slack MCP server (karbassi/slack-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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