# Output tokens never cache

> Output tokens cost ~5x input and never cache - cut free-form prose first. Rewriting inside a cached prefix mid-session can raise the bill, not lower it.

URL: https://bartlomiejkrupa.dev/notes/output-tokens-never-cache/
Category: rule | Updated: 2026-10-07
Summary: Output tokens cost ~5x input and never cache - cut free-form prose first. Rewriting inside a cached prefix mid-session can raise the bill, not lower it.

Output tokens bill at full price every call and never enter a prompt cache. On current Claude tiers they cost about 5x input (6x on GPT-5.5). That makes them the first place to cut when spend rises - not the stable prefix you depend on for cache hits.

## Why trimming input can backfire

A 2026 study of 2,848 provider-billed Claude Code runs ([arXiv 2607.12161](https://arxiv.org/abs/2607.12161)) found that removing 38% of raw tool-output tokens raised total cost 6.8%: cache traffic was ~87% of input-side cost, so compression could reach only a small slice of the bill, and it changed the agent's trajectory with extra retrieval and testing turns that each re-sent the cached prefix. The rule: trim what is never cached; never rewrite inside a [cache-stable prefix](/notes/cache-stable-prefix) mid-session.

## Rule

1. Cap `max_tokens` to the longest useful answer.
2. Demand a schema (`LABEL | reason`, JSON, enum) so the model cannot invent essay structure.
3. Mask or drop older turns instead of compressing text already inside a cached block.
4. If you must slim tool output, do it before it enters a cacheable block.

Illustrative Sonnet 5 arithmetic ($2/$10): 300 calls/day through a cached 1,500-token system prompt cost ~$0.09/day in cache reads; the same 300 calls each padding 300 tokens of prose cost ~$0.90/day in output.

Playbook: [Trim output, not the cache](/articles/trim-output-not-the-cache). Pair with [Don't break the prompt cache](/articles/dont-break-the-prompt-cache).

Sources:
- Token Reduction Is Not Cost Reduction: https://arxiv.org/abs/2607.12161
- Pricing: https://platform.claude.com/docs/en/about-claude/pricing
- API pricing: https://developers.openai.com/api/docs/pricing