Microsoft’s AI business looks like a triumph. Its shares jumped after earnings last week, and the AI unit is growing fast. A new filing shows how much of that rests on a single partner.

Microsoft booked $24.1 billion in revenue from OpenAI during the year to June, Bloomberg reported. The company disclosed the figure in a filing last week. That is most of its AI business. By Bloomberg’s estimate, OpenAI is more than half of Microsoft’s actual AI sales, and probably around 70%.

It is the clearest picture yet, and the first time Microsoft has spelled the figure out. Investors have pressed the company on exactly this. One accounting researcher, Olga Usvyatsky, suggested the disclosure may be tied to OpenAI’s plans for an eventual listing.

How the 70% is built

The 70% is an estimate, not a hard number. Microsoft has only ever disclosed its total AI business twice. It did not update the figure at its latest earnings. Bloomberg assumed the unit kept growing at the 123% rate Microsoft reported in March. That would put the AI business around $34 billion for the year. Set the disclosed $24.1 billion against it, and OpenAI is roughly two thirds.

The money is not quite what it sounds. Under their agreement, OpenAI pays Microsoft for cloud computing and for the cost of building models. It also hands over a share of its revenue. Much of the $24.1 billion is OpenAI’s own compute bill, routed through Azure and booked as Microsoft income.

Against Microsoft’s total revenue, OpenAI shrinks to under 10%, closer to 7%. The concentration only jumps out when you narrow the lens to the AI unit.

Even the internal tools run on OpenAI

On the same day the number circulated, Microsoft gave a smaller sign of the same dependence. Jay Parikh runs its CoreAI engineering group. In a memo, he told staff to default to OpenAI’s flagship GPT-5.6 Sol in GitHub Copilot, CNBC reported. They should use it most of the time.

The reason he gave was money. “Shifting more workloads to OpenAI models helps us get greater value from our token investment,” Parikh wrote. Microsoft holds intellectual-property rights to OpenAI’s models through 2032. Pointing its own engineers at Sol squeezes more from that stake.

It is also part of a wider retreat from tokenmaxxing. That was the brief era when developers ran up big AI bills without watching the output. The memo was first reported by 404 Media. It lands as cheaper open-weight models, many from China, pull spending the other way.

The diversification that hasn’t landed

None of this is for lack of trying. Microsoft has built its own MAI models and offers cloud customers more than 11,000 of them. It has reached for alternatives too, including a $5 billion bet on Anthropic. GitHub Copilot alone, now past 50 million users, serves models from Anthropic, Google, Moonshot, xAI, and Microsoft itself.

And yet the filing says roughly 70% of the AI revenue still traces to OpenAI. The effort to spread the risk has not moved the number that matters.

Engine or exposure

Whether that reads as strength or fragility depends on what the money is. KeyBanc analyst Jackson Ader wants to know where it comes from. Is it services sold to OpenAI, or the benefit of Microsoft’s own investment? “The more of that revenue comes from services to OpenAI,” he said, “the more favorably I’m going to look at it.”

The bears are blunter. The writer Ed Zitron notes Microsoft has spent more than $260 billion on capital projects since 2022. He argues much of its AI revenue is OpenAI’s compute bill dressed as growth. If OpenAI stumbles, Microsoft is left with the sunk cost and the empty data centres.

There is no sign of a stumble. The AI unit is still growing fast, which is why the stock climbed. But the filing settles a long argument with an uncomfortable answer. After years of work and hundreds of billions in spending to become more than OpenAI’s landlord, Microsoft, for now, mostly is one.

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