In his ElixirConf EU 2025 presentation [1], Chris McCord demonstrated Phoenix.new [2], an LLM-augmented VS Code in a browser that knows how to build Elixir/Phoenix apps.

McCord began by reviewing the current scripted and templated Phoenix code generator. He then showed how a tool-invoking LLM agent could be implemented in one screen of Elixir code. The bulk of his talk demonstrated generating apps with the Phoenix.new tool. It was interesting to see the tool generate development plan and begin coding to implement the plan.

I was fascinated to hear McCord explain and critique the LLM agent as it generated a Phoenix app.

"[It's] going to come up with a plan for our app and codify that into a Markdown file."

"Now it's going to take our high-level plan and make it into an expanded plan for us and itself."

"You can say, 'I changed the plan; execute the plan.'"

"We just recursed in the server and we did a req post. It's doing a req post right now and the tokens are coming back!"

"The agent just invoked a tool, which is 'I'm going to create a file.'"

"Hopefully it does a 'surgical modify tool,' which makes it not have to modify the whole file."

"A run-time error! Oh, no! Live coding is terrible. Oh, wait. It knows I need to add some missing functions here."

"You would be like, 'Oh, no, the page broke. What happened? I'm going to look at this error.' We just send that thing back to the chat completions endpoint and magic comes out and fixes the error."

"It didn't change a web file. We should fix that."

"This is an issue that humans hit; Steff and I need to fix that."

"It added the input at the bottom. That's not ideal."

"The agent decided to idle here. ... The way I implemented this is everything the agent responds with is a tool call and it has an idle tool and that's what it invokes when it is done."

[1] https://www.youtube.com/watch?v=ojL_VHc4gLk

[2] https://phoenix.new/