Quick Take

This comparison evaluates OpenAI’s GPT-5.5 Instant, released in June 2026, against Z AI’s GLM-5.1 (Reasoning), released in April 2026. While both models represent recent advancements in the LLM landscape, GLM-5.1 (Reasoning) establishes a clear lead in technical benchmarks and operational transparency.

Benchmark Read

GLM-5.1 (Reasoning) demonstrates superior capabilities across the board. It holds an intelligence index of 40.2 and a coding index of 55.8, compared to GPT-5.5 Instant’s 28.9 and 39.4, respectively. In specific benchmarks, GLM-5.1 (Reasoning) scores 0.868 on GPQA and 0.28 on HLE, outperforming GPT-5.5 Instant’s 0.823 and 0.186. While GPT-5.5 Instant leads slightly in SciCode (0.486 vs 0.438) and LCR (0.64 vs 0.623), GLM-5.1 (Reasoning) provides a broader suite of performance data, including strong results in IFBench (0.763), TerminalBench Hard (0.432), and TAU2 (0.977).

Cost and Speed

Cost efficiency is a major differentiator. GLM-5.1 (Reasoning) is priced at a blended rate of $2.15/1M tokens, significantly cheaper than GPT-5.5 Instant’s $11.25/1M. Furthermore, Z AI provides full transparency regarding speed, with an output of 81.318 tok/s and a time to first token of 0.844s. Conversely, these performance metrics remain unknown for GPT-5.5 Instant, complicating integration planning for latency-sensitive applications.

Best Fit

GLM-5.1 (Reasoning) is best suited for developers requiring high-performance reasoning and coding capabilities at a competitive price point. Its documented speed makes it ideal for production environments where latency is a critical factor. GPT-5.5 Instant may serve as a niche alternative, though its higher cost and lack of published performance data make it less attractive for high-volume enterprise use.