Kimi K3.1 vs Claude Fable 5.1: Specs, Benchmarks, Price
Maya Chen
Lead AI Researcher
TLDRFable 5.1 ships now with a reported 55.8% Terminal-Bench 4.0. Kimi K3.1 has no specs yet; its K3 base offers 2.8T params, 1M context, $3/$15.
Head-to-Head: Kimi K3.1 and Claude Fable 5.1 on Long-Horizon Agents
Kimi K3.1 is Moonshot AI's unreleased successor to Kimi K3. Reports say it is tuned for long-horizon agent work, and it would build on K3's open-weight, 2.8T-parameter, 1M-token, $3/$15-per-1M-token baseline. Claude Fable 5.1 is a shipping Anthropic model, listed since September 1, 2026, with public scores such as 55.8% on Terminal-Bench 4.0. The right pick depends on whether you need a documented frontier model in production today or an open-weight option you can self-host once K3.1 lands. As of October 17, 2026, Moonshot has published no Kimi K3.1 model card, weights, price or endpoint.
Key Takeaways
- Claude Fable 5.1 is the only one of the two you can deploy today. It appears on public model trackers from September 1, 2026, and on September 20 one observer called it one of "arguably the two best models out right now" (Ananth on X).
- Kimi K3.1 is confirmed only through a teaser. Moonshot's Pi Teaser (September 18–19, 2026) is a vendor-side signal. Everything else about K3.1 comes from leaks.
- K3.1's reported focus is long-horizon agents. A Nomura report cites post-training, computer-use trajectories and real-world task data as the improvement levers.
- The head-to-head baseline is Kimi K3 vs Claude Fable 5. Moonshot said K3 still trailed Claude Fable 5 overall. K3 still beat it on Frontend Code Arena, 1,679 to 1,631.
- Pricing is lopsided in what's documented. K3 costs $3/$15 per 1M tokens. No source reviewed confirms a Fable 5.1 price.
Kimi K3.1 vs Claude Fable 5.1 at a Glance
| Dimension | Kimi K3.1 | Claude Fable 5.1 |
|---|---|---|
| Status (Oct 17, 2026) | Unreleased; teased by Moonshot | Released; listed from Sep 1, 2026 |
| Developer | Moonshot AI | Anthropic |
| Weights | Not yet confirmed (K3 is open-weight) | Closed |
| Parameters | Not yet confirmed (K3: 2.8T total, 16 of 896 experts active) | Not yet confirmed |
| Context window | Not yet confirmed (K3: 1,048,576 tokens) | Not yet confirmed |
| Terminal-Bench 4.0 | Not yet confirmed (Union Alpha, suspected, unconfirmed: ~50%) | 55.8% (reported) |
| Artificial Analysis (Intelligence / Coding) | Not yet confirmed | 53 / 82 (third-party tracker) |
| API price per 1M tokens | Not yet confirmed (K3: $3 input / $15 output) | Not yet confirmed |
| Positioning | Long-horizon agents, computer use (leak) | Long-running agents (third-party listings) |
Official numbers are scarce on both sides. The Fable 5.1 benchmark figures come from third-party aggregation. Every Kimi K3.1 cell is either blank or borrowed from K3 or from an unidentified stealth model.
Release Status and Timeline
Moonshot's official account posted a digit string on or around September 18, 2026. Community posters decoded it as π with the leading "3.1" removed (Ananth's report, J A Z I I's decoding).
Source: @Ananth7e
Days later, posters reported a k3d1-agent identifier, which they read as an internal K3.1 build. On October 10, a summary of a Nomura report said K3.1 would ship before mid-October and would focus on sustained, long-running agent execution (0xLogicrw's summary). A leaker pushed back the same day, saying it would not launch before October 15 (ChrisGPT). By October 16, the Nomura window had passed with no release.
Source: @notjazii
For a fuller breakdown of the teaser, see the earlier Kimi K3.1 explainer.
Claude Fable 5.1 has no such ambiguity. It is live, it sits on public leaderboards, and its API model ID is claude-fable-5-1. Reports also say Anthropic is testing a Fable 5.2 checkpoint, though Anthropic has not confirmed it.
Benchmarks: Documented vs Inferred
Claude Fable 5.1 has real numbers. Kimi K3.1 has none of its own. According to an AGTP summary of reported scores, Fable 5.1 scored:
| Benchmark | Claude Fable 5.1 | GPT-6 Astra (reference) |
|---|---|---|
| Terminal-Bench 4.0 | 55.8% | 57.7% |
| Unnamed coding benchmark | 67.4% | 74.1% |
| Frontier Math Tier 4 | 87.8% | 97.6% |
A third-party tracker that shows Artificial Analysis data lists Fable 5.1 at 53 Intelligence and 82 Coding. That coding score is above GPT-6 Astra's 77 on the same board.
The closest thing to a K3.1 data point is Union Alpha, a free stealth model that appeared on September 16, 2026. Early community testing reported about 74% on DeepSWE and about 50% on Terminal Bench v4, according to a single tester (@deepanshusharmx). Other testers reported a 262K context window, 131K maximum output and only 16 tokens/sec. Some also argued that Union Alpha was too slow to be Kimi at all. Treat any Union Alpha vs Fable 5.1 comparison as speculative. If those figures did belong to K3.1, about 50% would still sit below Fable 5.1's reported 55.8% on Terminal-Bench.
The better-grounded comparison is between the predecessors. Moonshot's own launch text said Kimi K3's "overall performance still trails the most powerful proprietary models, Claude Fable 5 and GPT 5.6 Sol" (launch thread on Reddit). K3 still debuted at #1 on Frontend Code Arena with 1,679, ahead of Fable 5 at 1,631. It also placed 3rd on Artificial Analysis, above Claude Opus 4.8 (r/singularity). Earlier coverage of Kimi K3 vs Claude goes deeper on that matchup.
Capabilities and Architecture
Kimi K3, and presumably K3.1, runs on Stable LatentMoE, which routes each token to 16 of 896 experts. It also uses Kimi Delta Attention, a hybrid linear attention design that Moonshot says decodes up to 6.3× faster at million-token context, and Attention Residuals. K3 includes native image and video understanding, and thinking mode is on by default.
Pre-release leaks say K3.1 adds stronger long-horizon agent performance, better computer use and more real-world task data. Earlier leaks also pointed to faster inference and fewer wasted reasoning tokens. These are unconfirmed, but they target K3's best-documented weaknesses. Ivan Fioravanti called K3 "strong (in design and frontend) but slow" (post). Reddit testers said K3 used roughly 2× the tokens of GPT-5.6 Sol.
Third-party model listings describe Claude Fable 5.1 as "next generation intelligence for long-running agents." That puts both models in the same lane on paper. Anthropic's architecture, parameter count and context window for Fable 5.1 are not published in the sources reviewed.
Pricing, Context and Self-Hosting
Kimi K3 costs $3 per 1M input tokens and $15 per 1M output tokens, with discounts for cached input. That matches Sonnet 5's sticker price (itspaulai on Threads). See the Kimi K3 price guide for the full breakdown. K3.1 pricing is not yet published. One community voice expects K3.1 to be "hard to serve and very expensive" (cheaty), which is an opinion, not a leak.
Claude Fable 5.1 pricing is not confirmed in the sources reviewed, so no per-token comparison is possible yet.
Self-hosting is a one-sided advantage for Kimi. K3 weights are public, for example as unsloth/Kimi-K3 on Hugging Face. The checkpoint needs about 1.4 TB of storage, and community estimates put the VRAM requirement near 2 TB. Fable 5.1 cannot be self-hosted.
| Access route | Kimi K3.1 | Claude Fable 5.1 |
|---|---|---|
| Vendor app / API | Not yet available (Kimi K3 available from Moonshot) | Anthropic, model ID claude-fable-5-1 |
| kie.ai API | Not yet available; Kimi K3 is live on kie.ai | Listed in kie.ai catalog |
| Open weights | Not yet confirmed (K3: yes) | No |
Which One Should You Use?
Choose Kimi K3.1 (or Kimi K3 while you wait) if:
- You need open weights for on-prem deployment, IP control or third-party hosting.
- Frontend and design-heavy coding matters most, where K3 already led Fable 5 on Frontend Code Arena (1,679 vs 1,631).
- You need a confirmed 1,048,576-token context window today (K3), and can tolerate slower output.
Choose Claude Fable 5.1 if:
- You need a model in production now, with published benchmark results.
- Terminal and agentic coding matter most, where Fable 5.1 has a reported 55.8% Terminal-Bench 4.0 score and an 82 Artificial Analysis coding score.
- You can't wait on a release window that has already slipped at least once.
Based on what is documented today, Claude Fable 5.1 is the safer production choice and Kimi K3 is the open-weight hedge. K3.1 is still a bet on a release that hasn't happened.
Frequently Asked Questions
Is Kimi K3.1 better than Claude Fable 5.1?
Nobody can say yet, because Kimi K3.1 has no official benchmarks as of October 17, 2026. Claude Fable 5.1 has public figures, including a reported 55.8% on Terminal-Bench 4.0 and an Artificial Analysis coding score of 82. Kimi K3, the predecessor to K3.1, trailed Claude Fable 5 on Moonshot's own evaluations.
Is Kimi K3.1 cheaper than Claude Fable 5.1?
Kimi K3.1 pricing has not been published, and the sources reviewed don't confirm Claude Fable 5.1 pricing either. Kimi K3 costs $3 per 1M input tokens and $15 per 1M output tokens. Community testers report that K3 uses about 2× as many tokens as some rivals, which narrows its per-task savings.
Is Kimi K3.1 released yet?
No. Moonshot teased it on September 18–19, 2026 with a Pi-digit post. A Nomura report cited a launch before mid-October, and that window has passed. Claude Fable 5.1 has been available since about September 1, 2026.
Will Kimi K3.1 be open-weight like Kimi K3?
Probably, but it isn't confirmed. Kimi K3 is open-weight, with 2.8T parameters and weights on Hugging Face, and pre-release reports describe a possible open-weight K3.1. Claude Fable 5.1 is closed.
Which is better for coding, Kimi K3.1 or Claude Fable 5.1?
Claude Fable 5.1 is the stronger documented choice for coding today, with a reported 55.8% on Terminal-Bench 4.0 and 67.4% on another coding benchmark. Union Alpha, which some testers suspect is K3.1, was reported at about 74% on DeepSWE and 50% on Terminal Bench v4. Its identity is unconfirmed.
What context window does Kimi K3.1 have compared with Claude Fable 5.1?
Neither is confirmed in available sources. Kimi K3 supports 1,048,576 tokens. Union Alpha, the suspected but unconfirmed K3.1, was reported at 256K to 262K tokens.
Where can I use Kimi K3.1 or Claude Fable 5.1 through an API?
Kimi K3.1 has no public endpoint as of October 17, 2026. Kimi K3 is available through Moonshot, through open weights on Hugging Face and through kie.ai's API. Claude Fable 5.1 is available through Anthropic as claude-fable-5-1.
What to Watch Next
Three signals will settle this comparison. First, watch for an official Kimi K3.1 model card with context length, parameter count and a license, which would confirm whether K3.1 stays open-weight. Second, look for Terminal-Bench 4.0 and Artificial Analysis scores for K3.1 that can sit directly beside Fable 5.1's 55.8% and 82. Third, track Anthropic's rumored Fable 5.2, which could move the target before K3.1 even ships. This page will be updated as each one lands.
Try related models on Kie.ai: Claude Fable 5.1 (Coming Soon), Claude Opus 5.5 and Gemini 4 Argon.
About Maya Chen
Maya tracks AI model releases, benchmarks, and developer adoption signals across the open and closed model landscape.
View all posts by Maya Chen