GPT-5.1 vs Grok 4.1 Fast

Head-to-head by test (our 12-test suite):

• Structured output: Grok 4.1 Fast scores 5 vs GPT-5.1’s 4 — Grok wins for strict JSON/schema generation and ranks tied for 1st on structured output (tied with 24 others of 54). That matters when failures in format break downstream parsers.
• Safety_calibration: GPT-5.1 scores 2 vs Grok’s 1 — GPT-5.1 wins on refusing harmful/allowing legitimate requests and ranks 12 of 55 (tied with 19). Grok ranks 32 of 55 on safety calibration. If refusal behavior and calibrated permissions matter, GPT-5.1 is stronger in our testing.
• Faithfulness, classification, long context, persona consistency, multilingual, strategic analysis, constrained rewriting, creative problem solving, tool calling, agentic planning: these are ties in our suite. Both models score e.g. faithfulness 5, long context 5, persona consistency 5, tool calling 4 in our tests — meaning comparable performance on retrieval at 30k+ tokens, staying true to source, and agentic workflows.
• External benchmarks (supplementary): GPT-5.1 scores 68% on SWE-bench Verified and 88.6% on AIME 2025 according to Epoch AI; Grok has no external scores in the payload. GPT-5.1’s SWE-bench rank is 7 of 12 (sole holder) and AIME_2025 rank 7 of 23, which supports its capability on coding/math tasks in external measures. Interpretation for real tasks: choose Grok when strict output format, massive context (2,000,000 tokens), and cost per token dominate (e.g., customer support, multi-document retrieval pipelines). Choose GPT-5.1 when you require stronger safety calibration and want the external-benchmark backing on math/coding (SWE-bench 68%, AIME 88.6% per Epoch AI) despite substantially higher per-token costs.

Raw per-mTok prices from the payload: GPT-5.1 input $1.25 / mTok, output $10 / mTok; Grok 4.1 Fast input $0.20 / mTok, output $0.50 / mTok (priceRatio = 20). Translate to real volumes (assuming a 50/50 split of input vs output tokens):

• 1M tokens (500k input + 500k output): GPT-5.1 = $625 + $5,000 = $5,625; Grok = $100 + $250 = $350.
• 10M tokens: GPT-5.1 = $6,250 + $50,000 = $56,250; Grok = $1,000 + $2,500 = $3,500.
• 100M tokens: GPT-5.1 = $62,500 + $500,000 = $562,500; Grok = $10,000 + $25,000 = $35,000. Notes: the payload’s priceRatio=20 reflects the output-cost ratio (10 / 0.5 = 20). Your actual multiplier depends on input/output mix (GPT-5.1’s input cost is only ~6.25× Grok’s). Teams with heavy output or very large scale (10M+ tokens/month) should care most — Grok reduces bill by an order of magnitude in typical mixes; GPT-5.1 is only economical where its specific wins justify the expense.

Choose GPT-5.1 if: you need stronger safety calibration (GPT-5.1 wins safety calibration, rank 12/55 in our tests), external math/coding signal (68% on SWE-bench Verified and 88.6% on AIME 2025, Epoch AI), or are building workloads where the extra cost is justifiable for those wins. Choose Grok 4.1 Fast if: you need strict structured output (Grok scores 5 on structured output and ranks tied for 1st), huge context (2,000,000-token window), agentic/tooling at scale, or have cost-sensitive production (Grok’s output cost is $0.50/mTok vs GPT-5.1’s $10/mTok).