Gemini 3.1 Pro vs Gemini 3.8 Flash
Reading a 26.2K-token pull request costs $0.0524 on Gemini 3.1 Pro, 2.7× the $0.0197 it costs on Gemini 3.8 Flash. Both sit in the mid-range price tier, so the choice between them is about review quality more than budget. The benchmark cannot separate their recall: Gemini 3.1 Pro minus Gemini 3.8 Flash is −1 pt (95% interval −14 pts to +13 pts).
Side by side
| Figure | Gemini 3.1 Pro | Gemini 3.8 Flash |
|---|---|---|
| Price tier | Mid-range | Mid-range |
| Bugs caughtshare of known bugs | 67% | 68% (better) |
| False alarmsper clean PR | 0.8 (better) | 1.0 |
| Cost per reviewmeasured | $0.2499 | $0.0862 (better) |
| Cost per bug caught | $0.4134 | $0.1166 (better) |
| Exact linecatches on the expected lines | 89% | 94% (better) |
| Review timemean | 349s (better) | 427s |
| Typical review calllist price | $0.0814 | $0.0287 (better) |
| Input / 1M tokens | $2.00 | $0.75 (better) |
| Output / 1M tokens | $12.00 | $3.75 (better) |
| Context window | 1M | 1M |
Head to head
The known bugs both were scored on, by who caught each in at least one run.
By language
| Model | All | TypeScript6 bugs | Go4 bugs | Java4 bugs | Rust4 bugs | C#3 bugs | JavaScript3 bugs | Kotlin3 bugs | PHP3 bugs | Python3 bugs | Ruby3 bugs | Swift3 bugs | C++2 bugs | C1 bug |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Gemini 3.1 Pro | 67% | 67% | 38% | 78% | 88% | 56% | 33% | 67% | 67% | 67% | 100% | 100% | 50% | 50% |
| Gemini 3.8 Flash | 68% | 83% | 75% | 43% | 75% | 78% | 67% | 0% | 50% | 83% | 100% | 100% | 50% | 50% |
Of the 42 known bugs both were scored on, both caught 27 and neither caught 6. Gemini 3.1 Pro caught 5 bugs that Gemini 3.8 Flash missed: 2 in Java, 2 in Kotlin and 1 in Rust. Gemini 3.8 Flash caught 4 bugs that Gemini 3.1 Pro missed: 1 in C#, 1 in Go, 1 in JavaScript and 1 in TypeScript, most of them logic bugs.
On pull requests with no known bug, Gemini 3.1 Pro raised 0.8 false alarms per review and Gemini 3.8 Flash 1. A measured review cost $0.2499 on Gemini 3.1 Pro, 2.9× the cost of $0.0862 on Gemini 3.8 Flash.
| Pull request | Language · kind | Gemini 3.1 Pro | Gemini 3.8 Flash |
|---|---|---|---|
| kotest/kotestRefactor eventually to ensure wall-clock time usage | Kotlin · Error handling | 2/2 | 0/1 |
| ktorio/ktorKTOR-9171 Redesign ByteReadChannel.readUTF8Line API | Kotlin · Logic | 1/1 | 0/1 |
| mockito/mockitoAdd core API to enable Kotlin singleton mocking | Java · API misuse | 1/1 | 0/1 |
| jhy/jsoupStore cached child els in Attributes, not field | Java · Concurrency | 1/2 | 0/4 |
| nushell/nushellreorganize nushell configuration | Rust · Error handling | 1/2 | 0/1 |
| TanStack/queryfix(query-core): fix hydration bugs for already resolved promises | TypeScript · Logic | 0/1 | 1/1 |
| dotnet/orleansfix(serialization): cap codec allocations | C# · Logic | 0/1 | 1/1 |
| go-chi/chifeat: middleware.ClientIP, a replacement for middleware.RealIP | Go · Security | 0/1 | 1/1 |
| sindresorhus/p-throttleAdd `weight` option | JavaScript · Logic | 0/1 | 1/1 |
Defects caught over all runs: a PR reviewed three times with one known bug shows 3/3 when it was caught every time.
Every bug each was scored on: Gemini 3.1 Pro and Gemini 3.8 Flash.