Model comparison
Claude Fable 5.1 vs Claude Sonnet 5.5
ClaudeCompare published specifications, vendor API list prices and measured Borg performance. Both models are included in Borg.
Claude Fable 5.1
Claude Sonnet 5.5
Included in Borg
At a glance
| Specification | Claude Fable 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Provider | Anthropic | Anthropic |
| Release | — | 2026-09-28 Vendor source |
| Context | 1,000,000 tokens Vendor source | 1,000,000 tokens Vendor source |
| Modalities | Input: text, image; output: text | Input: text, image; output: text |
| Reasoning / thinking modes | Yes / Not published | Yes / Not published |
| Open weights | No | No |
| Vendor API list price / 1M tokens | $10 input / $50 output | $2 input / $10 output |
| Borg | Included in Borg subscription | Included in Borg subscription |
AnthropicNot published by the vendor: release.
Measured on Borg
Metrics are not directly comparable to vendor benchmark claims. Models served different requests; this is observed gateway traffic, not a controlled head-to-head test.
Time to first token (gateway proxy)
Output tokens per second (whole turn)
| Metric | Claude Fable 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Time to first token (gateway proxy) p50 | 6015 ms (2192 samples) | 2640 ms (35971 samples) |
| Tokens per second (end to end) p50 | 55.9 output tokens per second (7156 samples) | 76.7 output tokens per second (86888 samples) |
| Request success rate | 99.9% (6283 samples) | Not published |
| Coding bench pass rate | Not published | Not published |
Measurement window, samples and methodology
The fleet reference is the median of published model medians, not a request-weighted result. Prompt lengths, reasoning settings and traffic differ between models.
Claude Fable 5.1: Measured on Borg: SearchAtlas AI Gateway production request logs. 30-day window ending 2026-10-08; 7924 requests. Minimum 100 samples per published request metric.
Claude Sonnet 5.5: Measured on Borg: SearchAtlas AI Gateway production request logs. 30-day window ending 2026-10-08; 94296 requests. Minimum 100 samples per published request metric.
Completion speed covers the whole turn including queueing and reasoning time; it is not a streaming tokens-per-second figure.
- Time to first token (gateway proxy): time until the first streamed response event reaches the gateway; our closest available measure of time to first token
- Tokens per second (end to end): tokens per second, end to end: provider-reported output tokens divided by the whole turn's elapsed time, including queueing and reasoning; not a streaming per-token rate
Vendor benchmarks
Vendor-reported scores may use different settings; do not treat them as a controlled head-to-head test.
How to choose
Claude Fable 5.1 is listed for long-horizon coding agents tackling difficult repositories, whereas Claude Sonnet 5.5 is listed for well-scoped bug fixes and code reviews. Choosing between Claude Fable 5.1 and Claude Sonnet 5.5 requires checking published specifications against observed Borg traffic for your workflow.
Key differences
| Spec | Claude Fable 5.1 | Claude Sonnet 5.5 |
|---|---|---|
| Context window | 1,000,000 tokens Vendor source | 1,000,000 tokens Vendor source |
| Reasoning modes | Yes / Not published | Yes / Not published |
| Release date | Not published | 2026-09-28 Vendor source |
| API price input / 1M | 10 | 2 Lower listed rate |
| API price output / 1M | 50 | 10 Lower listed rate |
| Time to first token (gateway proxy) | 6015 ms (2192 samples) | 2640 ms (35971 samples) Lower median |
| Output tokens per second (whole turn) | 55.9 output tokens/s (whole turn) (7156 samples) | 76.7 output tokens/s (whole turn) (86888 samples) Higher measured rate |
| Request success | 99.9% (6283 samples) | Not published |
| Coding bench | Not published | Not published |
FAQ
How do Claude Fable 5.1 and Claude Sonnet 5.5 compare for coding?
Claude Fable 5.1 has a Borg coding benchmark of not published; Claude Sonnet 5.5 has not published. The Claude Fable 5.1–Claude Sonnet 5.5 coding comparison cannot establish an overall winner from this small test alone.
How quickly do Claude Fable 5.1 and Claude Sonnet 5.5 start responding on Borg?
Borg median first response for Claude Fable 5.1 is 6,015 ms; for Claude Sonnet 5.5 it is 2,640 ms. The Claude Fable 5.1–Claude Sonnet 5.5 gateway medians are observations, not guaranteed latency for individual requests.
What are the API prices for Claude Fable 5.1 and Claude Sonnet 5.5?
The listed API rate for Claude Fable 5.1 is $10 input / $50 output per 1M tokens; for Claude Sonnet 5.5 it is $2 input / $10 output per 1M tokens. Neither Claude Fable 5.1 nor Claude Sonnet 5.5 has its Borg subscription charge represented by those provider API rates. Claude Fable 5.1 vendor source · Claude Sonnet 5.5 vendor source
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