Claude Mythos 5 vs Grok 4.5

What I tested — same prompt, two models

I run the same fixed prompt set through every new model the day it drops — same string, one shot, single HTML file out — and I score the result 0–10 on whether it ran, how close it hit the brief, and how good it looked. Below is what came out when I gave the exact same prompts to Claude Mythos 5 and Grok 4.5, side by side, on 0 shared tasks inside the Agent Operating System.

Both models were given identical prompts inside the Agent Operating System — no help, no iteration, no "best of N" tricks. I run each prompt once, save the HTML file the model produces, and score it 0–10 on whether it ran, how close it hit the brief, and how good it looked. The scoring is mine. The verdicts below are pulled from my source comparison guides at agentos.guide where I publish every score and the reasoning behind it.

Claude Mythos 5 · Not currently dispatched from Agent OS — no public API. Tracked for news coverage only.

Grok 4.5 · Benched one-shot on the same GoldieBench game prompts as the field, with the threejs-game-director patterns baked into each prompt; weak builds iterated by Grok 4.5 itself (the model authors every fix, never hand-patched). Wired into the Agent OS as the newest engine.

Strengths & weaknesses I logged

Pricing & context — the spec sheet

The verdict — which should you pick?

Not enough scored shared tasks yet for a head-to-head average. The live demos for both are on the matrix above — play them and form your own opinion.

If you only run one of these inside your stack, the head-to-head average above is the call. If you can run both, my honest play is to wire Claude Mythos 5 and Grok 4.5 both into the Agent Operating System and dispatch each from the kanban by task type — vetted enterprise partners with mythos-class access → Claude Mythos 5, one-shot end-to-end app + game builds from a single prompt → Grok 4.5. That's the same setup I run for the 4,000+ founders inside the AI Profit Boardroom.

FAQ — Claude Mythos 5 vs Grok 4.5

Which is better, Claude Mythos 5 or Grok 4.5?

On Goldie Bench, Claude Mythos 5 averages no scored verdicts yet across the shared tasks, with 0 gold, 0 silver, 0 bronze overall. Grok 4.5 averages no scored verdicts yet, with 12 gold, 34 silver, 2 bronze. Not enough scored shared tasks yet to call a winner.

How much does Claude Mythos 5 cost vs Grok 4.5?

Claude Mythos 5: Mythos 5 is the un-released sibling of Fable 5. Same Mythos class, restricted to vetted partners. Made headlines mid-June 2026 when reports of a White-House-driven ban over China access dominated AI press. Grok 4.5: ~4.2x fewer output tokens than Opus 4.8 on SWE-Bench Pro (15,954 vs 67,020 avg) and served at ~80 TPS, so real cost/latency is well below the sticker. Free for a limited time in Grok Build + Cursor.

What's the context window for Claude Mythos 5 vs Grok 4.5?

Claude Mythos 5 has a 200,000 tokens context window. Grok 4.5 has a xAI's smartest model, built for coding + agentic tasks; trained alongside Cursor. Default model in Grok Build. Benched here via OpenRouter (x-ai/grok-4.5). context window.

When should I pick Claude Mythos 5 over Grok 4.5?

Pick Claude Mythos 5 for: Vetted enterprise partners with Mythos-class access; (For everyone else: read the Mythos ban coverage instead of waiting for access). The trade-off is the weaknesses we logged on the bench: No public API access — can't be benched against the open field; Restricted distribution makes it effectively unavailable to most builders.

When should I pick Grok 4.5 over Claude Mythos 5?

Pick Grok 4.5 for: One-shot end-to-end app + game builds from a single prompt; Cost/latency-sensitive agentic coding loops (token-efficient + fast); Office-work automation (Excel/PowerPoint/Word via Grok Build). The trade-off is the weaknesses we logged on the bench: Raycaster/FPS (doom) under-renders + walks out of bounds one-shot; needed multiple self-fix passes; Occasional TDZ/init bug blanks a build to black (racing) — recovered by the model itself in one pass; Not available in the EU until mid-July 2026.

How does Goldie Bench score Claude Mythos 5 vs Grok 4.5?

Every demo on this page was built by Julian Goldie inside the Agent Operating System — same fixed prompt for both models, one shot, single HTML file out. Each result gets a 0–10 score on whether it ran, how close it hit the brief, and how good it looked. The highest score on each task gets gold; second gets silver; third gets bronze. See methodology for full provenance.

Related comparisons

Other head-to-heads using the same scoring system:

Full model pages: Claude Mythos 5 · Grok 4.5 · back to the leaderboard