[Bug] Claude Opus 4.8 ignores MCP tool definitions, hallucinates parameters
Resolved 💬 4 comments Opened May 29, 2026 by rhgg2 Closed Jul 3, 2026
Bug Description
Opus 4.8 consistently ignores MCP tool definitions. Rather than loading the MCP schema, Claude prefers to guess plausible sounding parameter names, and then go into a long debugging loop, often blaming the tools or the harness, rather than checking what the correct calling conventions actually are. This is very noticeable and a clear change in behaviour from Opus 4.7.
Environment Info
- Platform: darwin
- Terminal: iTerm.app
- Version: 2.1.156
- Feedback ID: d4ddeb98-453d-4d5a-bd6b-56f6902d1535
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