Gets instant compiler errors and warnings without running a build.
**Prefer this over `cargo check`, `npm run build`, `tsc`, `go build`.**
## Usage
The tool infers scope from `filePath`:
**Workspace-wide diagnostics:**
```json
{}
```
**Single-file diagnostics:**
```json
{"filePath":"src/main.rs"}
```
## Parameters
- `filePath` — optional path to an existing file, absolute or relative to the workspace root. When omitted, checks the workspace.
## Output
`diagnostics` lists errors and warnings keyed by file path (relative to the workspace root), errors first:
```yaml
diagnostics:
src/main.rs:
- '2:18 error[E0308]: mismatched types'
```
If the required language server cannot start, the tool returns the startup error. For a missing TypeScript server, the error includes local and global npm installation commands.