rationale:

Resolution Basis and Current Baseline

The forecast target requires a mutually exclusive share of worldwide enterprise LLM API spend that sums with competitors to roughly 100%. This rules out adoption breadth metrics—like Datadog's finding that OpenAI is used by 63% of organizations datadoghq.com or an a16z CIO survey showing 56% wallet share 2 sources—because these involve overlapping multi-model usage that sums well above 100% datadoghq.com. The most appropriate anchor is a Menlo Ventures-style spend share, which put OpenAI at 27% in late 2025, trailing Anthropic's 40% 4menlovc.com. Revenue triangulation suggests OpenAI's true current API share may be even lower, perhaps in the high-teens to low-20s. While OpenAI reached a $40B overall run rate in mid-2026 3 sources, its direct API and developer platform line represents only about 15–20% of this total sacra.com, with much of its growth coming from consumer subscriptions and seat-based enterprise products.

Structural Headwinds in the API Market

Looking toward end-2028, OpenAI faces significant structural pressure on its dollar share of API spend. Enterprises are rapidly shifting toward multi-model architectures, routing production tokens across hundreds of models to optimize for cost and specific use cases rather than defaulting to a single provider 3 sources. Open-weight and Chinese models are capturing growing volume at the lower end of the market 2 sources, eroding frontier-vendor pricing power. Additionally, Anthropic maintains strong entrenchment in token-heavy agentic and coding workloads 2 sources, with a highly API-concentrated revenue mix sacra.com. Google is also expected to leverage its TPU cost advantage and Cloud distribution to command a 20–30% share, while broader enterprise AI budgets focus heavily on usage efficiency and cost control gartner.com.

Factors Supporting OpenAI's Resilience

Despite these headwinds, OpenAI remains a formidable competitor capable of reclaiming share. Ramp payments data from mid-2026 indicates that while Anthropic leads in business penetration, OpenAI's API growth has been reaccelerating 44 sources. OpenAI has been shipping models rapidly—with GPT-5.6 class endpoints already carrying a significant share of spend ramp.com—and cutting prices aggressively openai.com, which drives token volume even if it compresses revenue per token. Furthermore, the true size of OpenAI's enterprise footprint may be understated if massive Azure/OpenAI embedded deployments capture high-dollar workloads that do not fully reflect in direct API revenue estimates. Anthropic's headline run rates may also be somewhat inflated by gross reseller accounting futuresearch.ai.

Forecast Distribution and Key Uncertainties

The central expectation is that OpenAI remains a top-two provider, but structurally settles below its early dominance as the market fragments. However, balancing this projection against forecasts for competing providers' API market shares and broader revenue-scaling expectations supports a robust baseline, placing the median at 23.5%. This sits slightly below the late-2025 Menlo benchmark, reflecting ongoing competition from Anthropic, Google, and open-source alternatives. The distribution is kept wide to account for extreme historical volatility in model leadership cycles. A fat right tail (reaching 32.0% at the 75th percentile and stretching to 43.0% at the 90th percentile) accounts for the possibility of a major OpenAI model/API comeback, or a resolver interpretation that leans closer to large-enterprise CIO surveys or heavily weights Azure-bundled API spend. The left tail (anchored by a 10th percentile of 11.0% and a 25th percentile of 17.5%) reflects the risk of a continued squeeze by Google's ecosystem and cheap open-weight inference, though the broader revenue-scaling expectations across the market keep these lower bounds well-supported.