nostics

Every error condition becomes a typed Diagnostic (extends Error) with a stable code, docs URL, and actionable fix. Serializable via toJSON().

Diagnostic: name (the code), message/why (interpolated text), fix?, docs?, sources? ('file:line:column'), cause?, toJSON(). Throw it, catch it with instanceof Diagnostic, send toJSON() across process boundaries.

defineDiagnostics

Returns one callable handle per code. Calling a handle builds a fresh Diagnostic, fires every reporter in order, and returns it. throw the return value to raise (reporters still run, so a thrown diagnostic also reports).

import { createConsoleReporter, defineDiagnostics } from 'nostics'

const diagnostics = /*#__PURE__*/ defineDiagnostics({
  docsBase: (code) => `https://nuxt.com/e/${code.replace('NUXT_', '').toLowerCase()}`,
  reporters: [/*#__PURE__*/ createConsoleReporter()],
  codes: {
    NUXT_B1001: {
      why: 'Could not compile template.',
      fix: 'Check the template for syntax errors.',
    },
    NUXT_B2011: {
      why: (p: { src: string }) => `Invalid plugin "${p.src}". src option is required.`,
      fix: 'Pass a string path or an object with a `src` to `addPlugin()`.',
    },
    NUXT_W9001: { why: 'message', docs: false }, // per-code: string overrides docsBase, false opts out
  },
})

• docsBase string | (code) => string | undefined: string appends /${code.toLowerCase()}; function returns the full URL (or undefined to omit).
• codes: each definition needs why (string | (params) => string, the only required field, becomes Error.message); optional fix (string | (params) => string) and docs (string | false).
• reporters: fired on every call; optional. Their options types are intersected; required reporter options become required at the call site. Omit it (or pass []) for a catalog whose codes are only ever thrown: the thrown Diagnostic already carries the message, so a console reporter would print it once and surface it again from the uncaught error, a visible duplicate. Keep report-only warnings and fatal throws in separate catalogs when one needs a reporter and the other does not.
• Param inference: params from why and fix are intersected and required at the call site. If why needs { src } and fix needs { date }, the call requires { src, date }.

Call sites

diagnostics.NUXT_B1001() // no params: report only
diagnostics.NUXT_B2011({ src: '/plugins/bad.ts' }) // params first
diagnostics.NUXT_B2011({
  src,
  cause: originalError,
  sources: ['nuxt.config.ts:42:3'],
}) // runtime fields merge in
diagnostics.NUXT_B2011({ src }, { method: 'error' }) // reporter options second
throw diagnostics.NUXT_B2011({ src }) // raise

cause/sources go in the params object; sources matters most for build/config diagnostics where the JS stack points inside the library. Catch with if (err instanceof Diagnostic) then read .name, .message, .fix, .docs.

Reporters

(diagnostic: Diagnostic, options?: Opts) => void. Declaring a required options type makes the second call-site argument required and typed.

import type { DiagnosticReporter } from 'nostics'
const sentryReporter: DiagnosticReporter = (d) =>
  sentry.captureMessage(d.message, { tags: { code: d.name } })
const audited: DiagnosticReporter<{ priority: number }> = (d, o) =>
  audit.log({ name: d.name, priority: o.priority })
// → audited makes diagnostics.X({...}, { priority: 1 }) required and type-checked.

Formatters

formatDiagnostic output, detail order fixed fix → sources → see, missing fields omitted:

[NUXT_B2011] Invalid plugin `/plugins/bad.ts`. src option is required.
├▶ fix: Pass a string path or an object with a `src` to `addPlugin()`.
├▶ sources: nuxt.config.ts:42:3
╰▶ see: https://nuxt.com/e/b2011

Vite plugins (@nostics/unplugin, dev dependency)

@nostics/unplugin/strip-transform (library authors, build optimization) and @nostics/unplugin/dev-server-collector (app developers, dev-time collection). Both unplugin-based: .vite(), .webpack(), .rollup(), etc.

• nosticsStrip marks defineDiagnostics() /*#__PURE__*/ and wraps bare diagnostic expression statements with a NODE_ENV guard so they tree-shake out of production. Option packageName? (default 'nostics'). Throws/returns/assignments stay (they are behavior). For tracking: relative imports, export the catalog directly, no factory wrappers or deep barrels.
  • The plugin is optional. The same production output happens with no build transform if the catalog is annotated by hand: put /*#__PURE__*/ before defineDiagnostics( and before each reporter factory call inside it (as in every example here), and dev-guard each report-only call site (process.env.NODE_ENV !== 'production' && diagnostics.CODE(p)). Always write the annotations in source; reach for the plugin when report-only call sites are unguarded and you want stripping without touching them.
• nosticsCollector listens for createDevReporter() diagnostics over the Vite WebSocket and writes them as NDJSON via createFileReporter. Vite-only. Options logFile? (default .nostics.log), debug? (default !!process.env.DEBUG).

// vite.config.ts
import { nosticsStrip } from '@nostics/unplugin/strip-transform'
import { nosticsCollector } from '@nostics/unplugin/dev-server-collector'
export default defineConfig({
  plugins: [nosticsStrip.vite(), nosticsCollector.vite()],
})

// src/diagnostics.ts — pair the collector with createDevReporter()
import { createConsoleReporter, defineDiagnostics } from 'nostics'
import { createDevReporter } from 'nostics/reporters/dev'
export const diagnostics = /*#__PURE__*/ defineDiagnostics({
  reporters: [/*#__PURE__*/ createConsoleReporter(), /*#__PURE__*/ createDevReporter()],
  codes: {
    /* ... */
  },
})

Production builds

• Report-only diagnostics (bare diagnostics.X()) should disappear: nosticsStrip or hand annotations drop them, then the unused catalog tree-shakes.
• Surviving diagnostics (throw/return/assigned/argument) stay, and each keeps the whole catalog reachable, so every why/fix ships. Not every library throws in production: if yours only reports, stripping is enough, stop here.

When a library does throw in production, pick defineProdDiagnostics at definition time with a NODE_ENV ternary, so a consumer bundler drops the dev branch (all catalog text):

import { defineDiagnostics, defineProdDiagnostics } from 'nostics'
export const diagnostics =
  process.env.NODE_ENV === 'production'
    ? /*#__PURE__*/ defineProdDiagnostics({ docsBase })
    : /*#__PURE__*/ defineDiagnostics({
        docsBase,
        reporters: [
          /* ... */
        ],
        codes: {
          /* text */
        },
      })

The accessed code becomes the instance name, docs still derives from docsBase, why points to the docs URL when one exists (empty otherwise), no why/fix text ships. No reporters by default (so a surviving throw doesn't also log and then resurface as the uncaught error); pass reporters to keep prod telemetry. nosticsStrip tracks this ternary like a direct catalog export.

Conventions

• Codes are stable, fully-qualified PREFIX_XNNNN (B build, R runtime, C config, D deprecation). Never reuse or reassign a published code.
• Always provide why; provide fix whenever the solution is known (the most actionable field for humans and agents). Use parameterized templates for runtime values, not string concatenation outside the factory.
• why is the diagnosis, fix is the remedy — split them, don't overlap them. why states only what is wrong; fix states only what to do. The reporter prints both, so any wording that appears in both is dead weight. When a single source sentence carries both ("A hash must start with '#'. Prefix it with '#'."), cut it in two — diagnosis to why, remedy to fix — rather than pasting the whole thing into why and echoing it in fix. fix accepts a param function too ((p) => ...), so move value-bearing remedies (use "#${p.hash}") into it instead of leaving them in why.
• Pass cause when re-raising; pass sources when the JS stack doesn't reflect the user's source.
• Split large catalogs by domain (diagnostics/build.ts, runtime.ts, config.ts, re-exported from index.ts), each defineDiagnostics() sharing docsBase with its own code range.

References

• Migrating an existing library to nostics (replacing console.warn/console.error/warn()/thrown Errors with diagnostic codes, without changing runtime behavior): follow references/migration.md start to finish.
• Building the error-code documentation site (page template, deployment, agent optimization): references/documentation-site.md.