[Bug] Opus 4.8 malformed tool invocation formatting with 1M context window
Open 💬 0 comments Opened Jul 5, 2026 by eiz27
Bug Description
Using Opus 4.8 (1M context), tool calls occasionally get emitted with
broken formatting. Instead of the proper "antml:invoke" tool-call format,
the output sometimes has the literal string "court" prepended, or raw
"<invoke name=...>" / "<parameter ...>" tags printed into the message body.
When this happens the harness can't parse the tool call, so the tool never
runs and the turn stops there (a no-op / stall). Reproduced multiple times
within the same session.
Environment Info
- Platform: darwin
- Terminal: cursor
- Version: 2.1.201
- Feedback ID: 0e7cbb24-3d72-4bdc-a60e-c55ba54f43e8
Errors
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