build(web): fail vite build on a browser-externalized Node built-in (#1769)

.github/workflows/main.yml

@@ -53,6 +53,15 @@ jobs:
         # running the integration suite twice.
         run: npm run coverage
 
+      - name: Verify the browser-externalized-builtin build gate (#1769)
+        # Runs a real `vite build` with a Node built-in forced into the browser
+        # graph and asserts the build FAILS via the #1769 gate. The unit tests
+        # cover the detection logic against a captured message; only this real
+        # build catches the risk the issue calls out — that the Vite warning
+        # phrasing drifts across releases, silently disabling the message-keyed
+        # gate. It restores the mutated entry afterward (see the script).
+        run: npm run verify:build-gate
+
       # Playwright chromium is installed BEFORE the smokes because
       # `smoke:web:browser` (the headless-browser boot smoke, #1615) drives the
       # prod web bundle in chromium — restoring/installing it here lets that

AGENTS.md

@@ -16,7 +16,9 @@ inspector/
 │   │   │                               #   web-server-config.ts (env parsing + initial-config payload + banner),
 │   │   │                               #   sandbox-controller.ts (MCP Apps sandbox HTTP server),
 │   │   │                               #   inject-auth-token.ts (embeds the API token into served index.html),
-│   │   │                               #   vite-base-config.ts (shared optimizeDeps exclusions)
+│   │   │                               #   vite-base-config.ts (shared optimizeDeps exclusions),
+│   │   │                               #   browser-externalized-builtin-gate.ts (build-gate logic that fails
+│   │   │                               #     `vite build` on a browser-externalized Node built-in — #1769)
 │   │   └── static/                     # sandbox_proxy.html (served by sandbox-controller for MCP Apps tab)
 │   ├── cli/                            # CLI client (tsup bundle, @inspector/core alias)
 │   ├── tui/                            # TUI client (Ink + React, tsup bundle)
@@ -210,16 +212,17 @@ gh project item-edit --project-id PVT_kwDOCt2Azc4BJVxt --id "$ITEM_ID" --field-i
 
 ### Mandatory pre-push gate
 - ALWAYS do `npm run format` before committing — the **root** `format` auto-fixes `core/` (`format:core`) and every client's scope in one shot. `validate` runs `format:check` (the non-fixing variant, including `format:check:core`) and will fail in CI on any unformatted file, so always run the auto-fixer first rather than letting `format:check` catch it.
-- **`npm run ci` is the mandatory pre-push command** — it mirrors `.github/workflows/main.yml` (minus `npm install`): `validate` → `coverage` → `smoke` → Storybook play-function tests (installs Playwright chromium if needed). It now runs **`npm run coverage`**, the per-file ≥90 gate (lines/statements/functions/branches) that CI enforces — so `npm run ci` is a true superset of GitHub CI, and passing it locally means CI's gates will pass. Expect several minutes. **`npm run validate`** remains the fast inner-loop check during development (unit tests only — no coverage gate, no smoke, no Storybook), but it is **NOT** an acceptable substitute for `npm run ci` before pushing: `validate` runs `test`, not `test:coverage`, so it does **zero** coverage gating. Skipping the gate is how a push passes every fast local check and still fails CI (this exact gap broke PR #1601 on a function-coverage regression).
+- **`npm run ci` is the mandatory pre-push command** — it mirrors `.github/workflows/main.yml` (minus `npm install`): `validate` → `coverage` → `verify:build-gate` (the #1769 browser-externalized-builtin build gate) → `smoke` → Storybook play-function tests (installs Playwright chromium if needed). It now runs **`npm run coverage`**, the per-file ≥90 gate (lines/statements/functions/branches) that CI enforces — so `npm run ci` is a true superset of GitHub CI, and passing it locally means CI's gates will pass. Expect several minutes. **`npm run validate`** remains the fast inner-loop check during development (unit tests only — no coverage gate, no smoke, no Storybook), but it is **NOT** an acceptable substitute for `npm run ci` before pushing: `validate` runs `test`, not `test:coverage`, so it does **zero** coverage gating. Skipping the gate is how a push passes every fast local check and still fails CI (this exact gap broke PR #1601 on a function-coverage regression).
 - ALWAYS do `npm run format` before committing, then **`npm run ci`** before pushing. From the repo root, `validate` runs the **`core/` gate first** (`validate:core`) and then chains the four per-client validations (`validate:web` → `validate:cli` → `validate:tui` → `validate:launcher`); each client delegates to its own `npm run validate` in its own folder (no coverage — fast). Every client is self-validating and the top level just chains them, building each client's bundle along the way (no cross-client build dependencies).
   - **`validate:core` is the shared-code format + lint gate (#1689).** Each client's `prettier`/`eslint` is scoped to its own dir, so nothing reached `core/` before — `validate:core` closes that: it runs `format:check:core` (`prettier --check "core/**/*.{ts,tsx}"`) + `lint:core` (`eslint "core/**/*.{ts,tsx}"` via the **root** `eslint.config.js`). Use `npm run format:core` to auto-fix. The root carries prettier/eslint as devDependencies for this; `core/` is isomorphic (browser + Node globals, no JSX today — the `{ts,tsx}` glob future-proofs against a `core/**/*.tsx`). The root `eslint.config.js` honors an `_`-prefix as the intentionally-unused marker (`argsIgnorePattern`/`varsIgnorePattern`/`caughtErrorsIgnorePattern: '^_'`).
   - **cli and tui now typecheck their `src` (#1689).** Their `build`/`test` run through esbuild (no type check), so each has a `typecheck` script (`tsc --noEmit -p tsconfig.json`) folded into `validate`. Their `tsconfig.json` matches `clients/web/tsconfig.app.json`'s module/lib *resolution* options — DOM lib, `moduleResolution: bundler`, and **no** `noUncheckedIndexedAccess` (web's app config does not extend `tsconfig.base`, so re-enabling it would surface `core/` issues web never gates) — so the imported `core/` sources are validated the same way web validates them. It does **not** mirror web's extra strictness flags (`noUnusedLocals`, `verbatimModuleSyntax`, ES2023 target, …), so cli/tui's own `src` is checked slightly more loosely than web's. `core/` itself still typechecks through web's `tsc -b`.
   - The one CLI nuance: `clients/cli`'s out-of-process `e2e.test.ts` spawns the built binary, so its `test` **builds first** via `pretest` (`test-servers:build && build`). To avoid building it twice, `clients/cli`'s `validate` folds that in — it is `format:check && lint && typecheck && test` with **no** separate `build` step (the other clients, whose tests don't spawn their bundle, keep an explicit `build`). `validate:web`/`validate:tui`/`validate:launcher` are the uniform `format:check && lint && (typecheck &&) build && test`.
   - **`npm run coverage`** is the per-file ≥90 gate and is now part of `npm run ci` — never treat it as optional before a push. It supersedes the old standalone `test:integration` step: web's `test:coverage` runs the `unit` **and** `integration` projects under v8 instrumentation, so `coverage` both enforces the ≥90 gate and exercises the same web integration paths CI covers.
-- **`smoke` is NOT part of `validate`** — it is included in `npm run ci`. It runs `smoke:launcher` (`--help` dispatch) plus the prod `smoke:cli` / `smoke:tui` / `smoke:web` / `smoke:web:browser`, and contains **no build commands** — it assumes the cli/tui/launcher bundles already exist (a full `validate` builds them; `smoke:web` builds `clients/web/dist` on demand). CI runs `validate`, then the `coverage` gate (which also covers the web integration project), then `smoke` (with Playwright chromium installed just before it, since `smoke:web:browser` needs it). Storybook is the only CI step left out (see below).
+- **`smoke` is NOT part of `validate`** — it is included in `npm run ci`. It runs `smoke:launcher` (`--help` dispatch) plus the prod `smoke:cli` / `smoke:tui` / `smoke:web` / `smoke:web:browser`, and contains **no build commands** — it assumes the cli/tui/launcher bundles already exist (a full `validate` builds them; `smoke:web` builds `clients/web/dist` on demand). CI runs `validate`, then the `coverage` gate (which also covers the web integration project), then `verify:build-gate` (the #1769 build gate — see below), then `smoke` (with Playwright chromium installed just before it, since `smoke:web:browser` needs it). GitHub CI runs this same chain as separate workflow steps, with the Storybook play-function tests last (see below).
 - `smoke:launcher` (`scripts/smoke-launcher.mjs`) runs the built launcher with `--help`, `--cli --help`, and `--tui --help`, asserting each exits 0 and prints that mode's usage banner (which also proves the launcher resolved and loaded the right client build). It's the cheap dispatch check before the heavier prod smokes below.
 - `smoke:web` (`scripts/smoke-web.mjs`) starts `mcp-inspector --web` (prod, no `--dev`) against the built `clients/web/dist` and asserts `GET /` serves the SPA (HTTP 200) with the injected `__INSPECTOR_API_TOKEN__`. Prod `--web` serves from `clients/web/dist`, which ships in the published package but is absent in a fresh checkout — the runner builds it on demand (`build:client` = `vite build`) on first launch, or exits with an actionable error if that build can't run (see `clients/web/server/ensure-web-build.ts` and the launcher README). `--dev` runs Vite directly and never needs `dist`. It shares the spawn/readiness/teardown helper (`scripts/lib/prod-web-server.mjs`) with `smoke:web:browser`, so the two can't drift.
 - `smoke:web:browser` (`scripts/smoke-web-browser.mjs`, #1615) goes a step further than `smoke:web`: it boots the same prod `--web` server and then actually **runs** the bundle in headless Chromium (Playwright — already a `clients/web` devDependency for the Storybook tests), asserting the app renders its first meaningful frame (the "Add Servers" control) with **no uncaught error**. `smoke:web` only checks the served HTML, so a Node built-in reaching the browser bundle slipped through it; this smoke catches that regression as a *class* (e.g. #1612). The mechanism is the uncaught error, not a magic string: under Vite the excluded module becomes an empty stub and the first *call* into it (e.g. `fs.readFileSync(...)` during a transitive module's init) throws a `TypeError` that aborts app mount. A *synchronous* such throw fires `pageerror`; its *async* twin (the same `TypeError` via `await`/`.then()`, or a failed dynamic import) is logged on the console channel as `Uncaught (in promise) …` / `Failed to fetch dynamically imported module` — the smoke hard-fails on both. The literal `Module "…" has been externalized` text is, **in a prod build**, a build-time warning (`vite build` / `npm run build`), not a runtime message, so the browser never sees it (under `npm run dev` Vite's stub is instead a `Proxy` that `console.warn`s that string at runtime); and an externalized import that is never *called* ships a harmless `{}` and is invisible here by design. Every *other* console error is printed as a diagnostic, not a failure (so a benign font-CDN or React-warning `console.error` doesn't flake CI). Playwright is resolved via `createRequire` based at `clients/web/package.json` — a bare `import("playwright")` would resolve relative to `scripts/`, not the cwd, so it can't be reached that way (it only appears to work when an ancestor `node_modules` carries playwright, and fails in CI, which has none). The npm script's `cd clients/web` exists only so `npx playwright install chromium` finds the local playwright bin (a no-op when already installed).
+- **The build gate for the browser-externalized-builtin class (#1769)** is the earlier, more complete companion to `smoke:web:browser`. A Vite plugin in `clients/web/vite.config.ts` (logic in `clients/web/server/browser-externalized-builtin-gate.ts`, unit-tested) turns Vite 8's *browser-externalization warning* (`Module "node:*" has been externalized for browser compatibility`) into a hard `vite build` error, so a Node built-in in the browser graph now **fails `npm run build` / `validate`** instead of shipping a `{}` stub. This catches **both** the *called-at-init* case (which `smoke:web:browser` also catches, but later/at runtime) **and** the *imported-but-never-called* case (the `{}` stub that is invisible to the runtime smoke "by design" — see above). Because rolldown **swallows a throw inside `onLog`** (the one hook where a thrown error doesn't abort — verified against vite@8.0.0), the plugin *records* the warning in `onLog` and re-throws in `buildEnd`. There is **no stable log `code`**, so the gate keys off the documented message phrasing; `npm run verify:build-gate` (`scripts/verify-build-gate.mjs`, in `npm run ci` and the GitHub workflow) runs a real build with a `node:fs` probe forced into `src/main.tsx` and asserts the build fails via the gate — the only check that catches the message phrasing **drifting** in a future Vite bump and silently disabling the gate. The gate is scoped to `vite build` (`apply: 'build'`) — never `vite dev` or the vitest projects — **and** to the browser (`client`) environment (`applyToEnvironment`), so a future SSR/node environment built from this config isn't failed for a legitimate `node:*` import; the Node runner build (tsup, `build:runner`) is a separate config where built-ins are legitimate. `smoke:web:browser` stays as the runtime backstop for crashes the build can't reason about.
 - `smoke:cli` (`scripts/smoke-cli.mjs`) drives `mcp-inspector --cli` through the built launcher against the bundled stdio test server via a temp `--catalog`: it asserts `tools/list` returns the server's tools (real connect over stdio), the default writable catalog is seeded empty on first run, a missing read-only `--config` errors without seeding, and `--catalog` + `--config` is rejected. `smoke:tui` (`scripts/smoke-tui.mjs`) launches `mcp-inspector --tui --catalog <temp>` and asserts the Ink app renders its first frame (the "MCP Servers" panel) within a timeout, then SIGTERMs it — a shallow boot/render check, not full interaction. **`smoke:tui` is local-only: it self-skips when `process.env.CI` is set**, because the Ink TUI needs a real TTY (raw mode) that headless CI lacks — so run it (via `npm run smoke`) on your own machine before pushing. Both build `test-servers/build` on demand if it's missing.
 - Storybook play-function tests (`clients/web` `test:storybook`) run in headless Chromium via `@vitest/browser-playwright` (~10s). They are part of `npm run ci` (which installs Playwright chromium first); kept out of `validate` because they need the browser binary and are slower than the unit suite.
 

README.md

@@ -35,7 +35,7 @@ inspector/
 │   ├── react/        # React hooks over the state stores
 │   └── storage/      # File I/O helpers for the OAuth persist backends
 ├── test-servers/     # Composable MCP test servers + fixtures used by integration tests
-├── scripts/          # Root build/verify tooling (install cascade, smokes, pack:verify)
+├── scripts/          # Root build/verify tooling (install cascade, smokes, verify-build-gate, pack:verify)
 ├── specification/    # Design/build specifications
 ├── AGENTS.md         # Contribution rules for agents AND humans (see below)
 └── README.md         # You are here
@@ -143,7 +143,8 @@ Each client self-validates from its own folder; the root scripts chain them. The
 | `npm run validate`    | Runs `validate:core` (the shared `core/` `format:check` + `lint` gate) first, then per client: `format:check` + `lint` + **`typecheck`** (cli/tui only) + `build` + fast unit tests. The quick inner-loop check.                                          |
 | `npm run coverage`    | The **per-file ≥90% gate** (lines/statements/functions/branches) under v8 instrumentation, per client. CI-enforced. For web this also runs the integration project and covers the shared `core/` runtime (including `core/json` and `core/client`).       |
 | `npm run smoke`       | End-to-end smokes through the built launcher (`--help` dispatch + prod cli/tui/web), plus a headless-Chromium boot smoke that runs the prod web bundle and asserts a clean first render (no uncaught error — sync exception or unhandled rejection, how a Node built-in reaching the browser bundle manifests).                               |
-| `npm run ci`          | **Mandatory pre-push command.** `validate` → `coverage` → `smoke` → Storybook. A true superset of GitHub CI.                                                                                                                                             |
+| `npm run verify:build-gate` | Runs a real `vite build` with a Node built-in forced into the browser graph and asserts the build **fails** via the #1769 gate (which turns Vite's browser-externalization warning into a hard error). Guards against the warning phrasing drifting in a Vite bump and silently disabling the gate. Part of `npm run ci`.                        |
+| `npm run ci`          | **Mandatory pre-push command.** `validate` → `coverage` → `verify:build-gate` → `smoke` → Storybook. A true superset of GitHub CI.                                                                                                                        |
 | `npm run pack:verify` | Publish smoke — see [Publishing](#publishing).                                                                                                                                                                                                          |
 
 Per-client scripts exist too (`validate:web`, `coverage:cli`, `smoke:tui`, …), plus root `validate:core` / `format:core` for the shared `core/` package. Run `npm run format` before committing — the root `format` fixes `core/` and every client; `validate` runs the non-fixing `format:check` and fails CI on any unformatted file.

clients/web/README.md

@@ -19,6 +19,7 @@ The `server/` directory holds the Node-only backend:
 - **`web-server-config.ts`** — env parsing, the `GET /api/config` payload, the startup banner.
 - **`inject-auth-token.ts`** — embeds the API token into the served `index.html` (see [Auth token](#auth-token)).
 - **`sandbox-controller.ts`** — the MCP Apps sandbox HTTP server; **`ensure-web-build.ts`** — builds `dist/` on demand for prod `--web`; **`vite-base-config.ts`** — shared `optimizeDeps` exclusions.
+- **`browser-externalized-builtin-gate.ts`** — Vite-agnostic build-gate logic that fails `vite build` when a Node built-in reaches the browser bundle (#1769); the thin Vite plugin wiring lives in `vite.config.ts`. It sits under `server/` (rather than `src/`) as the home for Node-only, build-time tooling — it's imported by the Vite config, never by the browser — alongside the other `vite-*` config helpers here.
 
 ## Development