{"id":7066,"date":"2026-11-10T07:32:51","date_gmt":"2026-11-10T07:32:51","guid":{"rendered":"https:\/\/i10xnews.kinsta.cloud\/claude-model-guide-opus-5-5-fable-haiku-subagents"},"modified":"2026-11-10T07:32:51","modified_gmt":"2026-11-10T07:32:51","slug":"claude-model-guide-opus-5-5-fable-haiku-subagents","status":"publish","type":"post","link":"https:\/\/i10x.ai\/news\/claude-model-guide-opus-5-5-fable-haiku-subagents","title":{"rendered":"Which Claude Model Should You Use? Anthropic&#8217;s Guide Says Start With Opus 5.5 \u2014 and Tune Effort First"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Hours after launching Claude Fable 5.2, <a href=\"https:\/\/i10x.ai\/news\/company\/anthropic\">Anthropic<\/a> published a practical answer to the question its own lineup raises: which <a href=\"https:\/\/i10x.ai\/news\/product\/claude\">Claude<\/a> model should you actually use? The <a href=\"https:\/\/claude.dev\/blog\/which-claude-model-should-you-build-with\/\" target=\"_blank\" rel=\"noreferrer noopener\">claude.dev guide<\/a>, written by Addy Osmani and dated November 9, recommends a surprisingly conservative default \u2014 not the new flagship, but Claude Opus 5.5 at medium effort \u2014 and spends most of its length on how to save money by mixing models and tuning effort.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">\u26a1 Quick Take<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>Default:<\/strong> &#8220;Start most work on Opus 5.5 at its default effort level, <code>medium<\/code>.&#8221; It is the default in Claude Code and costs half as much per token as Fable 5.2.<\/li><li><strong>Move down:<\/strong> Sonnet 5.5 for well-scoped work like bug fixes; Haiku 5.5 for classification, routing, extraction and subagent side tasks.<\/li><li><strong>Move up:<\/strong> Fable 5.2 for the hardest work in coding, finance and the sciences, when Opus 5.5 at <code>high<\/code> still falls short, or for long unattended runs.<\/li><li><strong>The order of fixes:<\/strong> first let the model verify its own work, then raise effort one level, then move to the next model up.<\/li><li><strong>The cost trick:<\/strong> run everything at low effort and re-run only failures at high \u2014 about 97% pass rate at $0.17 per task in Anthropic&#8217;s SWE-bench Pro test, versus 95.3% at $0.29 all on high.<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">What happened<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The <a href=\"https:\/\/x.com\/ClaudeDevs\/status\/2119898687803621427\" target=\"_blank\" rel=\"noreferrer noopener\">@ClaudeDevs account<\/a> announced the guide on the evening of November 9, saying it &#8220;breaks down when to use Fable 5.2, Opus 5.5, Sonnet or Haiku&#8221; and covers &#8220;when to adjust effort instead of switching models, and how to save cost by putting subagents on smaller models.&#8221; The post passed 10,000 likes, and developers quickly turned it into checklists and Claude Code configurations. The same advice appears in Anthropic&#8217;s <a href=\"https:\/\/www.anthropic.com\/claude-fable-and-mythos-5-2\" target=\"_blank\" rel=\"noreferrer noopener\">Fable 5.2 launch post<\/a>, which says that &#8220;for most coding and knowledge tasks, we recommend that users start with Claude Opus 5.5, our strongest daily driver,&#8221; and in the <a href=\"https:\/\/platform.claude.com\/docs\/en\/about-claude\/models\/choosing-a-model\" target=\"_blank\" rel=\"noreferrer noopener\">platform docs<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Background: four models, five effort levels<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Anthropic&#8217;s current generally available lineup has four models, all with 1M-token context windows and 128K-token maximum output. According to the guide, list prices per million input\/output tokens are $0.10\/$0.50 for Haiku 5.5 (prompts up to 100K tokens; $0.50\/$2.50 above), $2\/$10 for Sonnet 5.5, $4\/$20 for Opus 5.5 and $8\/$40 for Fable 5.2. Claude Mythos 5.2 is available only through Anthropic&#8217;s verification programs. Each model also has an &#8220;effort&#8221; parameter that trades capability for cost and latency. Default effort differs by model and surface: Opus 5.5 and Haiku 5.5 default to <code>medium<\/code> on the API, while Fable 5.2 and Sonnet 5.5 default to <code>high<\/code> (<code>medium<\/code> in Claude Code).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Context: Opus 5.5 recently drew complaints about a behavioral shift in early November, covered in <a href=\"https:\/\/i10x.ai\/news\/claude-opus-5-5-nerf-complaints-data\">Claude Opus 5.5 nerf complaints and the data<\/a>, and i10x reported earlier on <a href=\"https:\/\/i10x.ai\/news\/claude-haiku-5-5-fast-cost-effective-subagent-model\">Haiku 5.5 as a subagent model<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">The guide&#8217;s task table<\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Task<\/th><th>Starting model<\/th><th>Starting effort<\/th><\/tr><\/thead><tbody><tr><td>Everyday engineering, debugging, multi-PR features<\/td><td>Opus 5.5<\/td><td>medium, then high<\/td><\/tr><tr><td>Classification, routing, tagging, high-volume extraction<\/td><td>Haiku 5.5<\/td><td>medium<\/td><\/tr><tr><td>Subagents that search files, read logs or summarize<\/td><td>Haiku 5.5 or Sonnet 5.5<\/td><td>model default<\/td><\/tr><tr><td>Chat and interactive sessions<\/td><td>Sonnet 5.5 (Haiku 5.5 for simple chat)<\/td><td>low or medium<\/td><\/tr><tr><td>Well-scoped coding: bug fixes, spec-driven work<\/td><td>Sonnet 5.5<\/td><td>medium, then high<\/td><\/tr><tr><td>Computer use and vision-heavy workflows<\/td><td>Opus 5.5<\/td><td>medium<\/td><\/tr><tr><td>Major refactors, migrations, leading many subagents<\/td><td>Fable 5.2<\/td><td>medium, then high<\/td><\/tr><tr><td>Deep research, scientific or quantitative analysis<\/td><td>Fable 5.2<\/td><td>medium, then high<\/td><\/tr><tr><td>Not sure yet<\/td><td>Opus 5.5<\/td><td>medium<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">How the advice works in practice<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Effort before model.<\/strong> Effort is &#8220;a finer dial for both cost and capability,&#8221; the guide says, but &#8220;past <code>high<\/code>, one more effort level can cost as much as a bigger model.&#8221; Its SWE-bench Pro numbers for Opus 5.5: <code>medium<\/code> scored about 2.5 points lower than <code>high<\/code> for about 70% of the cost; <code>low<\/code> about 8 points lower for a third of the cost; <code>xhigh<\/code> about 1.4 points higher for 2.5 times the cost. Higher effort &#8220;won&#8217;t give a model knowledge or skill it doesn&#8217;t have.&#8221;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Price per task, not per token.<\/strong> On Anthropic&#8217;s Terminal-Bench 3.0 runs, Fable 5.1 at <code>max<\/code> scored 58.0% while Opus 5.5 at <code>high<\/code> scored 58.9% &#8220;with half the tokens.&#8221; On FrontierCode 2.0, Fable 5.2 at <code>high<\/code> roughly matched Opus 5.5 at <code>max<\/code> for about half the cost \u2014 so &#8220;before you raise Opus 5.5 past <code>high<\/code> on a hard task, test Fable 5.2.&#8221; On one 20-problem benchmark, two problems carried 43% of the spend.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Subagents on smaller models.<\/strong> In Claude Code, a subagent definition can set <code>model: haiku<\/code> or <code>model: sonnet<\/code>, and <code>CLAUDE_CODE_SUBAGENT_MODEL<\/code> sets the default. The guide recommends small models only for work &#8220;where a mistake is cheap to spot, like finding files, running tests and reading logs,&#8221; while code edits and judgment calls stay on the main model. One notable change: the built-in Explore subagent now runs on the session&#8217;s model instead of always on Haiku, so the guide suggests defining a custom <code>Explore<\/code> subagent with <code>model: haiku<\/code>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Switching costs.<\/strong> The prompt cache belongs to the model that wrote it, so switching models mid-conversation re-pays cache writes. Thinking blocks are only partly readable across models \u2014 &#8220;a move from Fable 5.2 to any other generally available model drops its reasoning.&#8221;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Advisors and orchestration.<\/strong> The guide is cautious on its advisor tool (beta): in one internal pairing, an Opus 5.5 executor with a Fable 5.1 advisor scored 1.7 points higher &#8220;for about 2.1 times the money.&#8221; Bulk orchestration did better: on a defect-search benchmark over about 22 million tokens of code, an Opus 5.5 lead with Sonnet 5.5 workers cost about a third less than Opus 5.5 alone and scored about the same.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Pros and cons<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">For users<\/h3>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>Pro:<\/strong> Clear defaults reduce guesswork, and the advice helps Pro and Max subscribers stretch usage limits by not running everything on the biggest model.<\/li><li><strong>Con:<\/strong> The recommendations assume Claude Code and API controls that casual chat users don&#8217;t see.<\/li><\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">For developers<\/h3>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>Pro:<\/strong> Concrete numbers, config snippets and a compatibility checklist (for example, forced <code>tool_choice<\/code> returns a 400 error on Sonnet 5.5, Opus 5.5 and Fable 5.2) make migrations safer.<\/li><li><strong>Con:<\/strong> Many figures come from Anthropic&#8217;s internal runs, and the guide itself warns that multi-model setups must beat every single-model point on your own cost-vs-score chart.<\/li><\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">For companies<\/h3>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>Pro:<\/strong> Low-effort-first with high-effort retries and cheaper worker models offer measurable savings for large agent fleets.<\/li><li><strong>Con:<\/strong> Routing adds operational complexity \u2014 cache misses on switches, dropped reasoning and different API quirks per model.<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">What X is saying<\/h2>\n\n\n\n<blockquote class=\"twitter-tweet i10x-x-embed\"><p lang=\"en\" dir=\"ltr\">Which Claude model should you build with? Our new guide breaks down when to use Fable 5.2, Opus 5.5, Sonnet or Haiku. It also covers when to adjust effort instead of switching models, and how to save cost by putting subagents on smaller models.<\/p>&mdash; ClaudeDevs (@ClaudeDevs) <a href=\"https:\/\/x.com\/ClaudeDevs\/status\/2119898687803621427\" target=\"_blank\" rel=\"noreferrer noopener\">November 9, 2026 \u00b7 View on X<\/a><\/blockquote>\n\n\n\n<blockquote class=\"twitter-tweet i10x-x-embed\"><p lang=\"en\" dir=\"ltr\">before you blame fable 5.2 for eating your limit, here&#x27;s what anthropic says each claude model is for \ud83d\udc40<\/p>&mdash; Aditya Umale (@AdityaUmale17) <a href=\"https:\/\/x.com\/AdityaUmale17\/status\/2119988129348403342\" target=\"_blank\" rel=\"noreferrer noopener\">November 10, 2026 \u00b7 View on X<\/a><\/blockquote>\n\n\n\n<blockquote class=\"twitter-tweet i10x-x-embed\"><p lang=\"en\" dir=\"ltr\">Claude Code&#x27;s built-in Explore subagent used to always run on Haiku. Now it runs on your main model. If that&#x27;s Opus 5.5, or you just switched to Fable 5.2, Explore is burning big-model quota for nothing every time it reads files or searches code. So define your own subagent named Explore and set model: haiku.<\/p>&mdash; Vox (@Voxyz_ai) <a href=\"https:\/\/x.com\/Voxyz_ai\/status\/2119943123250078141\" target=\"_blank\" rel=\"noreferrer noopener\">November 10, 2026 \u00b7 View on X<\/a><\/blockquote>\n\n\n\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/x.com\/AdityaUmale17\/status\/2119988129348403342\" target=\"_blank\" rel=\"noreferrer noopener\">@AdityaUmale17<\/a> distilled the guide&#8217;s &#8220;cheapest trick&#8221;: &#8220;run everything on low effort, rerun only the failures on high.&#8221; <a href=\"https:\/\/x.com\/Voxyz_ai\/status\/2119943123250078141\" target=\"_blank\" rel=\"noreferrer noopener\">@Voxyz_ai<\/a> shared a long prompt asking Fable 5.2 to audit a Claude Code setup against the guide and to &#8220;Show me the recommendations first. Don&#8217;t change anything yet.&#8221; Other posts circulated more elaborate setups pairing Sonnet sessions with Haiku subagents and an Opus advisor; i10x could not verify every command-line flag in those posts against Anthropic&#8217;s documentation.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What Grok says<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Asked to summarize the guide, <a href=\"https:\/\/x.com\/grok\/status\/2120047821638291964\" target=\"_blank\" rel=\"noreferrer noopener\">Grok<\/a> answered (in Chinese; i10x translation): default to Opus 5.5 at medium effort, &#8220;the best value, first choice for everyday complex coding and knowledge work&#8221;; Haiku 5.5 for high-concurrency simple tasks such as classification, extraction and subagents; Sonnet 5.5 for well-scoped daily tasks at about half Opus&#8217;s cost; and Fable 5.2 for the hardest technical projects, deep research and large refactors.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Critical analysis<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">It is unusual for a vendor to launch its most expensive model and, the same day, tell most customers not to start with it. That candor is the guide&#8217;s strength \u2014 and partly self-interest: routing users away from Fable also eases load during the launch promotion and keeps heavy users from hitting limits. The core methodology, however, is sound and vendor-neutral: verification first, effort second, model third; measure cost per finished task; keep judgment on the strongest model.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The weak point is evidence. The guide&#8217;s numbers are mostly Anthropic&#8217;s own runs on Anthropic&#8217;s own benchmarks or internal tasks, and several (the advisor results) are explicitly near &#8220;run-to-run noise.&#8221; It also quietly documents friction that makes multi-model setups harder than the X threads suggest: cache invalidation, dropped reasoning across tiers, and different 400-error rules per model.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What&#8217;s still open<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li>Will Anthropic publish the underlying evaluation data for the cost-per-task comparisons?<\/li><li>Will Claude Code restore Haiku as the default for the Explore subagent, or keep it tied to the session model?<\/li><li>How will the advisor tool perform once it leaves beta?<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">\u270d\ufe0f About the analysis<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This independent, research-based i10x analysis draws on the claude.dev guide, Anthropic&#8217;s platform documentation and Fable 5.2 launch post, and public posts on X through November 10, 2026. Related: <a href=\"https:\/\/i10x.ai\/news\/anthropic-claude-fable-5-2-mythos-5-2-launch\">Claude Fable 5.2 and Mythos 5.2 launch<\/a> and <a href=\"https:\/\/i10x.ai\/news\/anthropic-single-agent-loop-subagents-guidance\">Anthropic&#8217;s single-agent loop and subagents guidance<\/a>.<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"https:\/\/claude.dev\/blog\/which-claude-model-should-you-build-with\/\" target=\"_blank\" rel=\"noreferrer noopener\">claude.dev: Which Claude model should you build with?<\/a><\/li><li><a href=\"https:\/\/platform.claude.com\/docs\/en\/about-claude\/models\/choosing-a-model\" target=\"_blank\" rel=\"noreferrer noopener\">Claude Platform docs: Choosing the right model<\/a><\/li><li><a href=\"https:\/\/platform.claude.com\/docs\/en\/models\/overview\" target=\"_blank\" rel=\"noreferrer noopener\">Claude Platform docs: Models overview<\/a><\/li><li><a href=\"https:\/\/www.anthropic.com\/claude-fable-and-mythos-5-2\" target=\"_blank\" rel=\"noreferrer noopener\">Anthropic: Introducing Claude Fable 5.2 and Claude Mythos 5.2<\/a><\/li><li><a href=\"https:\/\/x.com\/ClaudeDevs\/status\/2119898687803621427\" target=\"_blank\" rel=\"noreferrer noopener\">@ClaudeDevs on X<\/a><\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udd2d i10x Perspective<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Model choice is becoming an engineering discipline rather than a brand preference. Anthropic&#8217;s guide is essentially a playbook for cost-aware agent design: a capable default, cheap workers for checkable tasks, and the top model reserved for work that genuinely needs it. Teams that build evaluation harnesses for their own tasks \u2014 with outcome checks and per-task cost tracking \u2014 will get far more from this generation than teams that simply switch to the newest model and hope.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A new claude.dev guide by Addy Osmani tells developers to start most coding and knowledge work on Claude Opus 5.5 at medium effort, drop to Sonnet 5.5 or Haiku 5.5 for scoped or high-volume jobs, and move to Fable 5.2 only for the hardest projects. Its core advice: give the model a way to verify its work, then raise effort, and only then switch models.<\/p>\n","protected":false},"author":2,"featured_media":7065,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"paper_title":"","paper_url":"","pdf_url":"","doi":"","arxiv_id":"","authors":"","institutions":"","paper_date":"","venue":"","peer_reviewed":false,"code_url":"","data_url":"","funding_coi":"","figure_license":"","paper_license":"","replication":"","study_models":"","study_sample":"","footnotes":"","_yoast_wpseo_primary_category":"24"},"categories":[14,3,24,22],"tags":[],"companies":[39],"products":[51],"class_list":["post-7066","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-agents","category-anthropic","category-claude","category-coding","company-anthropic","product-claude"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Which Claude Model to Use: Anthropic&#039;s Opus 5.5 Guide<\/title>\n<meta name=\"description\" content=\"Which Claude model to use? 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