Free JSON Schema to Protobuf Converter (StopLight Alternative)

Converting JSON Schema to Protocol Buffers shouldn't require a $149/year subscription or uploading your API definitions to a server.

I built a free, browser-based tool that does exactly this — and it runs entirely client-side. Your schemas never leave your device.

Why This Exists

When I was migrating a REST API to gRPC, I needed to convert my existing JSON Schema definitions into .proto files. Most online tools:

• Upload your schema to their server (privacy risk for proprietary APIs)
• Require signup or payment
• Don't handle nested objects or discriminated unions properly
• Generate incomplete or incorrect output

So I built a simple, free converter that handles the common cases:

What It Supports

• Basic types — strings, integers, numbers, booleans mapped to their Protobuf equivalents
• Nested objects — converted to nested Protobuf messages
• Arrays — mapped to repeated fields
• Enums — converted to Protobuf enum blocks
• Discriminated unions — oneOf with discriminator becomes Protobuf oneof
• $ref references — generates proper import statements

How It Works

Pure JavaScript, zero dependencies. Everything runs in your browser — there's no backend, no API calls, no tracking.

Built as part of a series of free developer tools for API engineers who need simple utilities without the overhead of full-featured (and expensive) platforms like StopLight.

Update

This specific tool has since been retired and is no longer hosted. For other free, browser-based developer tools from the same project, see tools.jackgreen.top.