Best Local LLM for Mac Studio M5 Ultra (2026): 256GB

Bottom Line (August 25, 2026)

The M5 Ultra matters for one reason: Apple put the 256GB tier back on the order page.

Ready to buy? See the tested hardware list with current prices.

Our previous guidance on this site has been that if you wanted more than 96GB of unified memory, you had to buy used. Apple removed the 512GB M3 Ultra option in March 2026 and the 256GB option on May 5, leaving exactly one configuration for new buyers. That was a real constraint and it shaped every Mac recommendation we have made since.

That constraint is gone as of this morning.

What you should not do is buy one today on the strength of a spec sheet. The machine does not ship until September 22. Nobody outside Apple has run a model on it.

What Apple actually confirmed

These figures come from Apple’s own specifications page and launch-day coverage. We have not measured any of them.

Two details worth flagging.

The base M5 Ultra ships a 64-core GPU, not 80. The M3 Ultra was 80-core across the board. If you compare the base M5 Ultra to an M3 Ultra on GPU count alone, the new machine looks like a downgrade. It is not — bandwidth went up 47% and that is what governs token generation — but prompt processing is compute-bound, and that is where the GPU core count actually shows up. Buyers who feed the model long prompts should look hard at the 80-core bin.

Apple also moved the SSD to a PCIe Gen 6 architecture it claims is up to twice as fast. For local AI this is a model-load-time improvement, not an inference improvement. Loading a 150GB model off disk is a real annoyance and this helps it. It does nothing for tok/s.

The price

The $4,000 memory upgrade is the number to sit with. It is a flat charge on any M5 Ultra, and it is the entire reason this launch is interesting for local AI.

Put differently: you are paying $4,000 for 160GB of additional unified memory, or $25 per GB. Judged as a RAM purchase that is indefensible. Judged as the only way to run a 358B-parameter model on one quiet machine that draws well under 300W, it is a different calculation — and it is the calculation this site has always made for Ultra-tier Macs.

What fits in 256GB

This part we can speak to with confidence, because model sizes are known quantities and we have covered this tier before.

Reserve roughly 15 to 20 percent of unified memory for the OS and KV cache. Plan against about 205 to 215GB of usable model space.

• gpt-oss 120B at Q8 — about 120GB. Fits with enormous context headroom. Still the production-reliable pick, still the cleanest tool-call JSON of any open-weight model.
• Qwen3-VL 235B at Q4 — roughly 130GB. Comfortable. Vision plus a 235B parameter count is the strongest multimodal option that runs locally.
• GLM-4.7 at 358B Q3 — roughly 155GB at 128K context. This is the model that justifies the tier. It does not fit in 128GB at any useful quantization.
• Llama 4 Maverick at Q4 — about 95 to 100GB. On a 128GB machine this barely fits and you cap context at 16K to 32K. On 256GB you run it with real KV cache room, which changes what the model is useful for.
• DeepSeek V4 Flash — the MIT weight drop on July 31, 2026 ended the cloud-only situation. 284B total, 13B active, roughly 80GB at Q4. Fits easily.

The honest summary: the jump from 128GB to 256GB buys you GLM-4.7 and comfortable context on everything else. If neither of those matters to you, the 96GB machine at $5,499 is the better buy and it is not close.

Projected performance — read this section carefully

There are no M5 Ultra benchmarks. The hardware ships September 22, 2026.

What follows is arithmetic, not measurement. We took measured M3 Ultra 256GB results and scaled them by the bandwidth ratio, 1200 ÷ 819 = 1.47.

Treat these as ceilings you will not reach. Bandwidth scaling systematically overestimates real gains, for three reasons:

1. Prompt processing is compute-bound, not bandwidth-bound. It scales with GPU cores. The base M5 Ultra has fewer GPU cores than the M3 Ultra it replaces. On long-context work, the base bin could plausibly process prompts more slowly than the machine it succeeds.
2. MoE dispatch does not scale linearly with bandwidth. GLM-4.7 and DeepSeek V4 Flash are mixture-of-experts models. Their routing overhead is latency-sensitive in ways a bandwidth ratio does not capture.
3. Sustained thermal behaviour is unknown. A 30-second benchmark and a model serving an agent for eight hours are different workloads, and we have no data on the second one.

A realistic expectation is somewhere in the range of 25 to 40 percent real-world improvement over M3 Ultra, not the 47 percent the bandwidth ratio implies. We will publish measured numbers once we have hardware in hand.

Should you buy one?

Buy now if: you were blocked by the 96GB ceiling and you have been waiting to buy new rather than hunt a used 256GB M3 Ultra. That was a genuinely bad position and this fixes it. Order the 36-core/80-core bin, not the base.

Wait if: you already own a 256GB or 512GB M3 Ultra. Your machine is not obsolete. A projected 25 to 40 percent gain does not justify $9,499 before anyone has measured it. Revisit in October.

Skip entirely if: your models fit in 128GB. Nothing here changes your calculation. The M5 Max at 614 GB/s covers that tier at a fraction of the price, and we have written about it separately.

Consider waiting for October if: you want 512GB. It is not orderable today, and if the 256GB tax is $4,000 then the 512GB tier will price accordingly. Apple has not announced that number.

The 512GB question

Apple says the 512GB configuration arrives in late October 2026 and it is not available to pre-order today.

We are not going to speculate on its price or on what it unlocks. What we will say is that 512GB puts full-size frontier open-weight models in reach at usable quantizations for the first time on a single consumer machine, and that this is the configuration worth watching if you are building a serious private inference box. We will cover it when Apple publishes real numbers.

What we will do next

This guide goes up on announcement day because the 256GB tier returning is news that changes buying advice across this whole site. Several of our existing guides — including our Mac Studio 256GB guide and our M3 Ultra guide — open by telling readers that 96GB is the ceiling for new purchases. That is now wrong, and we are updating them.

We will replace every projected figure in this article with a measured one after September 22.

Before you order parts, check the tested hardware list for current prices by tier.