Migrating from the Claude API to the Gemini API (gemini-3.1-flash-lite): 16 call sites into one function

Written by Koshin (web engineer and app engineer) · published 2026-10-09

Development log of Torecadash / October 2026

On the server side of my iOS app Torecadash, AI is used for things like deciding “which product an article is about.” On July 25, 2026, I fully migrated this AI from Anthropic’s Claude (Haiku 4.5) to Google’s Gemini (gemini-3.1-flash-lite). What I decided, how I unified the calls, and what I added after the migration are written from my hand-over notes.

This site has a page for Torecadash (Works). This is a development log. I do not write amounts of money.

What triggered it

According to my notes, the reason was that I reached Anthropic’s API usage limit and got an unplanned charge. Since then only Gemini has run, and I removed the Anthropic SDK and key from the code and from the secret store (Secret Manager). In a check on July 30, I confirmed there are 0 mentions of Anthropic in the code.

How I chose the model

I chose gemini-3.1-flash-lite. My notes also keep the reason for what I did not choose.

Gemini 2.5 Flash-Lite, which is cheaper, was scheduled to be shut down on October 16, 2026, so I passed on it (according to my notes. When I reviewed the official deprecation list on October 8, 2026, this model showed “no shutdown date announced.” I have not confirmed whether the entry changed later).
The shutdown date of the one I chose, gemini-3.1-flash-lite, is May 7, 2027. At the time of migration (July 25) that was still about nine and a half months away.

If you choose only by price, it will be shut down soon and you migrate again. I added “when will it be shut down” to the selection criteria.

16 call sites into one function

There were 16 places that call AI (deciding the sales format, matching products, discovering listings and so on). I unified them to go through one function (createGeminiInteraction) at the top of functions/index.js. Inside it calls client.interactions.create() from @google/genai.

Callers pass arguments close to the original Anthropic form (system_instruction, max_output_tokens in generation_config, input) as they are. What comes back is text (output_text). I made it a form where only the inside is swapped, with almost no rewriting of callers.

Web search had no “call count cap” setting

In four places (discovering listings, discovering sellers, a Rakuten Books lookup and a list-price lookup), the AI does a web search. With Gemini I use tools: [{type: "google_search"}] (grounding with Google Search).

Here was a pitfall. Anthropic’s web search had max_uses (a cap on the number of searches), but Gemini has no equivalent setting, and the model searches as many times as it thinks necessary. My notes say: “the absence of this setting can itself be a cause of unexpected cost.”

So the day after migration (July 26), I added a circuit breaker with a daily counter inside createGeminiInteraction (up to 300 calls a day). When the limit is reached, it returns an error to the caller. In price review, I designed this error to be excluded and kept distinct from “could not find the list price” (I wrote about that in another article, in Japanese).

A switch to stop it

I added one more thing: AI_CALLS_PAUSED, a switch that stops all AI calls from one place. My notes include a reflection from July 25: “I thought I had stopped the calls, but they had not actually stopped, and the automatic job running every 10 minutes kept producing errors.” I decided that when the budget is about to be exceeded, I first set this to true and stop right away.

A note when adding new processing

Functions that call AI (Cloud Functions) must always have secrets: ["GEMINI_API_KEY"]. If you forget it, deployment succeeds and then it fails with a permission error.

What I have not verified

This article is written from the AI section of my hand-over notes. A before-and-after accuracy comparison of decisions is not part of the material. So I do not judge here whether Gemini is better or worse.

What I learned

If you gather calls in one place, the place to stop them is one place too.

Pass every call through one function. The migration, the safety limit and the stop switch can all live in that one place.
Count “there is no setting” as a risk. When you move to a service with no cap on call count, build the cap on your own side.
Check that the stop switch really stops. Thinking you stopped but not having stopped is the most expensive case.

This article is written from my hand-over notes (AI API information, July 25 and 26, 2026).

This is the English version of the original Japanese article.