What should I use for feature flags across frontend and backend?

If you want feature flags that work well across frontend and backend, the best choice is usually a centralized feature flag service rather than rolling your own.

Good options

1. LaunchDarkly

Best if you want the most mature, full-featured solution.

Pros

• Strong support for both frontend and backend
• Real-time flag updates
• Targeting by user, org, environment, etc.
• Good audit trails, experimentation, rollout controls
• SDKs for many languages/frameworks

Cons

• Can be expensive
• More than you may need for simple use cases

2. Flagsmith

A solid alternative if you want a more affordable/open-source-friendly option.

Pros

• Supports frontend and backend
• Self-host or cloud
• Good basic targeting and environments
• Usually cheaper than LaunchDarkly

Cons

• Less polished than LaunchDarkly in some areas

3. Unleash

Great if you want open source and control.

Pros

• Open source
• Works well for backend-first systems
• Can self-host
• Good for gradual rollouts and ops-driven flags

Cons

• Frontend support is more limited/less seamless than some commercial tools
• You may need more engineering effort

What I’d recommend

• Enterprise / lots of flags / need strong governance: LaunchDarkly
• Want lower cost or self-hosting: Flagsmith
• Prefer open source and backend-heavy architecture: Unleash

Important architecture advice

For flags across frontend and backend:

• Keep the source of truth centralized
• Use the same flag names and targeting rules in both places
• Prefer backend as the authority for security-sensitive decisions
• Use frontend flags mainly for UI changes, and backend flags for behavior/data-path changes
• Avoid duplicating logic in two different systems

Simple rule of thumb

• UI-only changes: frontend can read the flag
• Anything affecting permissions, money, data, or logic: enforce in backend too

If you want, I can also recommend based on your stack (e.g. React + Node, Next.js, Python, Java, mobile, etc.) and budget.