Use case
The best API for coding agents
For coding agents the deciding cost is cached input, not output: an agent resends the same system prompt, tool schema and file context on every step, so a model with a cheap cached-input rate can cost several times less than a nominally cheaper one without it. GLM-5.2 at $0.82 per million input and $0.16 cached is the strongest fit in this catalogue.
A coding agent is a loop, not a conversation. Each step resends the system prompt, the tool definitions and a slab of file context, then asks for a small diff. The input-to-output ratio is often 50:1 or worse, which inverts the usual pricing intuition — output rate barely matters and input rate is almost the whole bill.
That is why cached-input pricing decides this workload. The repeated prefix is served from the model's KV cache at a fraction of the standard input rate, and for an agent that prefix is most of every request. A model with no published cached rate is not cheaper; it is a model whose largest line item is unpriced.
What actually matters here
- Cached-input rate
- The agent's repeated prefix is the bulk of its tokens. This number, not the headline input rate, is what the monthly bill is mostly made of.
- Context window
- Enough room for the system prompt, tool schemas and several files at once. Running out mid-task forces re-planning, which costs a full round trip.
- Tool-calling reliability
- An agent that returns malformed tool calls burns a retry every time. That failure is invisible in a price comparison and expensive in production.
- Licence
- Agents get embedded in products that ship. A permissive licence removes a question legal will otherwise ask late.
What it costs, at a working agent, ~2,000 steps/month
Worked from this catalogue’s published rates at 400M input and 8M output tokens a month. Your figure will differ; the arithmetic will not.
| Model | Input | Output | Per month | At official rates |
|---|---|---|---|---|
| GLM-5.242% | $0.82/M | $2.55/M | $348.40 | $595.20 |
| Kimi K340% | $1.85/M | $9.00/M | $812.00 | $1,320.00 |
| DeepSeek V4 Pro37% | $0.28/M | $0.55/M | $116.40 | $180.96 |
Excludes cached-input savings, which on a repeated-prefix workload typically reduce the input column substantially. Cached rates are published per model on the pricing page.
Questions
- Does cached input really change the total that much?
- On an agent workload it is usually the largest single factor. If 80% of your input is a repeated prefix billed at roughly a fifth of the standard rate, the effective input cost falls by about two thirds — a bigger swing than the difference between most models' headline rates.
- Can I pin an agent to one model?
- Yes. Send the exact model id and no substitution happens. Fallback only applies if you ask for it, which matters for an agent whose prompts were tuned against one model's behaviour.