broadcast_flex_message

THE RISK

Medium Risk

Why broadcast_flex_message needs a policy

This tool creates and sends messages to an audience (all followers of the LINE account), which is a reversible write operation. It's not Execute because it doesn't run arbitrary code or trigger external operations based on arguments—it sends a predefined message. It's not Destructive because messages can be managed.

From the tool's definition Tool broadcasts a flex message to all users via LINE Official Account. The verb 'broadcast' and description 'via LINE to all users' indicate message creation and delivery to a potentially large audience.

Risk signalsBulk/mass operation — affects multiple targets

Documented attack patterns abuse exactly the kind of access broadcast_flex_message gives an agent:

POLICY

How to control broadcast_flex_message

PolicyLayer is an MCP gateway — it sits between your AI agents and Line Bot, and nothing reaches the server without passing your rules. This is the rule we recommend for broadcast_flex_message:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "broadcast_flex_message": {
      "limits": [
        {
          "counter": "broadcast_flex_message_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

broadcast_flex_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.

Create a free account and register Line Bot — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

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EXPLORE

Related tools and policies

More Line Bot tools

Destructive cancel_rich_menu_default Cancel the default rich menu. Destructive delete_rich_menu Delete a rich menu from your LINE Official Account. Write broadcast_text_message Broadcast a simple text message via LINE to all users who have followed your LINE Official Write create_rich_menu Create a rich menu based on the given actions. Generate and upload a rich menu image based Write push_flex_message Push a highly customizable flex message to a user via LINE. Supports both bubble (single c Write push_text_message Push a simple text message to a user via LINE. Use this for sending plain text messages wi Write set_rich_menu_default Set a rich menu as the default rich menu. Read get_follower_ids Get a list of user IDs of users who have added the LINE Official Account as a friend. This

All 12 Line Bot tools →

Write tools on other servers

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about broadcast_flex_message

What does the broadcast_flex_message tool do? +

Broadcast a highly customizable flex message via LINE to all users who have added your LINE Official Account. It is categorised as a Write tool in the Line Bot MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on broadcast_flex_message? +

Register the Line Bot MCP server in PolicyLayer and add a rule for broadcast_flex_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 Line Bot. Nothing to install.

What risk level is broadcast_flex_message? +

broadcast_flex_message is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit broadcast_flex_message? +

Yes. Add a rate_limit block to the broadcast_flex_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.

How do I block broadcast_flex_message completely? +

Set action: deny in the PolicyLayer policy for broadcast_flex_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.

What MCP server provides broadcast_flex_message? +

broadcast_flex_message is provided by the Line Bot MCP server (@line/line-bot-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Line Bot tool call.

Start from Line Bot, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

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