Three differences that matter
Memory off by default vs. on by default
You cannot A/B two prompts when the model half-remembers the previous nine attempts at both.
Versioned GPTs vs. edited GPTs
An upgrade is a decision with a date and an author, not a side effect of saving.
Evidence vs. output
“The page said this on 14 August” is checkable, not asserted.
When to use which
You want an assistant that knows you. Memory is the feature, not the problem, and Custom GPTs are a genuinely good way to hand a workflow to a colleague. When you are the only person who has to trust the output, accumulated context makes it better, and there is nothing to reproduce.
A second person depends on the answer, or the same answer is needed twice. Note what is not being compared: point an agent at GPT-5 and OpenAI answers the question — the model is the same. What you gain is a named, frozen configuration, per-person control over who may change it, and a run you can reproduce a year later. Ambient memory is precisely what makes that impossible.
Side by side
| ChatGPT | Coldcache | |
|---|---|---|
| What it is | An assistant you talk to | An instrument your team runs |
| Cross-chat memory | On by default | Off by default, per-version opt-in |
| Editing a saved agent | Rewrites it for everyone | Publishes the next version |
| Run against an old configuration | Not possible | Any published version, any time |
| Diff two configurations | No | Config diff and output diff, side by side |
| Regression fixtures | No | Accepted output hashes, checked on every new version |
| Audit log of changes | No | Append-only: publishes, stable moves, deprecations |
| Model choice | Their models | GPT-5, Claude or Gemini — you name the exact model id |
| Who can change an agent | Whoever can edit the GPT | Per person, per agent: read, write or approve |
Same models, same web, same ceiling on how good an answer can get. The entire difference is whether, a year from now, you can name what produced it, prove it has not changed, and run it again. That is not a smaller claim than intelligence. It is the one that survives an audit.
Pin an agent. Run it in a year. Get the same thing back.
Claude, ChatGPT and Gemini included — no provider account to open. Build one agent, publish v1, run it, then change the prompt and diff the two.