# How `native check` Performs Markup Validation and Reports File:Line:Column Errors in Native SDK Apps > Learn how native check validates markup in Vercel Native SDK apps. Get precise file:line:column error diagnostics without building. - Repository: [Vercel Labs/native](https://github.com/vercel-labs/native) - Tags: how-to-guide - Published: 2026-07-18 --- **`native check` scans the `src/` directory for `.native` markup files, validates them via `markup_cli.checkFiles`, and emits precise `file:line:column` diagnostics that let developers jump straight to the source of any error—without ever building the app.** As implemented in `vercel-labs/native`, the `native check` command provides fast, build-free validation for Native SDK applications. Its markup validation pipeline checks TypeScript cores, `.native` files, and the `app.zon` manifest while surfacing every issue with exact file, line, and column coordinates. ## The `native check` Markup Validation Pipeline In `tools/native-sdk/main.zig`, the `native check` entry point orchestrates a six-stage pipeline that validates the whole app without producing a build. ### Locate the App Manifest First, `native check` looks for an `app.zon` file in the current directory—or the directory you pass—to confirm it is inside a Native SDK app. If the file is missing, the command aborts immediately with a **MissingManifest** error. ### Run the TypeScript Core Checker If the app uses a TypeScript core, `native check` runs the **@native-sdk/core subset checker** before any markup validation. This ensures TypeScript diagnostics are collected and reported alongside markup issues. ### Collect Markup Files The command invokes **collectMarkupFiles**, defined in `tools/native-sdk/markup.zig`, to recursively walk `src/` and gather every file whose extension matches a markup file—specifically `.native`. The function returns a list of absolute paths that are fed into the validator. ### Validate Markup Files The collected paths are passed to `markup_cli.checkFiles`. The validator returns a **CheckOutcome** structure containing the count of failures, warnings, and whether the **model-contract** was checked. This stage is where markup syntax and semantics are evaluated against the app's contract. ### Validate the Manifest After markup checks, `app.zon` is parsed and validated using `tooling.manifest.validateFile` from `src/tooling/manifest.zig`. Any manifest problems are printed, and if validation fails, the command exits with an error. ### Report Results and Strict Mode Finally, `native check` prints a one-line summary such as: ```bash checked 3 markup files against the model contract, app.zon ``` If you pass the **`--strict`** flag, warnings are promoted to errors, causing the command to fail even when only warnings are present. ## File:Line:Column Error Format in Markup Diagnostics When the markup compiler finds a problem, it prints diagnostics in a **file:line:column** format that editors and terminals can parse directly. According to the `vercel-labs/native` README: > "`native check` validates every view … with **file:line:column** errors that teach." The underlying diagnostics are generated by `src/primitives/canvas/ui_markup.zig`. Inside the markup compiler, the `printDiagnostic` logic attaches source locations to each node and formats them as `file:line:column` when emitting errors. A typical CLI output looks like this: ```bash # Example error output src/app.native:12:5: unknown tag "buttonn" (did you mean "button"?) ``` This pinpoint output eliminates guesswork and allows IDEs to jump directly to the offending line and column. ## Running `native check` from the Command Line You invoke the validator from inside an app directory. The following examples show the standard and strict modes: ```bash # Validate TS core, markup, and app.zon native check # Promote warnings to errors native check --strict ``` Because validation runs without a build step, the feedback loop is nearly instantaneous. ## Key Source Files Behind `native check` Understanding the architecture is easier when you know which files handle each responsibility: - **`tools/native-sdk/main.zig`** — The CLI entry point that orchestrates the core checker, markup validation, and manifest verification. - **`tools/native-sdk/markup.zig`** — Implements `collectMarkupFiles` and the `CheckOutcome` structure; invokes the markup validator via `checkFiles`. - **`src/primitives/canvas/ui_markup.zig`** — The markup compiler that parses `.native` files and emits diagnostics with file:line:column source locations through `printDiagnostic`. - **`src/tooling/manifest.zig`** — Parses and validates `app.zon`, providing manifest-level diagnostics. ## Summary - **`native check`** validates an entire Native SDK app without building it. - The pipeline runs the TS core checker, collects `.native` files from `src/`, validates markup via `markup_cli.checkFiles`, and verifies `app.zon` through `tooling.manifest.validateFile`. - Failures and warnings are aggregated in a `CheckOutcome`, and results are summarized on the command line. - Diagnostics use a precise **file:line:column** format generated by `src/primitives/canvas/ui_markup.zig`, enabling direct navigation to errors. - The **`--strict`** flag elevates warnings to errors for more stringent CI/CD gates. ## Frequently Asked Questions ### What does `native check` validate besides markup? `native check` validates three layers: the TypeScript core (if the app uses one), every `.native` markup file under `src/`, and the `app.zon` manifest. Any failure in these layers causes the command to exit with an error. ### How does `native check` locate markup files to validate? It calls `collectMarkupFiles` in `tools/native-sdk/markup.zig`, which recursively walks `src/` and selects every file with a `.native` extension. These absolute paths are then passed to the markup validator. ### What happens when `--strict` is passed to `native check`? When `--strict` is used, all warnings collected in the `CheckOutcome` are promoted to errors. This guarantees that even minor markup or manifest issues will fail the command, which is useful for enforcing quality gates in continuous integration. ### Where are the file:line:column diagnostics generated? The diagnostics are generated inside `src/primitives/canvas/ui_markup.zig` by the markup compiler. Its `printDiagnostic` logic maps each AST node back to its source location and formats the output as `file:line:column` so that editors and terminals can resolve the exact point of failure.