claude-code - 💡(How to fix) Fix [Bug] Claude Opus 4.8 ignores MCP tool definitions, hallucinates parameters

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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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