Elixir New Added 2 October 2026

Elixir: protocol Jason.Encoder not implemented for %MyApp.User{}

Jason will encode maps but not structs it has not been told about, because a struct could contain secrets or other private fields. The surprise is that a plain map encodes fine. The struct only turns up when a Repo result or an association ends up in a json/2 call. Elixir 1.18 has a built-in JSON module, and it needs @derive JSON.Encoder in the same way.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Choose the fields deliberately on the schema
defmodule MyApp.User do
  use Ecto.Schema
  @derive {Jason.Encoder, only: [:id, :name, :email]}
  schema "users" do
    field :name, :string
    field :email, :string
    field :password_hash, :string
  end
end

# Elixir 1.18+ with the built-in JSON module
# @derive {JSON.Encoder, only: [:id, :name]}

# Or render an explicit map in the JSON view
def show(%{user: u}), do: %{id: u.id, name: u.name}

How to diagnose Elixir errors

Elixir errors are shaped by the actor model: a GenServer.call timeout does not mean the server crashed, it means the server was busy for longer than the caller was willing to wait. The right question is usually "what is that process doing?" rather than "why did the call fail?". Because supervisors restart failed processes automatically, transient errors can also hide in the logs while the system appears healthy.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Attach to a running node with iex --remsh and inspect the process: Process.info(pid, :message_queue_len). A growing mailbox means the process is the bottleneck.
Use :observer.start() to see the supervision tree, process memory and message queues visually.
Move long-running work out of handle_call. Use handle_cast, a Task, or a dedicated pool so the GenServer stays responsive.
Check restart intensity. A supervisor that exceeds max_restarts takes down its own supervisor, producing a cascade that looks like an unrelated failure at the top.
For compile errors in dependencies, run mix deps.compile --force and check that any required native toolchain (make, gcc, erlang headers) is installed.

Tools worth reaching for

:observer.start()
iex --remsh
Process.info/2
mix deps.tree
:recon

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

hexdocs.pm hexdocs.pm

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