Expert won't start in Zed: Hex.State was given as a child to a supervisor but it does not exist

[cblavier]

Hey there, I can no longer start expert from Zed editor on my system (whatever the project)
It used to work perfectly for weeks and I don't know what environment change is messing with the LSP.

The error:

10:39:17.161 [notice] Application hex exited: exited in: Hex.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Hex.State was given as a child to a supervisor but it does not exist

My environment is :

expert_erts-15.2.7_0.1.0-rc.2
erlang 28.2
elixir 1.19.4-otp-28

The full stacktrace from Zed LSP logs:

[Project file:///Users/cblavier/code/phenixdigital/phenix_displays] Using path: /Users/cblavier/Library/Application Support/.burrito/expert_erts-15.2.7_0.1.0-rc.2-ba10ae8/erts-15.2.7/bin:/Users/cblavier/Library/Application Support/.burrito/expert_erts-15.2.7_0.1.0-rc.2-ba10ae8/bin:/Users/cblavier/go/bin:/opt/homebrew/opt/mysql-client/bin:/Applications/Docker.app/Contents/Resources/bin/:/Applications/Postgres.app/Contents/Versions/18/bin:/Users/cblavier/.cargo/bin:/Users/cblavier/.local/bin:/Users/cblavier/bin:/Users/cblavier/.local/share/mise/installs/erlang/28.2/bin:/Users/cblavier/.local/share/mise/installs/elixir/1.19.4-otp-28/bin:/Users/cblavier/.local/share/mise/installs/elixir/1.19.4-otp-28/.mix/escripts:/Users/cblavier/.local/share/mise/installs/node/24.13.0/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/opt/pmk/env/global/bin
[Project file:///Users/cblavier/code/phenixdigital/phenix_displays] Found elixir executable at /Users/cblavier/.local/share/mise/installs/elixir/1.19.4-otp-28/bin/elixir
[Project file:///Users/cblavier/code/phenixdigital/phenix_displays] Found erl executable at /Users/cblavier/.local/share/mise/installs/erlang/28.2/bin/erl
[Project file:///Users/cblavier/code/phenixdigital/phenix_displays] Finding or building engine
sent request server -> client window/workDoneProgress/create
sent notification server -> client $/progress
Building engine: 
10:39:17.155 [error] beam/beam_load.c(594): Error loading function 'Elixir.Hex.State':print_invalid_config_error/2: op bs_add p x x u x:
  please re-compile this module with an Erlang/OTP 28 compiler or update your Erlang/OTP version
Building engine: 
10:39:17.158 [error] beam/beam_load.c(594): Error loading function 'Elixir.Hex.State':print_invalid_config_error/2: op bs_add p x x u x:
  please re-compile this module with an Erlang/OTP 28 compiler or update your Erlang/OTP version
Building engine: 
10:39:17.161 [notice] Application hex exited: exited in: Hex.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Hex.State was given as a child to a supervisor but it does not exist
            (elixir 1.19.4) lib/supervisor.ex:814: Supervisor.init_child/1
            (elixir 1.19.4) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.19.4) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.19.4) lib/supervisor.ex:800: Supervisor.init/2
            (elixir 1.19.4) lib/supervisor.ex:724: Supervisor.start_link/2
            (kernel 10.4.2) application_master.erl:299: :application_master.start_it_old/4
Building engine: 
10:39:17.163 [notice] Application inets exited: :stopped
Building engine: 
10:39:17.164 [notice] Application ssl exited: :stopped
Building engine: 
10:39:17.164 [notice] Application public_key exited: :stopped

10:39:17.164 [notice] Application asn1 exited: :stopped

10:39:17.164 [notice] Application crypto exited: :stopped
Building engine: ==> engine
Building engine: Could not start Hex. Try fetching a new version with "mix local.hex" or uninstalling it with "mix archive.uninstall hex.ez"
Building engine: ** (MatchError) no match of right hand side value:

    {:error,
     {:hex,
      {:bad_return,
       {{Hex.Application, :start, [:normal, []]},
        {:EXIT,
         {%ArgumentError{
            message: "The module Hex.State was given as a child to a supervisor but it does not exist"
          },
          [
            {Supervisor, :init_child, 1, [file: ~c"lib/supervisor.ex", line: 814]},
            {Enum, :"-map/2-lists^map/1-1-", 2, [file: ~c"lib/enum.ex", line: 1688]},
            {Enum, :"-map/2-lists^map/1-1-", 2, [file: ~c"lib/enum.ex", line: 1688]},
            {Supervisor, :init, 2, [file: ~c"lib/supervisor.ex", line: 800]},
            {Supervisor, :start_link, 2, [file: ~c"lib/supervisor.ex", line: 724]},
            {:application_master, :start_it_old, 4,
             [file: ~c"application_master.erl", line: 299]}
          ]}}}}}}

    (hex 2.3.1) lib/hex.ex:5: Hex.start/0
    (mix 1.19.4) lib/mix/hex.ex:64: Mix.Hex.start/0
    (mix 1.19.4) lib/mix/dep/loader.ex:193: Mix.Dep.Loader.with_scm_and_app/5
    (mix 1.19.4) lib/mix/dep/loader.ex:145: Mix.Dep.Loader.to_dep/4
    (elixir 1.19.4) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2
    (mix 1.19.4) lib/mix/dep/loader.ex:368: Mix.Dep.Loader.mix_children/3
    (mix 1.19.4) lib/mix/dep/loader.ex:321: anonymous fn/5 in Mix.Dep.Loader.mix_dep/3
    (mix 1.19.4) lib/mix/project.ex:557: Mix.Project.in_project/4

Thanks for your help!

[mhanberg]

We are actively working to future proof this, but for now can you try running mix local.hex --force in that project? I believe that its using a globally installed hex (which is what happens when you do mix local.hex typically and dont mess with MIX_HOME environment variable.

[cblavier]

This command did not solve the issue.

However I was able to fix it by upgrading my current project's elixir and erlang versions from 1.19.4 to 1.19.5 and 28.2 to 28.3.1 which are the versions installed globally on the system (not homebrew but mise --global)

So there may be a kind of environment leakage between system and project tools?

[urcadox]

I have the same issue and no amount of reinstalling elixir, erlang, expert, or hex has changed anything.

My logs look very similar to my eyes, but just in case here they are:

2026-02-26 11:56:10.895 [info] Expert language-client is already running. Restarting.
2026-02-26 11:56:10.900 [info] [Error - 11:56:10 AM] handling server specific request XPGenLSP.Requests.Shutdown
2026-02-26 11:56:10.900 [info] [Info  - 11:56:10 AM] Shutting down
2026-02-26 11:56:10.901 [info] handled request client -> server shutdown in 3ms
2026-02-26 11:56:11.000 [info] Could not attach default Logger handler: {:handler_not_added, {:invalid_config, :logger_std_h, %{formatter: {Logger.Formatter, %Logger.Formatter{template: ["\n", :time, " ", :metadata, "[", :level, "] ", :message, "\n"], truncate: 8096, metadata: [:instance_id], colors: %{error: :red, enabled: false, info: :normal, warning: :yellow, debug: :cyan, notice: :normal, alert: :red, critical: :red, emergency: :red}, utc_log?: false}}}}}

2026-02-26 11:56:11.077 [info] handled request client -> server initialize in 37ms
2026-02-26 11:56:11.077 [info] [Info  - 11:56:11 AM] Server initialized, registering capabilities
2026-02-26 11:56:11.078 [info] sent request server -> client client/registerCapability
2026-02-26 11:56:11.081 [info] [Info  - 11:56:11 AM] [Project file:///home/urcadox/dev/phenix_marvel] Starting project
2026-02-26 11:56:11.081 [info] handled notification client -> server initialized in 4ms
2026-02-26 11:56:11.084 [info] sent request server -> client window/workDoneProgress/create
2026-02-26 11:56:11.085 [info] sent notification server -> client $/progress
2026-02-26 11:56:11.247 [info] [Info  - 11:56:11 AM] [Project file:///home/urcadox/dev/phenix_marvel] Using path: /home/urcadox/.local/share/.burrito/expert_erts-15.2.7_0.1.0-rc.2/erts-15.2.7/bin:/home/urcadox/.local/share/.burrito/expert_erts-15.2.7_0.1.0-rc.2/bin:/home/urcadox/.local/share/mise/installs/erlang/28.3.1/bin:/home/urcadox/.local/share/mise/installs/elixir/1.19.5-otp-28/bin:/home/urcadox/.local/share/mise/installs/elixir/1.19.5-otp-28/.mix/escripts:/home/urcadox/.local/share/mise/installs/node/24.13.0/bin:/home/urcadox/.local/share/mise/installs/pipx-mistral-vibe/2.2.1/bin:/home/urcadox/.local/share/mise/installs/python/3.13.3/bin:/home/urcadox/.local/share/mise/installs/usage/2.0.3:/usr/bin:/usr/local/sbin:/usr/local/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/home/urcadox/go/bin/:/home/urcadox/.bin/
2026-02-26 11:56:11.247 [info] [Info  - 11:56:11 AM] [Project file:///home/urcadox/dev/phenix_marvel] Found elixir executable at /home/urcadox/.local/share/mise/installs/elixir/1.19.5-otp-28/bin/elixir
2026-02-26 11:56:11.247 [info] [Info  - 11:56:11 AM] [Project file:///home/urcadox/dev/phenix_marvel] Found erl executable at /home/urcadox/.local/share/mise/installs/erlang/28.3.1/bin/erl
2026-02-26 11:56:11.247 [info] [Info  - 11:56:11 AM] [Project file:///home/urcadox/dev/phenix_marvel] Finding or building engine
2026-02-26 11:56:11.247 [info] sent request server -> client window/workDoneProgress/create
2026-02-26 11:56:11.248 [info] sent notification server -> client $/progress
2026-02-26 11:56:11.607 [info] Building engine: 
11:56:11.599 [error] beam/beam_load.c(596): Error loading function 'Elixir.Hex.State':print_invalid_config_error/2: op bs_add p x x u x:
  please re-compile this module with an Erlang/OTP 28 compiler or update your Erlang/OTP version
2026-02-26 11:56:11.607 [info] Building engine: 
11:56:11.604 [error] beam/beam_load.c(596): Error loading function 'Elixir.Hex.State':print_invalid_config_error/2: op bs_add p x x u x:
  please re-compile this module with an Erlang/OTP 28 compiler or update your Erlang/OTP version
2026-02-26 11:56:11.613 [info] Building engine: 
11:56:11.610 [notice] Application hex exited: exited in: Hex.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Hex.State was given as a child to a supervisor but it does not exist
            (elixir 1.19.5) lib/supervisor.ex:814: Supervisor.init_child/1
            (elixir 1.19.5) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.19.5) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.19.5) lib/supervisor.ex:800: Supervisor.init/2
            (elixir 1.19.5) lib/supervisor.ex:724: Supervisor.start_link/2
            (kernel 10.5) application_master.erl:299: :application_master.start_it_old/4
2026-02-26 11:56:11.614 [info] Building engine: 
11:56:11.613 [notice] Application inets exited: :stopped
2026-02-26 11:56:11.615 [info] Building engine: 
11:56:11.614 [notice] Application ssl exited: :stopped
2026-02-26 11:56:11.615 [info] Building engine: 
11:56:11.614 [notice] Application public_key exited: :stopped
2026-02-26 11:56:11.616 [info] Building engine: 
11:56:11.614 [notice] Application asn1 exited: :stopped
2026-02-26 11:56:11.616 [info] Building engine: 
11:56:11.614 [notice] Application crypto exited: :stopped
2026-02-26 11:56:11.616 [info] Building engine: ==> engine
2026-02-26 11:56:11.616 [info] Building engine: Could not start Hex. Try fetching a new version with "mix local.hex" or uninstalling it with "mix archive.uninstall hex.ez"
2026-02-26 11:56:11.621 [info] Building engine: ** (MatchError) no match of right hand side value:

    {:error,
     {:hex,
      {:bad_return,
       {{Hex.Application, :start, [:normal, []]},
        {:EXIT,
         {%ArgumentError{
            message: "The module Hex.State was given as a child to a supervisor but it does not exist"
          },
          [
            {Supervisor, :init_child, 1, [file: ~c"lib/supervisor.ex", line: 814]},
            {Enum, :"-map/2-lists^map/1-1-", 2, [file: ~c"lib/enum.ex", line: 1688]},
            {Enum, :"-map/2-lists^map/1-1-", 2, [file: ~c"lib/enum.ex", line: 1688]},
            {Supervisor, :init, 2, [file: ~c"lib/supervisor.ex", line: 800]},
            {Supervisor, :start_link, 2, [file: ~c"lib/supervisor.ex", line: 724]},
            {:application_master, :start_it_old, 4,
             [file: ~c"application_master.erl", line: 299]}
          ]}}}}}}

I'm using Expert from VS Code, here are some details:

• Expert extension 0.3.1 (nightly disabled, so it's using the latest RC)
• Elixir 1.19.5-otp-28
• Erlang 28.3.1

Thanks for your work and your help!

[doorgan]

Can you try with the latest rc? This should be fixed now

[spicychickensauce]

I had the exact same problem with rc.2.

With rc.3, it does get further, but it still seems to get stuck. Now it seems to get stuck at "Preparing engine" and "Starting engine node"

After restarting zed, this is my log now:

handled request client -> server initialize in 80ms
Server initialized, registering capabilities
sent request server -> client client/registerCapability
[app] Starting project
handled notification client -> server initialized in 11ms
handled notification client -> server workspace/didChangeConfiguration in 4ms
sent request server -> client window/workDoneProgress/create
Opened file:///home/user/Code/elixir/app/lib/app/knowledge/opening.ex
handled notification client -> server textDocument/didOpen in 4ms
sent notification server -> client $/progress
sent request server -> client window/workDoneProgress/create
sent notification server -> client $/progress
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
[app] Using path: /home/user/.local/share/.burrito/expert_erts-15.2.7_0.1.0-rc.3/erts-15.2.7/bin:/home/user/.local/share/.burrito/expert_erts-15.2.7_0.1.0-rc.3/bin:/home/user/AUR_builds/git-subrepo/lib:/home/user/.mix/escripts:/home/user/.npm-global/bin:/home/user/.local/share/mise/installs/elixir/1.19.5-otp-28/bin:/home/user/.local/share/mise/installs/elixir/1.19.5-otp-28/.mix/escripts:/home/user/.local/share/mise/installs/deno/2.6.3/bin:/home/user/.local/share/mise/installs/deno/2.6.3/.deno/bin:/home/user/.local/share/mise/installs/erlang/28.3/bin:/home/user/.local/share/mise/installs/node/24.9.0/bin:/home/user/.local/share/mise/installs/python/3.11.14/bin:/home/user/.cargo/bin:/home/user/.local/share/mise/installs/usage/2.3.2:/home/user/.local/share/mise/installs/uv/0.9.8/uv-x86_64-unknown-linux-musl:/home/user/bin:/usr/bin:/home/user/bin:/home/user/.volta/bin:/home/user/.volta/bin:/home/user/bin:/home/user/.volta/bin:/usr/local/bin:/usr/local/sbin:/var/lib/flatpak/exports/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/usr/lib/rustup/bin
[app] Found elixir executable at /home/user/.local/share/mise/installs/elixir/1.19.5-otp-28/bin/elixir
[app] Found erl executable at /home/user/.local/share/mise/installs/erlang/28.3/bin/erl
[app] Preparing engine
Received request textDocument/codeAction before engine for app was initialized. Ignoring.
handled request client -> server textDocument/codeAction in 23ms
Received request textDocument/codeLens before engine for app was initialized. Ignoring.
handled request client -> server textDocument/codeLens in 2ms
[app] Engine build output: Namespacing 12 files:
  New: 4
  Changed: 8
  Deleted: 4
[app] Engine build output: Rewriting 0 app files
[app] Engine build output: Rewriting .beam files
[app] Engine build output:  Found 0 protocols
 Found 0 app beam files
Finished namespacing .beam files
[app] Engine build output: Rewriting 0 scripts
[app] Engine build output: Rebuilding 0 boot files
[app] Engine build output: Rewriting 0 config scripts.

On the first time with the new rc, I also saw that it downloaded and compiled elixir-sense, which id didn't do with rc.2 where it got stuck earlier

[doorgan]

Does it get stuck and not get you any more logs?
Can you run expert engine clean --force? I suspect you might have some broken engine build from the previous broken runs

[urcadox]

With the new rc I'm in the same boat as @spicychickensauce.
It's stuck after Engine build output: Rewriting 0 config scripts; the status bar still says "Preparing engine".

I'm not sure how I'm supposed to run expert engine clean --force? Where should I find the expert binary?

I have tried closing VS Code, removing ~/.local/share/Expert/, ~/.local/share/.burrito/expert_erts-15.2.7_0.1.0-rc.3/ and .expert in my project, then restarting VS Code. After a little while, it's getting stuck again after logging Engine build output: Rewriting 0 config scripts.

[spicychickensauce]

Exact same as @urcadox, I didn't know where the expert binary is as it's managed via zed. Then I did rm -rf .expert/ && rm -rf ~/.local/share/Expert/ and restarted and now it's stuck again.
The process is also not using any CPU, it seems to be waiting for something

[mhanberg]

I'm not sure how I'm supposed to run expert engine clean --force? Where should I find the expert binary?

The vscode extension should print the location of the binary it downloaded, you can use that. We are still working out a way to run those from the command palette

[mhanberg]

@doorgan i find this line curious

app] Engine build output: Namespacing 12 files:
New: 4
Changed: 8
Deleted: 4

It should basically always namespace every single file, presumably because they are all new for a versioned release (not building locally).

[mhanberg]

Also potentially maybe hitting what #370 aims to fix.

[urcadox]

The vscode extension should print the location of the binary it downloaded, you can use that. We are still working out a way to run those from the command palette

Oh right.

❯ /home/urcadox/.config/Code\ -\ OSS/User/globalStorage/expertlsp.expert/expert_linux_amd64 engine clean --force
Deleted /home/urcadox/.local/share/Expert/0.1.0-rc.3/ex-1.19.5-erl-16.2
Deleted /home/urcadox/.local/share/Expert/0.1.0-rc.3/tooling

It does not fix the issue sadly.

By the way, I also noticed that I had multiple beam processes hanging around, I closed VS Code, cleaned everything again, killed all these processes, tried again, but no luck.

Here is the complete log when starting from scratch: expert.log

[doorgan]

I also wonder if the engine builder process it may be receiving a line like Rewriting config files\nengine_path:/...\n
This:

[app] Engine build output: Namespacing 12 files:
New: 4
Changed: 8
Deleted: 4

shows we're not receiving lines cleanly cut at \n so I wonder if we should split on newlines to get the paths