Small CLAUDE.md improvements

Author: cpsievert

CLAUDE.md

@@ -164,4 +164,12 @@ When implementing a new LLM provider, follow this systematic approach:
 
 ## Connections to ellmer
 
-This project is the Python equivalent of the R package ellmer. The source code for ellmer is available in a sibling directory to this project. Before implementing new features or bug fixes in chatlas, it may be useful to consult  the ellmer codebase to: (1) check whether the feature/fix already exists on the R side and (2) make sure the projects are aligned in terms of stylistic approaches. Note also that ellmer itself has a CLAUDE.md file which has a useful overview of the project. 
+This project is the Python equivalent of the R package ellmer. The source code for ellmer is available in a sibling directory to this project. Before implementing new features or bug fixes in chatlas, it may be useful to consult  the ellmer codebase to: (1) check whether the feature/fix already exists on the R side and (2) make sure the projects are aligned in terms of stylistic approaches. Note also that ellmer itself has a CLAUDE.md file which has a useful overview of the project. 
+
+## Differences from ellmer
+
+One important difference to note is that in `ellmer::chat_openai()` uses the completions API, while `chatlas.ChatOpenAI()` uses the responses API. Look to `ellmer::chat_openai_responses()` and `chatlas.ChatOpenAICompletions()` for the "non-default" API.
+
+## Access to gh CLI
+
+If ellmer or chatlas issues or PRs are referenced, try using the `gh` CLI tool to gain necessary context. They live at `tidyverse/ellmer` and `posit-dev/chatlas`