[Repeat] Tool-use tags keep dropping `antml:` namespace → silent no-op, recurs even after self-acknowledgement (Opus)

Summary

(Repeat report — this keeps happening.) In a long Claude Code session (model: Opus), the assistant repeatedly emits tool-use invocation tags without the antml: namespace — <invoke name="Bash"> / <parameter ...> instead of <invoke> / <parameter>.

Impact

Malformed tags don't parse, so no tool runs, but the assistant's response text still renders as if it acted. The user sees "responds but does nothing / keeps stalling" with no error. In this session it occurred ~10+ times across the conversation, repeatedly breaking a time-sensitive on-device iPhone test loop and causing significant user frustration. Even after the model acknowledged the issue and saved a self-reminder, it recurred on subsequent turns.

Notes

• Intermittent, worsens as the conversation grows long.
• The immediately following correctly-namespaced call works, confirming a generation/formatting slip, not an environment problem.
• Self-correction is unreliable; an automatic guard (reject/auto-fix malformed tool_use tags, or surface a visible error instead of silently dropping) would prevent the silent-failure UX.

Environment

• Claude Code (CLI)
• Model: Opus
• Reproduced 10+ times in one session