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Next.Js TypeScript Plugin Diagnostics Not Reported By CLI / CI Type Checking

Next.js provides a TypeScript language-service plugin that reports framework-specific diagnostics in the editor, but these diagnostics are not available when running `tsc` or any Next.js CLI command. This means CI type checking can miss Next.js-specific type errors (e.g., invalid metadata export types), leading to false confidence. A framework-aware CLI type check command (e.g., `next check`) should be added.

highConfidence 92%Next.js Affected V16.3.1-Canary.13

Origin Analysis

The Next.js TypeScript plugin is implemented as a language-service plugin, which is only loaded by editors (VS Code) that support TypeScript plugin loading. The TypeScript compiler (`tsc`) does not support loading language-service plugins (see microsoft/TypeScript#16607), and Next.js does not provide a wrapper CLI command that uses the TypeScript LanguageService API to load its plugin and emit diagnostics. Therefore, Next.js-specific diagnostics are invisible to command-line/CI type checking.

1. Create a Next.js project with the `next` TypeScript plugin enabled in tsconfig.json (e.g., `"plugins": [{ "name": "next" }]`). 2. Write a page that exports a `metadata` object with an incorrect type (e.g., string instead of Metadata). 3. Observe the diagnostic in VS Code (ts(71008)). 4. Run `pnpm tsc` or `next build`; no error is reported. 5. Run `pnpm next check` or `pnpm next tsc`; the command does not exist (or is misinterpreted as project directory).

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This fix relies on the internal API of the Next.js TypeScript plugin and the 'next/typescript' export, which may change across Next.js versions. Running the LanguageService manually can be slower than `tsc` because it does not leverage incremental compilation caches. It may also load a different TypeScript version than the one used by the project, causing compatibility issues; ensure the script uses the project's installed TypeScript (via `require('typescript')`) or bundle a compatible version. If the plugin's `create` function expects a different shape of the language service host, the service may fail or produce incomplete diagnostics. Rollback is straightforward: remove the script and revert to `tsc`, but be aware that Next.js-specific diagnostics will again be missed. For production, prefer an official Next.js implementation that is properly tested across platforms and TypeScript versions.