DeepSeek V3.1 vs Step 3.5 Flash
DeepSeek V3.1 (2025) and Step 3.5 Flash (2026) are frontier reasoning models from DeepSeek and StepFun. DeepSeek V3.1 ships a 64k-token context window, while Step 3.5 Flash ships a 256k-token context window. On SWE-bench Verified, Step 3.5 Flash leads by 8.4 pts. On pricing, Step 3.5 Flash costs $0.10/1M input tokens versus $0.27/1M for the alternative. This comparison covers specs, pricing, API access, capabilities, benchmarks, input and output token costs, and production fit for coding and agent workloads.
Step 3.5 Flash is ~170% cheaper at $0.10/1M; pay for DeepSeek V3.1 only for coding workflow support.
Decision scorecard
Local evidence first| Signal | DeepSeek V3.1 | Step 3.5 Flash |
|---|---|---|
| Best for | multimodal apps and provider-routed production | reasoning-heavy apps |
| Decision fit | Coding, Agents, and Vision | Coding, Agents, and Long context |
| Context window | 64k | 256k |
| Cheapest output | $1/1M tokens | $0.30/1M tokens |
| Provider routes | 8 tracked | 1 tracked |
| Shared benchmarks | 1 shared | SWE-bench Verified leader |
Decision tradeoffs
- DeepSeek V3.1 has broader tracked provider coverage for fallback and route flexibility.
- DeepSeek V3.1 uniquely exposes Vision, Multimodal, and Structured outputs in local model data.
- Local decision data tags DeepSeek V3.1 for Coding, Agents, and Vision.
- Step 3.5 Flash holds a shared-benchmark lead on SWE-bench Verified, ahead by 8.4 points.
- Step 3.5 Flash has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Step 3.5 Flash has the lower cheapest tracked output price at $0.30/1M tokens.
- Step 3.5 Flash uniquely exposes Reasoning in local model data.
- Local decision data tags Step 3.5 Flash for Coding, Agents, and Long context.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output route or tier on this page.
DeepSeek V3.1
$466
Cheapest tracked route/tier: Novita AI
Step 3.5 Flash
$155
Cheapest tracked route/tier: OpenRouter
Estimated monthly gap: $311. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- No overlapping tracked provider route is sourced for DeepSeek V3.1 and Step 3.5 Flash; plan for SDK, billing, or endpoint changes.
- Step 3.5 Flash is $0.70/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Vision, Multimodal, and Structured outputs before moving production traffic.
- Step 3.5 Flash adds Reasoning in local capability data.
- No overlapping tracked provider route is sourced for Step 3.5 Flash and DeepSeek V3.1; plan for SDK, billing, or endpoint changes.
- DeepSeek V3.1 is $0.70/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Check replacement coverage for Reasoning before moving production traffic.
- DeepSeek V3.1 adds Vision, Multimodal, and Structured outputs in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-08-21 | 2026-01-29 |
| Context window | 64k | 256k |
| Parameters | 671B total, 37B active (MoE) | 196B (11B active) |
| Architecture | Mixture of Experts | Mixture of Experts |
| License | MITOSI-approved | Proprietary |
| Openness | Open source | Proprietary |
| Weights | Unknown | Not released |
| Code | Unknown | Unknown |
| Commercial use | Commercial use: permitted | Commercial use: conditional |
| Knowledge cutoff | - | - |
Pricing and availability
| Pricing attribute | DeepSeek V3.1 | Step 3.5 Flash |
|---|---|---|
| Input price | $0.27/1M tokens | $0.10/1M tokens |
| Output price | $1/1M tokens | $0.30/1M tokens |
| Providers |
Capabilities
| Capability | DeepSeek V3.1 | Step 3.5 Flash |
|---|---|---|
| Vision | Yes | No |
| Multimodal | Yes | No |
| Reasoning | No | Yes |
| JSON / Tool use | No | No |
| Structured outputs | Yes | No |
| Code execution | Yes | No |
| IDE integration | No | No |
| Computer use | No | No |
| Parallel agents | No | No |
Benchmarks
| Benchmark | DeepSeek V3.1 | Step 3.5 Flash |
|---|---|---|
| SWE-bench Verified | 66.0 | 74.4 |
Continue comparing
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Last reviewed: 2026-06-29. Data sourced from public model cards and provider documentation.