Google Just Made Thinking Mandatory. They Forgot to Say Who Owns It.
On February 19, Google released Gemini 3.1 Pro. Buried in the API documentation is a change that most developers overlooked:
Thought Signatures are now mandatory.
In Gemini 3 Pro, capturing the model's internal reasoning trace was optional — a developer could request it or ignore it. In 3.1 Pro, under Function Calling Strict mode, the API requires thought signatures to be preserved and cycled back. Skip them, and you get a 400 error. The model won't run.
Google is telling developers: you must keep the receipts of how the model thinks.
But Google didn't finish the sentence. The API requires you to capture thought signatures. It doesn't say where to store them. It doesn't say who owns them. It doesn't say what happens to them after the session ends. It doesn't say whether the platform can use them, sell them, or train on them.
A mandatory artifact with no defined custody. The thinking is required. The record of thinking is nobody's responsibility.
RE was built for exactly this gap.
Since submission in February, RE's geminiService.ts has been capturing thought signatures from the Gemini API and writing them into the cryptographic audit log. Every thought trace is serialized as part of the evidence chain — timestamped, hash-chained, stored as an RFC 5322 email object.
When we built this, thought signatures were optional. We captured them anyway, because a governance protocol that only records outputs but ignores reasoning is like a courtroom that records verdicts but not arguments. The verdict tells you what was decided. The argument tells you why — and whether the reasoning was sound.
Now Google has validated that position. Thought signatures aren't optional anymore. The model's reasoning process is officially part of the API contract. But the storage, ownership, and custody of that reasoning? Still undefined.
RE's answer has been the same since Update #1: the record lives in your email. The evidence chain is an RFC 5322 object — the same format your bank statements and legal correspondence use. It lives in your Gmail archive, on infrastructure you already control. The model can't modify it. The platform can't delete it. It's yours.
This isn't RE chasing Google's roadmap. This is Google arriving at a requirement that RE's architecture already satisfies.
In Update #7, we described what happens when the model upgrades: the ground doesn't change, the eyes reading it do. Update #8 is the complement: Google changed the ground. They made thinking a first-class artifact. But they left the question of custody unanswered.
RE answers it. The thinking belongs to the person who asked the question. The evidence chain is where it lives. The email is how you keep it.
Thirty billion Workspace interactions happen every day across three billion users. Every one of those interactions now generates mandatory thought signatures. Where do they go?
RE knows where they should go. Home.
— Che, Solo developer, Project RE, Taipei Taiwan
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