The scenario
You’ve picked the right model for your email routing task. Emails get classified correctly and customers are happy. Then you notice something in your logs: you spend more on input tokens than output tokens. The question worth asking is: does your prompt need to be that long?This guide uses
POST /v1/traces/cost to compare dollar cost for two prompts with the same model and output size.The hypothesis
Verbose system prompts feel safer, but every word ships on every request. For straightforward classification, a shorter prompt might cost less with the same accuracy. Price both versions before you rewrite production traffic.Test configuration
Keep the model fixed (gpt-oss-20b) and vary only the system prompt.
Count tokens for each prompt style, then call the Pricing API with the same completion_tokens estimate.

Prompts tested
Concise prompt (~225 input tokens):Price each prompt
cost_breakdown.total from each response.
At scale, multiply by your daily request volume.
What to track
- Cost per request from the API
- Latency in your stack (shorter prompts often decode faster)
- Accuracy on a fixed email set
Results
Both prompts hit 100% accuracy on the test emails in the original experiment. The concise prompt used roughly half the input tokens and responded faster. Extra formatting and guidelines did not improve routing quality.
Winner for this test
The concise prompt saves about 23% per million requests and responds 8% faster with the same accuracy. There is no quality tradeoff on this workload.Example in action
Both prompts route this sales inquiry correctly:Subject: Enterprise plan pricing
From: cto@fastgrowth.ioHi there,A 200-person company is looking to upgrade from its current solution. Could you send information about:
Mike Chen, CTO
From: cto@fastgrowth.ioHi there,A 200-person company is looking to upgrade from its current solution. Could you send information about:
- Enterprise plan features
- Volume discounts
- Implementation timeline
- API rate limits
Mike Chen, CTO
Sales team

The takeaway
Shorter prompts cut input tokens on every call. In this case, the concise version is 23% cheaper and 8% faster with 100% accuracy.
POST /v1/traces/cost with your real token counts before and after a prompt edit to see the dollar delta.
Pull live rates first with the quickstart or price search.