How to Use Claude Sonnet 4.5 in Your Coding Agent
Claude Sonnet 4.5 is Anthropic’s mid-tier model tuned for coding and tool use. It sits below Opus on raw reasoning but above it on price-to-performance, which is why it is the default for most coding agents. Plugging it into an existing agent takes one model-string change.
What is Claude Sonnet 4.5?
Sonnet 4.5 is a general-purpose model from Anthropic with strong tool-calling and code generation. It supports a large context window, accepts images, and exposes the same messages API as other Claude models. In the Claude lineup it is the pragmatic middle: more capable than Haiku, cheaper than Opus.
How do you connect it to an agent?
The wiring is minimal:
- Take an Anthropic API key and export it as
ANTHROPIC_API_KEY. - Point your agent at the model string for Sonnet 4.5.
- Define your tools in the standard schema — Sonnet handles parallel tool calls well.
- Set a max output token cap to keep responses bounded.
Because the API is stable across Claude versions, switching from Haiku or Opus is a one-line edit. Tools like droid let you swap the backing model in config without reinstalling.
How does Sonnet 4.5 price against Opus?
Sonnet costs a fraction of Opus per million tokens — roughly one-fifth on input. On a typical coding task burning 50k tokens, Sonnet runs a few cents while Opus runs closer to twenty. For 90% of day-to-day edits the quality gap does not show up, which makes Sonnet the sensible default and Opus the escalation path.
When should you still pick Opus?
Reach for Opus on architecture decisions, subtle concurrency bugs, and large refactors where a wrong plan wastes more than the token premium. A good split: Sonnet writes the change, Opus reviews the plan on high-stakes tasks. This mirrors the hybrid routing that keeps agent costs sane.
What are the limits?
Sonnet can over-narrate — it explains more than needed, which inflates output tokens. It also sometimes edits more files than requested unless you scope the task. Both are prompt-fixable: ask for the diff only, and name the target files explicitly.
Quick Summary
- Claude Sonnet 4.5 is Anthropic’s mid-tier coding model, below Opus on cost.
- Connect via the Anthropic API; swapping models is a one-line change.
- Costs roughly one-fifth of Opus on input tokens.
- Best default for routine edits; escalate to Opus for hard planning.
- Scope tasks tightly — Sonnet can over-narrate and over-edit.
Last updated September 2026.
