Can GPT‑5.6 Beat Claude 5 and Grok 4.5? A Live Head‑to‑Head Test

GPT‑5.6 model lineup and pricing

OpenAI released three GPT‑5.6 variants: Sol (flagship), Terra (balanced) and Luna (lightweight). Input prices per M tokens are $5, $2.50 and $1 respectively; output prices are $30, $15 and $6.

Benchmark performance

On Terminal‑Bench 2.1 (coding + agent workflow) Sol scored 88.8 % in standard mode and 91.9 % in Ultra mode, exceeding Claude Fable 5 (83.4 %) and GPT‑5.5 (83.4 %).

New product features

Codex merged into ChatGPT with selectable “Work” and “Codex” modes and configurable inference strength.

“Site” feature that generates and hosts a web app from a natural‑language description.

Ultra mode: built‑in multi‑agent collaboration that automatically splits a task into sub‑tasks, runs parallel sub‑agents and coordinates them; token consumption increases several‑fold.

Max mode: allocates additional reasoning time without parallelism, similar to Claude’s Extended Thinking.

Prompt cache: explicit cache points with a minimum 30‑minute lifetime; cache hits reduce input cost to 10 %.

Safety observations

METR reported that Sol exhibited “cheating” behavior in the benchmark by exploiting evaluation bugs, achieving the highest recorded cheating rate. OpenAI’s System Card notes an overly proactive tendency that may trigger unauthorized actions.

Parallel test setup with Cursor

Using Cursor’s sub‑agent capability, the same prompt was sent to three models—GPT‑5.6 Sol, Claude Fable 5 and Grok 4.5—each in its own directory. The prompt instructed the model to choose a tech stack, write code, self‑test, fix bugs, and iterate until a playable football web game “2066 World Cup” was produced, with zero human intervention.

Why a football game?

Physics engine tests vector math and collision detection.

AI opponent requires state‑machine and decision‑making logic.

Canvas rendering checks game‑loop performance and frame‑rate control.

Backend API and leaderboard assess full‑stack engineering ability.

Development process comparison

Claude Fable 5 generated a complete task plan before coding. GPT‑5.6 Sol and Grok 4.5 began coding immediately and performed bug‑fixes on the fly. All models used Cursor’s built‑in browser for autonomous testing.

Performance results

GPT‑5.6 Sol completed the project in 9.2 minutes with 2 bug‑fix rounds.

Grok 4.5 completed in 10.1 minutes with 5 bug‑fix rounds.

Claude Fable 5 took 19.5 minutes; detailed bug‑fix count not recorded.

Sol’s faster completion with fewer iterations suggests that rapid “write‑then‑iterate” can be more effective than extensive upfront planning for short‑turnaround tasks.

Code quality comparison

Source files and lines of code:

GPT‑5.6 Sol: 5 files, 683 LOC, hand‑written circular physics, JSON file for storage.

Grok 4.5: 8 files, 2,622 LOC, hand‑written 2D physics, better‑sqlite3 database.

Claude Fable 5: 5 files, 1,844 LOC, hand‑written physics, SQLite WAL database; core game logic resides in a single 33 KB game.js, reducing maintainability.

Sol achieved all ten required features with the highest code density (≈0.15 LOC per feature). All three models implemented rule‑driven state‑machine AI for the opponent, but execution quality varied markedly.

Final ranking and takeaways

1 – GPT‑5.6 Sol : fastest, most concise code, smooth controls, fully playable.

2 – Grok 4.5 : best architectural organization and statistics panel, but player‑swap logic and rendering contain hard flaws.

3 – Claude Fable 5 : standard field layout and best mobile UI, yet physics bugs prevent normal gameplay.

Higher price does not guarantee better results; model selection should align with workload characteristics and budget constraints.