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+---
+name: dev-toolbar-review
+description: Use when reviewing PRs that touch packages/dev-tools/, packages/common/posthog-client.ts,
+  or packages/common/feature-flags.tsx. Covers environment guards, flag override cookies,
+  telemetry event subscription, and SSE stream safety.
+---
+
+# Dev Toolbar Review Guide
+
+Review checklist for PRs touching the dev toolbar (`packages/dev-tools/`) and its
+integration points in `packages/common/`. The toolbar surfaces telemetry events and
+allows feature flag overrides during local development (expanding to staging/preview).
+
+## When This Applies
+
+PRs modifying any of these paths need growth eng review:
+
+- `packages/dev-tools/**` (owned by `@supabase/growth-eng` in CODEOWNERS)
+- `packages/common/posthog-client.ts` (flag override reads, event subscription)
+- `packages/common/feature-flags.tsx` (flag override merge logic)
+- App-level mounting: `DevToolbarProvider`/`DevToolbar`/`DevToolbarTrigger` in `apps/studio/`, `apps/www/`, `apps/docs/`
+
+Note: `posthog-client.ts` and `feature-flags.tsx` are NOT in CODEOWNERS for growth-eng,
+so PRs touching only those files won't auto-request review. Watch for these in the PR feed.
+
+## Review Checklist
+
+### 1. Environment Guards
+
+**Files:** `packages/dev-tools/index.ts`, `DevToolbar.tsx`, `DevToolbarTrigger.tsx`, `DevToolbarContext.tsx`
+
+The toolbar uses two layers of protection:
+- **Build-time tree-shaking** in `index.ts`: `process.env.NODE_ENV !== 'development'` ternaries that replace components with noops/stubs so the implementation is eliminated from production bundles.
+- **Runtime guards** in components: `IS_LOCAL_DEV` checks — `DevToolbar` and `DevToolbarTrigger` return `null` to hide themselves, while `DevToolbarProvider` passes children through (`<>{children}</>`) to preserve the component tree.
+
+**Check for:**
+- Guards being removed or broadened. The toolbar is expanding to staging and preview deploys but must remain invisible in production.
+- Tree-shaking ternaries in `index.ts` staying intact — these are the primary production safety mechanism.
+- New components or exports that bypass the existing guard pattern.
+
+### 2. Flag Override Cookies
+
+**Files:** `packages/dev-tools/DevToolbar.tsx`, `packages/common/posthog-client.ts`, `packages/common/feature-flags.tsx`
+
+The toolbar writes two cookies that override feature flags locally:
+- `x-ph-flag-overrides` — PostHog flag overrides
+- `x-cc-flag-overrides` — ConfigCat flag overrides
+
+These are read by:
+- `posthog-client.ts:getFeatureFlag()` — checks the PostHog override cookie before querying the SDK
+- `feature-flags.tsx` — merges both override cookies into the flag store during initialization
+
+**Check for:**
+- Cookie name changes (must stay in sync across writer and all readers)
+- Changes to the merge/precedence logic in `feature-flags.tsx` (currently: `vercel-flag-overrides` first, then `x-cc-flag-overrides` takes precedence in local dev)
+- Override cookies being read outside the `IS_LOCAL_DEV` / `isLocalDev` guard — overrides must never affect production flag evaluation
+- Changes to `parseOverrideValue` or `valuesAreEqual` in `packages/dev-tools/utils.ts` that could cause type coercion bugs
+
+### 3. Telemetry Event Subscription
+
+**Files:** `packages/common/posthog-client.ts`, `packages/dev-tools/DevToolbarContext.tsx`
+
+The toolbar subscribes to client-side PostHog events via `posthogClient.subscribeToEvents()`.
+The PostHog client calls `emitToDevListeners()` after `capturePageView`, `capturePageLeave`,
+and `identify`. Note: `captureExperimentExposure` calls `posthog.capture()` directly
+without emitting to dev listeners — experiment exposure events are invisible in the toolbar.
+
+**Check for:**
+- Changes to `emitToDevListeners` or `subscribeToEvents` that could introduce side effects on the actual capture path (e.g., throwing errors, blocking, mutating event data)
+- The listener set (`devListeners`) being iterated synchronously in a way that could delay event dispatch
+- New PostHog client methods that capture events but don't call `emitToDevListeners` (gap in toolbar visibility)
+
+### 4. SSE Server Telemetry Stream
+
+**Files:** `packages/dev-tools/DevToolbarContext.tsx`
+
+The toolbar connects to `${apiUrl}/telemetry/stream` via Server-Sent Events to display
+server-side telemetry. Uses exponential backoff on connection errors.
+
+**Check for:**
+- Changes to the SSE endpoint URL or `session_id` cookie handling
+- Reconnection logic changes that could cause excessive retries or connection leaks
+- Note: the stream endpoint lives in the platform repo — cross-repo changes need coordinated review
+
+### 5. App-Level Mounting
+
+**Provider + toolbar panel** (`DevToolbarProvider`, `DevToolbar`):
+- `apps/studio/pages/_app.tsx`
+- `apps/www/pages/_app.tsx`, `apps/www/app/providers.tsx`
+- `apps/docs/features/app.providers.tsx`
+
+**Trigger button** (`DevToolbarTrigger`) — rendered separately in nav/header components:
+- `apps/studio/components/layouts/Navigation/LayoutHeader/LayoutHeader.tsx`
+- `apps/www/components/Nav/index.tsx`
+- `apps/docs/components/Navigation/NavigationMenu/TopNavBar.tsx`
+
+**Check for:**
+- Provider being added or removed from an app
+- `apiUrl` prop changes (must point to the correct platform API)
+- Rendering order changes that could affect the toolbar's access to PostHog context
+
+## What Doesn't Need Growth Review
+
+Changes that are purely UI/UX within the toolbar panel itself — styling, layout, copy
+changes, drag behavior, popover positioning — don't need growth eng review unless they
+also touch the integration points above.