Redis session store: Session struct contains non-serializable fields (PIDs, atoms)

[emilwojtaszek]

Problem

When using the Redis session store, sessions fail to serialize because Anubis.Server.Session contains non-serializable fields:

** (Protocol.UndefinedError) protocol Jason.Encoder not implemented for #PID<0.123.0>

The name field in the Session struct can be:

• A PID (e.g., #PID<0.123.0>)
• An atom (e.g., :my_server)
• A tuple (e.g., {:via, Registry, {MyRegistry, "key"}})

These are valid GenServer.name() types but cannot be JSON-encoded.

Current Workaround

I had to implement a custom Jason.Encoder for Anubis.Server.Session:

defimpl Jason.Encoder, for: Anubis.Server.Session do
  def encode(session, opts) do
    %{
      id: session.id,
      protocol_version: session.protocol_version,
      initialized: session.initialized,
      name: nil,  # PIDs can't be deserialized anyway
      client_info: session.client_info,
      client_capabilities: session.client_capabilities,
      log_level: session.log_level,
      pending_requests: session.pending_requests
    }
    |> Jason.Encode.map(opts)
  end
end

This works because:

1. PIDs/atoms can't be meaningfully restored after a restart anyway
2. The name is reset when the session is restored (see session.ex restore logic)

Questions

1. Should Anubis.Server.Session implement Jason.Encoder natively in the library?
2. Or should the Redis store handle serialization differently (e.g., using :erlang.term_to_binary instead of JSON)?
3. Is there a better approach I'm missing?

Environment

• anubis_mcp: 0.16.0
• Redis store: Upstash (with SSL via PR feat(redis): add redix_opts for SSL/TLS support #59)
• Elixir: 1.15+

[zoedsoupe]

interesting, this is a bug actually since on storing session data we should remove both name/pids and terms that are dynamic to the BEAM runtime.

and yes, it would be more robust if we implement a encode/decode protocol for the structure, however for JSON specifically this carries more complexity since we ideally should (i dunno?) support the main JSON libraries (Jason, Poison and the native JSON module), which is out of the scope of the library since we only use the native JSON module. anubis-mcp is to be used with elixir 1.18+

[zoedsoupe]

so what i need to review the session store implementation and check:

1. if we're correctly filtering/removing all the non serializable terms from session before storing it
2. propose a better way/more robust way to solve this issue

if you can, it would be nice to have a PR for that