Phoenix Channels: Prevent auto-subscription to raw client topic

TL;DR

Clients join a simple topic like "daily_agent_stats:total_inbound_calls".
On the server I expand it using workspace_id + agent_id from params.
But Phoenix still subscribes them to the raw topic too → I want to stop that and only keep the expanded subscription.

Hey folks

I’m building a Phoenix channel for agent analytics stats.
Here’s a simplified version of my use case:

What I want

• Clients connect with workspace_id and agent_id as query params.

• Then they only subscribe with a simplified topic (no need to repeat IDs).

• On the server, I expand that simplified topic into the full PubSub key and subscribe internally.

Example

Channel:

defmodule MyAppWeb.AgentChannel do
  use Phoenix.Channel

  # Client joins something like: "daily_agent_stats:total_inbound_calls"
  def join("daily_agent_stats:" <> stat_key, _payload, socket) do
    workspace_id = socket.assigns[:workspace_id]
    agent_id = socket.assigns[:agent_id]

    # Expand topic internally
    full_topic = "agent/analytics/daily_agent_stats:#{workspace_id}:#{agent_id}:#{stat_key}"

    IO.puts("Subscribing to #{full_topic}")
    Phoenix.PubSub.subscribe(MyApp.PubSub, full_topic)

    {:ok, socket}
  end

  def join(topic, _payload, _socket) do
    {:error, %{reason: "Invalid topic #{topic}"}}
  end
end

UserSocket:

defmodule MyAppWeb.UserSocket do
  use Phoenix.Socket

  channel "daily_agent_stats:*", MyAppWeb.AgentChannel

  def connect(%{"workspace_id" => ws_id, "agent_id" => agent_id}, socket, _connect_info) do
    {:ok, assign(socket, :workspace_id, ws_id) |> assign(:agent_id, agent_id)}
  end

  def id(_socket), do: nil
end

Client connection (via wscat):

wscat -c "ws://localhost:4000/v1/agent/websocket?workspace_id=c8f4a3a0-835d-4ee2-9c71-0c18550680e5&agent_id=47dbbab6-1abf-4392-a42f-dbde107c2793"

Join a topic without IDs:

{"topic":"daily_agent_stats:total_inbound_calls","event":"phx_join","payload":{},"ref":1}

The Issue:

Even though I subscribe manually inside join/3, Phoenix still also assigns the socket to the original "daily_agent_stats:total_inbound_calls" topic.

So the socket ends up listening on two topics:

• "daily_agent_stats:total_inbound_calls"

• "agent/analytics/daily_agent_stats:<workspace_id>:<agent_id>:total_inbound_calls"

I want to disable the first (auto) subscription and only rely on the expanded one.

My Question:
Is there a way to tell Phoenix not to subscribe the socket to the raw topic passed in by the client, so I can fully control subscriptions in join/3?

Any guidance on best practices here would be super helpful.