[BUG] Code blocks silently drop lines containing certain XML tags (rendering + copy code)

What's Wrong?

Fenced code blocks in the chat silently drop lines containing certain XML tags. The lines are missing both from the on-screen rendering and from the output of the "copy code" button. The underlying content generated by the model is correct — when the assistant writes the same snippet to a file on disk, the file is complete, valid XML — so this is purely a rendering/clipboard issue in the app.

This makes the app return non-wellformed XML for common tasks (e.g. Maven pom.xml snippets), and is very confusing to debug: the model sees its own output as valid, while the user sees and copies garbage.

What Should Happen?

Every line of a fenced code block should render on screen and be included in the "copy code" clipboard output, regardless of whether the line looks like an HTML/XML tag.

Steps to Reproduce

1. In Claude for Windows, ask the assistant to output this Maven snippet in a fenced ```xml code block:

``xml
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-source-plugin</artifactId>
<version>3.3.1</version>
<executions>
<execution>
<id>attach-sources</id>
<goals>
<goal>jar-no-fork</goal>
</goals>
</execution>
</executions>
</plugin>
``

1. Look at the rendered code block and/or click the "copy code" icon and paste the clipboard contents.

Actual result: two lines are missing — <goals> and <goal>jar-no-fork</goal> — while the closing </goals> line remains, so the block renders as:

      <id>attach-sources</id>
      </goals>

Notes:

• HTML-entity-escaped versions of the same lines (<goals>) render correctly, suggesting the code-block renderer is HTML-sanitizing tag-like text instead of treating it as literal code.
• Reproduced consistently across multiple messages in the same session.
• Screenshot showing the truncated rendering next to the app version: see comment below.
• This appears to be a recurring bug, previously reported with other tag names (e.g. <context>) in #70613, #31635 and #43581 — all closed for inactivity rather than fixed. Filing fresh since it still reproduces on current version.

Is this a regression?

I don't know.

Claude Code Version

Claude for Windows (desktop app) Version 1.21459.0 (f7518f). Update to 1.21459.3 was pending at the time of reporting.

Claude Model

claude-sonnet-5 and claude-fable-5 (bug is in the app's rendering, not model output — file written to disk by the model is correct).

Platform

Anthropic API

Operating System

Windows 11 Enterprise 10.0.26200

Terminal/Shell

Not applicable — Claude for Windows desktop app chat UI.