Kimi K2 Thinking vs Step 3.5 Flash
Kimi K2 Thinking (2025) and Step 3.5 Flash (2026) are frontier-tier reasoning models from Moonshot AI and StepFun. Kimi K2 Thinking ships a 256k-token context window, while Step 3.5 Flash ships a 256k-token context window. On pricing, Step 3.5 Flash costs $0.10/1M input tokens versus $0.60/1M for the alternative. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit.
Step 3.5 Flash is ~500% cheaper at $0.10/1M; pay for Kimi K2 Thinking only for provider fit.
Decision scorecard
Local evidence first| Signal | Kimi K2 Thinking | Step 3.5 Flash |
|---|---|---|
| Best for | reasoning-heavy apps and provider-routed production | reasoning-heavy apps |
| Decision fit | RAG, Long context, and Classification | Long context |
| Context window | 256k | 256k |
| Cheapest output | $2.50/1M tokens | $0.30/1M tokens |
| Provider routes | 7 tracked | 1 tracked |
| Shared benchmarks | 0 rows | 0 rows |
Decision tradeoffs
- Kimi K2 Thinking has broader tracked provider coverage for fallback and procurement flexibility.
- Kimi K2 Thinking uniquely exposes Structured outputs in local model data.
- Local decision data tags Kimi K2 Thinking for RAG, Long context, and Classification.
- Step 3.5 Flash has the lower cheapest tracked output price at $0.30/1M tokens.
- Local decision data tags Step 3.5 Flash for Long context.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output route or tier on this page.
Kimi K2 Thinking
$1,105
Cheapest tracked route/tier: Fireworks AI
Step 3.5 Flash
$155
Cheapest tracked route/tier: OpenRouter
Estimated monthly gap: $950. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- Step 3.5 Flash is $2.20/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Structured outputs before moving production traffic.
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- Kimi K2 Thinking is $2.20/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Kimi K2 Thinking adds Structured outputs in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-01-01 | 2026-01-29 |
| Context window | 256k | 256k |
| Parameters | 1T (32B active) | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Proprietary | Open Source |
| Knowledge cutoff | - | - |
Pricing and availability
| Pricing attribute | Kimi K2 Thinking | Step 3.5 Flash |
|---|---|---|
| Input price | $0.60/1M tokens | $0.10/1M tokens |
| Output price | $2.50/1M tokens | $0.30/1M tokens |
| Providers |
Capabilities
| Capability | Kimi K2 Thinking | Step 3.5 Flash |
|---|---|---|
| Vision | No | No |
| Multimodal | No | No |
| Reasoning | Yes | Yes |
| Function calling | No | No |
| Tool use | No | No |
| Structured outputs | Yes | No |
| Code execution | No | No |
| IDE integration | No | No |
| Computer use | No | No |
| Parallel agents | No | No |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on structured outputs: Kimi K2 Thinking. Both models share reasoning mode, so the practical split is not just feature count. Use those differences to decide whether the page is about raw model quality, agentic coding support, multimodal ingestion, or predictable structured API behavior.
For cost, Kimi K2 Thinking lists $0.60/1M input and $2.50/1M output tokens on the cheapest tracked provider, while Step 3.5 Flash lists $0.10/1M input and $0.30/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Step 3.5 Flash lower by about $1.01 per million blended tokens. Availability is 7 providers versus 1, so concentration risk also matters.
Choose Kimi K2 Thinking when provider fit and broader provider choice are central to the workload. Choose Step 3.5 Flash when provider fit and lower input-token cost are more important. For production, rerun your own prompts through the exact provider, region, and tool stack you plan to ship. This keeps the decision grounded in measurable tradeoffs instead of brand-level assumptions. It also helps separate model capability from provider packaging, which can change cost and latency.
FAQ
Which has a larger context window, Kimi K2 Thinking or Step 3.5 Flash?
Kimi K2 Thinking supports 256k tokens, while Step 3.5 Flash supports 256k tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Which is cheaper, Kimi K2 Thinking or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. Kimi K2 Thinking costs $0.60/1M input and $2.50/1M output tokens. Step 3.5 Flash costs $0.10/1M input and $0.30/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is Kimi K2 Thinking or Step 3.5 Flash open source?
Kimi K2 Thinking is listed under Proprietary. Step 3.5 Flash is listed under Open Source. License labels affect whether you can self-host, redistribute weights, or rely only on hosted APIs, so confirm the upstream license before deployment.
Which is better for reasoning mode, Kimi K2 Thinking or Step 3.5 Flash?
Both Kimi K2 Thinking and Step 3.5 Flash expose reasoning mode. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface.
Which is better for structured outputs, Kimi K2 Thinking or Step 3.5 Flash?
Kimi K2 Thinking has the clearer documented structured outputs signal in this comparison. If structured outputs is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run Kimi K2 Thinking and Step 3.5 Flash?
Kimi K2 Thinking is available on Fireworks AI, GCP Vertex AI, NVIDIA NIM, AWS Bedrock, and OpenRouter. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
Continue comparing
Last reviewed: 2026-05-29. Data sourced from public model cards and provider documentation.