inspector/clients/web/README.md

MCP Inspector Web Client

The browser incarnation of the Inspector: a Vite + React + Mantine single-page app backed by a small Node (Hono) server. The SPA is presentational — it renders data and fires callbacks; all MCP state comes from the shared @inspector/core hooks. The backend proxies MCP connections, serves the built SPA, and exposes /api/*.

This README covers what's specific to the web client. For the repo-wide picture (the @inspector/core shared package, the "dumb components" philosophy, the top-level validate/coverage/ci scripts, and publishing), see the root README.

Two halves: src/ (browser) and server/ (Node)

Path	Runs in	Purpose
src/	browser	The React SPA — components, hooks, theme, entry (main.tsx).
server/	Node	The dev/prod backend wiring (never imported by the browser).

The server/ directory holds the Node-only backend:

• vite-hono-plugin.ts — mounts the Hono /api/* middleware onto the Vite dev server (so npm run dev has a live backend).
• server.ts — the standalone Hono production server (serves dist/ + /api/*).
• run-web.ts / start-vite-dev-server.ts — entry points the launcher calls for prod --web and --web --dev.
• web-server-config.ts — env parsing, the GET /api/config payload, the startup banner, the default origin allow-list.
• resolve-bind-host.ts — the shared bind-host guard (refuses an all-interfaces HOST unless DANGEROUSLY_BIND_ALL_INTERFACES), used by both bind points (web-server-config.ts + vite.config.ts); see Host binding & the origin allow-list.
• inject-auth-token.ts — embeds the API token into the served index.html (see Auth token).
• sandbox-controller.ts — the MCP Apps sandbox HTTP server; app-origin-controller.ts — the dedicated app-origin server for _meta.ui.domain (see MCP App dedicated origins); 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

npm run dev        # Vite dev server + Hono /api middleware, HMR

For the launcher-driven prod/dev flows (npm run web / web:dev), see the root README — those run the built launcher.

Build

npm run build      # tsc -b  →  vite build  →  build:runner

Two artifacts come out, both of which ship in the published package:

• dist/ — the browser SPA (vite build). Served statically by the prod backend.
• build/ — the Node prod-server runner (build:runner = tsup --config tsup.runner.config.ts), which bundles server/ + @inspector/core into one ESM file and externalizes npm deps.

build:client runs only the vite build half when you just need fresh dist/.

Component layers

Components live under src/components/ in four layers, smallest to largest:

Layer	Count	What it is
elements/	~31	Leaf presentational pieces (badges, buttons, toggles) over Mantine primitives.
groups/	~63	Composite pieces (cards, panels, modals, control bars).
screens/	~11	Full tab screens (Tools, Resources, Servers, monitoring screens…).
views/	1	InspectorView — the top-level layout that composes the screens.

Every screen and element has a *.stories.tsx (see Storybook). Styling follows the Mantine-first rules in AGENTS.md — theme variants and component props over CSS, --inspector-* tokens over raw colors.

Non-component code: src/lib vs src/utils

Two grab-bag directories, split by one rule: utils = functions that compute; lib = things that instantiate, adapt, or touch the environment. If it does I/O or wraps a subsystem, it's lib; if it's a pure transform, it's utils.

• src/utils/ — pure, side-effect-free functions (no DOM/window/sessionStorage I/O, no subsystem ownership), trivially unit-testable with no mocks. Examples: jsonUtils, schemaUtils, toolUtils, maskSecrets, inspectorTabs, deepLink, mcpNetworkHeaders. Carve-outs that stay utils:
  • Diagnostic logging (console.warn/console.error) doesn't count as a side effect.
  • Importing from @inspector/core — neither a type-only import nor re-exporting core's pure functions/constants is a subsystem dependency (what makes a module lib is wrapping core's stateful runtime).
  • Pure domain types + their constructors (customHeaders) — there is no types/ sub-bucket inside lib/utils.
• src/lib/ — infrastructure / stateful adapters: modules that compose subsystems, wrap the @inspector/core runtime, or produce side effects. Examples: environmentFactory, remoteOAuthStorage, oauthResume (sessionStorage), browserTabVisibility (DOM listeners), clearServerOAuthState, downloadFile.

The top-level src/types/ is a separate sibling — ambient .d.ts module stubs, not the place for new domain types (the one that lingers there, dead navigation.ts, is tracked for removal in #1785).

Nothing enforces the boundary — no path alias keys off it, and the coverage include in vite.config.ts lists both directories, so a move between them is coverage-neutral. It's a human-legible import-time signal. See AGENTS.md for the full rule (including the whitelist caveat — a module placed outside components/lib/utils/server falls out of the ≥90 gate).

MCP Apps screen automation contract

The Apps screen exposes a small, stable set of data-testid / data-* attributes so an automated driver (deep-link auto-open, CI review harness) can waitForSelector on a deterministic signal instead of sleeping. Treat these as a public contract — drivers depend on them staying stable:

Attribute	Where	Meaning
data-testid="apps-form"	Apps content card	The container that carries the status/error attributes below.
data-app-status	on apps-form	Renderer lifecycle: idle (nothing running) → loading (bridge building / ui/initialize in flight) → ready (view fired notifications/initialized) → error (bridge factory threw/rejected). Poll for ready.
data-app-error	on apps-form	The failure reason string when data-app-status="error" (e.g. no connected client); absent otherwise.
data-testid="apps-error"	error panel	Rendered below the frame when the app fails to load (factory throw/reject); shows the reason so the failure isn't a silent blank frame.
data-testid="open-app"	Open App button	Launches the selected app.
data-testid="apps-stage"	Stage-partial button	Snapshots the current form values for progressive-render testing.
data-testid="apps-messages"	messages panel	ui/message submissions from the running view.
data-testid="apps-logs"	app-logs panel	notifications/message log entries (default-expanded).

App-rendered elicitations (#1854) render through the same AppRenderer but outside the Apps screen — one modal per request, from AppElicitationHost — and carry their own pair:

Attribute	Where	Meaning
data-testid="app-elicitation"	the elicitation modal	One per in-flight app-rendered elicitation. Absent means the request was answered by the native elicitation form instead, which is what a driver asserts to prove the negotiation gate held.
data-app-elicitation-status	on app-elicitation	The same AppRendererStatus for that modal's app. ready is when the host forwards the elicitation/create through its bridge.

scripts/smoke-web-elicitation.mjs drives both halves against the public fixture (test-servers/configs/app-elicitation-http.json and its -native- sibling).

The renderer lifecycle itself is AppRendererStatus (loading | ready | error) reported via AppRenderer's onAppStatusChange; the screen maps it to data-app-status. Resource-read failures (malformed/404 UI resource) are surfaced as a toast via the bridge factory's onResourceError; because the app never reaches ready in that case, a driver times out on data-app-status and reads the toast.

Deep-link auto-connect

A driver (launcher, CLI --print-handoff, CI review harness) can reach a connected inspector with a single navigate by encoding the target in the URL query string. Parsing + security gating live in src/utils/deepLink.ts (parseDeepLink), and a returned DeepLink is proof the link passed validation.

http://127.0.0.1:6274/?serverUrl=<url>&transport=http|sse&autoConnect=<token>

Param	Meaning
serverUrl	The MCP server URL. Restricted to http: / https: (a crafted javascript: / data: / file: value is rejected). Canonicalized via URL.href so it matches the OAuth store's key form.
transport	http (streamable-HTTP, the default) or sse. Unknown values fall back to http.
autoConnect	CSRF gate. Must equal the per-launch MCP_INSPECTOR_API_TOKEN. The token is random per launch and only known to whatever started the server, so a third-party-minted link cannot satisfy it — this is the same exposure surface as the existing ?MCP_INSPECTOR_API_TOKEN= param. Without a match the link is ignored.

The deep link upserts a stable deep-link catalog row (so a reload reconnects to the same row instead of accumulating duplicates) and connects. Connection-level outcomes are surfaced on the AppShell.Header as a machine-readable contract, so a driver can waitForSelector and read the failure reason without scraping a transient toast:

Attribute	Where	Meaning
data-testid="connection-status"	header	The element carrying the attributes below.
data-status	on connection-status	The live ConnectionStatus (disconnected → connecting → connected / error). Poll for connected.
data-error-message	on connection-status	Why the last connect failed (handshake error, OAuth-start failure, deep-link automation failure); absent when there is no error.
data-deeplink	on connection-status	parsed (a valid deep link drove this load), rejected (deep-link params present but the token/serverUrl gate failed), or none. Lets a driver distinguish "no deep link" from "rejected" — both otherwise leave data-status idle.

Landing on a rendered app

Three further params extend the deep link to pre-select — and optionally auto-open — an MCP App, so a driver reaches a rendered widget with zero clicks:

…&openApp=<toolName>&appArgs=<base64url(JSON)>&autoOpen=<token>

Param	Meaning
openApp	The app-tool name. Once the connection is up and the tool appears in the app list, the inspector switches to the Apps tab and pre-selects it.
appArgs	base64url(JSON) object of form values. Merged over the tool's schema defaults (collectSchemaDefaults) so a required-with-default field isn't left blank — which would otherwise disable "Open App". Malformed / non-object values fall back to {}.
autoOpen	Same CSRF gate as autoConnect — must equal the session token. When set, "Open App" fires automatically (a tool call from a URL), so the token gate is mandatory. Without a match the app is pre-selected but not opened.

The app-side render lifecycle is observable through the MCP Apps screen automation contract above (data-app-status="ready"), so a driver can waitForSelector the whole connect → open → ready chain deterministically.

MCP App dedicated origins (_meta.ui.domain)

By default an MCP App is rendered by handing its HTML to the sandbox proxy as srcdoc, inside an iframe sandboxed without allow-same-origin. That is the isolation model from #1565 and it stays the default — but it gives the app document an opaque origin, so every request it makes carries Origin: null. An app whose backend allowlists origins (CORS, an OAuth callback, an API-key allowlist) can't work that way, which is what the spec's _meta.ui.domain exists to solve: a server uses it to ask its host for a stable, dedicated origin.

The Inspector's host-specific contract. The spec makes domain's format host-dependent ("servers MUST consult host-specific documentation"), and the Inspector owns no domain infrastructure — it cannot serve my-app.example.com. So it treats the field as a request, not an address:

• Any non-empty domain string opts the resource in. The value itself is not parsed, matched, or reserved — declare whatever your production host expects.
• The Inspector answers with a real HTTP origin on loopback: http://<host>:<MCP_APP_ORIGIN_PORT>, default 6278 — in the same 627x family as the web port 6274 and the sandbox port 6275, so the three forward together. It is not 6276: that is the fixed loopback OAuth callback the CLI and TUI listen on, which OAuth apps pre-register and which therefore cannot move. 6277 is skipped as v1's retired proxy port.
• The app document is served from there under an unguessable path, its per-app CSP delivered as a real response header (stronger than the <meta> the srcdoc path relies on) plus a frame-ancestors that admits only the two origins in its real ancestor chain — the sandbox proxy that frames it, and the Inspector page that frames the proxy. Both are required, not belt-and-braces: frame-ancestors is checked against every ancestor, so omitting the Inspector's own origin blocks the frame outright (see appDocumentEmbedders).
• The inner iframe is granted allow-same-origin on this path only — that is what makes the origin real rather than opaque. It is not a weakening of #1565: the listener is on its own port, so the app is cross-origin to both the sandbox proxy and the Inspector, and same-origin policy blocks the reach either way. The proxy refuses the grant outright if the URL's origin equals its own, and the grant is never reachable from the server-supplied sandbox string, which is still stripped.

It is one shared origin, not one per app. Every domain-declaring app is served from the same port, keyed by path. That delivers the property the field is for — a real, allowlistable origin — without minting a port or a DNS name per app, at the cost of not being a per-app isolation boundary: two such apps share localStorage, sessionStorage, and cookies for that origin.

Every failure falls back rather than blanking the app. No app-origin listener, a port that never bound, an older backend with no POST /api/app-document route, a network error — each renders the app the default (opaque-origin) way and logs a console warning naming _meta.ui.domain. Losing the real origin degrades what the app can reach; losing the app itself would be worse.

Forward 6278 too if you need this off loopback (a container, a tunnel). As with the sandbox port, a taken port falls back to an OS-assigned one with a loud warning — the app still renders, but the origin an app's backend was told to allowlist is then wrong, which is what the warning tells you.

Theme (src/theme/)

Each customized Mantine component has a Theme<Name>.ts file (Button.ts, Text.ts, …, ~21 total) exporting a Theme<Name> constant; the barrel index.ts re-exports them and theme.ts assembles the MantineProvider theme. Theme files hold app-wide defaults and variants (flat CSS-in-JS); only pseudo-selectors, nested child selectors, keyframes, and native-HTML styling belong in App.css. Element components import from @mantine/core (never from theme/) — the theme layer is applied transparently by the provider.

cssVariables.ts is the third piece, beside the component files and App.css. It holds overrides for the CSS variables MantineProvider injects at runtime, which App.css cannot reach: the provider appends its generated <style> after the stylesheet imports, so a :root rule there loses on source order at equal specificity. cssVariablesResolver is the supported seam. It is passed at all three MantineProvider sites — the app (main.tsx), the Storybook preview, and renderWithMantine — so the running app, the stories, and the tests cannot disagree about a token's value. It currently corrects --mantine-color-error, whose Mantine defaults fail WCAG AA in both schemes at the size input error text renders.

Code editing (JsonObjectInput)

Payloads whose values may be arbitrary JSON — _meta is the case that forced it (#1910) — are edited with Ace (react-ace + ace-builds, declared in this client because they render React) rather than the key/value rows used for headers and env, which cannot express an object value. Ace brings code folding, brace auto-closing, and per-line error annotation from its JSON worker.

Three integration details are load-bearing:

• The worker is imported as ?url so Vite emits it as an asset. Without it Ace fetches worker-json.js from a path that does not exist in a bundled app and silently loses its annotations.
• The gutter's colors are overridden in App.css, keyed off Ace's cssClass (ace-github / ace-github-dark — not the theme-github_dark module name). Ace's own themes are 1.89:1 and 4.13:1 there, both under AA, and folding needs the gutter so it cannot simply be hidden.
• The label and error are wired to Ace's hidden textarea by hand. Input.Wrapper associates a Mantine input through context; Ace renders its own DOM, so the id, aria-invalid and aria-describedby are set on the textarea in an effect.

Testing it is split by necessity. Ace's input path does not work under happy-dom — userEvent.type reaches the textarea and produces no edit — so a keyboard test in the unit project passes while asserting nothing. Unit tests drive the editor instance through src/test/aceEditor.ts; real keyboard behaviour lives in the Storybook play functions, which run in Chromium.

Testing

Tests run under three Vitest projects (configured in vite.config.ts), each in the right environment:

Project	Env	Scope	Script
unit	happy-dom	Components, hooks, utils (*.test.tsx beside the source)	npm test
integration	node	@inspector/core + transports + auth, spawning the real stdio test server (src/test/integration/**)	npm run test:integration
storybook	real Chromium	Story play functions as interaction tests	npm run test:storybook

• npm test runs the fast unit project (happy-dom). test:watch for the loop.
• Integration tests run in a real Node env (no happy-dom, 30s timeouts) and spawn test-servers/build/test-server-stdio.js as a subprocess, so pretest/the coverage script build the test servers first (test-servers:build).
• npm run test:coverage runs unit and integration under v8 instrumentation and enforces the per-file ≥90% gate (lines/statements/functions/branches) — the same gate CI runs. Genuinely-unreachable branches are annotated with a justified /* v8 ignore … */, not waved through.

Integration tests live under src/test/integration/ mirroring the core/ layout; anything placed there is picked up by the integration project automatically. Render components with renderWithMantine (src/test/renderWithMantine.tsx) so they get the project theme.

Storybook

npm run storybook        # dev server on :6006
npm run build:storybook  # static build
npm run test:storybook   # run every story's play function in headless Chromium

Storybook is first-class here because the components are presentational — each renders against fixture props. Play functions double as interaction tests and run headless in real Chromium via @vitest/browser-playwright + @storybook/addon-vitest (the storybook Vitest project). They're part of npm run ci (which installs the Chromium binary first) but kept out of the fast validate loop since they need the browser.

Auth token

The dev/prod backend guards every /api/* route with x-mcp-remote-auth: Bearer <MCP_INSPECTOR_API_TOKEN>. The browser recovers the token, in priority order (see App.tsx getAuthToken()): the window.__INSPECTOR_API_TOKEN__ global injected into index.html on every page load (server/inject-auth-token.ts), then a ?MCP_INSPECTOR_API_TOKEN=… query param, then sessionStorage. Injection is a no-op when auth is disabled (DANGEROUSLY_OMIT_AUTH). See the root AGENTS.md for the full rationale.

Host binding & the origin allow-list

Both the prod backend (server/web-server-config.ts) and the dev Vite server (vite.config.ts) resolve their bind host through one shared guard, server/resolve-bind-host.ts. It binds 127.0.0.1 by default and refuses an all-interfaces host (0.0.0.0, ::, empty, or any equivalent spelling — 0, 0x0, 0.0, ::0, ::ffff:0.0.0.0, … are all folded to the wildcard and refused) — which would expose the process-spawning backend to the whole network, the exposure DNS-rebinding attacks target — unless DANGEROUSLY_BIND_ALL_INTERFACES=true is set. The Docker image sets that flag (a container must bind 0.0.0.0 to be reachable through -p); a bare HOST=0.0.0.0 anywhere else exits with an actionable error.

The default is the IPv4 loopback address, deliberately not the name localhost (#1951). server.listen(port, host) resolves a name through dns.lookup and binds the single address it gets back; since Node 17 the default result order is verbatim, so on a glibc Linux host — whose /etc/hosts maps localhost to both 127.0.0.1 and ::1 — that first address is ::1. HOST=localhost there binds IPv6 loopback only and refuses every IPv4 client. On a desktop this is invisible (the browser resolves localhost the same way and lands on ::1 too); it breaks wherever the connection is made by something that pins 127.0.0.1 — a VS Code dev container's port forwarder, ssh -L 6274:127.0.0.1:6274, a container healthcheck. Binding the address takes the resolver out of the decision. Browsing to http://localhost:PORT still works, since browsers fall back across address families and the loopback allow-list covers all three forms. An explicit HOST=localhost is still honored as typed — the single-family bind is then your choice.

The backend's /api/* routes also enforce an origin allow-list (allowedOrigins) as DNS-rebinding protection. When left to default on a loopback host, it expands to all three interchangeable loopback origin forms for the port — http://localhost:PORT, http://127.0.0.1:PORT, and http://[::1]:PORT — because localhost resolves to either IPv4 or IPv6 loopback and Node/Vite may bind the IPv6 form, so the browser can legitimately arrive at http://[::1]:PORT. Set ALLOWED_ORIGINS (comma-separated) to override; entries are canonicalized (new URL(o).origin), so a trailing slash / uppercase host / explicit :80 still match. Each entry must include the scheme — http://localhost:6274, not localhost:6274 (a scheme-less value is dropped with a warning). ALLOWED_ORIGINS replaces the default list (it does not merge), so list every origin you'll browse from, including the loopback forms you still want (http://localhost:PORT, http://127.0.0.1:PORT, http://[::1]:PORT) — otherwise local access stops working. A blank ALLOWED_ORIGINS does not disable the check — it falls back to the default (fail closed); there is no env knob to turn origin validation off.

Hosting on a network

The guard blocks only the wildcard all-interfaces addresses. Binding a specific IP or hostname is allowed with no opt-in — that's a single, deliberate exposure, unlike the wildcard which binds every interface at once (the pattern DNS-rebinding exploits). To serve the Inspector on a LAN or the internet:

• Bind a specific address. HOST=192.168.1.50 (a LAN IP) or a public IP works directly: the default origin allow-list follows the bind host, so allowedOrigins becomes http://<that-host>:PORT and a browser hitting that address is accepted with no extra config. (The host is canonicalized the way a browser is — so HOST=127.1 advertises http://127.0.0.1:PORT, an IPv6 bind host is bracketed as http://[2001:db8::1]:PORT, and the port is dropped when it's the http default :80 — matching what the browser sends.)
• Behind TLS or a reverse proxy, the browser's Origin becomes the public origin (e.g. https://inspector.example.com, often without a port), which won't match the auto-derived http://<bind-host>:PORT. Set ALLOWED_ORIGINS to the real public origin(s): ALLOWED_ORIGINS=https://inspector.example.com.
• Using the 0.0.0.0 wildcard (opt-in via DANGEROUSLY_BIND_ALL_INTERFACES=true, as the Docker image does): a wildcard bind also serves loopback, so the default allow-list is the loopback trio plus the canonical wildcard origins (http://0.0.0.0:PORT, http://[::]:PORT), and local access works out of the box — docker run -p 127.0.0.1:6274:6274 browsed at http://localhost:6274 connects with no extra config. Reaching it at a non-loopback address (a LAN IP, a public hostname) still needs ALLOWED_ORIGINS — but since that replaces the default, keep the loopback forms in the list if you also browse locally: ALLOWED_ORIGINS=http://localhost:PORT,http://127.0.0.1:PORT,http://192.168.1.50:PORT,https://inspector.example.com.

The bind-host guard and the ALLOWED_ORIGINS allow-list apply to both the prod server and --dev. Note that in --dev the Vite dev server additionally enforces its own server.allowedHosts Host-header check, whose default accepts loopback and IP-literal hosts. The host you bind is auto-allowed (Vite adds the resolved server.host — which this config sets from HOST — to the allow-list), so HOST=<hostname> works out of the box under --dev too. What needs an explicit server.allowedHosts entry is reaching the dev server at a different name than the one bound — e.g. a wildcard bind reached by hostname, or a reverse-proxy domain. For those, prefer the prod server (mcp-inspector --web) or add the host to server.allowedHosts.

MCP Apps caveats. The MCP Apps sandbox runs on a separate port — MCP_SANDBOX_PORT, defaulting to a fixed 6275 (#2008; it was OS-assigned before, which meant it changed every run and so could never be named in a forwardPorts / -p / tunnel config written ahead of time). For the Apps tab to work off loopback, that sandbox port must be independently reachable from the browser — expose/forward 6275 alongside 6274, or set MCP_SANDBOX_PORT to pick another. If the port is already taken the sandbox falls back to an OS-assigned one and warns, so a second Inspector still gets a working Apps tab locally — but the forwarded port is then wrong, which is what the warning tells you. (Under a 0.0.0.0 wildcard bind the sandbox URL is advertised as localhost, which is reachable — a wildcard bind serves loopback — so only the port needs handling.) Also note the sandbox iframe is gated by a frame-ancestors CSP, and a bracketed IPv6 literal is not a valid CSP host-source — so MCP Apps requires browsing the app at a name or IPv4 (localhost, 127.0.0.1, a hostname, a LAN IPv4), not a bare http://[::1]:… address. Finally, the sandbox URL is always http:// — so behind TLS (an https:// app page) the browser blocks the http://…/sandbox iframe as mixed content and MCP Apps can't render; the Apps tab needs a plain-http app origin today.

In every case, exposing the Inspector beyond loopback also means anyone who can reach it can drive its backend — keep authentication on (do not set DANGEROUSLY_OMIT_AUTH) and prefer a specific bind address over the wildcard.

HTTP proxy support

The web backend connects to remote MCP servers through the shared Node fetch (core/mcp/node/proxyFetch.ts), which honors the conventional proxy environment variables: HTTPS_PROXY / HTTP_PROXY (and their lowercase forms) select the proxy, and NO_PROXY exempts hosts. Routing uses undici's own fetch bound to its EnvHttpProxyAgent, imported lazily only when a proxy variable is set, so runs without a proxy configured pay no cost.

The proxy sits at the bottom of the fetch stack rather than wrapping the transport's fetch, and both undici halves must come from the same copy — see HTTP proxy support in the CLI README for why (#2067).