Tool-use invoke blocks intermittently render as plain text (antml: prefix dropped) - regression since ~2026-06-13

Summary

Since around Friday 2026-06-13, tool-use blocks intermittently lose the antml: namespace prefix on the invoke tag. When that happens the whole tool call is rendered as plain text in the transcript instead of being executed.

Impact

• Tool calls silently fail: no execution, no error, just text output that looks like a tool call.
• Observed identically across multiple independent sessions on the same model/harness, so it does not look like user-side input error.

When it started

Around 2026-06-13 (Friday). Heavy tool use during April and May with the same usage patterns had zero occurrences.

Trigger correlation

Long non-ASCII (Korean) text plus special characters (arrows, emoji, pipes, quotes) in tool parameters appears to raise the drop frequency. Short ASCII bodies are reliable. The underlying symptom is the missing prefix, not the body content itself.

Workaround

Keep the prefix and use short ASCII tool bodies.

Environment

Claude Code CLI, Claude Opus 4.8 (1M context), macOS.