Claude Sonnet 4.5

“Sonnet 4.5 was the right pick for two quarters; now the only strategic question is migration timing to 4.6.”

From October 2025 through February 2026 this was the default workhorse. With Sonnet 4.6 now available at the same price with 1M context and large OSWorld/ARC gains, the strategic question is migration timing, not continued use. Anthropic keeps 4.5 fully supported, but new deployments should standardize on 4.6. The risk in staying is incrementally dated training data and the eventual deprecation cycle. For production traffic, plan a controlled rollout to 4.6 over a quarter and treat 4.5 as exit-only.

“Sonnet 4.5's legacy is the agent tooling — checkpoints, the Agent SDK, VS Code — that still anchors the ecosystem.”

Sonnet 4.5's market significance is less about benchmarks than about the agent infrastructure it launched: the Claude Code checkpoint system, the native VS Code extension, context editing, memory tools, and the Claude Agent SDK all shipped with it. That tooling built the ecosystem moat that 4.6 and 4.7 now inherit. As a standalone product its differentiation has faded, but its strategic footprint as the model that operationalized Claude agents remains large.

“Same $3/$15 as 4.6 with no cost downside to migrating — and a soft cost reason to move on document-heavy work.”

Pricing is identical to Sonnet 4.6 at $3/$15, so the cost case for staying or moving is a wash on rate card, and cache/batch discounts match. The one budget consideration is the 200k context, which forces chunking workflows that can use more total tokens than equivalent work on 4.6's 1M window. Net: no financial reason to delay migration, and a soft financial reason to accelerate it on document-heavy pipelines.

“The checkpoint system and VS Code extension shipped here first — but for browser agents, 4.6's computer use wins.”

For builders, Sonnet 4.5 was the daily driver for Q4 2025 and into early 2026. Tool use is identical to later models, the Claude Code checkpoint system shipped here first, and the VS Code extension is mature. The honest gap is computer use and OSWorld — Sonnet 4.6 is meaningfully better, so for any browser-control or agentic workflow, 4.6 is the right move. Pure coding is close enough between 4.5 and 4.6 that you can stay on 4.5 if your prompt suite depends on its behavior; otherwise migrate.

“Fast and high-quality — on casual chat most users won't notice the gap to 4.6 at all.”

For a consumer chat product, Sonnet 4.5 delivered a strong experience: fast latency, high conversation quality, calibrated refusals. End users will not perceive a meaningful gap between 4.5 and 4.6 on casual chat. Where 4.6 wins on user-visible quality is hard reasoning, computer use, and very long sessions where context limits matter. Safety calibration is mature. The January 2025 reliable cutoff is starting to feel dated for current events — web search mitigates but does not eliminate it.

“A fine 2025 model whose ARC-AGI-2 of 13.6% shows just how far the goalposts moved in one release.”

Sonnet 4.5 was genuinely strong at release and the coding numbers held up, so there is no deception in the original claims. The skeptical point is generational: ARC-AGI-2 at 13.6% versus 58.3% on Sonnet 4.6 shows how quickly the frontier moved, and the 200k context plus January 2025 cutoff now look dated next to 4.6 at identical price. The AIME "100% with tools" framing also flatters the model — the no-tools 87% is the honest number. There is no cost or capability case for new work here; it is a maintenance model.