1. What changed, and who can use it?
The official changelog dates 12-hour caching to October 20, Terra to October 26, eligible subscription API credits to October 28, and GPT-6 Rosalind to October 30. Rosalind has specialized research access requirements; that announcement does not imply access for a general developer account.
We focus on the general application decisions: Terra, reusable context and credit accounting. The calculations and acceptance framework below are our analysis. We have not called paid APIs or run a comparative model benchmark for this article.
2. Evaluate Terra on a complete task
The model card positions Terra for visual reasoning, computer use and writing. Tool calling requires Responses. Standard prices per million tokens are $1 for input, $0.05 for cached reads and $5 for output. Requests above 272K input tokens have different long-context rates.
Our suggested evaluation unit is a finished business task: extract fields from a product screenshot, then reconcile them against a catalogue. Hold the image, output contract, tool permissions and stopping conditions fixed. Count invalid structures, failed actions and every retry. A cheaper successful request can still produce a more expensive finished task when recovery becomes frequent.
3. Organize input by how often it changes
The cache guide documents prompt_cache_options.ttl: 30m writes cost 1.25 times ordinary input; 12h writes cost 2.5 times. These are total write rates. A lifetime is a minimum retention interval, not a deletion deadline. Earlier models have separate parameter rules.
A useful application layout is stable instructions and tool contracts, versioned reference material, session context, then the current question. Put changing timestamps and request identifiers after the reusable section. Give each reference release an explicit version in your own logs. Customer history still needs application access controls; a cache accounting key is not an authorization system.
Rules / tools → Reference version → Session history → Current question
Stable reuse candidate Short-lived Variable4. Worked example: ten uses of a 20,000-token prefix
Assume Standard Terra pricing, one prefix write followed by nine full hits, no rewrites, and total request length below the long-context threshold. This calculation covers only prefix input. Outputs, changing suffixes, tools, storage and other charges are excluded.
| Strategy | Calculation | Ten-call total |
|---|---|---|
| Ordinary input throughout | 10 × $0.02 | $0.200 |
| One 30-minute write | $0.025 + 9 × $0.001 | $0.034 |
| One 12-hour write | $0.050 + 9 × $0.001 | $0.059 |
Under those assumptions, the default strategy beats uncached input at two total calls; the longer lifetime needs three. If the shorter cache keeps serving hits, extending its lifetime raises cost. If it actually expires and requires repeated writes, that conclusion can reverse. Measure the reuse count and gap distribution for each reference version before choosing a lifetime.
5. Keep resource consumption and credits in separate ledgers
The subscription credit rules require eligibility and a link to an API organization you own. Credits expire and have scope restrictions. They do not increase ChatGPT or signed-in Codex limits, and exclude API-key Codex usage.
Record the cost before credits as well as the cash payable after credits. The first measures architecture efficiency; the second supports cash planning. A temporary grant can conceal an inefficient workflow until it expires. Before budgeting a subscription benefit, verify its ownership and actual activation rather than treating an advertised maximum as an issued balance.
6. Design an evaluation that can reject the migration
First compare cache strategies on the same model. Then hold caching fixed while comparing models. Changing both together obscures whether a difference came from model behavior, context reuse or retries. Include stable references, changed references, delayed repeats and recovery cases.
- Record acceptance, every attempt, input and output volume, cache reads and writes, tool fees and cost before credits for each task.
- Measure time to first output and full task duration; inspect slower cases as well as averages.
- Replay one reference version, then replace it and check whether obsolete material still influences results.
- Keep the previous route available when lower cost comes with more failures.
Cache diagnostics can help explain prefix differences, but classification is best effort. Check actual read/write token usage and total cost. Expand deployment only after the evidence supports your own acceptance criteria.