MCP Inspector

A developer tool for inspecting Model Context Protocol (MCP) servers. It ships as a single package, @modelcontextprotocol/inspector, that provides three ways to inspect a server:

Web — a Vite + React + Mantine single-page app with a Node backend.
CLI — a scriptable command-line client for automation, CI, and fast agent feedback loops.
TUI — an interactive terminal UI built with Ink.

All three run through one global mcp-inspector binary:

npx @modelcontextprotocol/inspector          # web UI (default)
npx @modelcontextprotocol/inspector --cli    # CLI
npx @modelcontextprotocol/inspector --tui    # TUI

Upgrading from v1? Read the v1 → v2 migration guide — CLI flags, the new --config vs. --catalog split, the Node engine bump, and what no longer ships.

Repo status. This is the v2 line of the Inspector. Active development happens on v2/main (the develop branch — all v2 PRs target it), which is merged into main at milestone releases; main is the default branch and holds the latest released v2, published to the npm latest tag. The legacy v1 line lives on v1/main — security fixes only, published straight from that branch to the npm v1-latest tag (npx @modelcontextprotocol/inspector@v1-latest). See AGENTS.md for branch/board conventions.

Project layout

v2 is not an npm workspace. Each client under clients/* keeps its own package.json and node_modules; shared code lives in core/ and is consumed via a @inspector/core build-time alias (no package.json of its own). A single npm install at the root cascades installs into every client (see Setup).

inspector/
├── clients/
│   ├── web/          # Web client (Vite + React + Mantine). src/ = browser app; server/ = Node dev/prod backend
│   ├── cli/          # CLI client (tsup bundle, @inspector/core alias)
│   ├── tui/          # TUI client (Ink + React, tsup bundle)
│   └── launcher/     # Shared launcher — provides the `mcp-inspector` bin, dispatches to web/cli/tui
├── core/             # Shared code consumed via the `@inspector/core` alias (no package.json)
│   ├── auth/         # OAuth: providers, discovery, storage, mid-session recovery (browser/node/remote backends)
│   ├── client/       # Install-level client config (`client.json`): browser-safe parse/validate + Node load/save, remote backend, secrets
│   ├── json/         # JSON + parameter/argument conversion utilities
│   ├── logging/      # Silent pino logger singleton
│   ├── mcp/          # InspectorClient runtime, state stores, transports, config import
│   ├── node/         # Node-only shared helpers: version reader, hostUrl (host normalize/canonicalize + all-interfaces/loopback detection)
│   ├── 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, verify-build-gate, verify-format-coverage, verify-dep-lockstep, pack:verify)
├── docs/             # Task-oriented guides (v1→v2 migration, server configuration, MCP App review, launcher/config plan)
├── specification/    # Design/build specifications
├── AGENTS.md         # Contribution rules for agents AND humans (see below)
└── README.md         # You are here

Each client has its own README with client-specific detail: web · cli · tui · launcher.

Task-oriented guides live under docs/:

Migrating from v1 to v2 — the v1 → v2 map: CLI flag mapping, --config vs. --catalog semantics with before/after examples, the Node engine bump (>=22.7.5 → >=22.19.0), env-var renames, and the sub-packages that no longer ship.
MCP server configuration — which server(s) the Inspector connects to: --catalog vs. --config, ad-hoc targets, the -- separator, the file format and its Inspector-specific per-server fields. Shared by all three clients; the cli and tui READMEs delegate their server-options sections to it.
Reviewing an MCP App — the CLI-first → one-shot-web recipe for automated App-tool review: --app-info probe → deep-link navigate → rendered widget, plus OAuth handoff and proxy support.
Launcher and config consolidation — why the launcher runs a client in-process rather than spawning it, and how the shared config processor fits in.

Setup

Requires Node >=22.19.0.

npm install     # root install; postinstall cascades into every client

Fresh clone: run npm install at the repo root.
After a pull that changes a client's dependencies: re-run npm install at the root to re-sync every client.

The cascade (scripts/install-clients.mjs) is dev-only — it exits early when the package is installed as a dependency, and the published tarball ships only each client's build/, so end users are unaffected. Set INSPECTOR_SKIP_CLIENT_INSTALL=1 to skip it.

Where a dependency is declared. The MCP SDK packages (@modelcontextprotocol/client, core, server, server-legacy, ext-apps) live in the root package.json only — never in a client's. Node resolution walks up, so the root install is on every client's chain, and the root manifest is already what the published tarball resolves against. Declaring them per client installs a second copy that can drift from the root's, which is how two versions of ext-apps (and of the transitive v1 @modelcontextprotocol/sdk) ended up in the tree before #1970 — and a second copy of client/core is the failure vitest.shared.mts carries a dedupe workaround for. The same root-only placement holds for anything reached solely through root-owned code with no manifest of its own (test-servers/src, core/), and vitest.shared.mts aliases those to the repo root — express and yaml, both reached through test-servers/src, are the two today. Whether such a package is a dependency or a devDependency follows from who consumes it at runtime, not from where it is declared: anything core/ imports at runtime must be a root dependency, because the client builds externalize npm packages and a published install resolves them from the root manifest, where devDependencies are absent. express is test-only and is a devDependency; yaml currently sits in dependencies.

Running during development

For day-to-day web iteration, run Vite directly from the web client (fast HMR, no launcher build needed):

cd clients/web && npm run dev

The launcher-driven scripts below run the built launcher, so build first (npm run build):

npm run web        # prod web launcher against clients/web/dist
npm run web:dev    # web launcher in --dev mode (Vite)

The `@inspector/core` shared package

core/ holds the logic shared by all three clients so that web, CLI, and TUI behave identically. Its entry point is the InspectorClient class (core/mcp/), which owns the connection to an MCP server, the request/response lifecycle, and a set of state stores; core/react/ exposes React hooks over those stores that both the web and TUI (Ink) React trees consume. OAuth (core/auth/) is factored into isomorphic logic plus browser/node/remote backends so the same flows work in the browser, in Node, and against a remote backend.

core/ intentionally has no package.json — it is not published on its own. Each client bundles it in via a @inspector/core alias:

CLI / TUI: esbuildOptions.alias in their tsup.config.ts maps @inspector/core → the repo core/ directory, and noExternal: [/^@inspector\/core/] inlines it into the bundle.
Web: the same alias in clients/web/vite.config.ts for the browser app and the Node backend runner.

Publishing core/ as its own package (e.g. for third parties to build on) is deliberately deferred — see issue #1636.

Web client: "dumb components" + Storybook

The v2 web client is built from presentational ("dumb") components — they accept data and callbacks as props and contain only display logic, with no direct data fetching or client state. State comes from the @inspector/core hooks, wired in near the top of the tree. This keeps components isolated, testable, and documentable.

That approach is what makes Storybook first-class here: every screen and element component has a *.stories.tsx file (96+ stories) that renders it against fixture props. Storybook play functions double as interaction tests, run headless in CI (npm run ci:storybook, Chromium via Playwright).

Styling follows a strict Mantine-first convention (theme variants and component props over CSS classes, --inspector-* CSS custom properties over raw color literals). The full rules live in AGENTS.md under React instructions — read them before touching web UI. Element components live in clients/web/src/components/elements/; theme variants in clients/web/src/theme/.

Test servers

test-servers/ provides composable MCP servers used by the integration and smoke suites, so tests exercise a real server over a real transport instead of mocks. A server is assembled from presets (fixture factories in test-servers/src/preset-registry.ts — tools, resources, prompts, tasks, elicitation, sampling, OAuth, …) and can be driven two ways:

In-process — import the factories (createTestServerHttp, createEchoTool, …) and run the server inside the test's event loop (used by the HTTP integration paths).
As a subprocess — test-servers/build/test-server-stdio.js is spawned as a real stdio child (used by the CLI smoke and stdio integration tests).

Configure a server declaratively with a JSON config (see test-servers/configs/*.json) selecting presets, then load it via --config. Because the servers are spawned as real subprocesses, the build output must exist first:

npm run test-servers:build   # (from clients/web) → tsc -p test-servers, emits test-servers/build/

The Vite alias @modelcontextprotocol/inspector-test-server (in clients/web/vite.config.ts) points at test-servers/build/index.js so getTestMcpServerPath() resolves to a real .js path.

Serving the modern protocol era

A streamable-HTTP server can also serve the modern (2026-07-28) protocol era via the SDK's createMcpHandler:

Set transport.modern in the JSON config — true for dual-era stateless serving, or { "legacy": "reject" } for modern-only strict.
Or pass modern on the ServerConfig for an in-process createTestServerHttp.

This is what lets an Inspector connection negotiating protocolEra: "auto" | "modern" reach the modern leg (populated server/discover, sessionless). See test-servers/configs/modern-http.json.

Showcase configs

Each config below is a ready-made server for exercising one feature by hand. Load one with --config, and unless noted, connect with Protocol Era = Modern.

Config	Demonstrates	Issue
`mcp-app-http.json` (legacy era)	An MCP App (UI resource + app tool) in the Apps tab	#1859
`modern-mrtr-http.json`	A single MRTR round-trip	—
`mrtr-showcase-http.json`	Every MRTR preset in one server	—
`modern-network-http.json`	Network tab: `Mcp-*` headers + error taxonomy	#1628
`xmcpheader-modern-http.json`	Tools tab: `x-mcp-header` mirroring and exclusions	#1632
`pagination-http.json`	Page-by-page list fetching	#1721
`structured-output-http.json`	Tools tab: a result's `structuredContent` section	#1908
`duplicate-tool-names-http.json`	A `tools/list` that repeats a tool name	#1957
`advertised-extensions-http.json`	Tool registration gated on advertised extensions	#1739
`logging-{legacy,modern}-http.json`	Logging, both eras	#1629
`subscriptions-{legacy,modern}-http.json`	Resource subscriptions, both eras	#1630
`tasks-{legacy,modern}-http.json`	Tasks, both eras	#1631

MCP Apps

mcp-app-http.json serves the mcp_app_demo tool (_meta.ui.resourceUri) alongside its mcp_app_demo_widget UI resource, so the Apps tab has a real App to render. It is a plain streamable-HTTP server — connect with the default (legacy) protocol era, not Modern.

Open the Apps tab, select mcp_app_demo, give it a title and click Open App: the widget renders inside the sandbox iframe and exercises the host-side UI protocol surface — host-context render, size-changed, ui/message, and a log line into the App logs panel. Because the widget is served through the sandbox proxy page, this config is also what reproduces #1859 (a missing clients/web/static/sandbox_proxy.html surfaces here as a "Sandbox not loaded" message in place of the widget) — a failure that only ever appeared in an installed package, never in the repo.

For the scripted version of the same flow (--app-info probe → deep link → rendered widget), see Reviewing an MCP App.

MRTR

modern-mrtr-http.json serves the mrtr_confirm tool (preset mrtr_confirm, createMrtrTool) over the modern leg. Its handler returns inputRequired(...) embedding a form elicitation, so invoking it produces a real round-trip: input_required → the client fulfils the embedded elicitation and retries with a new id → complete.

The Inspector drives MRTR manually (inputRequired: { autoFulfill: false }), so the embedded elicitation pauses at the pending-request modal (tagged "input_required") for you to answer, then the retry completes. Useful for eyeballing both that pending-request UX and the Protocol view's MRTR conversation grouping.

mrtr-showcase-http.json bundles every MRTR preset in one server:

Preset	Behavior
`mrtr_confirm`	Single round
`mrtr_two_step`	Two elicitation rounds via `requestState`
`mrtr_sample`	Embedded sampling → the Sampling panel
`mrtr_roots`	Embedded `roots/list`, auto-answered silently from configured roots (no modal)
`mrtr_edge`	An `inputRequests`-only round, then a `requestState`-only round
`mrtr_loop`	Never completes → trips the `MRTR_MAX_ROUNDS` bound

The legacy collect_elicitation preset calls server.elicitInput, which errors on the 2026-07-28 leg — server→client requests aren't allowed there. MRTR is the modern replacement.

Network tab — standardized headers and error taxonomy

modern-network-http.json covers SEP-2243 / SEP-2575. It serves a get_weather tool whose city argument carries an x-mcp-header: "City" annotation, so a modern client mirrors it to Mcp-Param-City.

It also serves four trigger_* tools that the modern leg's spec-error injector (transport.modern.injectSpecErrors: true) answers with a real HTTP status plus JSON-RPC error body:

Tool	Response
`trigger_header_mismatch`	`400` / `-32020`
`trigger_missing_capability`	`400` / `-32021`
`trigger_unsupported_version`	`400` / `-32022` (with `data.supported`)
`trigger_method_not_found`	`404` / `-32601`

Open the Network tab to see the mirrored Mcp-* headers highlighted, sentinel values decoded, and each error rendered distinctly.

Mcp-Param-* mirroring is built by the Inspector, not the SDK. The SDK only mirrors inside client.callTool(), and skips it in the browser (detectProbeEnvironment() !== "browser"). The Inspector routes tools/call through client.request() to drive MRTR manually, so it builds the mirrored headers itself (#1846) — on every client, web included, since the web client's upstream request is issued by the Node backend rather than the browser. So get_weather is callable from web, CLI, and TUI alike, in both the plain and "Run as task" forms.

`x-mcp-header` in the Tools tab

xmcpheader-modern-http.json serves:

echo — plain tool.
get_weather — a valid x-mcp-header: "City" annotation on its city argument.
invalid_header_tool — an annotation using the header name "Bad Header". The space makes it an invalid RFC 9110 token, so the whole tool definition is invalid.
trigger_invalid_params — answered with a real -32602 Invalid params error whose message is not about a missing tool.

Open the Tools tab: get_weather's detail panel shows a "Mirrored request headers (SEP-2243)" section (city → Mcp-Param-City), and invalid_header_tool appears struck-through under an "Excluded (SEP-2243)" divider with the reason on hover. A conforming Streamable HTTP client MUST drop it from tools/list; the Inspector surfaces why.

Under SDK v2 a tools/call rejecting with -32602 renders as a distinct error panel rather than an isError result — headed "Unknown Tool" when the message names a missing tool, or "Invalid Parameters" otherwise (run trigger_invalid_params).

Page-by-page fetching

pagination-http.json serves 12 tools, 12 resources, and 12 prompts (presets numbered_tools / numbered_resources / numbered_prompts, count: 12) with a maxPageSize of 4 each, so every list paginates into three pages.

Turn on "Fetch Lists One Page at a Time" (Server Settings — the paginatedLists setting, or the Paginated switch in a list sidebar) and the lists load page 1 only (4 items) with a Load next page control and an N pages loaded status. Each click fetches the next 4 and appends them; Refresh resets to page 1. With the switch off (the default), the same lists auto-aggregate all three pages on connect.

Structured output

structured-output-http.json serves list_items (nested structuredContent — objects inside arrays inside an object, the shape from #1908), get_temp (a flat three-key payload), and echo (no outputSchema at all). It is a plain streamable-HTTP server — connect with the default (legacy) protocol era.

Run list_items from the Tools tab: the result panel shows the content[] text summary ("Found 2 items.") and a collapsible Structured Output section rendering the schema-validated payload as pretty-printed, copyable JSON. That section is what v2 was dropping — a tool declaring an outputSchema returns its real data there, and the text block usually only summarizes it. Run echo to confirm the section is absent when a result carries no structuredContent.

Duplicate tool names

duplicate-tool-names-http.json serves get_weather, get_temp, echo, and add, then repeats get_weather and echo at the end of tools/list with the same name and a (duplicate) title (duplicateToolNames). No preset can produce this shape — the SDK's registerTool rejects a repeated name — but a real server can and does, and the Inspector has to render it faithfully.

Connect (default legacy era), open the Tools tab, and type get into Search tools: the list must narrow to exactly the three get_* rows. On the broken build it kept a stale echo row, because the sidebar keyed rows by tool.name alone and the colliding keys orphaned a child during reconciliation (#1957).

The duplicated copies are appended rather than placed beside their twin on purpose. React matches a leading run of same-key children first, so a head-adjacent duplicate happens to line up and the defect hides; separating the pair is what makes it observable — and it is also the realistic shape, two tool sources concatenated.

Advertised extensions

advertised-extensions-http.json serves echo (always) and a get_weather tool gated on the io.modelcontextprotocol/tasks extension (extensionGatedTools): the tool is registered but starts disabled, and the server enables it on notifications/initialized only when the client declared that extension in its capabilities.extensions.

Connect — the Inspector advertises the Tasks extension by default, so the Tools list shows both echo and get_weather.
Open Server Settings → Advertised Extensions, uncheck Tasks (io.modelcontextprotocol/tasks), and reconnect.
The client now advertises no extensions, the server never enables get_weather, and the Tools list shows only echo.

This is the debugging knob for a server legitimately changing tool registration based on what the client advertises. Legacy stateful leg only — the modern per-request leg has no persistent oninitialized.

Logging, both eras

logging-legacy-http.json and logging-modern-http.json both serve logging: true plus a send_notification tool that emits a notifications/message at a chosen level. The legacy one is a plain streamable-HTTP server; the modern one sets transport.modern: true.

Legacy — the Logs tab gives a session-scoped Set Active Level selector + Set button. Calling send_notification streams the log into the panel.
Modern — the same tab instead shows Log Level per Request. Pick a level to opt in and the client stamps _meta["io.modelcontextprotocol/logLevel"] on every subsequent request (verify in the Network tab's request body). Calling send_notification streams the log over the request's SSE response. Set it back to Off and the same call is silently gated — the request omits the logLevel key, so the log never arrives.

That gating is faithful to the spec ("a server MUST NOT emit notifications/message for a request that didn't opt in") because send_notification emits through the SDK's request-scoped, threshold-aware extra.log (ctx.mcpReq.log). On the modern leg it reads the per-request logLevel opt-in from the request envelope and drops the message when the client didn't opt in or the level is below the requested severity; on legacy it honors the session level from logging/setLevel. Because it emits through the request's notify, the modern response upgrades to SSE and the log rides the originating request's stream.

Resource subscriptions, both eras

subscriptions-legacy-http.json and subscriptions-modern-http.json both serve three numbered_resources with subscriptions: true. The legacy one also serves an update_resource tool; the modern one sets transport.modern: true.

Legacy — open a resource in the Resources tab and click Subscribe. The client sends resources/subscribe and the Subscriptions section lists the URI with no stream chrome. Call update_resource with that URI and the server updates the content and emits notifications/resources/updated, stamping the subscribed tile's last-updated time.
Modern — the same Subscribe instead sends subscriptions/listen (its filter carries resourceSubscriptions plus the resourcesListChanged opt-in) and resolves on notifications/subscriptions/acknowledged. The Subscriptions section then shows a stream-status badge (Connecting… → Listening) in its header, and reconnects by re-listing if the long-lived stream drops.

The modern config deliberately omits update_resource. The SDK's modern leg is stateless/per-request (createMcpHandler(() => createMcpServer(config))), so the tool would run against a throwaway server instance — the content change wouldn't persist for the next resources/read, and its resources/updated wouldn't reach the separate listen stream. More confusing than useful.

So the live update-notification round-trip is demonstrated on the legacy (stateful-session) server, and the modern server is for the subscribe/listen/badge behavior. The Inspector's receive path is era-transparent, so a real stateful modern server that routes resources/updated onto the listen stream drives the subscribed tile the same way.

Tasks, both eras

Legacy (tasks-legacy-http.json) advertises capabilities.tasks (tasks: { list, cancel }) with the simple_task / progress_task / elicitation_task presets. Run one of those tools with Run as task on, and the Tasks tab lists it (populated via tasks/list), polls tasks/get, fetches the payload with the blocking tasks/result, and cancels with tasks/cancel.

Modern (tasks-modern-http.json) sets transport.modern: true and tasksExtension: true, advertising the io.modelcontextprotocol/tasks extension (SEP-2663) and serving modern_task / modern_input_task. The Tasks tab is gated on the negotiated extension, not capabilities.tasks.

Run modern_task as a task — the tools/call returns a CreateTaskResult (resultType: "task", visible in the Protocol/Network tabs), the client polls tasks/get (no tasks/list), and the completed task inlines its result (no blocking tasks/result).
Run modern_input_task — the task moves to input_required, surfacing an embedded elicitation through the pending-request modal. Answering it sends tasks/update with the inputResponses, and the next poll completes.

SDK v2 removed all tasks support and era-gates the tasks/* spec methods out of the modern era on both sides. So the Inspector drives the extension itself — the resultType: "task" frame is rewritten at the transport into a CallToolResult carrying the handle, and tasks/get / update / cancel ride a raw-wire request channel with the full modern envelope. The test server serves tasks/* from an Express interceptor ahead of the SDK handler, since the SDK's modern leg would answer them -32601.

The Tasks tab's Refresh re-polls the handles already known to the client — modern has no server-side task list.

Building

npm run build     # builds all clients: web → cli → tui → launcher

Individual clients: build:web, build:cli, build:tui, build:launcher. The web build produces both the browser SPA (clients/web/dist, Vite) and the Node prod-server runner (clients/web/build, tsup).

Testing & the quality gate

Each client self-validates from its own folder; the root scripts chain them. There is no aggregate root test script — use validate (fast) or coverage (the gate).

Script	What it does
`npm run validate`	Runs the three durable guards first — `verify:format-coverage` (every tracked source file is format-gated), `verify:typecheck-coverage` (every one lands in a tsconfig project), `verify:dep-lockstep` (no dependency the shared sources directly import skews across installs) — then `test:scripts` (the guards' own parser unit tests), then `validate:core` (the shared `core/` `format:check` + `lint` gate), then per client: `format:check` + `lint` + `typecheck` (cli/tui/launcher; web typechecks via `tsc -b` inside its `build`) + `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 two headless-Chromium smokes: a 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), and an MCP Apps smoke (`smoke:web:app`) that drives connect → open app → `data-app-status="ready"` against a composable App server, covering the sandbox proxy and UI-protocol bridge.
`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 verify:format-coverage`	Parses the `format:check` globs out of every `package.json` (only those reachable from `validate`), enumerates all tracked source files, and fails listing any not covered by a glob — the durable guard for the "every first-party source file is format-gated" invariant (#1792). Runs first in `validate`.
`npm run test:scripts`	Table-driven unit tests (`node --test`) for the guard's own pure parsers (`scripts/lib/npm-scripts.mjs` + the exported helpers of `verify-typecheck-coverage.mjs` and `verify-dep-lockstep.mjs`), one case per rule they encode. Runs in `validate` — and `verify:typecheck-coverage` guards this gate in turn (reachable from `validate`, non-empty test set, every test file matched by the `test:scripts` glob), since `node --test` silently skips a file its glob misses and still exits 0.
`npm run verify:typecheck-coverage`	The typecheck-coverage analog of the above (#1791): for each Node client (auto-discovered from disk — enrolled via its `typecheck` script's projects, or for a `tsc -b` client like `clients/web` via its `tsconfig.json` `references`) it runs those projects with `tsc --listFilesOnly`, unions them, and fails listing any tracked `.ts`/`.tsx`/`.mts`/`.cts` under the client that lands in no project (so a new top-level config/helper can't silently go untypechecked). It also requires, deny-by-default, the first-party TS no client owns (`test-servers/src`, the root `vitest.shared.mts`, all of `core/`, and any new top-level location) to land in some client project's tsc pass — so a `core` `*.tsx` web's projects don't reach is caught too. Also asserts the gate is wired (each client's typecheck pass — its `typecheck` script, or web's `tsc -b` — is reachable from its `validate`, and the root chain runs each client's `validate`). Runs in `validate`.
`npm run verify:dep-lockstep`	Guards the "one version per install-crossing dependency" invariant (#1896). v2 is not a workspace, so a client's test project compiles the shared first-party TypeScript — `core/`, `test-servers/src`, and the root-owned `vitest.shared.mts`, all of which resolve their dependencies from the root install — alongside the client's own sources, putting the same package in one `tsc` program twice. At the same version that's harmless; skewed, TypeScript must relate two structurally-distinct copies of every type, which for a recursive-generic surface is exponential (zod `4.3.6` vs `4.4.3` exhausted the 4GB tsc heap in `clients/web`). Derives its candidate set from what those shared sources import, compares the committed lockfiles' top-level entries, and fails deny-by-default on any skew not in the annotated `TOLERATED_SKEW` allowlist — and an allowlisted package is tolerated only within a major version. Runs in `validate`.
`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.

Per-client scripts exist too (validate:web, coverage:cli, smoke:tui, …), plus root validate:core / format:core for the shared core/ package, format:scripts for the root scripts/ tooling, and format:shared / lint:shared for the root "shared" surface (test-servers/src/**, vitest.shared.mts, the root eslint.config.js). Run npm run format before committing — the root format fixes core/, the root scripts/, the shared surface, and every client; validate runs the non-fixing format:check and fails CI on any unformatted file.

For the full testing rules — the ≥90% per-file gate, where test files live, the unit vs. integration vs. storybook projects, and the v8 ignore policy — see AGENTS.md.

Publishing

The root @modelcontextprotocol/inspector package ships as one tarball with a single version number — no separate -web / -cli / -tui / -core packages. npm run build builds every client, then prepack runs before npm publish. Runtime dependencies are declared on the root package.json; client builds bundle @inspector/core and externalize npm packages resolved from the root install.

What ships, and the packaging invariants

The root package.json "files" allowlist is the source of truth for the tarball. A few non-obvious entries exist because they are read at runtime or were silently dropped by npm's packlist — do not remove them without re-running npm run pack:verify:

No source maps. The client bundlers set sourcemap: false (clients/{cli,tui}/tsup.config.ts, clients/web/tsup.runner.config.ts); Vite and the launcher's tsc already emit none. Maps are ~half the unpacked size and aren't needed at runtime — debug via npm run dev on the source.
clients/web/build ships via clients/web/.npmignore. clients/web/.gitignore lists build/, and npm's packlist honors that nested .gitignore over the root "files" allowlist — so the prod web-server runner was silently missing from the tarball while clients/web/dist slipped through (its .gitignore only lists dist-ssr). clients/web/.npmignore overrides the .gitignore for publishing so both build/ (runner) and dist/ (SPA) ship. The other clients don't need this — none ship a nested .gitignore.
clients/web/static ships the MCP Apps sandbox proxy. clients/web/static/sandbox_proxy.html is a committed source file (not a build artifact), read from disk at runtime by clients/web/server/sandbox-controller.ts as <runner dir>/../static/sandbox_proxy.html. It was missing from the root "files" allowlist entirely, so every published build failed the Apps tab with "Sandbox not loaded" (#1859) while working fine in the repo. Because the path is resolved relative to clients/web/build, the directory must ship at that exact location — pack:verify asserts both the tarball entry and the installed-on-disk path.
A dependency that renders React is bundled, not externalized. An externalized package resolves its own react from wherever npm placed it in the consumer's tree, which is not necessarily where the bundle resolves ours — npm places a package beside a React satisfying its peer range, and those ranges are looser than ours. ink-form and ink-scroll-view declare ">=18", so a project holding React 18 satisfies them and gets them hoisted while the Inspector's React 19 nests underneath: two React copies, and the TUI dies with TypeError: Cannot read properties of null (reading 'useState') the moment a tool test form or a scroll view mounts (#1952). Both are therefore inlined by clients/tui/tsup.config.ts and are not root dependencies: the tarball ships their code inside clients/tui/build/index.js rather than having consumers install them. Bundling also pins their transitive deps to what this repo's install resolved (notably ink-select-input@6 via overrides, which npm ignores for a package installed as a dependency). ink is the one exception, on cost: bundling it works but adds ~1.4 MB (react-reconciler and yoga-layout come along, plus a createRequire banner for the inlined CJS), so it stays external — not because its ">=19" peer makes it safe, which it does not. What keeps that tolerable is the root react range: "^19.0.0" is deliberately open to the whole major so npm can dedupe our React with whatever React 19 a consumer pins, leaving an external ink on the same copy the bundle uses. Narrowing that range reopens the bug for the renderer itself — clients/tui/__tests__/tsupConfig.test.ts pins it to ink's peer floor, and guards the rest of the split; see the TUI README.
A single version number, read from the root package.json. The Inspector ships as one package with one version, so only the root package.json carries a version — the four clients/*/package.jsons deliberately have none. Every Node client (CLI, TUI, and the web backend) resolves the version through the shared readInspectorVersion() reader in core/node/version.ts, which walks up to the root manifest (always present in the tarball). No client package.json is read at runtime, so none needs to ship. The web browser can't read the filesystem; it gets its version from the backend via GET /api/config (see #1639).

`npm run pack:verify` — publish smoke against the real tarball

The smoke:* scripts run against the in-repo build tree, which is not the published package. npm run pack:verify (scripts/pack-and-verify.mjs) closes that gap: it builds, npm packs the publishable tarball (asserting no source maps ship and that the runtime-required files are present), installs the tarball into a clean throwaway consumer — a fresh temp directory where it runs a real npm install <tgz> (pulls runtime deps, runs postinstall), exactly as npx @modelcontextprotocol/inspector would — and drives the installed mcp-inspector bin end to end: --help dispatch, a real --cli tools/list over stdio, and a prod --web boot that must serve / from the shipped dist. It catches "works in --dev, breaks under npx …" path/packaging failures. It requires network access (the install pulls deps), so it is a local / release check, not part of the fast validate/ci loop.

Cutting a release

Publishing is automated by two release-gated jobs in .github/workflows/main.yml (github.event_name == 'release', both needs: build):

publish — the npm package. Runs npm run pack:verify as the pre-publish gate, asserts the release tag matches the root package.json version, then npm publish --access public --provenance — a single npm publish (v2 is not an npm workspace, so there is no v1-style publish-all/--workspaces), with a signed provenance attestation via GitHub OIDC (id-token: write, environment: release, NPM_TOKEN).
publish-github-container-registry — the container image (see Docker).

A v2 release is cut from main, after the milestone's work has been merged there from v2/main — not from v2/main itself. (The v1 line releases independently from v1/main to the v1-latest tag and never touches main; see Repo status.)

Because there is one version number (only the root package.json has one — the clients carry none, so there is nothing to keep in sync and no check-version step), the release flow is just:

npm version <major|minor|patch>   # bumps the root package.json + tags
git push --follow-tags
# then draft & publish a GitHub Release for that tag → triggers `publish`

The release's target commit selects which workflow runs, so this only publishes when a release is cut from a commit carrying this (v2) workflow.

Docker

A container image is published to GHCR (ghcr.io/modelcontextprotocol/inspector, linux/amd64 + linux/arm64) by the release workflow. The Dockerfile is a two-stage build: the first stage installs and npm packs the publishable tarball; the second stage npm install -gs that tarball, so the image ships the exact same artifact as npm, with a clean mcp-inspector bin.

# run the web UI (reads the auth token from the container logs)
docker run --rm -p 127.0.0.1:6274:6274 ghcr.io/modelcontextprotocol/inspector

# or build the image locally
docker build -t mcp-inspector .
docker run --rm -p 127.0.0.1:6274:6274 mcp-inspector

Keep the 127.0.0.1: prefix on the published port. A bare -p 6274:6274 publishes on every host interface, putting the Inspector on your local network. The container's HOST=0.0.0.0 is a separate concern — it governs the container's interfaces, not the host's — so the DANGEROUSLY_BIND_ALL_INTERFACES opt-in that guards a wildcard bind outside a container does not cover this. It matters more here than for an ordinary web app: the backend spawns processes on request, GET / embeds the API token into the served HTML, and a request arriving with no Origin header skips the origin allow-list entirely — so for any non-browser client the API token is the only guard. Publishing wider needs a real access-control boundary in front of the Inspector — a reverse proxy that authenticates, an SSH tunnel, a private network. Setting your own MCP_INSPECTOR_API_TOKEN does not substitute: GET / discloses whatever token is in use, so a custom one is harvested exactly as easily as a generated one.

Keeping the servers you add. The Inspector saves your server list to $HOME/.mcp-inspector/mcp.json, which in the image is /home/node/.mcp-inspector/mcp.json — inside the container's writable layer, so --rm discards it and every run starts with an empty list. Mount a volume there to keep it:

docker run --rm -p 127.0.0.1:6274:6274 \
  -v mcp-inspector-data:/home/node/.mcp-inspector \
  ghcr.io/modelcontextprotocol/inspector

The same volume also persists OAuth tokens and stored state, so an authorized server stays authorized across runs. Use -e MCP_CATALOG_PATH=/some/other/path.json to put the catalog somewhere else — mount a volume covering whatever directory you point it at. If you bind-mount a host directory instead of a named volume (-v "$PWD/inspector-data:/home/node/.mcp-inspector"), the directory keeps its host ownership, so on Linux add --user "$(id -u):$(id -g)" or chown it to uid 1000 — otherwise the non-root node user can't write and adding a server fails with EACCES.

Upgrading from an image before this fix? Earlier images did not create /home/node/.mcp-inspector, so Docker created the volume's mount point as root and the non-root node user couldn't write to it. An empty volume repairs itself on the first run of a current image (Docker applies the image directory's ownership to an empty volume), but one that already has files in it keeps its old root ownership and still fails with EACCES. Fix it once:

docker run --rm -u 0 --entrypoint chown \
  -v mcp-inspector-data:/data ghcr.io/modelcontextprotocol/inspector \
  -R node:node /data

The image defaults to --web bound to 0.0.0.0:6274 with browser auto-open disabled; override the args to run another mode (docker run --rm ghcr.io/modelcontextprotocol/inspector --cli …). Pass -e MCP_INSPECTOR_API_TOKEN=… to set a known token (otherwise one is generated and printed in the logs), or -e DANGEROUSLY_OMIT_AUTH=true to disable auth. Binding 0.0.0.0 (all network interfaces) is refused by default outside a container — it exposes the process-spawning backend to the local network — so the image opts in explicitly with DANGEROUSLY_BIND_ALL_INTERFACES=true (already set in the Dockerfile); a bare HOST=0.0.0.0 without that flag exits with an error. If you remap the published port (-p 127.0.0.1:8080:6274), the browser's origin (http://localhost:8080) no longer matches the in-container port, so set -e ALLOWED_ORIGINS=http://localhost:8080,http://127.0.0.1:8080 (or run -e CLIENT_PORT=8080 -p 127.0.0.1:8080:8080) or connects will 403. ALLOWED_ORIGINS replaces the default list rather than merging, so list every loopback form you'll browse from (see the web README). The image runs as the non-root node user and has a HEALTHCHECK that probes the web UI — it assumes the default --web mode, so add --no-healthcheck when running --cli/--tui (which have no web server).

Contributing — `AGENTS.md` and `CLAUDE.md`

AGENTS.md is the contract for changing this codebase, and it applies to humans and AI agents alike. It is not agent-only boilerplate — it holds the project's real conventions: the issue-and-board workflow, branch/label rules, the TypeScript and Mantine/React standards, the testing and coverage requirements, and the mandatory pre-push gate. Read it before making changes, and keep it up to date when you change structure, tooling, or rules.

CLAUDE.md is the entry point the Claude Code agent loads automatically; it simply includes AGENTS.md and this README, so both agents and humans work from the same source of truth. If you use a different agent that reads AGENTS.md, you get the same rules.

A key rule worth surfacing here: all work is issue-driven. Before starting, find or create a tracking issue on the v2 project board; open PRs against v2/main with Closes #<issue>. The exact recipes (labels, board IDs, statuses) are in AGENTS.md.

License

MIT.

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Name		Name	Last commit message	Last commit date
Latest commit History 2,743 Commits
.claude		.claude
.github		.github
clients		clients
core		core
docs		docs
scripts		scripts
specification		specification
test-servers		test-servers
.dockerignore		.dockerignore
.gitattributes		.gitattributes
.gitignore		.gitignore
.mcp.json		.mcp.json
.npmignore		.npmignore
AGENTS.md		AGENTS.md
CLAUDE.md		CLAUDE.md
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
CONTRIBUTING.md		CONTRIBUTING.md
Dockerfile		Dockerfile
README.md		README.md
SECURITY.md		SECURITY.md
eslint.config.js		eslint.config.js
package-lock.json		package-lock.json
package.json		package.json
tsconfig.base.json		tsconfig.base.json
vitest.shared.mts		vitest.shared.mts

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MCP Inspector

Project layout

Setup

Running during development

The @inspector/core shared package

Web client: "dumb components" + Storybook

Test servers

Serving the modern protocol era

Showcase configs

MCP Apps

MRTR

Network tab — standardized headers and error taxonomy

x-mcp-header in the Tools tab

Page-by-page fetching

Structured output

Duplicate tool names

Advertised extensions

Logging, both eras

Resource subscriptions, both eras

Tasks, both eras

Building

Testing & the quality gate

Publishing

What ships, and the packaging invariants

npm run pack:verify — publish smoke against the real tarball

Cutting a release

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Contributing — AGENTS.md and CLAUDE.md

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`npm run pack:verify` — publish smoke against the real tarball

Contributing — `AGENTS.md` and `CLAUDE.md`