Free estimator · GPT-5.5 leaves ChatGPT, ChatGPT Work and Codex on 2026-10-14 · API unaffected
GPT-5.5 retirement October 14: model migration cost estimator
The GPT-5.5 retirement on October 14, 2026 removes the model from ChatGPT, ChatGPT Work and Codex on all plans. The API is not affected. Every workspace default, custom agent, scheduled task and script that names gpt-5.5 has to move, and the cost is not the code change. It is the regression testing: proving every prompt and tool call still behaves on the replacement. Enter your agent count, prompts per agent, eval coverage and target model. You get engineering hours, the eval gap to close first, the run-cost delta, and whether you make the date.
| Re-point + smoke test | 1560 |
|---|---|
| Prompt / tool regression | 9360 |
| Config surfaces | 1560 |
| Missing evals | 6551.999999999999 |
- Monthly run cost on GPT-5.6 Sol
- $5,000 (+$0 vs GPT-5.5 at $5,000)
- Tokenizer adjustment
- none
- Retirement scope
- ChatGPT, ChatGPT Work, Codex on all plans; API unaffected
- Eval coverage benchmark
- Only 52% of teams run offline evals (LangChain, 2026-03)
146 hours, about $19,032, to move 6 agents to GPT-5.6 Sol; readiness by 2026-10-14: at risk.
Assumptions and sources (15)
| Constant | Value | Basis |
|---|---|---|
| Retirement date and scope | GPT-5.5 leaves ChatGPT, ChatGPT Work and Codex on 2026-10-14; API unaffected | sourcedOpenAI model release notes |
| Re-point + smoke test | 2 h per agent | estimateStudio estimate, to be replaced by measured data |
| Prompt / tool regression | 1.5 h per prompt | estimateStudio estimate |
| Config surfaces | 1 h per surface per agent | estimateStudio estimate |
| Minimal eval suite | 12 h per agent for ~30 cases | estimateStudio estimate; rationale from the 3-level production testing framework |
| Capacity | 6 productive hours per engineer per day until 2026-10-14; at risk within 25% | estimateStudio estimate |
| GPT-5.5 price | $5 in / $30 out per MTok | sourcedSECONDARY: CloudZero tracker 2026-09-10 (openai.com returned 403) |
| GPT-5.6 Sol price per MTok | $5 in / $30 out | sourcedSECONDARY: CloudZero GPT-5.6 pricing tracker (updated 2026-09-10). openai.com/api/pricing returned HTTP 403 on 2026-09-19 |
| GPT-5.6 Terra price per MTok | $2 in / $12 out | sourcedSECONDARY: CloudZero GPT-5.6 pricing tracker (updated 2026-09-10). openai.com/api/pricing returned HTTP 403 on 2026-09-19 |
| Claude Sonnet 5 price per MTok | $2 in / $10 out (introductory price made permanent) | sourcedAnthropic pricing page, fetched 2026-09-19 |
| Claude Opus 5 price per MTok | $5 in / $25 out | sourcedAnthropic pricing page, fetched 2026-09-19 |
| Gemini 3.6 Flash price per MTok | $0.75 in / $3.75 out ($1.50 / $7.50 / $0.15 after 2026-12-31) | sourcedGoogle Gemini API pricing page, fetched 2026-09-19 |
| Token split | 70% input / 30% output | estimateStudio estimate |
| Claude tokenizer | ~30% more tokens for the same text on Claude 4.7+ | sourcedAnthropic pricing page, 2026-09-19 |
| Eval coverage benchmark | 52% run offline evals | sourcedLangChain State of Agent Engineering, 2026-03-03 |
Key takeaways
- GPT-5.5 leaves ChatGPT and Codex on 2026-10-14. The API keeps it. Anything that names the model in a UI or config has to move.
- Re-pointing takes hours. Proving the agent still works takes days, unless the eval suite already exists.
- GPT-5.6 Sol is priced the same as GPT-5.5 ($5 / $30). Terra and Claude Sonnet 5 are cheaper if evals hold.
- Only 52% of teams run offline evals. That is why retirements become incidents.
What happens to GPT-5.5 on October 14, 2026
OpenAI retires GPT-5.5 from ChatGPT, ChatGPT Work and Codex on all plans on 14 October 2026 (OpenAI release notes). The OpenAI API is not affected. Codex users signed in with ChatGPT are pointed to GPT-5.6 Sol. Workspace defaults, custom agents, scheduled tasks and scripts that select gpt-5.5 stop working as configured.
GPT-5.5 shipped to the API on 2026-04-24. Six months from launch to retirement is the new normal, and it is a line item. TheoSym ships every production agent with the model id behind a router and an eval suite that gates the swap, so the next retirement is a pull request, not a project.
How model migration cost is calculated
Four lines. Re-pointing and a smoke test per agent. Regression per prompt and tool definition, because a model change alters tone, tool-call formatting and refusal behaviour. One hour per configuration surface per agent. And the eval suites you do not have yet, at about twelve hours each for a minimal 30-case set.
The last line is usually the biggest. Without an eval you are not migrating, you are guessing. LangChain's 2026 survey found only 52% of teams run offline evals (State of Agent Engineering). The per-agent hours here are studio estimates; the AI Factory workproofs show measured runs.
- Hours = agents x (2 + 1.5 x prompts + surfaces) + missing evals x 12
- Cost = hours x engineer rate
- Ready = days to Oct 14 x 6 h x engineers >= hours
GPT-5.5 to GPT-5.6 Sol vs Claude Sonnet 5 vs Gemini 3.6 Flash
Sol is the like-for-like: $5 in and $30 out, identical to GPT-5.5, so the swap is not a price change (CloudZero tracker; secondary source, openai.com returned 403 on 2026-09-19). Terra at $2 and $12 halves the bill if the task tolerates it. Claude Sonnet 5 at $2 and $10 is cheaper still on list, with about 30% more tokens on the Claude 4.7+ tokenizer, which the estimator applies. Gemini 3.6 Flash is the floor at $0.75 and $3.75 through December.
The right target is the cheapest model that passes your evals, not the cheapest on the price list. That comparison is a standard step in TheoSym's AI consulting engagements. Regulated decisioning agents route to QGI, where a model swap has to leave the decision output reproducible.
When do you need a model migration checklist?
Now, if any surface names gpt-5.5, and before the next retirement in any case. A working checklist: inventory every place a model id is set; write or complete the eval per agent; run the candidate model against it; compare cost per task; swap behind a router; monitor the online evals for a week (Developers Digest checklist).
Model churn is a control problem, not a vendor problem. An agent whose behaviour you cannot regression-test is an agent you do not control, whatever model sits under it. Dr. Sam Sammane and the TheoSym team publish shipped work in the press center; bring your inventory to the strategy call.
GPT-5.5 retirement October 14: questions people ask
What happens to GPT-5.5 on October 14, 2026?+
OpenAI retires GPT-5.5 from ChatGPT, ChatGPT Work and Codex on all plans on 14 October 2026. The OpenAI API is not affected. Codex users signed in with ChatGPT should switch to GPT-5.6 Sol, and any workspace defaults, custom agents, scheduled tasks and scripts that select gpt-5.5 must be updated.
How much does it cost to migrate an AI agent to a new model?+
The engineering cost is dominated by regression testing, not the code change. Re-pointing an agent takes hours; verifying that every prompt and tool call still behaves takes days unless an eval suite already exists. The estimator prices both, plus the run-cost difference of the replacement model.
Will GPT-5.6 Sol cost more than GPT-5.5?+
No. Sol is priced identically at $5 per million input tokens and $30 per million output, so the swap is not a price increase. Terra ($2 / $12) and Luna ($0.20 / $1.20) are cheaper tiers if your evals show they hold quality for the task. Prices are from a secondary tracker dated 2026-09-10.
How do I avoid this problem at the next model retirement?+
Keep a repeatable offline eval suite per agent, abstract the model id behind a router, and gate every model swap on eval pass rates. Only about half of teams run offline evals today, which is why retirements become incidents. Eval-gated swaps across providers are what a production harness provides.
Are the per-agent hours in this estimator measured?+
No. Two hours to re-point, 1.5 hours per prompt to regress, one hour per configuration surface and twelve hours per missing eval suite are studio estimates, labelled as such under assumptions. They are meant to size the work before October 14, not to quote it. Replace them with your own timings as soon as you have one migration done.
Next step
Turn the estimate into a scoped plan
Send the calculator result. You get one reply from a human within one business day, with an eval plan for the workflow and a real scope.
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