Top 3 Feature Flag Config Alternatives

You are running flags out of a config file rather than a hosted platform, which for a small team is a perfectly reasonable decision. What you need is a JSON block with a dozen flags in it, each with a default state and a rollout percentage, formatted consistently so the next person to add one does not invent a different shape.

Hand-writing that is fine for three flags and error-prone by twelve. The failure modes are boring and expensive: a trailing comma that breaks the parse, a rollout value entered as 50 where the code expects 0.5, a flag added with enabled where every other entry uses default. None of these are hard problems and all of them cost an afternoon when they reach production.

How to judge a config building approach

Does it know your shape, or just JSON? A generic editor validates syntax. It has no opinion about whether your flag entries are consistent with each other.

Does it prevent the common mistakes? Trailing commas, wrong types, and inconsistent keys are the three that actually happen.

Can you read the result? Config that a human can scan is config that gets reviewed properly.

Does the data leave your machine? Flag names frequently telegraph unreleased features.

The comparison

Facts checked August 2026; plans can change.

JSON Editor Online

JSON Editor Online is the most capable general tool in this space — formatting, beautifying, comparing two documents, and querying data, in a mature interface a lot of developers already know. The comparison feature is genuinely useful for flag configs specifically, because diffing yesterday’s config against today’s is how you catch an unintended change.

It is a JSON tool, not a flag tool. It will happily let you write a flag entry with a completely different key structure from the one above it, because both are valid JSON, and that inconsistency is the actual problem you were trying to avoid.

OnFlyTools JSON Creator

OnFlyTools lets you build JSON objects and arrays visually with drag-and-drop, real-time validation, and schema generation, which removes the syntax errors entirely — you cannot produce a trailing comma through a visual builder.

The trade-off is speed. Once you know the shape you want, clicking through a builder is slower than typing, and for a repetitive structure like a flag list the repetition is the tedious part rather than the syntax.

JSON Crack

JSON Crack renders JSON as a node graph, which is an unusually good fit for reviewing a config someone else wrote. A nested flag structure that reads as a wall of braces becomes something you can actually see the shape of.

It is built for visualising rather than authoring. As a review step before merging a config change it is excellent; as the place you write the config it is not what it is for.

Feature Flag JSON Builder

Ours is shaped around the specific structure. Add flag names, default states, and rollout percentages, and it generates a ready-to-use feature flag configuration JSON — every entry with the same keys, in the same order, with values in a consistent form. The consistency is the whole point; the JSON generation is incidental. It runs entirely in your browser, so unreleased flag names stay on your machine.

The limitations are worth being direct about. It produces a config in a common shape, not in your platform’s schema — if you are feeding this into a specific SDK or a bespoke internal format, you will be adapting the output rather than pasting it. It also generates a static file: there is no evaluation logic, no targeting rules, no user segmentation, and nothing that serves the flags at runtime. That is the difference between a config file and a flag platform, and if you need targeting by user attribute, a file is not going to be enough for long.

Which one to pick

• If you need to diff two versions of a config, JSON Editor Online does that best.
• If you want to eliminate syntax errors entirely, OnFlyTools’ visual builder prevents them structurally.
• If you are reviewing a config someone else wrote, JSON Crack makes the shape visible.
• If you want a consistent flag config generated from a list of flags, use ours.

How to do it with Feature Flag JSON Builder

1. Open the Feature Flag JSON Builder.
2. Add each flag with its name, default state, and rollout percentage.
3. Copy the JSON and adapt the keys to your platform’s schema if it differs.
4. Diff it against the existing config before merging. More developer tools are in the tools directory.

You might also need

• Feature Toggle Naming Generator — for consistent names before you build the config.
• Feature Flag Config Diff — for checking what changed between two versions.

Frequently asked questions

Is there a free feature flag config builder that doesn’t need an account?

Yes — the JSON tools above are free to use, JSON Crack is open source, and ours requires no account because the site has no signup at all.

Should feature flags live in a config file or a platform?

A file is reasonable when flags are few, changes are infrequent, and a deploy to flip one is acceptable. Once you need runtime toggling, per-user targeting, or audit trails, a platform earns its cost. Martin Fowler’s article on feature toggles covers how toggle categories differ in what infrastructure they demand.

What does a rollout percentage actually do?

It defines what proportion of users see the flag as on, usually by hashing a stable user identifier so the same user gets a consistent result. The important detail is stability: a rollout that reassigns users on every evaluation produces a flickering experience rather than a gradual release.

Final thought

Generate the config rather than hand-writing it, then diff before merging. The syntax errors are the visible failure; the inconsistent keys are the one that bites in production.

Try the free Feature Flag JSON Builder