DeepSeek V3.1 Terminus vs GPT-5.1

Summary of our 12-test comparison (scores shown are from our testing):

• Faithfulness: GPT-5.1 5 vs DeepSeek 3 — GPT-5.1 wins and ranks tied for 1st of 55 models, indicating better stick-to-source behavior in our tests (fewer hallucinations).
• Classification: GPT-5.1 4 vs DeepSeek 3 — GPT-5.1 wins and is tied for 1st of 53 models, so routing and categorization are stronger in our runs.
• Tool calling: GPT-5.1 4 vs DeepSeek 3 — GPT-5.1 wins and ranks 18 of 54; expect better function selection and argument accuracy with GPT-5.1 in agentic flows.
• Constrained rewriting: GPT-5.1 4 vs DeepSeek 3 — GPT-5.1 wins (rank 6 of 53), useful when compressing content into hard limits.
• Safety calibration: GPT-5.1 2 vs DeepSeek 1 — GPT-5.1 wins (rank 12 of 55), meaning it refused harmful prompts more appropriately in our tests.
• Persona consistency: GPT-5.1 5 vs DeepSeek 4 — GPT-5.1 wins and is tied for 1st, so it better maintains character and resists injection attacks in our samples.
• Structured output: DeepSeek 5 vs GPT-5.1 4 — DeepSeek wins and is tied for 1st of 54 models, showing superior JSON/schema compliance in our runs.
• Strategic analysis, creative problem solving, long context, agentic planning, multilingual: ties across both models (scores 4–5). Notably, both score 5 for long_context and rank tied for 1st on long-context retrieval at 30K+ tokens. External benchmarks (Epoch AI): GPT-5.1 scores 68% on SWE-bench Verified (rank 7 of 12) and 88.6% on AIME 2025 (rank 7 of 23) — we cite these as supplementary evidence of coding/math strength for GPT-5.1. DeepSeek has no external SWE/MATH scores in the payload. In short: GPT-5.1 is stronger on factual fidelity, classification, tool workflows and safety in our tests; DeepSeek is best for strict structured outputs and offers comparable long-context performance at far lower cost.

Pricing (per 1,000 tokens): DeepSeek V3.1 Terminus input $0.21 / output $0.79; GPT-5.1 input $1.25 / output $10.00. Assuming equal input and output volume (1M input + 1M output tokens/month): DeepSeek costs $1,000 ($210 input + $790 output) while GPT-5.1 costs $11,250 ($1,250 input + $10,000 output). At 10M/10M tokens/month: DeepSeek $10,000 vs GPT-5.1 $112,500. At 100M/100M: DeepSeek $100,000 vs GPT-5.1 $1,125,000. The gap matters for high-volume apps (10M+ tokens/mo): GPT-5.1 delivers accuracy gains but at ~11x the per-million cost; cost-sensitive startups, large-scale embedding/ingestion pipelines, or apps with predictable JSON outputs will prefer DeepSeek for unit economics.

Choose DeepSeek V3.1 Terminus if: you need strict JSON/schema compliance, long-context retrieval, or have heavy volume and must minimize cost (DeepSeek costs $1,000 per 1M in+out vs GPT-5.1 $11,250 in our equal-volume example). Choose GPT-5.1 if: you prioritize faithfulness, classification accuracy, tool calling, persona consistency or improved safety behavior and can absorb much higher per-token fees (GPT-5.1 input $1.25 / output $10 per 1k tokens).