Slack: Deny Channel Creation

What this policy does

Direction: ingress (tool_pre_invoke) Default: allow, with a targeted deny on channel-creation calls Package: slack.ingress.deny_channel_create

What it does

Blocks Slack channel-creation tool calls at ingress. Every other Slack tool — and every non-Slack tool — passes through untouched. A blocked call returns a clear denial reason instead of creating a channel.

Why ingress

Creating a channel is a write with a permanent side effect on the workspace. The violation is fully determined by the request (the tool name alone), so denying at ingress prevents the channel from ever being created.

How it matches

Two conditions must both hold for a call to be denied:

• Slack-server scoping. The first hyphen-separated segment of the tool name starts with slack. This matches slack-..., slack-prod-..., slack-mcp-..., etc., so the policy keeps working regardless of how the Slack MCP server is named on a given gateway.
• Create-channel detection. The (lowercased) tool name ends with one of the known create-channel suffixes. Both common naming families are covered:
  • verb-first shapes: -create-conversation, -create-channel, and their _-separated and concatenated variants;
  • Slack-API-mirroring shapes: -conversations.create, -conversations-create, -conversations_create.

Tool names on the gateway are prefixed with the configured MCP server name and that prefix is not standardized, which is why this matches on suffix rather than an exact name. Confirm the exact tool name your gateway emits with the dump-input debug technique before relying on this in production, and add any missing shape to create_channel_suffixes.

Examples

Denied (channel creation)

{
  "input": {
    "action": "tool_pre_invoke",
    "resource": { "name": "slack-mcp-conversations-create", "type": "tool" },
    "payload": {
      "name": "slack-mcp-conversations-create",
      "args": { "name": "incident-2026-06" }
    }
  }
}

allow = false, reason = "Creating Slack channels is not permitted via this gateway. ...".

Allowed (any other Slack tool)

{
  "input": {
    "action": "tool_pre_invoke",
    "resource": { "name": "slack-mcp-slack-post-message", "type": "tool" },
    "payload": {
      "name": "slack-mcp-slack-post-message",
      "args": { "channel": "C123", "text": "hello" }
    }
  }
}

allow = true, no reason.

Composition

Single-purpose and default allow := true, so it composes cleanly with other Slack ingress policies (e.g. block-secrets) on the same pipeline.

Known limitations

• Suffix list, not an exhaustive catalog. Only the create-channel tool shapes listed in create_channel_suffixes are matched. If a Slack MCP server exposes channel creation under a different tool name, add its suffix.
• No identity-based exemptions. All callers are treated the same. To allow a specific admin/break-glass user to create channels, gate a separate allow if branch on input.subject.claims.

Policy source (Rego)

package slack.ingress.deny_channel_create

# Default-allow: only deny Slack channel-creation calls.
default allow := true

# -----------------------------------------------------------------------------
# Tool matching
# -----------------------------------------------------------------------------

# Slack channel-creation tool names vary by MCP server implementation. Cover
# the common shapes via endswith on the lowercased resource name.
create_channel_suffixes := {
    # Verb-first ("create conversation/channel") — a common Slack MCP shape.
    "-create-conversation",
    "-create_conversation",
    "-createconversation",
    "-create-channel",
    "-createchannel",
    "-create_channel",

    # Slack-API-mirroring shape (conversations.create) — kept for MCP servers
    # that expose the underlying REST path more literally.
    "-conversations-create",
    "-conversations.create",
    "-conversations_create",
}

# Slack-server detection: the tool name's first hyphen-separated segment starts
# with "slack". Matches "slack-...", "slack3-...", "slack-prod-...", etc., so
# the policy keeps working regardless of how the MCP server is named on a
# given gateway.
is_slack_tool if {
    name := lower(input.resource.name)
    server_name := split(name, "-")[0]
    startswith(server_name, "slack")
}

is_create_channel if {
    is_slack_tool
    name := lower(input.resource.name)
    some suffix in create_channel_suffixes
    endswith(name, suffix)
}

# -----------------------------------------------------------------------------
# Deny rule
# -----------------------------------------------------------------------------

allow := false if {
    is_create_channel
}

reason := "Creating Slack channels is not permitted via this gateway. Contact your InfoSec team if this needs to change." if not allow

Canonical source: policy.md on GitHub · raw · raw on this site (.md)

Used in these guides

• Slack hygiene for autonomous AI agents

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