The default has changed. As of late April 2026, OpenAI’s GPT-5.1 is the new API flagship, replacing GPT-5 as the default reasoning engine for API calls, including adaptive reasoning workloads. This isn’t a preview or a beta. It’s a production routing change that affects every application currently calling the GPT-5 endpoint without a pinned model version.
According to reporting on OpenAI’s April model wave, GPT-5.1 ships with a configurable latency toggle, letting developers trade reasoning depth for throughput depending on the workload. That’s a meaningful architectural choice, not a checkbox feature. Teams running latency-sensitive pipelines should test the toggle behavior before assuming it performs identically to GPT-5 at their specific call volume.
The deprecation clock is running
OpenAI has stated its intent to sunset the Assistants API in H1 2026, per its community communications. That’s not a distant horizon for teams with production applications built on the Assistants API, it’s an active migration project that needs to be on the roadmap now. The migration path leads to the Responses API, which is also the interface that GPT-5.1-Codex reportedly uses for autonomous coding workflows. Codex is described as supporting configurable reasoning effort settings, though this characterization comes from a secondary source only, treat it as reported until OpenAI’s own documentation confirms the specifics.
Efficiency variants for high-volume workloads
GPT-5.4 mini and GPT-5.4 nano round out the April release wave. Both are positioned as efficiency variants targeting high-volume, lower-cost API workloads, consistent with OpenAI’s established naming convention for mid-tier models. OpenAI’s announcement describes GPT-5.4 as 33% less likely to produce false individual claims and 18% less likely to contain full-response errors, compared to GPT-5.2. Those figures come from OpenAI’s own announcement, they’re self-reported benchmarks without independent evaluation published to date. Independent assessment is pending.
What’s still unresolved
Context window specifications for GPT-5.1 are contested across sources. The highest-authority source fragment available points to 128K tokens, while secondary sources cite figures ranging from 196K to higher. Until OpenAI’s API documentation is checked directly, no specific figure should be relied on for architectural planning. Similarly, benchmark scores beyond the 33%/18% self-reported figures have not been confirmed against a primary source in this reporting cycle.
What to watch
The H1 2026 Assistants API sunset is the most time-sensitive signal here. If OpenAI holds that timeline, teams that haven’t started their Responses API migration within the next few weeks are entering the risk window. The GPT-5.4 mini and nano pricing details will matter for any organization currently modeling API cost at volume, that data hasn’t been confirmed yet. And independent benchmark evaluation for this model family remains pending; Epoch AI and third-party evaluators haven’t published assessments of GPT-5.1 as of this reporting date.
TJS synthesis
This April release wave is less about capability breakthroughs and more about API-layer infrastructure decisions that developers need to make now. The flagship change is the least urgent item, applications calling the endpoint without a pinned model version will be routed to GPT-5.1 automatically. The deprecation is the urgent item. Any team that built on Assistants API made a reasonable bet at the time; that bet now has an expiration date. The efficiency variants signal that OpenAI is actively competing on cost at volume, which has pricing implications for the broader API market. Treat the self-reported benchmarks as directional until independent evaluation arrives.