Elixir New Added 2 October 2026

Ecto: (Ecto.Query.CastError) value "abc" in where cannot be cast to type :id

A value went into a query uncast, usually a path or query parameter straight from Phoenix, and it does not match the field's type. Ecto raises before the database ever sees the query. It is a 500 in production when it should have been a 404 or 400, so the fix is to validate at the edge, not to rescue the exception.

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

# Cast at the edge and handle the failure
case Ecto.Type.cast(:id, params["id"]) do
  {:ok, id} -> Repo.get(User, id)
  :error -> {:error, :not_found}
end

# UUID primary keys
case Ecto.UUID.cast(params["id"]) do
  {:ok, uuid} -> Repo.get(User, uuid)
  :error -> {:error, :not_found}
end

# Phoenix: let FallbackController turn {:error, :not_found} into a 404

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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This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.