Claude Fable 5.1 or Claude Sonnet 5.5?
Choose Claude Fable 5.1 when ambitious coding, research and long-running asynchronous projects is the closer match. Choose Claude Sonnet 5.5 when full-stack software development, automated ui testing and production agent workflows matters more. Test both on the same real tasks before moving a production workflow.
Ambitious coding, research and long-running asynchronous projects
Full-stack software development, automated UI testing and production agent workflows
Claude Fable 5.1
Anthropic's frontier model for demanding reasoning and long-horizon agentic work.
Official specification ↗Claude Sonnet 5.5
Anthropic's workhorse model with enhanced computer use and sub-second token generation.
Official specification ↗Side-by-side comparison
| Category | Claude Fable 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Provider | Anthropic | Anthropic |
| Type | Model · Frontier reasoning model | Model · Balanced reasoning model |
| Pricing summary | $10 input, $0.25 cache read and $50 output per 1M tokens | $2 input, $0.20 cache read and $10 output per 1M tokens |
| Input price | $10 / 1M tokens | $2 / 1M tokens |
| Cached input | $0.25 / 1M tokens | $0.20 / 1M tokens |
| Output price | $50 / 1M tokens | $10 / 1M tokens |
| Context window | 1M tokens | 500K tokens |
| Maximum output | 128K tokens | 128K tokens |
| Knowledge cutoff | June 2026 | August 2026 |
| Reasoning | Adaptive, always on; high default effort | Adaptive thinking with custom effort |
| Relative speed | Slower | High throughput |
| Input modalities | Text and image | Text and image |
| Output modalities | Text | Text |
| Tools and integrations | Claude API tool use, computer use and agent workflows | Claude API tool use, desktop computer use, Claude Code |
| API / access | Claude Messages API | Claude Messages API |
| Best fit | Ambitious coding, research and long-running asynchronous projects | Full-stack software development, automated UI testing and production agent workflows |
| Company | Anthropic | Anthropic |
| Product category | Model · Frontier reasoning model | Model · Balanced reasoning model |
| Current pricing note | $10 input, $0.25 cache read and $50 output per 1M tokens | $2 input, $0.20 cache read and $10 output per 1M tokens |
| Best suited to | Ambitious coding, research and long-running asynchronous projects | Full-stack software development, automated UI testing and production agent workflows |
| Key capabilities | Long-horizon agents; Repository-scale coding; Multistep research; Vision and document work | Sub-second latency; Enhanced desktop computer use; 500K context; Claude Code default |
| Last verified | 2026-09-08 | 2026-10-15 |
| Official product source | https://platform.claude.com/docs/en/models/fable-5-1/overview | https://platform.claude.com/docs/en/models/sonnet-5-5/overview |
How we reached this verdict
We looked at the things that affect everyday use: what each product can do, how its plans are structured, where it fits into your existing workflow, and what the provider currently documents. Benchmark results are useful context, but they are not a universal winner score because test settings and tools vary.
Common questions
Claude Fable 5.1 or Claude Sonnet 5.5: which should I choose?
Choose Claude Fable 5.1 for ambitious coding, research and long-running asynchronous projects. Choose Claude Sonnet 5.5 for full-stack software development, automated ui testing and production agent workflows. Validate the decision with your own prompts and total task cost.
Is Claude Fable 5.1 cheaper than Claude Sonnet 5.5?
Pricing units can differ. Claude Fable 5.1: $10 input, $0.25 cache read and $50 output per 1M tokens. Claude Sonnet 5.5: $2 input, $0.20 cache read and $10 output per 1M tokens. Check tool charges, plan limits, caching and long-context rules before comparing totals.
Which is better for coding and agent work?
Capability depends on the selected product surface, tools and harness. Claude Fable 5.1 is positioned for ambitious coding, research and long-running asynchronous projects; Claude Sonnet 5.5 is positioned for full-stack software development, automated ui testing and production agent workflows. Run the same repository task on both.
How do their context windows compare?
Claude Fable 5.1 lists 1M tokens; Claude Sonnet 5.5 lists 500K tokens. A larger limit does not by itself guarantee better retrieval or lower cost.
Official sources
Use these provider pages to confirm current features, pricing and availability.
AI tools change quickly. We checked this page on 2026-09-08. Before you subscribe, confirm the latest price, regional availability and usage limits on the provider website.