As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/v/yt-mehul-mohan-new-sonnet-5-5-is-opus-5-level/

# NEW Sonnet 5.5 Is Opus 5 Level

Mehul Mohan, 29 September 2026, YouTube. 21,319 views as of 24 October 2026.
Kind: Community.
Watch: https://www.youtube.com/watch?v=VcQIW6rdOMY

## Why it is here

Found 2026-09-29 by YouTube search "sonnet 5.5 review" (sorted by upload date). Listed as: 15,341 views, length 18:40, published "13h ago" (so the date above is approximate).

## Description (written by Gemini from the video)

**Summary**  
Software engineer Mehul Mohan reviews Anthropic’s release of Claude Sonnet 5.5, analyzing its benchmark performance, pricing structure, and positioning within the Claude 5.5 family. He demonstrates using Sonnet 5.5 in Claude Code to implement a privacy toggle on his custom financial trading dashboard, highlighting the model's high coding speed alongside subtle instruction-following lapses compared to Claude Opus 5.5.

**What is shown**  
* **[00:00]** Anthropic's announcement post on X detailing Claude Sonnet 5.5's release, speed improvements, and reduced token costs.  
* **[01:30]** Anthropic's release blog post and launch documentation overview.  
* **[02:25]** Evaluation benchmark table comparing Sonnet 5.5, Sonnet 5, Opus 5.5, and GPT-6 Sol across Terminal-Bench 4.0, FrontierCode 1.1, CursorBench 4.0, GDPval-AA, Humanity’s Last Exam, and OSWorld 2.1.  
* **[04:09]** System card footnote detailing why Sonnet 5.5 scored lower at Max effort than Xhigh effort on FrontierCode due to Claude Code review subagent timeout issues.  
* **[05:37]** Sponsor walkthrough of the Nebius Token Factory catalog and playground.  
* **[08:15]** Cost and speed pricing table comparing Sonnet 5.5 ($2/$10 per 1M tokens) to Opus 5.5 ($4/$20 per 1M tokens) and cache read/write rates.  
* **[09:31]** Side-by-side animated coding test generating an HTML/JS canvas simulation of a 400-starling murmuration.  
* **[10:35]** Walkthrough of the presenter's personal Interactive Brokers trading dashboard.  
* **[12:54]** Claude Code CLI terminal logs showing a prompt to add a privacy mode switch, Sonnet 5.5's unrendered implementation, and its subsequent fix after reviewing a user-submitted screenshot.  
* **[14:01]** Whiteboard diagramming illustrating the "instruction following gap" between Opus 5.5 (100% completion) and Sonnet 5.5 (95% completion requiring manual correction).  
* **[16:03]** Anthropic playbook article ("Building with Claude Sonnet 5.5" by Addy Osmani) outlining workload recommendations between Sonnet and Opus.  
* **[17:41]** Mehul's post on X summarizing "sonnet is the new opus / opus is the new fable."

**Claims & numbers**  
* The presenter highlights Anthropic's claim that Sonnet 5.5 runs more than 30% faster and costs up to 30% less for most tasks compared to Sonnet 5 [01:35].  
* On Terminal-Bench 4.0 agentic coding, Sonnet 5.5 scores 70.6% compared to Sonnet 5 (10.3%) and Opus 5.5 (66.4%) [02:25].  
* On FrontierCode 1.1, Sonnet 5.5 achieves 46.2% at Max effort and 52.1% at Xhigh effort, versus Sonnet 5 (42.4%), Opus 5.5 (54.4%), and GPT-6 Sol (49.3%) [02:26].  
* On CursorBench 4.0, Sonnet 5.5 scores 55.5% versus 34.1% for Sonnet 5 and 57.8% for Opus 5.5 [02:26].  
* Sonnet 5.5 scores 1844 on GDPval-AA v2.1 and 80.1% on OSWorld 2.1 computer use [03:49].  
* Claude Sonnet 5.5 API pricing is set to $2 per million input tokens, $10 per million output tokens, $0.20 per million cache reads, and $2.50 per million cache writes, representing half the cost of Opus 5.5 on inputs, outputs, and cache writes [08:15, 08:48].  
* The presenter estimates that 96% to 97% of heavy agentic token consumption consists of cache reads [08:40].  
* The presenter claims Sonnet 5.5 typically achieves 95% of complex agentic tasks cleanly but regularly requires human intervention on the final 5%, whereas Opus 5.5 completes tasks with 100% reliability in his experience [14:18–14:45].  
* The presenter notes OpenAI DevDay is scheduled for the following day with anticipated personal AI assistant announcements [18:08].

**Notable quotes**  
* "Sonnet 5.5 scores more than Opus 5.5, which is a very, very interesting observation." [02:30]  
* "Opus 5.5 is probably the best model ever... in the history of all AI models that I have personally used." [12:00]  
* "Sonnet 5.5 is sort of like, it gets to 95%, right? You have to go ahead and push it at the rest of the 5%. With Opus 5.5, what I have seen is that this is happening at 100% every time." [14:18]

**Assessment**  
An authentic developer review and hands-on appraisal. The presenter tests the model on real-world personal codebases via Claude Code and provides transparent terminal logs showing genuine errors and self-corrections alongside official benchmark comparisons.

_Described by gemini-3.8-flash on 2026-09-29 from the video's audio and frames._

## How AI was used to make it

AI made a small part of this video (Subject built by AI).
- Claude Sonnet 5.5 coded an HTML/JS canvas simulation of starling murmuration demonstrated on screen
- Claude Sonnet 5.5 implemented code changes for a trading dashboard privacy toggle via Claude Code
Tools: Claude Sonnet 5.5, Claude Code
(Classified by gemini-3.8-flash from our description; high confidence. Overview: https://postcutoff.com/ai-video/)

## Related

- 2026-09-28: [Anthropic releases Claude Sonnet 5.5](https://postcutoff.com/e/2026-09-28-claude-sonnet-5-5/)
- 2026-07-24: [Anthropic releases Claude Opus 5](https://postcutoff.com/e/2026-07-24-claude-opus-5/)

## More videos on this

- 2026-09-29: ["Build what you think you look like": Claude Sonnet 5.5 vs GPT-6 Astra](https://postcutoff.com/v/siyabuilt-sonnet-5-5-vs-gpt-6-astra-what-you-look-like-x/), Siya (@siyabuilt), X, made by AI, 7,259,229 views as of 27 October 2026.
- 2026-09-28: [Introducing Claude Sonnet 5.5](https://postcutoff.com/v/claude-introducing-sonnet-5-5/), Claude, YouTube, official, 4,403,927 views as of 27 October 2026.
- 2026-09-28: [Fall foliage simulator made with Sonnet 5 vs Sonnet 5.5 (official @claudeai launch-thread video)](https://postcutoff.com/v/claudeai-sonnet-5-5-fall-foliage-simulator-x/), Claude (@claudeai), X, made by AI, 755,645 views as of 26 October 2026.
- 2026-07-31: [Claude Opus 5 is a freak](https://postcutoff.com/v/yt-ai-search-claude-opus-5-is-a-freak/), AI Search, YouTube, community, 691,630 views as of 9 October 2026.

What else happened that month, day by day: https://postcutoff.com/timeline/2026/09/