Hi! Context: I worked with a large knowledge vault (millions of words, thousands of files). Everything organised to the letter. Nothing to mess with. Goal was to transform a part of the data (<40%) using other LLM provider. I trusted Claude Code (Opus 5 High) with a deterministic environment created in detail over months of trial and error. Even a Gemini Flash model could follow these. But Claude recommended to use Opus on High due to the complexity of the vault. The job was simple: - find files based on criteria - extract headers and body - create new documents with from each of the headers - upload to different provider for a specific task - replace older headers with newly transformed data from each of these files - Simply put: extract piece of puzzle, process piece of puzzle, put new piece of puzzle back. Easy. It made an absolute mess and I was dumb enough to believe it: - Invented relationships between files from filenames. - Mixed old and new outputs. - Created broken sequences like L-A and E-B instead of A B C D. Adding L-A before C. But L-A was F. - Duplicated some content and omitted other parts. - Changed a complete rebuild into partial work without disclosure. - Marked its own broken output as verified and complete. - Used misleading counts that could not prove coverage. - Produced confident, invented explanations when challenged. - Minimized mistakes and shifted attention toward the user’s decisions. The instructions were deterministic and clear. Claude’s implementation was chaos. My mistake was trusting the completion report before checking the final files with another LLM provider. After 3 days of forensic work, I still cannot reliably separate good output from plausible-looking invented material. I stoped using Opus 5. Even asking Sonnet 5 to create a coherent document about what it did, it is unable to finish paragraphs without gibberish talk and word salads. And I think this is also because of their watermarking system (ex: using "checks out" instead of "done"). Constantly speaking like a confused GenZ who didn't attend school but wants to look original.