inspector/.github/copilot-instructions.md

Copilot review instructions — MCP Inspector

AGENTS.md is the source of truth. This file is a review-focused distillation of it, kept in sync by hand — see "Keep documentation files up to date" in AGENTS.md. Where the two disagree, AGENTS.md wins, and the drift is a bug worth flagging in review.

The Inspector ships as one package with three clients (Web, CLI, TUI) over a shared core/, consumed via the @inspector/core build-time alias. v2 is not an npm workspace: the root and each clients/* keep their own package.json and node_modules.

TypeScript

• Never use any. Not in types, not in casts, not in generics.
• Never suppress errors to satisfy the linter or compiler — no disabling no-unused-vars / no-explicit-any in config, and no // @ts-nocheck or // @ts-ignore (@typescript-eslint/ban-ts-comment rejects these across every surface).
• Avoid double casts (as unknown as T). They erase all type safety and usually mean the real type is being worked around. Prefer a type guard, a narrower single cast, or fixing the underlying type. If genuinely unavoidable (a documented gap in a third-party type, or bridging structurally-identical shapes TS can't relate), it must carry an inline comment justifying why it's safe and why nothing better exists. An unjustified as unknown as is not acceptable in review.
• Prefer inference, type guards, and precise annotations over assertions.
• An _ prefix is the intentionally-unused marker (argsIgnorePattern / varsIgnorePattern / caughtErrorsIgnorePattern).
• Never float a promise. @typescript-eslint/no-floating-promises is error in all five ESLint scopes, so every promise must be awaited, .catch(…)-terminated, returned, or explicitly discarded with void. Prefer holding and settling it — a floated rejection surfaces in an unrelated file as an SDK-internal stack, which is how two un-held callTool calls failed the whole npm run ci chain (#1947). Reach for void only when the callee already owns its failures (it ends in a catch) or the caller genuinely cannot await (a sync effect body, an Ink key handler), and say which in a comment. In Storybook play functions, expect(...) from storybook/test is instrumented and returns a promise — it must be awaited.

React and UI (web client)

The web client is built from presentational ("dumb") components — they take data and callbacks as props and hold display logic only. No data fetching or client state inside them; state comes from the @inspector/core hooks wired near the top of the tree. A component that reaches for a store or fetches directly is a review finding.

Styling is Mantine-first, in this strict order of preference: component props → theme variants → CSS classes (last resort).

• Never use inline styles.
• Never use raw color literals — no hex (#ddd), no rgba(). Use the --inspector-* CSS custom properties from App.css :root (e.g. c: 'var(--inspector-text-primary)'). If no token fits, add one to :root first.
• Avoid div and bare HTML for layout. Use Mantine Box, Group, Stack, Flex, Paper.
• Never add a CSS class when the styles can be component props or a theme variant. Flat CSS properties (margin, padding, background, border, color, font-size) belong in the theme (src/theme/<Component>.ts, via Component.extend()). App.css may contain only what the theme cannot express: @keyframes, pseudo-selectors (:hover, :focus), cross-component hover relationships, nested child selectors for third-party HTML output, and styles for native elements (img, iframe).
• When a theme variant needs a class for nested/pseudo selectors, assign it via classNames in the theme extension — never a manual className in JSX for theme-styled components.

The .withProps() rule

Declare a named subcomponent constant via .withProps() whenever an inline Mantine element carries two or more static props. This applies to single-use elements too — "it's only used once" is not an exemption.

• Static = a literal configuring styling, layout, or behavior: size="sm", c="dimmed", fw={500}, gap="xs", justify="space-between", variant="light", withBorder, readOnly, striped.
• Not counted: dynamic props (value, on*, children, key, ref, anything whose value is a variable) — pass these at the call site; and per-instance content/accessibility literals (label, description, placeholder, title, aria-label, role) — these never by themselves trigger extraction.

const CardContent = Group.withProps({
  flex: 1,
  align: "flex-start",
  justify: "space-between",
  wrap: "nowrap",
});

Legitimate exceptions (each stays inline, with a one-line comment saying why):

• Box — does not support .withProps(). Use Group/Stack/Flex/Text/Paper/UnstyledButton/Image instead, chosen by purpose. A Box that genuinely needs a non-flex primitive (component="iframe", display="grid") stays inline.
• Accordion — a compound, multiple-discriminated generic; .withProps() loses its JSX call signature and fails to type.
• Headless, non-factory() components such as Transition — no Styles API, so no .withProps static at all.
• data-* attributes — not part of a component's typed props, so excess-property-checked out of a withProps literal. Pass at the call site.
• Anything that isn't a Mantine factory component — a react-icons glyph, another library's component, or a first-party plain export function.

State and effects

• Never reset or re-sync local state from a prop inside a useEffect. useEffect(() => setX(prop), [prop]) paints the stale value first and renders twice; it is an error under react-hooks/set-state-in-effect.
• Use useValueChange(value, onChange) (src/hooks/useValueChange.ts) — React's documented "adjusting state during render" pattern. It does not fire on the first render; seed the state with useState. The comparison is Object.is, so pass a referentially stable value — a primitive key (id/name/URI) or a memoized one, never a fresh object literal.
• The onChange runs during render, so it must be pure — setState and nothing else. No fetches, DOM writes, logging, ref mutation, or parent callbacks; a render can be replayed or abandoned.
• Effects remain correct for real external-system synchronization (DOM measurement, rAF, subscriptions, timers).

Theme files vs. element components

Both exist and do different jobs. Theme files (src/theme/<Component>.ts) customize a Mantine primitive app-wide. Element components (src/components/elements/) add domain semantics on top of primitives.

• Element components import from @mantine/core, not from src/theme/ — the theme layer is applied transparently by the provider.
• Never push domain-specific variant logic into theme files (annotation types, transport types, …). Domain variants belong to the element component that owns those semantics.

Where code goes (web client)

utils = functions that compute; lib = things that instantiate, adapt, or touch the environment. If it does I/O or wraps a subsystem it's lib; if it's a pure transform it's utils.

• src/utils/ — pure, side-effect-free. Also: pure shared domain types and their constructors; diagnostic console.warn/error does not count as a side effect; type-only imports from @inspector/core, and re-exporting pure functions/constants from core, are both fine.
• src/lib/ — infrastructure, integration, stateful adapters: composes subsystems, wraps the core runtime, touches DOM / window / sessionStorage, or produces side effects.
• Cross-directory imports go one way: lib → utils, never the reverse.
• src/types/ is only for ambient .d.ts module stubs — not a home for new domain types.
• ⚠️ The coverage include is a whitelist naming components / hooks / theme / lib / utils / server (plus the core/* runtime). A module placed outside those directories silently falls out of the ≥90% gate. Flag new top-level files or new grab-bag directories.

Dependency placement

• The MCP SDK packages (@modelcontextprotocol/client, core, server, server-legacy, ext-apps) belong in the root package.json only. Adding one to a clients/*/package.json is a review finding: Node resolution walks up, so the root install already serves every client, and a per-client entry installs a second copy that drifts (it produced two versions of ext-apps, and of the transitive v1 SDK, at once — #1970) and reintroduces the duplicate-copy failure vitest.shared.mts has a dedupe workaround for.
• The v1 @modelcontextprotocol/sdk is not a dependency of this repo and must not become one. It is a peer of ext-apps, present in lock files only.
• Dependencies reached only through root-owned code with no manifest (test-servers/src, core/) are declared at the root and aliased to the repo root in vitest.shared.mts — as express and yaml are — not to <client>/node_modules like the other pins there.
• A dependency that renders React components must be bundled into the client that uses it, and is then not a root dependency. An externalized package resolves its own react from wherever npm placed it in the consumer's tree, beside a React satisfying its peer range — looser than ours, which is all it takes to split React. ink-form/ink-scroll-view declare ">=18", so a consumer's React 18 satisfies them, the TUI ends up with two React instances, and it crashes on the first hook (#1952). Both are inlined via noExternal in clients/tui/tsup.config.ts. ink is the one exemption, justified by cost (~1.4MB) — never by a peer range: flag any claim that ">=19" keeps npm from misplacing it, which is false and was in this repo once. What keeps it safe is the root react range staying open to the whole major (^19.0.0) so npm can dedupe with a consumer's pinned React 19; treat narrowing that range as reopening the bug. clients/tui/__tests__/tsupConfig.test.ts enforces the split, the root-declaration of exempt packages, and that range.
• Which section is a separate question from which manifest. A package core/ imports at runtime must be in root dependencies: client builds externalize npm packages, so a published install resolves them from the root manifest and devDependencies are absent there. Only test/build-only packages (express) belong in devDependencies. Flag a runtime core/ import added to devDependencies — it passes every local check and breaks the published package.
• A root-declared package that core/ imports at runtime must also be named in each client's bundler external list — clients/{cli,tui}/tsup.config.ts and clients/web/tsup.runner.config.ts, all three. Bundlers externalize what the client's manifest declares, and these packages are root-only by rule, so omitting them means they get bundled. For a CJS package inlined into an ESM bundle that is fatal: esbuild's Dynamic require of "path" is not supported shim throws at import time and the binary dies before parsing a flag (#2082, proper-lockfile). Flag a new root runtime dependency that is not added to all three. This has broken twice — undici slipped past #2082 because it was declared in clients/cli/package.json, so the CLI looked fine while the web and TUI bundles inlined 1.05MB of it, unloadably (#2067). npm run verify:bundle-externals now guards it from the built output; flag any change that would weaken or skip that guard.
• Pin a transitive dependency past its parent's declared range with an overrides entry — never with npm audit fix. When the advisory range has no upward escape inside the parent's range, npm audit fix resolves it by silently downgrading (esbuild 0.27.7 → 0.27.2 across three installs, #2058/#2062). Flag that in a diff. An override puts a bundler on a major it does not declare, so the gate for one is npm run build, not npm audit; a lockfile diff accompanying it should touch that package's entries only. Flag an override added without a note saying when it can be dropped.

Tests and the coverage gate

• All new or modified code needs tests. The per-file gate is ≥ 90% on all four dimensions — lines, statements, functions, and branches — enforced in CI for clients/{web,cli,tui,launcher} and the gated core/ runtime.
• Never lower the gate to accommodate an unreachable branch. Annotate at the source with a justified /* v8 ignore … -- <reason> */. Acceptable reasons: happy-dom-inherent paths (Mantine portal mounts, useMediaQuery fallbacks, typeof window SSR guards), React StrictMode effect-replay blocks, and provably-dead defensive guards. Anything else is a missing test.
• Suppress expected error output from the console in tests that exercise error paths.

Test placement

• Web: side-by-side by default — <Name>.test.tsx next to the source. A web-owned test under src/test/ instead of beside its source is a bug. src/test/ is only for what can't be co-located: tests of the repo-root core/ package (src/test/core/…, mirroring core's layout), the integration project (src/test/integration/… — placement is the manifest), and shared test infrastructure.
• CLI / TUI / launcher: all tests live in a top-level __tests__/ directory. A co-located src/**/*.test.* lands in no tsconfig project and fails verify:typecheck-coverage.

Rendering components in tests

• Always render through renderWithMantine from src/test/renderWithMantine.tsx. A hand-rolled bare MantineProvider skips the project theme and the helper's options, and drifts from every other test. (It does not reintroduce the timer-leak class — an older version of this rule said so; the leaked-timer net in setup.ts is global and covers every unit test regardless of how it renders.)
• For a forced color scheme, pass the option — renderWithMantine(ui, { colorScheme: "dark" }) — rather than a hand-rolled defaultColorScheme provider.
• Only when asserting mid-flight transition state, use renderWithMantineTransitions, passing settleMs derived from the component's real animation duration. Do not combine it with vi.useFakeTimers(), and use the unmount() it returns if the test unmounts the tree itself.

Gates and PR hygiene

• npm run format before committing; npm run ci before pushing (validate → coverage → verify:build-gate → verify:bundle-externals → smoke → Storybook). npm run validate is the fast inner-loop check and is not a substitute — it runs test, not test:coverage, so it does zero coverage gating.

• A dependency bump must land in every install that declares it. v2 is not a workspace — the root and each clients/* have their own node_modules, and a client's test project compiles core/ and test-servers/src (which resolve from the root) alongside the client's own sources. Bumping a shared dependency in one manifest only puts two versions of it in one tsc program; for a recursive-generic surface like zod that exhausts the tsc heap (#1896). verify:dep-lockstep fails the build on this — it derives its candidates from what each tsc program actually resolves (tsc --listFilesOnly, keeping packages that reach one program from two installs), so a package reached only through another package's .d.ts counts too (#1965) — so a PR bumping a package the shared sources pull in should update the root and every client that already lists it — not every client unconditionally, since a package absent from an install can't skew and adding it there would be a spurious dependency.

• A test or smoke must not touch real user state. The web smokes run against a throwaway catalog via the shared scripts/lib/prod-web-server.mjs helper, never the developer's ~/.mcp-inspector/mcp.json (#1977); the cli/tui smokes drive a temp --catalog; pack:verify's --web child sets its own MCP_CATALOG_PATH for the same reason (#2003 — its App deep link persists a server row). Anything that boots the web backend and then navigates it needs that isolation, not just the scripts named smoke:*. A new smoke spawning its own server, or teardown that removes a work dir without first awaiting stopChild (the #1801 race — child-cleanup.mjs exports both halves and both are required), should be flagged.

• The headless web smokes are engine-parameterized, not Chromium-only. smoke:web:browser / smoke:web:app / smoke:web:elicit take their engine from SMOKE_BROWSER (chromium — the default — firefox, webkit) and CI covers Chromium plus Firefox (#2086; WebKit runs locally but is out of the matrix pending #2132, a real Safari SSE hang it found), because the MCP Apps sandbox is built out of the primitives that diverge between engines: srcdoc CSP inheritance, nested sandboxed iframes, Permissions-Policy, cross-frame postMessage. Nothing else covers that — sandbox-csp.test.ts asserts a policy string, and no Storybook story reaches the sandbox at all. Flag a new browser-driven script that launches Playwright itself instead of going through scripts/lib/headless-browser.mjs, an unrecognized-engine path that falls back to Chromium rather than failing (it would claim coverage that never ran), or a launch-failure message naming the wrong engine. pack:verify is pinned to Chromium on purpose — it is a packaging check.

• Build output is never a gate target. Lint, format, and typecheck read first-party source only; everything a build writes (clients/*/build, clients/web/dist, storybook-static, coverage, test-servers/build, core/**/{build,dist}, *.tsbuildinfo) stays out via each scope's globalIgnores, format globs, and tsconfig include. Gating generated code reports defects in vendored third-party source that nobody can fix, and a rule promotion turns that warning into a validate failure (#2043). Flag a PR that adds a build location without ignoring it in the same change, that widens an ignore to silence a finding in first-party code, or that adds a build directory to a tsconfig include to make a generated .d.ts resolve. Note the coverage guards don't catch this — they assert source is covered, not that output is excluded.

• Lint has no warning tier. Every lint script runs --max-warnings 0, so a warning fails validate exactly as an error does (#2085) — a warn-level react-hooks/exhaustive-deps finding otherwise let a stale-closure bug pass the pre-push gate and reach review. Flag a PR that silences a finding to make the gate pass (widening a globalIgnores, dropping a rule, or an inline disable with no justification comment); the fix is the defect, not the message. A rule meant to be enforced should be set to error rather than left at warn and carried by the flag.

• Every PR references an issue, first body line Closes #<ISSUE_NUMBER>.

• Every PR carries exactly one version label, v1 or v2, matching its base branch.

• Commits carry a Signed-off-by: trailer. The DCO check is a hard merge gate and fails on a single unsigned commit; it matches the trailer against the author or committer, and skips only merge and bot commits. Use git commit -s — note format.signOff does not sign git commit (only format-patch). Repairing pushed commits means git rebase HEAD~<n> --signoff + git push --force-with-lease; remediation commits are not enabled on this repo.

• Update the relevant README.md / AGENTS.md when a change adds, removes, renames, or repurposes a file or folder, changes the structure or tech stack, or introduces a command, dependency, or architectural pattern.

What to prioritize in review

1. Correctness and security — this backend spawns local processes and proxies outbound requests, so anything touching auth, origin validation, host binding, or the proxy's SSRF controls deserves close reading.
2. Type-safety violations (any, suppressions, unjustified double casts).
3. Missing or thin tests against the ≥90% four-dimension gate, and modules placed outside the gated directories.
4. Mantine convention violations — inline styles, raw colors, unnecessary CSS classes, missing .withProps() extraction.
5. Docs that contradict the change.