Codex App Markdown renders ::Word as a header

What version of the Codex App are you using (From “About Codex” dialog)?

Version 26.513.31313 (2867)

What subscription do you have?

Business

What platform is your computer?

Darwin 23.6.0 arm64 arm

What issue are you seeing?

When the model output contains ::[A-Za-z]+ it is rendered in Markdown as a header (# ::[A-Za-z]+). This means classes like Net::HTTP get rendered as Net \n # ::HTTP, making it difficult to understand the response:

<img width="793" height="557" alt="Image" src="https://github.com/user-attachments/assets/397410c7-538c-4a19-8df1-09b417321dbb" />

This doesn't happen when the output is :Word or :::Word, as far as I've noticed it's only double colon.

This makes code-related conversations impossible to read, having to swap to TUI. The Codex TUI (codex-cli 0.132.0) is unaffected:

<img width="525" height="316" alt="Image" src="https://github.com/user-attachments/assets/66413a06-3632-441e-8f07-c46315336098" />

What steps can reproduce the bug?

Start a new chat with this prompt:

Return the following strings as-is:

Use `Net::HTTP` for HTTP requests and `File` for reading files

This is another line

Use Net::HTTP for HTTP requests and File for reading files

This is another line

What is the expected behavior?

The "Net::HTTP" string is not converted into a header. The markdown parser work the same as, for example, Github's own markdown parser or the observed codex-cli output:

Use Net::HTTP for HTTP requests and File for reading files
Use Net::HTTP for HTTP requests and File for reading files

Additional information

_No response_