Dev Toolbar Review Agent Skill

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.