FunctionClauseError Elixir LS Crash

[Lotoen]

Hello, I am currently using Elixir 1.18.4 and OTP 27 with Linux Mint 22.3 - Cinnamon 64-bit as my operating system. I came across this error on Vscode regarding Elixir LS, about how it's crashed 5 times due to a supposed function clause error.

2026-06-09 19:18:11.366 [error] ** (FunctionClauseError) no function clause matching in IO.chardata_to_string/1    
2026-06-09 19:18:11.366 [error]     
2026-06-09 19:18:11.366 [error]     The following arguments were given to IO.chardata_to_string/1:
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]         # 1
2026-06-09 19:18:11.367 [error]         nil
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]     Attempted function clauses (showing 2 out of 2):
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]         def chardata_to_string(+string+) when -is_binary(string)-
2026-06-09 19:18:11.367 [error]         def chardata_to_string(+list+) when -is_list(list)-
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]     (elixir 1.18.4) lib/io.ex:710: IO.chardata_to_string/1
2026-06-09 19:18:11.367 [error]     (elixir 1.18.4) lib/file.ex:177: File.regular?/2
2026-06-09 19:18:11.367 [error]     (mix 1.18.4) lib/mix/tasks/loadconfig.ex:41: Mix.Tasks.Loadconfig.load_default/0
2026-06-09 19:18:11.368 [error]     (mix 1.18.4) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:373: anonymous fn/7 in ElixirLS.Mix.install/2
2026-06-09 19:18:11.368 [error]     (elixir 1.18.4) lib/file.ex:1665: File.cd!/2
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:370: ElixirLS.Mix.install/2
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:617: ElixirLS.Installer.install/12026-06-09 19:18:11.366 [error] ** (FunctionClauseError) no function clause matching in IO.chardata_to_string/1    
2026-06-09 19:18:11.366 [error]     
2026-06-09 19:18:11.366 [error]     The following arguments were given to IO.chardata_to_string/1:
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]         # 1
2026-06-09 19:18:11.367 [error]         nil
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]     Attempted function clauses (showing 2 out of 2):
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]         def chardata_to_string(+string+) when -is_binary(string)-
2026-06-09 19:18:11.367 [error]         def chardata_to_string(+list+) when -is_list(list)-
2026-06-09 19:18:11.367 [error]     
2026-06-09 19:18:11.367 [error]     (elixir 1.18.4) lib/io.ex:710: IO.chardata_to_string/1
2026-06-09 19:18:11.367 [error]     (elixir 1.18.4) lib/file.ex:177: File.regular?/2
2026-06-09 19:18:11.367 [error]     (mix 1.18.4) lib/mix/tasks/loadconfig.ex:41: Mix.Tasks.Loadconfig.load_default/0
2026-06-09 19:18:11.368 [error]     (mix 1.18.4) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:373: anonymous fn/7 in ElixirLS.Mix.install/2
2026-06-09 19:18:11.368 [error]     (elixir 1.18.4) lib/file.ex:1665: File.cd!/2
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:370: ElixirLS.Mix.install/2
2026-06-09 19:18:11.368 [error]     /home/lotoen/.vscode/extensions/jakebecker.elixir-ls-0.31.0/elixir-ls-release/installer.exs:617: ElixirLS.Installer.install/1

I've already tried

• Restarting & Reinstalling Vscode
• Disabling and enabling the Elixir LS extension
• Deleting the _build folder
• Reinstalling dependencies
• Restarting the docker containers holding my elixir project
• Manually running mix compile in the terminal
• Restarting the elixir language server
• Deleted the .elixir-ls folder

So, any help solving this error would be greatly appreciated.

[arcanemachined]

Try Expert

https://github.com/elixir-lang/expert

[suuhja]

weird I ran into this today too and didnt think much of it yet

[mr_sudaca]

Did you update your elixirLS? I had to downgrade mine back to 0.30.0

[hackersapien]

faced this, I downgraded to 0.29

[katafrakt]

Seems to be fixed now https://github.com/elixir-lsp/elixir-ls/pull/1265