[Bug] Claude Opus 5 generates hallucinated responses not present in previous versions

Hey, I came across your issue about hallucinations in Claude Code responses.

I've been working with AI agents, LLM workflows, RAG, context management, and tool-based automation, and hallucinations can often be reduced by improving the context pipeline, retrieval strategy, tool usage, or prompt/agent architecture.

If you're still investigating the issue, I'd be happy to take a look at the workflow and share some ideas on where the responses may be going off track.

Iam an user. I want to know more about what Claude hallucination it is. Can you share some screenshot ?

Yes the hallucination I’m referring to is when Claude Opus 5 confidently generates details, code behavior, or conclusions that aren’t supported by the existing context, files, or tool output. I’ve seen this in agent workflows where it can introduce assumptions as if they were already established facts.

I’ll share a concrete screenshot/example so the regression is clear.

The key point is that it is much worse with 5, but really didn't happen much for me on 4.8.

I’m seeing the same pattern. The difference with 5 is significant enough that I’d be interested in digging into the workflow and identifying what’s triggering it. If you can share the setup/repro case, I can take a closer look and help isolate the cause and potential fix. If you need someone to handle the investigation and implementation, I’d be happy to help you fix that.

An example:

Scan method: regex with backreference so only same-expression pairs match, both orders, == nil || len()==0|<1|<=0 and != nil && len()>0|!=0|>=1, multi-line aware.

Found 3 things in a large codebase. Me:

this is surprisingly little

Opus 5:

Fair — my regex only matched simple dotted identifiers. Let me do it properly with an AST pass.

It's lazy, refuses work and invents results.