They must have been A/B testing this with 4.7 lately, I noticed it changed from its normal mode in a way that matches a lot the just released 4.8

Claude's 4.6 - 4.7 transition made me discover codex, and with gpt 5.5 there is no way i'm going back

> The Messages API now accepts system entries inside the messages array. Developers can update Claude’s instructions mid-task without breaking the prompt cache or routing the update through a user turn. This can be used in a given harness to update permissions, token budgets, or environment context as an agent runs.

Biggest deal imo

Opus 4.8 says to take the car. 4.7 said to walk.

“I want to wash my car. The carwash is 50m away. Should I take the car or go by foot?”

https://claude.ai/share/5f7f738a-5f29-48ff-9807-9a2dd37fb405

https://claude.ai/share/ecd14393-9d42-4527-ae0c-89f3d05216c8

Bash(echo "hello"; pwd) ⎿ hello /Users/username/Work/Github/project

Bash(echo test123) ⎿ test123

  Read 1 file, listed 1 directory (ctrl+o to expand)

 Bash(echo "checking output works")
  ⎿  checking output works

  Read 1 file (ctrl+o to expand)
  ⎿  API Error: 400 messages.3.content.56: `thinking`
     or `redacted_thinking` blocks in the latest
     assistant message cannot be modified. These
     blocks must remain as they were in the original
     response.

Very inspiring improvements. DIssapointing result for a code review i expected to see after my 30 min walk

    ln -s $HOME/.local/share/claude/versions/2.1.153 $HOME/.local/bin/claude

The table comparing eval scores shows the following:

Agentic Terminal Coding (Terminal-Bench 2.1) Opus 4.8 74.6% GPT 5.5 78.2%

Then, when you scroll all the way down to the bottom Footnotes section it says

"Terminal-Bench 2.1: We reported scores for all models using the Terminus-2 public harness. GPT-5.5’s reported score with the Codex CLI harness is 83.4%."

This made me laugh. Training Opus 4.7 on business skills caused it to sometimes exhibit dishonest behaviour, and not training 4.8 on those skills removed it. From the system card:

> 6.2.5 External testing from Andon Labs Andon Labs reviewed the behavior of Claude Opus 4.8 in their simulated Vending-Bench 2 retail-management evaluation, as reported in the Capabilities section of this system card (see Section 8.13.5). Although they did observe some unexpected capability failures, they did not find clear instances of the kind of concerning in-game behaviors that were discussed in other recent system cards.

> What might have led to these differences? We monitor and investigate the effects of different training environments on alignment; Claude Opus 4.7, for example, had training that focused on business skills and robustness against adversarial agents, but we discovered that this training inadvertently contributed to misaligned behavior including dishonesty. We therefore removed it for Opus 4.8.

> Thus, Opus 4.8 did not show the same misaligned behaviors as Opus 4.7 in Vending-Bench, but also had reduced business success due to being more susceptible to scammers and being less able to negotiate good deals with other agents. We are currently working on training to improve business capabilities while maintaining aligned and ethical behavior.

/model claude-opus-4-8

seems to work but idk why they never set it so you can see it in the /model list.

"what model are you

I'm Claude Opus (claude-opus-4-8), running in Claude Code."

Ugh...

Invalid request The request couldn't be completed. View details API Error: 400 messages.1.content.7: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.

I would rather not. 4.6 was fine. 4.7 got to be fine 1 week after the release. Now 4.8. No difference, same thing.

But the app is broken and nothing works. So now I have to regress to different clients and wait it out while it becomes workable again.

Initial testing feels better than 4.8 And the knowledge cutoff claim of January 2026 seems to check out since it was able to "remember" without search about the double-tap killing of a drug smuggler by the US Army in late December.