--- title: GPT-5.6 vs. Claude Fable 5 for Physical AI, which... description: JuliaHub ran a controlled benchmark of frontier LLMs (GPT-5.6 terra/sol/luna and Claude Fable 5) on five physics modeling and simulation problems using their... canonical: https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best? | daily.dev og:description: JuliaHub ran a controlled benchmark of frontier LLMs (GPT-5.6 terra/sol/luna and Claude Fable 5) on five physics modeling and simulation problems using their... og:url: https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd og:image: https://api.daily.dev/og/posts/1cJJrmxHD.png og:image:alt: GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best? og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best? **[Hacker News](https://daily.dev/sources/hn)** · 13 min read · 1 upvotes · 1 comments ## Summary JuliaHub ran a controlled benchmark of frontier LLMs (GPT-5.6 terra/sol/luna and Claude Fable 5) on five physics modeling and simulation problems using their Dyad AI agent harness. Claude Fable 5 achieved the highest difficulty-weighted score (0.889) and was the only model to sweep all 12 core trials, but costs 3–8x more per trial than the GPT-5.6 variants. GPT-5.6 Sol ranked second (0.814) at roughly a fifth of Fable's price, making it the value pick for routine work. Each model showed a distinct work style: Fable verifies aggressively before committing, Sol over-specifies (which cuts both ways), Luna iterates excessively without external references, and Terra economizes at the cost of margin. Crucially, the study found that swapping the agent harness (Dyad vs. a stock coding agent) moved scores by 0.366 points — more than double the 0.162-point gap between the best and worst frontier models — underscoring that the harness matters more than the model choice. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Community take How the wider developer community reacted, aggregated from 3 discussions and 19 comments across hackernews (as of 2026-08-03). **TL;DR:** The community is skeptical of this benchmark, raising concerns about missing baselines, incomplete model coverage, and potential bias toward promoting the authors' own harness; a few commenters find the results directionally interesting but note the data is already outdated. **Sentiment:** 20% positive · 40% mixed · 40% skeptical **The case for** - Gemini performs well on physics/circuit-design style textbook tasks, consistent with Google's historical multimodal strengths. - The cost-vs-performance comparison (e.g., $22.56 Sol run vs $124.76 Fable) gives useful practical signal. **The pushback** - Benchmark is already outdated, missing Kimi 3 and Opus 5 at time of posting. - Effort levels ('xhigh') are not comparable across models, making cross-model conclusions unreliable. - Missing important baselines and the article is perceived as vibe-coded and methodologically weak. - Seen as a 'benchmark to promote their own harness' rather than a neutral evaluation. - Codex and the full Claude family (Opus/Sonnet/Haiku) were omitted from the main comparison. **By community** - hackernews (skeptical): Commenters broadly question the benchmark's methodology, missing models, and potential self-promotion bias, while a few note interesting cost/performance data points. **Hottest debate:** Whether the benchmark's effort-level comparisons are meaningful given that 'xhigh' is not standardized across different models/harnesses. **Open questions** - How will world models (video-generative or otherwise) compare to LLMs on physical simulation tasks once mature? - When will updated benchmarks including Kimi 3, Opus 5, and Codex be published? - What is the actual performance of Opus, Sonnet, and Haiku on these tasks relative to the included models? **Highlights** > A really frustrating partial presentation, given an apparent lack of testing with a spread of efforts for each model. Given that there's no reason to believe that Fable's xhigh is comparable to GPT-sol's xhigh, or Opus xhigh, for that matter, it would be far more useful to see the effort level where these tasks no longer achieved their goals. > — [xnorswap on hackernews · 1 comments](https://news.ycombinator.com/item?id=49100265) > So Fable "won" but it cost $124.76 for marginal performance benefits over the $22.56 5.6 Sol run. > — [draginol on hackernews](https://news.ycombinator.com/item?id=49102155) > Yet another "benchmark to promote their own harness" > — [gizmodo59 on hackernews](https://news.ycombinator.com/item?id=49099227) > They were all benchmarked, but not included in the post to try to keep the amount of data from being excessive. (Results are also not surprising — Opus good, Sonnet struggles, Haiku fails.) > — [StefanKarpinski on hackernews](https://news.ycombinator.com/item?id=49103289) **Source threads** - [hackernews](https://news.ycombinator.com/item?id=49098388) · 39 points · 19 comments - [hackernews](https://news.ycombinator.com/item?id=49022265) · 2 points · 0 comments - [hackernews](https://news.ycombinator.com/item?id=48990514) · 1 points · 0 comments ## Community discussion Top comments from developers on daily.dev. **@marinkomijatovic** · 0 upvotes > The biggest takeaway is that agent design can matter more than model choice—better harnesses may deliver larger gains than upgrading to a more expensive frontier model. ## Similar posts on daily.dev - [GPT-5.6 Sol vs Claude Fable 5: Which Model Writes Better Plans?](https://daily.dev/posts/gpt-5-6-sol-vs-claude-fable-5-which-model-writes-better-plans--m7odzpdq7) · Kilo Blog · 1 upvotes · 0 comments - [Grok 4.5 vs Claude Fable 5 vs GPT-5.6 Sol: How I’m Testing Them Across 100 Real Tasks](https://daily.dev/posts/grok-4-5-vs-claude-fable-5-vs-gpt-5-6-sol-how-i-m-testing-them-across-100-real-tasks-hb5pgi2wo) · Medium · 0 upvotes · 0 comments - [$100 AI Music Video: Claude Fable 5 vs. GPT-5.6 Sol](https://daily.dev/posts/100-ai-music-video-claude-fable-5-vs-gpt-5-6-sol-yzb9rxhec) · Hacker News · 4 upvotes · 0 comments --- Tags: [#ai](https://daily.dev/tags/ai), [#ai-agents](https://daily.dev/tags/ai-agents), [#robotics](https://daily.dev/tags/robotics) [View this post on daily.dev](https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best?","url":"https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd"},"datePublished":"2026-07-29T17:15:04.020Z","dateModified":"2026-08-03T12:51:31.333Z","description":"JuliaHub ran a controlled benchmark of frontier LLMs (GPT-5.6 terra/sol/luna and Claude Fable 5) on five physics modeling and simulation problems using their...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/5af21c19d17ab50c471fe156d24aeca9?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/5af21c19d17ab50c471fe156d24aeca9?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Hacker News","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Hacker News","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/hn","url":"https://daily.dev/sources/hn"},"commentCount":1,"discussionUrl":"https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":1},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":1}],"keywords":"ai,ai-agents,robotics","timeRequired":"PT13M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Hacker News","item":"https://daily.dev/sources/hn"},{"@type":"ListItem","position":3,"name":"GPT-5.6 vs. Claude Fable 5 for Physical AI, which performs best?"}]} {"@context":"https://schema.org","@type":"WebPage","@id":"https://daily.dev/posts/gpt-5-6-vs-claude-fable-5-for-physical-ai-which-performs-best--1cjjrmxhd","comment":[{"@type":"Comment","text":"The biggest takeaway is that agent design can matter more than model choice—better harnesses may deliver larger gains than upgrading to a more expensive frontier model.","datePublished":"2026-07-30T13:19:31.624Z","url":"https://daily.dev/posts/1cJJrmxHD#c-iCtVE5N7X","author":{"@type":"Person","name":"Marinko Mijatovic","url":"https://daily.dev/marinkomijatovic","image":"https://lh3.googleusercontent.com/a/ACg8ocLwdsEqtnHQI7cm1bW_7ujRh-gg8m_bC2LH0WBcgHZ3PJAoNA=s96-c"}}]} ```