As GPT 5.6 is confirmed to launch tomorrow, today is pretty much the last day anyone will be excited about a GPT 5.5 equivalent model launch, and that is exactly what SpaceXAI did:

Cursor@cursor_ai

We've partnered with SpaceXAI to train Grok 4.5. It’s our most powerful model yet and the first we've built for more than software engineering.

5:57 PM · Jul 8, 2026 · 3.24M Views

605 Replies · 1.26K Reposts · 14.4K Likes

The new Grok 4.5 is a different weight class than the Composer series (1.5T) and despite the solid evals still performs very comparably to the current workhorse Opus and GPTs, although per OpenAI’s evals team even the mighty SWE-Bench Pro is now saturated/terminally flawed - leaving presumably a small list of successors including FrontierCode.

As for training and data disclosures, this is all the information we have.

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Top Story: Grok 4.5 release

xAI/“SpaceXAI” publicly launched Grok 4.5 as a new coding-and-agents-focused frontier model, positioned on capability-per-dollar rather than absolute benchmark supremacy.

• Elon Musk first said Grok 4.5 would be made public “tomorrow” based on strong beta feedback, calling it “Opus-class,” but faster, more token-efficient, and lower cost @elonmusk.

• Musk later framed Grok 4.5 internally as “roughly comparable to Opus 4.7, but much faster,” emphasizing usefulness to Tesla and SpaceX engineers over benchmark chasing @elonmusk.

• The official launch came from xAI’s account, describing Grok 4.5 as “our first model trained specifically for coding and agents,” trained with Cursor, and offering “frontier intelligence at leading speeds and cost efficiency” @SpaceXAI.

• Cursor said it partnered with xAI to train Grok 4.5, called it “our most powerful model yet,” and stressed that it was “the first we’ve built for more than software engineering” @cursor_ai.

• Cursor also announced in-product availability with “double usage for the first week” @cursor_ai.

• Cursor clarified that “Grok 4.5 and Composer are two different model weight classes,” and that Composer 2.5 would remain available with future models in that smaller class @cursor_ai.

• Early ecosystem support appeared immediately: Grok 4.5 became available in Grok Build/API/Cursor @milichab, day-0 support was announced for Hermes Agent @Teknium, and later live availability in Hermes Agent/Portal/OpenRouter/Grok subscriptions was confirmed @Teknium.

• Musk said the context window would likely move from 500k back to 1M “by next week” @elonmusk.

Officially, xAI’s message was not “best overall model,” but near-Opus quality with materially better economics and speed:

• “Opus-class model, but faster, more token-efficient and lower cost” @elonmusk

• “First model trained specifically for coding and agents” @SpaceXAI

• “Frontier intelligence at leading speeds and cost efficiency” @SpaceXAI

• “Most powerful model yet” and “first we’ve built for more than software engineering” @cursor_ai

This framing matters: xAI is explicitly targeting the coding-agent workflow market that has recently been dominated by Anthropic/OpenAI/Cursor-style tool-using systems, not just general chat.

The concrete numbers that surfaced:

• Official pricing: $2 / 1M input tokens, $6 / 1M output tokens @scaling01

• Artificial Analysis repeated the same price point and added:

  • cache hits discounted by 75% to $0.5 / 1M tokens

  • long inputs over 200k tokens cost double

  • 500k context window, down from Grok 4.3’s 1M

  • vision input retained

  • configurable reasoning retained @ArtificialAnlys

• Musk later said the context window would probably upgrade back to 1M soon @elonmusk.

Relative pricing comparisons cited by users:

• Grok 4.5: $2 in / $6 out

• GPT-5.6: $5 in / $30 out

• Opus 4.8: $5 in / $25 out @kimmonismus

One important spec surfaced via third-party reporting of Musk’s disclosure:

• Grok 4.5 is 3x larger than Grok 4.3 at 1.5T parameters @ArtificialAnlys

That is a notable jump, and likely central to why multiple observers interpreted 4.5 as xAI’s first entry into the true flagship coding-agent tier rather than an iterative refresh.

Artificial Analysis provided the most substantive external evaluation in the tweet set.

Key results:

• #4 on Artificial Analysis Intelligence Index, score 54, behind only Fable 5, GPT-5.5, and Opus 4.8 @ArtificialAnlys

• +16 points vs Grok 4.3 on the same index @ArtificialAnlys

• GDPval-AA v2 Elo 1543, also ranking #4, behind Anthropic’s latest Claude releases @ArtificialAnlys

• Top score on τ³-Banking: 33%, above 31% for GPT-5.5 (xhigh) @ArtificialAnlys

• Artificial Analysis Coding Agent Index score 76 in Grok Build, “on par with GPT-5.5 in Codex” and below Fable 5 in Claude Code @ArtificialAnlys

• Cost per Intelligence Index task: $0.31 @ArtificialAnlys

• Cost per GDPval task: $0.49 @ArtificialAnlys

• Cost per Coding Agent Index task: $2.59 @ArtificialAnlys

• Average output tokens per Intelligence Index task: ~14k, over 60% lower than Opus 4.8 @ArtificialAnlys

• Average total tokens per Coding Agent Index task: 1.9M, versus 7.2M for Fable 5 in Claude Code and 6.2M for GPT-5.5 in Codex @ArtificialAnlys

Artificial Analysis’ interpretation was clear: Grok 4.5 is near-frontier on capability, but unusually strong on efficiency, making it sit on the Pareto frontier for cost/performance.

Musk explicitly amplified the Artificial Analysis assessment @elonmusk.