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, endpoint overrides, mid-session recovery (browser/node/remote backends);
│   │                 #   plus per-server secret storage — the keychain/file/memory SecretStore
│   │                 #   implementations, the selection policy, and the descriptor the banner and UI report
│   ├── client/       # Install-level client config (`client.json`): browser-safe parse/validate + Node load/save, remote backend, secrets
│   ├── json/         # JSON + parameter/argument conversion utilities, the nullable-union
│   │                 #   schema collapse shared by the web and TUI form builders, and the
│   │                 #   tool-schema portability lint all three clients report from
│   ├── logging/      # Silent pino logger singleton
│   ├── mcp/          # InspectorClient runtime, state stores, transports, config import,
│   │                 #   and the RFC 6570 URI-template helpers the web form and TUI expand through
│   ├── 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. vite and @vitejs/plugin-react are root dependencies for the same reason, not by mistake — they look like build tooling, but clients/web/server/start-vite-dev-server.ts imports them at runtime for mcp-inspector --web --dev, and clients/web/tsup.runner.config.ts lists both as external, so a published install resolves them from the root manifest. Moving them to devDependencies would break --web --dev for consumers (and the on-demand vite build in ensure-web-build.ts) while passing every local check. It does mean they show up under npm audit --omit=dev, which is a feature: they really are in the production tree.

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
app-elicitation-http.json (legacy era)	An MCP App rendering a form elicitation	#1854
mcp-app-domain-http.json (legacy era)	An MCP App asking for a dedicated origin (_meta.ui.domain)	#2056
modern-mrtr-http.json	A single MRTR round-trip	—
mrtr-showcase-http.json	Every MRTR preset in one server	#1860
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
nullable-fields-http.json	Tools tab: nullable (anyOf + null) arguments	#1928
unportable-schemas-http.json (legacy era)	Tool schemas a real client rejects, flagged in all three clients	#1005
rfc6570-templates-http.json	Resources tab: RFC 6570 resource-template expansion	#1919
advertised-extensions-http.json	Tool registration gated on advertised extensions	#1739
oauth-custom-resource-metadata-http.json (legacy era)	OAuth discovery driven by the challenge's resource_metadata	#2071
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.

An App's dedicated origin

mcp-app-domain-http.json serves the same mcp_app_demo widget as above, with one addition: its UI resource declares _meta.ui.domain. Plain streamable-HTTP; connect with the default (legacy) protocol era.

That field is how a server asks its host for a stable, dedicated origin. Without one, an App renders into a srcdoc frame sandboxed without allow-same-origin, so its document has an opaque origin and every request it makes carries Origin: null — which no CORS policy, OAuth callback, or API-key allowlist can admit (#2056).

Open the Apps tab and run mcp_app_demo. The widget renders identically to mcp-app-http.json — the difference is not visual. Inspect the inner iframe in devtools: on this server it is served from http://127.0.0.1:6278/app-document/<id> and location.origin is that real origin, where on mcp-app-http.json it is about:srcdoc with an origin of null. (The host is whatever the Inspector bound to — 127.0.0.1 by default, an address rather than the name localhost, for the reason resolve-bind-host.ts documents.)

The spec makes domain's format host-dependent, and the Inspector owns no domain infrastructure — so it reads any non-empty value as a request rather than an address, and answers with a real loopback origin of its own. See MCP App dedicated origins for the full contract, including what the one shared origin does and does not isolate, and how every failure falls back to the default render rather than blanking the app.

App-rendered form elicitations

app-elicitation-http.json serves app_choose_option alongside the choose_option_app UI resource (ui://demo/choose-option.html). The tool sends a completely ordinary form elicitation/create — the only thing added is _meta.ui.resourceUri naming that app. Plain streamable-HTTP; connect with the default (legacy) protocol era.

Run app_choose_option from the Tools tab. The server's app renders in a modal instead of the built-in elicitation form, and clicking Option A, Decline, or Cancel returns the standard ElicitResult straight to the server, which echoes it into the tool result.

App rendering is selected only when all four conditions hold (#1854, per ext-apps#733 / SEP-3118):

1. the client advertises elicitation.form;
2. the client advertises extensions["io.modelcontextprotocol/ui"].mimeTypes including text/html;profile=mcp-app;
3. both the client and the server advertise the nested elicitation setting on that same extension;
4. the request carries a valid absolute ui:// URI in _meta.ui.resourceUri.

Only the web client advertises the nested client-side setting, and only because it has a sandbox renderer to back it; the CLI and TUI advertise the MIME type (they know what an App is) but never claim they can resolve an elicitation through one, so the same server falls back to their native prompts. Turning Server Settings → Advertised Extensions → MCP Apps UI off, or turning form elicitation off, removes the claim on web too.

Everything else falls back to the built-in elicitation form, by design: metadata that is absent or not an absolute ui:// URI, a resource that fails to load, a sandbox or bridge that fails to initialize, an app that did not advertise elicitation, a request that times out, and any result that is not a valid ElicitResult for the requested schema. An explicit decline or cancel is not a fallback — it is a completed elicitation and goes back to the server as-is.

The Inspector speaks the ext-apps#733 wire protocol but does not yet consume its helpers: the released @modelcontextprotocol/ext-apps (1.7.5) predates that PR. core/mcp/appElicitation.ts and clients/web/src/components/elements/AppRenderer/requestAppElicitation.ts mirror it exactly and are marked for deletion in favour of the package's own exports once a release containing it ships.

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_empty	Completes with an empty result — no content, no structuredContent
mrtr_loop	Never completes → trips the MRTR_MAX_ROUNDS bound

Run mrtr_empty and answer its single elicitation: the Protocol tab groups the exchange as an MRTR conversation ending COMPLETE, and the Results panel says "Empty result — The tool call completed successfully and returned no content." On the broken build that same result rendered as "No results yet", the panel's pre-run placeholder (#1860) — so a call the user had just watched succeed read as a call that never ran. An empty content array with no structuredContent is a legal CallToolResult, and the panel only ever mounts once a result exists, so the placeholder wording could not be true there. (The neighbouring half of the same gap — a result whose payload lives only in structuredContent — was closed by #1908.)

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.

Nullable arguments

nullable-fields-http.json serves record_shipment, whose four arguments are each declared with Zod's .nullish() — "optional and explicitly nullable". That compiles to anyOf: [<branch>, { "type": "null" }], so the real type (and, for the enum, its enum list) sits on a branch rather than at the top level. get_temp sits alongside it with a plain, non-nullable units enum for comparison. Plain streamable-HTTP — connect with the default (legacy) protocol era.

Open the Tools tab and select record_shipment: direction must render as a Select (envio / recebimento) with a clear button that sets it back to null, reference as a text input, quantity as a number input, and express as a checkbox. On the broken build every one of them fell through to the raw-JSON textarea, which re-escaped its own contents on each keystroke until the value was unusable (#1928). The tool echoes the arguments it received, so the result panel shows exactly what was sent.

The TUI had the same gap and is worth checking against the same server (--tui, then test record_shipment): direction is a select, quantity an integer field, express a boolean. Both clients now share one collapse step — normalizeNullableUnion in core/json/nullableUnion.ts — precisely so they cannot drift on which schemas they can render.

Unportable tool schemas

unportable-schemas-http.json serves four tools, three of whose advertised schemas carry constructs that are legal JSON Schema and are refused or mishandled by real MCP clients:

Tool	What it carries
get_temp	outputSchema.properties.data as a bare true — what Go's jsonschema package emits for interface{}, the case reported in #1005
echo	an array-form "type": ["null","boolean"], and an opts property that constrains nothing
add	a property pointing at a remote $ref
get_weather	nothing — left clean, so a flagged tool sits beside an unflagged one

Plain streamable-HTTP — connect with the default (legacy) protocol era. Every tool here is still runnable: the override replaces only the advertised schema, and the flagship bare-true rides get_temp (which returns structured content) rather than echo — see the ⚠️ caveat at the end of this section for why that placement matters.

The Inspector is where a server author looks first, so a construct that will fail downstream is named here rather than passed through silently. All three clients report the same verdict from core/json/schemaLint.ts, each with the room it has:

mcp-inspector --cli http://127.0.0.1:6603/mcp --method tools/list --strict   # exits 6

• CLI — --strict prints the full report (path, issue, suggested fix) on stderr and exits 6 on an error-severity finding; without it, one summary line. See Schema portability.
• TUI — the tools list marks get_temp with a red ! and echo/add with a yellow ?; the detail pane lists each finding under Schema Portability.
• Web — the Tools sidebar row carries the same flag as a hover-labelled icon, and selecting the tool shows a Schema portability section above the argument form.

This is deliberately not a JSON Schema validator. A census of 617 public servers (14,804 tool schemas) reported on that issue found 0 that fail the SDK's own ListToolsResultSchema.safeParse, so a conformance check would report nothing on essentially every real server. What bites is the narrower subset each consumer accepts, and each rule here is a construct known to be refused or degraded by a shipping client. The schemas are supplied through the test server's rawToolSchemas override, because the Zod-built presets cannot express any of them — which is the same reason a real server hits this only when its schemas come from another generator.

⚠️ If you add an outputSchema override of your own, put it on a tool that returns structured content. A conforming client validates a tool result against the advertised output schema, so an override on a preset that returns none makes every call to it fail with "declares an output schema but returned no structured content" — a confusing thing to hit from a fixture. That is why the bare true rides get_temp here and not echo.

RFC 6570 resource templates

rfc6570-templates-http.json serves two resource templates straight out of #1919 — events_by_topic (foobar://events/{topic}) and events_by_query (foobar://events{?topic}) — each echoing the URI it was matched against, plus a plain foobar://events resource (see below). Plain streamable-HTTP; connect with the default (legacy) protocol era.

Open the Resources tab and pick events_by_topic, then enter foo/bar. The request must go out as foobar://events/foo%2Fbar, and the result echoes back the URI the server matched. On the broken build the value was spliced in raw, so the slash created a second path segment and the SDK's matcher answered -32602 Resource not found: foobar://events/foo/bar — the exact failure in the issue. The same holds for ?, #, %, spaces, and non-ASCII text.

events_by_query is the half that was invisible: the old /\{(\w+)\}/g scan could not see an expression carrying an operator, so no topic input was rendered at all. It now appears, marked Optional — RFC 6570 drops the whole expression when the variable is undefined, so reading with the field blank requests foobar://events, and filling it in requests foobar://events?topic=foo%2Fbar. The URI preview beside the title shows the partially-expanded form as you type, leaving unfilled expressions standing as written.

The plain foobar://events resource is registered deliberately, not as filler. The SDK's UriTemplate.match() compiles {?topic} to a required \?topic=([^&]+), so a template alone cannot serve the blank read — match("foobar://events") returns null. A real server exposes the unfiltered collection as its own resource; the showcase does the same so that step actually resolves.

The web client and the TUI expand through one shared helper, core/mcp/uriTemplate.ts — the web Resources form directly, the TUI via InspectorClient.readResourceFromTemplate — and both derive their form fields from its parser too, which is the half that makes the sharing real: a form submits values under the names it rendered, so a parser that mangles a name silently drops the value at expansion time. (The CLI is not a consumer: it has no template form, and its resources/read passes the already-expanded --uri straight through.)

The SDK's UriTemplate is still used, but only to validate a template (constructing it is what rejects an unclosed expression). Its expander is not, because it is incomplete in five ways — each measured against the pinned SDK, not inferred:

Shape	SDK behavior
{a,b}	raw-joins the values — no encoding, operator prefix dropped
{;id}	; is missing from its operator list, so the variable parses as ;id
{id:3}	the prefix modifier is folded into the name, giving id:3
{+v} / {#v}	encodeURI mangles reserved [/] ([::1] → %5B::1%5D) and double-encodes pct-triplets (%2F → %252F)
{v}	encodeURIComponent leaves the sub-delims !'()* bare, which RFC 6570 requires encoded

The ; and :3 rows are the ones a user sees directly: on the SDK's parse the form renders fields literally labelled ;id and id:3. The +/# row is silent corruption rather than over-escaping — an IPv6 literal or an already-encoded path arrives at the server altered.

A template that cannot be expanded at all — an out-of-grammar modifier ({id:abc}), or an expression declaring no variable ({}, {a,}, {?}) — withholds the read rather than sending something. Pick events_malformed (foobar://events/{topic:abc}) to see it: Read Resource is disabled, the reason is printed under the form, and the preview shows the template as the server declared it. The alternative is worse than it looks: x://{} would otherwise expand to x:// with no inputs rendered, so the form's "everything required is filled" check passes vacuously and it reads a URI that is not the template the server published.

Literals are pct-encoded on expansion too (RFC 6570 §3.1): café/{var} sends caf%C3%A9/value, not raw UTF-8 in the path — something the SDK's expander does not do either. And the names a template may use are RFC 6570's varchar plus a labelled tolerance for - and ~: the conformance suite rejects {default-graph-uri}, but real servers publish such names and the SDK's matcher round-trips them, so the Inspector expands them and marks the variable conforming: false rather than refusing a resource that demonstrably works.

An undefined variable is what omits its expression — a variable defined as the empty string expands (x{?q} gives x?q=, x{;q} gives x;q, per RFC 6570 §3.2.7). The expander honors that distinction, so a caller such as readResourceFromTemplate can request either URI. Collapsing the two is a form concern, not a template one: both clients seed every declared variable with "" and a text input cannot express "defined but empty", so each form drops its blanks (definedValues) on the way in.

Requiredness is a property of the expression, not the variable: RFC 6570 drops undefined names from a multi-name expression, so {a,b} with only a filled is expandable and a form must not block it. requiredGroups returns one entry per non-omittable expression and hasRequiredValues asks that each be satisfied by any one of its names — which no per-variable flag can express once a name recurs across expressions ({a,b}{a,c} is satisfied by filling b and c).

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.

1. Connect — the Inspector advertises the Tasks extension by default, so the Tools list shows both echo and get_weather.
2. Open Server Settings → Advertised Extensions, uncheck Tasks (io.modelcontextprotocol/tasks), and reconnect.
3. 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.

OAuth resource_metadata at a non-default path

oauth-custom-resource-metadata-http.json is an OAuth-protected server (combined AS + resource, DCR enabled) that serves its RFC 9728 protected-resource metadata document only from /custom/protected-resource, and advertises it on every 401:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer resource_metadata="http://127.0.0.1:8082/custom/protected-resource"

The default /.well-known/oauth-protected-resource route is deliberately left unserved, so a client that ignores the advertised URL cannot discover the document at all. Plain streamable-HTTP — connect with the default (legacy) protocol era.

Add the server, click Connect, and watch the Inspector's first protected-resource metadata request in the Network tab: it must go to /custom/protected-resource. On the broken build the challenge's resource_metadata was parsed and then dropped before the SDK's auth() ever saw it (#2071), so discovery probed locations derived from the MCP server URL, 404'd, and authorization failed for any server that puts the document somewhere other than the well-known path.

The same server is worth running against --cli / --tui, which reach it by a different route: with no stored token in the legacy era the Inspector connects with no auth provider (so the SDK cannot open a browser before the callback server is listening), the 401 surfaces as the SDK's headerless UnauthorizedError, and the client calls authenticate() with no challenge in hand. The transport therefore observes every 401/403 passively, so the advertised URL is still available on that path.

The value now rides the normalized AuthChallenge as a string — it has to be serializable, because the web client's challenge crosses the remote-backend boundary as JSON — and is converted to a URL at the OAuth boundary, where it is handed to auth() as resourceMetadataUrl and to the CIMD pre-registration probe, which runs before auth() and would otherwise do its own default-location discovery. A malformed value is ignored rather than surfaced, matching the SDK's own WWW-Authenticate parser: discovery falls back to the default locations instead of failing the whole authorization on a bad header. The callback leg needs nothing extra — SDK auth() persists the URL in its discovery state, so it survives both the web full-page redirect and the CLI/TUI loopback callback.

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 reaching one tsc program from two installs skews across them) — 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 three 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, and an app-rendered elicitation smoke (smoke:web:elicit) that drives one end to end — call the tool, answer inside the sandboxed app, see the app's ElicitResult reach the server — and then the same tool against a server that never advertised the capability, which must fall back to the native elicitation form.
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:bundle-externals	Guards the must-not-bundle invariant (#2067): for each tsup-bundled client it reads the built build/ output and fails if any package that must stay external was inlined anyway. Candidates are the union of the client's own external array and the root manifest's dependencies — the latter because #2067 was a missing external entry, which a self-referential check would have passed. Detection is via esbuild's // <path> module banners, so it covers both shapes — a separate <pkg>-HASH.js chunk (what a dynamically import()ed CommonJS package produces) and a statically-imported package folded straight into index.js, which emits no chunk at all. A build with no banners fails as such rather than passing clean, so enabling minify cannot silently retire the check. undici was declared only in the root and clients/cli manifests, and tsup auto-externalizes only what the nearest manifest declares, so the web and TUI bundles inlined 1.05MB of it — and CommonJS inlined into an ESM bundle throws Dynamic require of "assert" is not supported on first use, from a specifier no user-side install can satisfy. Reads the output rather than the config because those two disagreed for four releases. 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, scripts/lib/tsc-program.mjs + the exported helpers of verify-typecheck-coverage.mjs and verify-dep-lockstep.mjs), one case per rule they encode, plus two suites over shared scripts/lib helpers that no smoke can check itself: resolve-node-bin.test.mjs — the cross-platform bin resolver (#1939), pinned against the real bin/exports shapes of the packages the scripts actually spawn — and announced-child.test.mjs — the spawn/readiness ownership helper (#2000), which drives real node -e children to prove a child that never announces is still published to the caller before the timeout throws, and so is reachable by teardown rather than orphaned. Two more do the same: mcp-app-flow.test.mjs covers the shared MCP Apps flow (#2003) — the deep link's two CSRF gates and appArgs encoding, plus driveAppFlow's failure branches against a stand-in page, all of which are dead code from the happy-path smokes' point of view and would otherwise surface only as opaque timeouts; and ensure-test-servers.test.mjs pins the #2111 invariant — that test-servers/build is rebuilt even when it already exists — which no smoke can assert about itself, since one driving a stale fixture reports a product failure rather than a staleness one. 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 actually enters each program (#1965) — every client tsconfig project listed with tsc --listFilesOnly via the shared scripts/lib/tsc-program.mjs, each resolved node_modules file mapped to its owning install, keeping the packages that reach one program from two installs (a package whose declarations arrive only through another package's .d.ts, as @modelcontextprotocol/sdk's do, is invisible to a scan of first-party imports). Prices each copy from the lockfile entry for the exact install path the program resolved, compares only the installs that met in one program, and fails deny-by-default on any disagreement not in the annotated TOLERATED_SKEW allowlist — empty today — with an allowlisted package tolerated only within a major version. Runs in validate.
npm run ci	Mandatory pre-push command. validate → coverage → verify:build-gate → verify:bundle-externals → 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.

Linting is type-aware. All five ESLint scopes (clients/{web,cli,tui,launcher} plus the root core/ + shared gate) enable @typescript-eslint/no-floating-promises at error, so a promise that is neither awaited, returned, .catch(…)-terminated, nor explicitly discarded with void fails lint — and therefore validate (#1959). The rule needs type information, so each scope's config names a parser project; the root scope's is tsconfig.lint.json, a lint-only project covering core/**, test-servers/src/**, and vitest.shared.mts, which have no tsconfig of their own. It emits nothing and changes no typecheck — but a new first-party TS location added to the root lint scope must be added to its include. See TypeScript instructions in AGENTS.md for when void is acceptable.

And lint has no warning tier. Every lint script runs with --max-warnings 0, so a warning fails validate exactly as an error does (#2085) — a warn-level react-hooks/exhaustive-deps finding had otherwise let a stale-closure bug pass the mandatory pre-push gate and reach review. Fix the finding rather than silencing it; if a rule genuinely must be waived, use its inline disable comment with a one-line justification.

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 the tarball entry, the installed-on-disk path, and (since #2003) that a widget actually loads through it on the installed bin. Presence and reachability are different properties: a rename with a stale reader ships a file that is there and unusable.
• 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, a prod --web boot that must serve / from the shipped dist, and — riding that same boot — an MCP App rendered in headless Chromium through the shipped sandbox proxy, connect → open app → data-app-status="ready" (#2003). That last step shares its flow with smoke:web:app via scripts/lib/mcp-app-flow.mjs rather than copying it, since the deep-link shape is the part that rots silently; the client comes from the install while the App test server stays a repo fixture. 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 three steps.

1. Bump on v2/main, before the milestone merge. The bump is part of the milestone's work, so it belongs on the develop branch and flows into main with everything else:

Substitute the real issue number and release below — the commands are written to be copy-pasteable as-is (a 2.2.0 → 2.3.0 minor bump):

git checkout -b v2/chore/2010-bump-2-3-0 v2/main
npm version minor --no-git-tag-version   # or major / patch; bump only, no tag
# PR → v2/main

⚠️ --no-git-tag-version is load-bearing. A bare npm version also tags, and the tag would land on a v2/main commit — but the release must be cut from main, so the tag has to point at the merge commit there (step 3). Tagging here creates a tag on a commit that is never released.

2. Merge v2/main → main through the usual milestone-merge branch. It now carries the bump, so the release lands on main with the version already correct.

Between steps 1 and 2 the two branches do differ, and that is expected, not drift: v2/main reads the version being built while main still reads the one currently released. What this ordering removes is post-release drift — once the milestone merge lands they agree again, and v2/main is never left behind main. If you see v2/main ahead of main, a release is in flight; if you see it behind, something went wrong.

3. Tag the main commit and draft the Release:

git fetch origin main
git tag 2.3.0 origin/main && git push origin 2.3.0
# then draft & publish a GitHub Release for that tag → triggers `publish`

⚠️ Tag origin/main, not your local HEAD. git checkout main && git pull resolves through whatever merge-or-rebase strategy you have configured, so a divergent local main can quietly produce or replay local commits. Tagging HEAD there tags a commit that is not on origin/main, and git push origin <tag> pushes only the tag — leaving a release whose commit was never published. Naming origin/main explicitly makes the tagged commit exactly what the remote branch points at, regardless of local state.

⚠️ No v prefix. This repo's release tags are bare x.y.z — 2.2.0, 2.1.0, 2.0.0 — so tag 2.3.0, not v2.3.0. Note npm's own tag-version-prefix defaults to v and the repo sets no .npmrc, so a bare npm version would have produced a v-prefixed tag that does not match the convention. Tagging by hand (step 3) is what keeps it right. The workflow's assert step strips a leading v before comparing, so a v-prefixed tag would still publish — it would just be inconsistent with every previous release.

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.

Why the bump goes on v2/main first (#2010). It used to happen on the milestone-merge branch, which is cut from main — so the bump existed only downstream of v2/main and nothing carried it back. v2/main sat at 2.0.0 through both the 2.1.0 and 2.2.0 releases. That is not cosmetic: a branch cut from a milestone-merge branch silently carries the bump into an unrelated PR (this happened on #2009, where a container bugfix arrived with a 2.0.0 → 2.2.0 diff), and anything reading the version in development — readInspectorVersion(), --version, GET /api/config — reported a version two releases old.

Do not "fix" a future drift by merging main back into v2/main. main carries the entire pre-v2 v1 history (retained through ec5d8e13 chore: replace main's tree with v2 — ~230 commits v2/main does not have), so a back-merge grafts all of it into the develop branch's log permanently in order to deliver a two-file change. Bumping first means there is nothing to back-merge.

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

Using the Apps tab? Publish 6275 too. The MCP Apps sandbox is a second listener the browser reaches directly, on MCP_SANDBOX_PORT (default 6275). Nothing else needs it, so the single-port commands above are fine for ordinary inspection — but the Apps tab renders a blank widget without it:

docker run --rm -p 127.0.0.1:6274:6274 -p 127.0.0.1:6275:6275 \
  ghcr.io/modelcontextprotocol/inspector

And 6278 if your app declares _meta.ui.domain. That is the spec field a server uses to ask its host for a stable, dedicated origin — without one the app runs at an opaque origin and its requests carry Origin: null, which no CORS / OAuth-callback / API-key allowlist can admit. The Inspector answers the request with a real loopback origin on a third listener, MCP_APP_ORIGIN_PORT (default 6278); apps that declare no domain never touch it. Publish it if you use one — this is the one failure that does not fall back: the backend publishes fine (its listener bound inside the container), so it hands the browser a URL on a port the browser cannot reach, and that app's frame stays blank. The cross-origin navigation failure is not observable from the page, so there is no opaque-origin fallback and no console warning here; those cover the failures the backend can see (no listener, a port that never bound, an older backend). See MCP App dedicated origins for the host-specific contract and its isolation trade-offs.

docker run --rm -p 127.0.0.1:6274:6274 -p 127.0.0.1:6275:6275 -p 127.0.0.1:6278:6278 \
  ghcr.io/modelcontextprotocol/inspector

Publish each on the same port number inside and out. The sandbox URL is handed to the browser via /api/config as http://localhost:<container port>/sandbox, so remapping it (-p 9000:6275) advertises a port the browser can't reach; use -e MCP_SANDBOX_PORT=9000 -p 127.0.0.1:9000:9000 instead. The same holds for the app origin: the URL a published app document is served from is built from the port the container bound, so remap with -e MCP_APP_ORIGIN_PORT=9001 -p 127.0.0.1:9001:9001 rather than with -p alone.

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.

Where secrets go, and how to make them survive (#1950). The Inspector keeps the values it deliberately does not write to mcp.json — an OAuth client secret, an enterprise IdP client secret, each stdio env: value — in the OS keychain. A container has no keychain (the published image has no D-Bus session), so on startup the Inspector probes for one and falls back, saying so in the logs and in a permanent footer at the bottom of the Client Settings and Server Settings dialogs. Which fallback you get depends on whether the directory it would write to is going to survive:

Situation	Store	Secrets survive a restart?
Keychain reachable (a normal desktop install)	OS keychain	Yes
Container, no volume on /home/node/.mcp-inspector	Memory	No — session only
Container with that volume, or any host without a keychain	~/.mcp-inspector/secrets.json, mode 0600	Yes

So the same volume that keeps your server list also switches secrets from session-scoped to durable — nothing extra to configure. The in-memory default for an unmounted container is deliberate: a file in the writable layer is discarded by --rm and by every image update, and promising durability it can't deliver is worse than declining to.

A file-backed store is unencrypted unless you give it a key. Set MCP_INSPECTOR_SECRET_KEY and the file is encrypted with AES-256-GCM (the passphrase is stretched with scrypt against a per-file random salt). Without it the file is still 0600, but the values are readable to anyone who can read the file — which the startup log and the settings footer both say, every session, in a warning tone:

docker run --rm -p 127.0.0.1:6274:6274 \
  -v mcp-inspector-data:/home/node/.mcp-inspector \
  -e MCP_INSPECTOR_SECRET_KEY="$MY_PASSPHRASE" \
  ghcr.io/modelcontextprotocol/inspector

Use a high-entropy passphrase — generated, not chosen. The random salt stops an attacker precomputing a table across files; it does nothing against guessing, and the scrypt cost is deliberately low because the derivation runs on every read and write. Anyone who obtains secrets.json can therefore test candidate passphrases quickly and offline, so treat this value like any other credential rather than like a memorable password.

Setting the passphrase later is safe — the next write upgrades an existing plaintext file in place. Until that write happens the existing values really are still readable, and the banner and footer keep saying so rather than reporting the file as encrypted the moment the variable appears. Changing or losing the passphrase is not safe: a file that can no longer be decrypted is read as empty and refuses to be written, rather than being silently replaced with a new one holding only your latest secret. Restore the original passphrase, or delete secrets.json and re-enter the values.

The Inspector writes the file 0600 and re-tightens it at startup if something loosened it. If it cannot — the file belongs to another user, or the mount is read-only — it says so in the log rather than continuing to describe the file as protected, since on that box the mode claim above is not true.

Two Inspectors, one file. Within a process, mutations are serialized per file path, so a web session's own concurrent saves cannot lose each other. Across processes — a CLI run beside a web session — each mutation takes an exclusive lock on secrets.json.lock for the whole read-modify-write, using proper-lockfile (the same library npm itself locks with). The lock expires 10 seconds after its holder stops refreshing it, so an Inspector that is killed mid-save does not leave the file unwritable.

Two running Inspectors are therefore genuinely serialized. What a lock file cannot make single-winner is the takeover of a lock whose holder died — that needs a compare-and-swap on a directory entry (renameat2) which Node does not expose, and it is what an earlier hand-rolled attempt failed three review rounds on. proper-lockfile does not close that race either. The window opens only after a holder dies without releasing.

The Inspector adds one thing on top: every lock-directory removal the library makes on its behalf — on release, and from its exit handler — is guarded by a check that the directory is still the one it created (by inode and birth time, which survive the library's own refresh but not a delete-and-recreate). That matters because those removals are otherwise unconditional, so a holder whose lock had been replaced would delete the winner's lock on the way out, turning one compromised writer into two unprotected ones. It also surfaces the takeover as a warning. Treat all of this as best-effort: the guard is still a check followed by an act, so it makes the destructive case rare rather than impossible, and it rests on filesystem metadata that not every filesystem reports.

Which is why, underneath the lock, each mutation still reads the file, applies its change, writes, then reads back and compares the whole map; if something wrote in between it re-applies onto what was left and retries, failing loudly after five lost rounds rather than returning as though the value were saved. That check is what still catches a clobber inside that window — and it covers what no lock can, since a lock only orders the writers that take it: an editor, a restored backup, or an Inspector older than this release.

If another process holds the lock and will not let go, the save fails rather than going ahead unlocked — waiting past the stale window first, so a crashed Inspector resolves itself rather than failing everyone else's saves. Writing alongside a writer you can see is the one case where degrading would lose the secret it was trying to protect.

It is also what covers the lock being unavailable. This store exists for boxes where the usual mechanism isn't there, so a directory that can't hold a lock file — a read-only $HOME, a mount owned by another uid — makes the save proceed unlocked with a warning, rather than turning every set into a failure on exactly the deployments the store was written for.

Three env vars affect where the file lands. MCP_INSPECTOR_SECRET_STORE=keyring|file|memory picks the store outright, bypassing the probe. MCP_INSPECTOR_SECRET_FILE names the file. Failing both, the file follows MCP_STORAGE_DIR — the same variable that relocates OAuth tokens and client.json — so mounting a volume at your configured storage directory is enough to make secrets durable there.

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.