Kimi K3.1 Pro vs Claude Sonnet 5.5: same price, 2.7x the bill

Kimi K3.1 Pro and Claude Sonnet 5.5 have identical prices on FastMetal: $2.11 per million input tokens and $10.55 per million output tokens. Both have a context window of about a million tokens and both take image input. The spec sheet won't pick one for you, so we ran both on the same prompts and measured what came back.

The short version:

• Kimi K3.1 Pro billed about 2.7x the output tokens. It used more on 34 of 36 prompts, and since the per-token price is the same, the output bill is 2.7x too.
• Kimi's median response time was 150 seconds; Sonnet 5.5's was 14. Sonnet 5.5 was faster on all 36 prompts.
• The visible answers were about the same length (0.94x). On the Japanese prompts, Kimi's answers were actually shorter. Nearly all of the extra tokens are reasoning you never see.

How we measured

We used the 36 prompts from our comparison set that both models have answered: 20 in English and 16 in Japanese, covering debugging, code review, SQL optimization, business email, translation, proofreading, meeting summaries and word problems. Each model got the identical prompt with default settings; we didn't change reasoning effort or anything else.

Kimi K3.1 Pro ran on October 27, 2026 and Claude Sonnet 5.5 on September 30, 2026, both through the FastMetal API. Output tokens are the billed figures our gateway recorded, reasoning included. Response times are wall-clock times on our route and will vary with load. Thirty-six prompts is a small sample, so read this as a trend.

Results

• Kimi used more output tokens on 34 of 36 prompts. The two exceptions both have short answers: a Japanese product-description task (1,329 vs 1,521) and counting the r's in "strawberry" (315 vs 332).
• Sonnet 5.5 answered faster on all 36.
• Across all 36 answers, Kimi wrote 110,191 characters and Sonnet 5.5 wrote 116,786. The token ratio is 2.72x but the character ratio is 0.94x. On the 16 Japanese prompts alone, Kimi's answers were 0.79x the length of Sonnet's.

In dollars, output alone came to about $2.28 for Kimi and $0.84 for Sonnet 5.5 across the 36 prompts. The prompts are short, so input barely moves those totals. Per prompt that's about $0.063 against $0.023. One call won't matter, but an agent that makes dozens of calls per task pays the 2.7x on every one.

The ratio held in both languages: Kimi averaged 5,981 output tokens on the English prompts to Sonnet's 2,191, and 6,012 to 2,213 on the Japanese ones.

The gap grows with the work

The biggest differences came on prompts with many steps or judgment calls:

• Code Review: Kimi 16,506 tokens in 560 s; Sonnet 5.5 3,003 tokens in 21 s
• Recipe Adaptation: Kimi 15,724 tokens in 565 s; Sonnet 5.5 4,874 tokens in 38 s
• Recipe adaptation (Japanese): Kimi 12,958 tokens in 305 s; Sonnet 5.5 2,355 tokens in 20 s

On that Japanese recipe prompt, Kimi's visible answer was 730 characters, shorter than Sonnet's 1,198, yet it billed more than five times the output tokens. Kimi K3.1 Pro reasons at length before it answers, and that reasoning is billed as output.

Short prompts narrow the gap. Counting the r's in "strawberry" took Kimi 315 tokens and 11 s, and Sonnet 5.5 332 tokens and 3 s.

The answers themselves were both good

Volume and speed differed a lot; quality, from the answers we read, did not.

Rewriting for clarity (a long Japanese sentence on the CAP theorem). Kimi returned just the rewrite, three sentences and 180 characters. Sonnet 5.5 returned a rewrite, an explanation of each change and a shorter alternative, 674 characters. Both corrected the original's "you can only have two of three" framing to the accurate version: during a network partition you choose between consistency and availability. The difference is that Sonnet explains why.

Translation with nuance (a Japanese product review into English). Kimi gave the translation alone and rendered 「これは贅沢な悩みかもしれません」 ("this may be a luxury worry") as "but that might be asking too much." Sonnet 5.5 wrote "that's probably a luxury problem" and added notes on each phrase. Both read naturally.

Japanese proofreading (a business notice with typos and grammar slips). Both fixed the doubled character in 「ありがとうございいます」 and the dropped い in 「予定してます」. Kimi separated real errors from style normalization and also flagged that 「3月25日の深夜2時」 ("2 a.m. on the night of March 25") can be read as two different dates. Sonnet 5.5 laid out its corrections in tables.

We read a handful of answers, not all 36. In those, Kimi's Japanese was natural and it tended to answer more briefly.

What the vendors say

As stated on each vendor's own page, checked October 27, 2026:

• Moonshot AI describes Kimi K3.1 Pro as a post-trained version of Kimi K3 and says the full weights will be released by November 4, 2026. Moonshot AI: Kimi K3.1 Pro
• Anthropic released Claude Sonnet 5.5 on September 28, 2026. Anthropic: Introducing Claude Sonnet 5.5

This test can't separate the model's own habits from its default reasoning settings.

Which one to use

• Default to Claude Sonnet 5.5. At the same per-token price it billed about a third of the output and answered about ten times faster on these 36 prompts. That matters most in chat and interactive tools, where waiting is the experience, and in agents that make many calls.
• Kimi K3.1 Pro's case isn't cost or speed. Its answers were concise and its Japanese natural, and with open weights announced you can expect to run the same model yourself later. It fits batch jobs where latency doesn't matter and you specifically want Moonshot's model.
• Kimi can stop thinking; Sonnet 5.5 can't. Our model pages list Kimi K3.1 Pro as supporting reasoning.enabled: false, while Claude Sonnet 5.5 always thinks. We measured both at their defaults, so we haven't tested what turning Kimi's reasoning off does to length or quality.
• Check the data policy. On our model pages, Claude Sonnet 5.5 is listed as "Not used for training"; Kimi K3.1 Pro is currently "Unknown — not declared".

The same per-token price can still mean very different bills and wait times. The reliable test is your own prompts: run a few dozen and look at output tokens and latency.

Both models' answers to the same prompts are side by side on the comparison page. Details are on the Kimi K3.1 Pro and Claude Sonnet 5.5 pages, and dollar prices on the pricing page. The API model ids are kimi-k3.1-pro and anthropic-claude-sonnet-5-5. For a similar measurement against OpenAI, see GPT-6.1 Sol vs Claude Sonnet 5.5.