push_flex_message

THE RISK

Medium Risk

Why push_flex_message needs a policy

This tool creates and sends messages to LINE users, which is a Write operation (creates data with reversible side effects—messages can be deleted/unsent by users or account owner). It is not Read (no data retrieval), Execute (no code execution or triggering external operations), Destructive (messages are not irreversibly deleted), or Financial (no money movement).

From the tool's definition Tool name 'push_flex_message' and description 'Push a highly customizable flex message to a user via LINE' indicate creation/transmission of message content. The verb 'push' combined with message delivery to users represents creating and sending data.

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

POLICY

How to control push_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 push_flex_message:

policy.json

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

push_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.

LIMIT THIS TOOL →

Free to start. No card required.

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_flex_message Broadcast a highly customizable flex message via LINE to all users who have added your LIN 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_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 push_flex_message

What does the push_flex_message tool do? +

Push a highly customizable flex message to a user via LINE. Supports both bubble (single container) and carousel. 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 push_flex_message? +

Register the Line Bot MCP server in PolicyLayer and add a rule for push_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 push_flex_message? +

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

Can I rate-limit push_flex_message? +

Yes. Add a rate_limit block to the push_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 push_flex_message completely? +

Set action: deny in the PolicyLayer policy for push_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 push_flex_message? +

push_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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