At 9:24 PM Eastern on September 13, 2026, someone on X asked what to expect from SpaceXAI in September apart from Grok 4.7. Elon Musk answered with a model nobody had named before: “Grok 4.8, which is a 2.5T model trained with our new C++ software stack, will finish training this week and start RL” (source). Twenty minutes later the post had 65,000 views. Two days earlier he had said Grok 4.7, the model before it, needs a few more days.
This page is the record for Grok 4.8: what has been said, by whom, when, and what xAI has published, which as of September 14 is nothing. It sits next to our Grok 4.7 tracker, which carries the earlier model’s four missed dates. Every quotation below is from a post read at its own timestamp; dates are UTC unless marked Eastern.
On this page · 12 sectionsOpen
Grok 4.8 exists in exactly one place as of September 14, 2026: a post by Elon Musk at 9:24 PM Eastern on September 13. xAI’s news page, model directory, API release notes and llms.txt contain no Grok 4.8, and no earlier Musk post uses the name.
His description, in full: a 2.5T model trained with xAI’s new C++ software stack, finishing training this week, then reinforcement learning. No date, no benchmark, no price, and no xAI document behind any of it.
It was posted while Grok 4.7 is still unreleased. The 4.7 target of September 11 passed; Musk said that day it needs a few more days. The successor was named two days later, in reply to a post asking what else to expect from xAI this month.
The C++ stack has its own timeline, all from Musk: a C training stack almost finished on May 28, the whole training and inference stack to be rewritten in C/C++ within about three months on June 29, and Grok 4.5 not yet on the new inference software on July 8. September 13 is the first time a numbered model is said to be trained on it.
Parameter counts, per Musk: a 0.5T model served all production traffic in May; the 1.5T V9-Medium became Grok 4.5 and 4.6; 4.7 is 2.1T; 4.8 is 2.5T. xAI has never published a parameter count for any Grok 4 model.
His own cadence puts a release weeks out: the 1.5T model finished training on May 25 and shipped as Grok 4.5 on July 16, 52 days later; the 2T run was to finish training the week of July 18 and Grok 4.6 shipped August 12, 25 days later. A model finishing training in mid-September lands, on those gaps, between early October and early November.
This is a living tracker. When xAI names Grok 4.8 on x.ai or docs.x.ai, a model id, a price or a launch post appears, the confirmed facts land here the same day next to a scorecard of every claim above.
§ 01What we actually know
| Claim | Status |
|---|---|
| Grok 4.8 exists and is in training | Musk on X, September 13 (9:24 PM Eastern); no xAI document |
| 2.5 trillion parameters | Musk on X, same post; unverified, no xAI spec for any Grok 4 model |
| Trained with a new C++ software stack | Musk on X, same post; the stack itself described by him since May 28 |
| Training finishes the week of September 14; RL follows | Musk on X, same post; no date for RL to end |
| Release date | Nothing from anyone |
| Benchmarks, price, context window, API id, model card | Nothing exists |
| Earlier mention of the name | None: a search of Musk’s own account for the name returns only the September 13 post |
| xAI surfaces (news, model directory, release notes, llms.txt) | No Grok 4.8, no Grok 4.7; Grok 4.6 is the released flagship (read 01:47 UTC, September 14) |
| Grok 4.7, the model before it | Unreleased; September 11 target missed; Musk: a few more days |
§ 02What Musk has said, word for word
The Grok 4.8 record is one post. The record for the stack it is said to run on is four months long, and every entry is his.
September 13, 9:24 PM Eastern (01:24 UTC September 14): “Grok 4.8, which is a 2.5T model trained with our new C++ software stack, will finish training this week and start RL” (source). The post is a reply; the question it answers reads “Apart from Grok 4.7, what are we even expecting from SpaceXAI in september”. Nothing about 4.7 in the answer.
May 28, 06:27 UTC: “SpaceX has almost finished writing V1.0 of an in-house AI training stack in C that exact-maps to 220k GB300s with 800G NICs, making heavy use of pipeline parallelism and getting as close to bare metal as possible.” (source)
May 28, 16:40 UTC: “Next will be writing the inference stack in C for simultaneous high-speed RL across a large block of GB300s.” and, in the same post, “(We do use a little C++ tbh, but not much)” (source)
June 29, 00:10 UTC: “Truly massive gains will come in ~3 months when the entire training and inference stack is written in C/C++ and massively simplified (most software layers will be deleted completely) and we exact-map Grok to work incredibly well on a GB300” (source)
July 8, 21:30 UTC: “Grok 4.5 is not yet using our internally developed C/C++ inference software that exact maps to the GB300 hardware. Doubling or more of the current speed is probably achievable.” (source)
Read together, the September 13 post is the first time the stack he has been describing since May is attached to a numbered model. Three months from June 29 is late September, so a model finishing training on the new stack in mid-September is inside his own window. Whether 4.7, whose initial training finished in August, was trained on the old stack or the new one, he has not said.
§ 03The stack that finally got a model
| Date (UTC) | What he said | What it implies |
|---|---|---|
| May 28 | A C training stack, V1.0 almost finished, mapped to 220,000 GB300s | Training side first |
| May 28 | Inference stack in C is next, for high-speed RL; a little C++ in use | RL is the target workload |
| Jun 29 | Entire training and inference stack in C/C++ in about three months | A late-September horizon |
| Jul 8 | Grok 4.5 is not yet on the new inference software | 4.5 shipped on the old stack |
| Sep 13 | Grok 4.8 is trained with the new C++ software stack | First model named on it |
The word changed along the way. May was a stack written in C, with a little C++ admitted; June was C/C++; September is a C++ software stack. That is a founder describing an engineering project in passing, not a spec sheet, and it should be read that way. What is consistent is the claim: fewer software layers, code mapped to one Nvidia system, and speed as the payoff.
§ 04The parameter ladder, all from one account
| Date (UTC) | Count | Attached to | Source |
|---|---|---|---|
| May 25 | 0.5T | The v8-small model serving all Grok production traffic at the time | Musk on X |
| May 25 | 1.5T | V9-Medium, finished training; became Grok 4.5 | Musk on X |
| Jul 18 | 2T | A model he tagged Grok 4.6 with a question mark | Musk on X |
| Jul 28 | 1.5T and 2.1T | Grok 4.6 is the 1.5T model; Grok 4.7 the 2.1T | Musk on X |
| Sep 13 | 2.5T | Grok 4.8 | Musk on X |
| Any date | Any count | Any Grok 4 model | xAI: none published |
Two cautions. The July 18 post attached a 2T model to Grok 4.6 with a question mark, and ten days later the 4.6 label went to the 1.5T model with the 2T-class run renamed 2.1T and called 4.7, so the founder’s own mapping of counts to names has moved once already. And total parameter counts for mixture-of-experts models describe memory, not speed; he said 4.7 would be “slightly slower to serve” than 4.6 for exactly that reason, and 2.5T on a new stack could be faster or slower than 2.1T on the old one. Nobody outside xAI can say.
§ 05What finishing training has meant before
The only calibration for “will finish training this week and start RL” is what happened the last two times he posted that shape of sentence.
| Model | The post | Posted (UTC) | xAI release | Gap |
|---|---|---|---|---|
| Grok 4.5 | “Grok foundation model V9-Medium (1.5T) has finished training.” then “2 to 3 weeks to public release.” | May 25 | July 16 | 52 days |
| Grok 4.6 | “2T Grok (Grok 4.6?) will finish training next week.” | Jul 18 | August 12 | 25 days |
| Grok 4.7 | “Initial training is complete” (from the August 12 post; SpaceX data added in supplemental training) | Aug 12 | Not released as of September 14 | 33 days and counting |
| Grok 4.8 | “will finish training this week and start RL” | Sep 13 | Not released | Open |
Apply those gaps to a training finish in the week of September 14 and Grok 4.8 lands between about October 9 and about November 6, if it follows either precedent. The May 25 post also promised “2 to 3 weeks to public release” for a model that took seven, so the shorter end of any estimate he gives has not held yet. None of this is a prediction. It is the arithmetic on his own record, and the record says the release comes later than the first sentence implies.
§ 06Where this leaves Grok 4.7
The honest reading of the September 13 post is that Grok 4.7 is still coming, still in RL, and now has a successor on its heels. Grok 4.5 shipped on July 16 and was superseded by 4.6 on August 12, 27 days later, so a short-lived 4.7 would not be new for xAI. The question for anyone waiting on 4.7 is whether xAI ships it as the flagship for a few weeks or folds the wait into 4.8. Nothing said so far answers that; the Grok 4.7 tracker grades every 4.7 date as it lands or slips.
§ 07What would confirm it
- A line on x.ai or docs.x.ai. A news post, a model page, a release note or a model id containing 4.8. Grok 4.6’s launch post arrived the same day as its API.
- Musk saying RL has started. His own post makes “start RL” the next milestone; a post confirming it dates the clock.
- A 4.7 release. Every 4.8 date depends on whether 4.7 ships first or gets skipped; the 4.7 tracker is the tell.
- A price relative to 4.6. Grok 4.6 is $2 per million input tokens and $6 output on xAI’s API. A larger model on a faster stack could move either way; the Grok Bot pricing page changes with it.
- Anyone outside xAI naming a benchmark. No leak, no sheet and no anonymous checkpoint has been attributed to 4.8. The last stealth model guessed to be a Grok, Ox Alpha, was GLM-5.3-Flash.
§ 08Where CellCog stands
Our conflict, declared: we build CellCog, a platform where a business hires AI employees that keep memory and work as a team, and we choose the models underneath them. Today that is Claude Fable 5.1 at Core and Max and Gemini 3.8 Flash at Flash. We do not route to Grok. We track xAI’s models the way an operator does, as candidate engines, and a Grok 4.8 that is fast, cheap and good at agentic work would get tested on our infrastructure the day it ships. Until then, this page records what has been said and refuses to round it up.
§ 09What we are watching for
- The xAI surfaces, several times a day: news feed, model directory, release notes, llms.txt.
- Musk’s account for a training-done or RL-started post, and for any date.
- Grok 4.7’s release, which resets the arithmetic above.
- The stack claim spreading to 4.7: if he says 4.7 was also trained on the new stack, the first-model-on-it framing here changes.
§ 10The tracker
This page updates when facts change, not when posts get louder. As of September 14, 2026: Grok 4.8 has been named once, by its founder, with a size, a stack and a stage, and by nobody else. No date, no spec, no benchmark, no price exist in any xAI document. When xAI moves, the confirmed facts and a graded scorecard of every claim above land here the same day.
§ 11Update log
This is a living page; when the story moves, the update lands here.
As of September 14, 2026: page opened. Only source is the September 13 post; xAI surfaces read at 01:47 UTC, zero mentions.
§ 12Sources
Elon Musk on X, each read at the post’s own timestamp: September 13, 9:24 PM Eastern (Grok 4.8), replying to Tech Dev Notes; May 25 (V9-Medium finished training); May 28, 06:27 UTC and 16:40 UTC (the C training and inference stack); June 29 (whole stack in C/C++); July 8 (4.5 not yet on the new inference software); July 18 (2T model); July 28 (1.5T and 2.1T); August 12 (4.7 initial training complete); September 11 (4.7 needs a few more days). xAI: news, model directory, Grok 4.6 launch post, read September 14, 2026, 01:47 UTC. Grok 4.5 and 4.6 release dates as recorded on our Grok 4.7 tracker from x.ai. CellCog routing statements reflect our configuration on September 14, 2026.
Q1Has xAI announced Grok 4.8?
No. We checked xAI’s news feed, its model directory, its API release notes and its llms.txt at 01:47 UTC on September 14: zero mentions of Grok 4.8, and zero of Grok 4.7. The only source is Musk’s post, and a search of his own account for the name returns only that post.
Q2What is the new C++ software stack?
Musk’s phrase for an in-house training and inference stack written in C and C++ to run as close to bare metal as possible on Nvidia GB300 systems. He first described a C training stack on May 28, said on June 29 that the entire training and inference stack would be in C/C++ within about three months, and said on July 8 that Grok 4.5 was not yet using the new inference software. Grok 4.8 is the first model he has said was trained on it.
Q3Is 2.5T a real specification?
It is Musk’s number, and his numbers are the only ones there are: 1.5T for the V9-Medium model behind Grok 4.5 and 4.6, 2.1T for Grok 4.7, 2.5T for Grok 4.8. xAI has not published a parameter count for any Grok 4 model, and total counts for mixture-of-experts models say little about active parameters or speed.
Q4Will Grok 4.7 ship before Grok 4.8?
Almost certainly, on what has been said: 4.7 finished initial training in August and is in late RL, while 4.8 has not finished training. The question the September 13 post raises is different: whether 4.7 becomes a short-lived release, the way 4.5 was replaced by 4.6 within four weeks.
Q5Should I wait for Grok 4.8?
For a model without a date, no. If you run on Grok today, Grok 4.6 is the released model and 4.7 is the one Musk says is days away. If you are choosing where your agents run, the model underneath matters less than the layer around it: memory, task board, inbox and approvals a person can see. CellCog runs Claude Fable 5.1 at Core and Max and Gemini 3.8 Flash at Flash; we do not route to Grok, and we would test 4.8 the day it ships.