JSON to Protobuf Converter

How to use

1. 1Paste a JSON value into the input on the left, or click Load sample JSON to see a worked example.
2. 2Read the generated proto3 schema on the right; one message is emitted per nested object shape, with the root named after the Root message name field.
3. 3Pick the integer type (int64 is the safe default, sint64 for often-negative values) and the decimal type (double keeps full precision, float halves the size).
4. 4Toggle Mark missing fields optional so keys absent from some array elements use the proto3 optional keyword, and Detect ISO timestamps to map date-time strings to google.protobuf.Timestamp.
5. 5Optionally add a package line, then click Copy proto or Download .proto to use the schema with protoc, buf, or your gRPC toolchain.

About this tool

JSON to Protobuf Converter turns any JSON value into a proto3 .proto schema entirely in your browser. Paste an API response, a config file, a fixture, or a copied object literal, and the tool parses it with the browser's native JSON parser, walks the resulting tree, and emits one message per object shape with a syntax = "proto3" header. Scalar values map to proto3 scalar types: strings to string, booleans to bool, whole numbers to int64 (or int32, uint64, or sint64 depending on the integer strategy you choose), and numbers with a fractional part to double (or float). Field names are converted to lower_snake_case to match the protobuf style guide, while message names are PascalCase, and every field is assigned a sequential field number starting at 1 in declaration order, which is the wire tag protobuf uses to identify fields. Nested objects are promoted into their own named messages referenced by name, so the file reads top-down from the root message and matches the structure of hand-written proto. JSON arrays become repeated fields, with the element type inferred from the union of every element seen, so a list of objects produces a single merged message and a list of numbers produces a repeated scalar. When the original JSON key cannot be expressed verbatim as a snake_case identifier, the tool appends a [json_name = "originalKey"] field option so the generated message round-trips the exact JSON your service already returns. Because proto3 has no top-level non-message values, a JSON document whose root is an array or a scalar is wrapped in a single-field message so the output stays valid. There are options for the integer type (uint64 is only used when no sampled value is negative, otherwise it falls back to int64), the decimal type, the placeholder type for empty arrays and mixed unions (a plain string or google.protobuf.Value from struct.proto), whether keys that are missing from some array elements get the proto3 optional keyword, the indent width, and an optional package declaration. ISO 8601 date-time strings can be detected and typed as google.protobuf.Timestamp with the import added automatically. The result is ready to copy or download as a .proto file you can feed to protoc, buf, or any gRPC toolchain. Parsing, type inference, and code generation all run locally on your device, so the JSON you paste here is never uploaded.

Free to use. Works in your browser. No signup, no login.