Anthropic's Claude joins just4o.chat | use Sonnet alongside 4o

Anthropic’s Claude models have a very particular energy. They’re calm, deliberate, and unusually good at staying grounded when the stakes are high. For a lot of people, Claude is the model you bring in when you need the answer to be not just clever, but careful. On just4o.chat, that personality isn’t hidden behind an abstract “Claude” toggle; you can explicitly choose between Claude 4.5 Sonnet and Claude 4.5 Haiku, and you can weave them into your workspace exactly where they make the most sense.

Claude 4.5 Sonnet is the one you give the hard work to. It’s built for serious coding sessions, multi-step agents, and long-running tasks that have to keep track of details over time. On just4o.chat, Sonnet feels less like a chatbot and more like a steady teammate: you can anchor it to a project as the dedicated “codebase partner,” let it orchestrate tool calls and refactors, and trust it to stay plugged into the same context over weeks of iteration. It’s also a natural fit for analytical roles—reviewing financial models, dissecting dense policy PDFs, or drafting documents that need to thread a careful needle between nuance and clarity.

Claude 4.5 Haiku plays a different role. It’s the fast, cost-efficient Claude you can keep on all day without worrying about burning through your budget. Haiku is what you point at chatty, real-time work: customer support flows, internal knowledge assistants, quick coding help, content clean-up, summarisation, and drafting. It gives you a lot of Claude’s judgement in a tighter, more responsive package, which makes it a perfect default for teams that want Anthropic’s “feel” everywhere, not just in the most critical flows.

The magic comes from where these models live. just4o.chat is the only place that treats Claude 4.5 Sonnet and Haiku as peers sitting right next to multiple GPT-4o checkpoints, the GPT-5 and 5.1 families, Gemini 2.5, Grok, and Echo. That means you can do things that are basically impossible in a single-vendor UI: let an older 4o checkpoint handle your longform writing because you like the way it sounds, hand your codebase to Claude Sonnet, let GPT-5 pressure-test the architecture, and keep Haiku in the background handling support and internal questions. All of it shares one memory layer and one project structure, and none of it is subject to invisible routing or mid-week model swaps.

In practice, teams quickly find their own pattern. A startup might run Haiku as the “front desk”—the first line for support, FAQs, and sales questions—then pull in Sonnet for complex implementation questions while a November-2024 4o checkpoint handles brand-sensitive copy. An engineering org might prefer GPT-5 for gnarly algorithmic discussions but lean on Claude Sonnet whenever an agent needs to touch production code. A solo creator might simply like Claude’s tone and keep Haiku as their default, occasionally bouncing ideas off an older 4o checkpoint when they want a different flavour of creativity.

Because just4o.chat is router-free, these choices actually stick. When you pick Claude 4.5 Sonnet for a persona, you’re not secretly getting some cheaper fallback next week. When you bookmark a project that uses a specific GPT-4o checkpoint and Claude Haiku together, it will still behave that way when you come back a month later. Claude isn’t an abstract brand; it’s a pair of precise tools that you can combine with OpenAI, Google, xAI, and Echo to build your own custom stack.

If you’ve only ever met Claude inside the first party app, using it on just4o.chat feels different. The same careful, Anthropic-style intelligence is there—but now it’s living next to the exact 4o checkpoints you loved, the latest GPT-5 reasoning models, Gemini’s multimodality, and Grok’s more opinionated voice, all in the same place, all under your control.