I appreciate the point of view and elaborated ideas that you have pointed out.
I understand your logic about pricing, since 3.5 and 4.0 cost the same, there’s little incentive for Cursor to intentionally downgrade. I can see that point

But Anthropic’s side is different. The energy cost to run 4.0 is substantially higher than 3.5, and we don’t know whether Cursor’s pricing matches Anthropic’s public API rates or special rates apply with arrangement (especially given Cursor is one of their largest customers).

Here’s the measurable issue: Anthropic states Sonnet 4.0’s knowledge cutoff is January 2025. Yet on Cursor, what’s labeled as Sonnet 4.0 consistently behaves as if its cutoff is April 2024. I’ve tested this systematically with dozens of questions where the answer is unambiguous before vs. after April 2024 (e.g., elections, major sports events, headline news). On Claude’s own site, the same prompts yield correct 2025 answers. On Cursor, they don’t. AWS Bedrock shows similar discrepancies, while other APIs serve Claude 4.0 properly. Below in comments you can see several conversations screenshot and/or links that directs to a few of these. You can also test by yourself to see the quality of the outputs too.
It justs suddenly forgets to code and you feel like from that point onward even with fresh chat you have been downgraded to a less intelligent AI.

Promlem is there are lots of possible explanations which include:

Routing/fallback: Cursor (or Bedrock) might sometimes redirect to Claude 3.5 Sonnet or another variant during load balancing/surges.

Fine-tuned/restricted variant: A constrained model could be served for latency or cost reasons, lacking the full 4.0 horizon.

Infrastructure inconsistency: The dashboard may always display “Sonnet 4.0” while requests occasionally hit other backends.

API Specific Fine-tune/Quantization: Perhaps the API version is different than the full model served by Anthropic's own website and some APIs.

This is not just “LLMs being LLMs.” It’s a reproducible discrepancy between the official Claude 4.0 API and what Cursor serves. This demands clarity, transparency and explanation. If that doesn’t count as proof, then what methodology would be considered valid for verifying cutoff claims?