Dynamic Pages API routes return 404 on Vercel when Pages Router i18n is enabled

### Link to the code that reproduces this issue https://github.com/adelrodriguez/nextjs-i18n-dynamic-api-repro ### To Reproduce 1. Open the public reproduction repository. 2. Use commit `b93f5cee3b789905cf27600d66fc7694f80daeb3`. 3. Deploy the repository to Vercel. 4. Use the Next.js framework preset. 5. Keep the default install command and build command. 6. Send these requests: ```bash REPRO_URL="https://nextjs-i18n-dynamic-api-repro-cv5wg0r69-personal-0d17b53c.vercel.app" curl -i "$REPRO_URL/api/ping" curl -i "$REPRO_URL/api/echo/hello" curl -i "$REPRO_URL/api/echo/%5Bslug%5D" ``` The first request returns `200`. The second request returns `404`. The response contains this header: ```text x-matched-path: /en/404 ``` The third request returns `200`. The response body is: ```json {"ok":true,"route":"dynamic","slug":"[slug]"} ``` The third request is a diagnostic request. It is not standard API use. ### Current vs. Expected behavior I expect `/api/echo/hello` to match the dynamic Pages API route at `pages/api/echo/[slug].ts`. I expect the route to return `200` and this body: ```json {"ok":true,"route":"dynamic","slug":"hello"} ``` On Vercel, Next.js `16.3.0` returns the localized `404` page. The static Pages API route in the same deployment returns `200`. The Vercel deployment contains a function named `api/echo/[slug]`. The encoded placeholder request starts this function. These results show that Vercel created the function but does not match a normal path segment to the function. The local production server returns `200` for both API routes. The local production build lists `ƒ /api/echo/[slug]`. ### Provide environment information ```bash Operating System: Platform: darwin Arch: arm64 Version: Darwin Kernel Version 25.6.0 Available memory (MB): 49152 Available CPU cores: 14 Binaries: Node: 24.2.0 npm: 11.3.0 Yarn: N/A pnpm: 11.16.0 Relevant Packages: next: 16.3.0 eslint-config-next: N/A react: 19.2.8 react-dom: 19.2.8 typescript: 6.0.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Dynamic Routes, Internationalization (i18n), Pages Router, Output ### Which stage(s) are affected? (Select all that apply) Vercel (Deployed) ### Additional context I tested each deployed request three times. Each test gave the same result. | Next.js | i18n | `/api/echo/hello` on Vercel | Encoded placeholder | Commit | Deployment | | --- | --- | --- | --- | --- | --- | | 16.2.12 | Enabled | `200` | `200` | [`bcd5da6`](https://github.com/adelrodriguez/nextjs-i18n-dynamic-api-repro/commit/bcd5da6f10107a10fafee9c62459b826c75c0c01) | [Open](https://nextjs-i18n-dynamic-api-repro-8ryra6til-personal-0d17b53c.vercel.app) | | 16.3.0 | Enabled | `404`, matched `/en/404` | `200` | [`b93f5ce`](https://github.com/adelrodriguez/nextjs-i18n-dynamic-api-repro/commit/b93f5cee3b789905cf27600d66fc7694f80daeb3) | [Open](https://nextjs-i18n-dynamic-api-repro-cv5wg0r69-personal-0d17b53c.vercel.app) | | 16.3.1-canary.7 | Enabled | `404`, matched `/en/404` | `200` | [`b439280`](https://github.com/adelrodriguez/nextjs-i18n-dynamic-api-repro/commit/b439280984a19594f09d23ec6e8f64c21b38efe1) | [Open](https://nextjs-i18n-dynamic-api-repro-kxrd7j8eu-personal-0d17b53c.vercel.app) | | 16.3.0 | Disabled | `200` | `200` | [`35a3f03`](https://github.com/adelrodriguez/nextjs-i18n-dynamic-api-repro/commit/35a3f03789ec898f759284516f2691d8361d2ac2) | [Open](https://nextjs-i18n-dynamic-api-repro-jvyjugppw-personal-0d17b53c.vercel.app) | The version tests show that the regression starts after `16.2.12` and is present in `16.3.0`. The latest canary version, `16.3.1-canary.7`, is also affected. The control test shows that the dynamic API route works with Next.js `16.3.0` when I remove only the `i18n` configuration. The public repository contains the full HTTP responses, the local build output, the `next info` output, and the Vercel function lists. The earlier report is #96931. An automated check closed it because it did not contain a public reproduction link. PR #94905 and commit `668cd3a4a7da71e4402abf9a818e89e0476f275e` changed adapter i18n handling for Pages API routes in this version range. This change is an investigation lead. The reproduction does not prove that this change caused the regression.