[Bug] Model emits malformed tool calls with corrupted invocation tokens

Bug Description
Repeated malformed tool calls — model emitted a corrupted invocation token and couldn't recover

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.174
• Feedback ID: b5b2563f-3111-45a8-a681-e2f006dfc88e

Errors
In a long session, the model started emitting malformed tool calls: instead of the correct namespaced antml:invoke opening tag, it output the literal string court + a bare <invoke>, which is unparseable. The harness rejected each with "malformed and could not be parsed," and court leaked into the visible chat.

The worse problem was non-recovery: the model repeated the identical broken pattern 5–6 times across turns, despite explicit "malformed" errors and the user repeatedly saying stop. It (1) copied its own prior broken output instead of resetting to the correct schema, (2) misread the rejections as display glitches and re-sent the same command, and (3) ignored the error text pointing at tag syntax. A "malformed tool call" should force re-deriving the invocation format, never re-emitting the same bytes.

Context: Fable 5 ↔ Opus 4.8 session with earlier auto- and manual model switches, in case this correlates with model handoffs or long-context state.