This is a Kotlin Multiplatform line-by-line transliteration port of GREsau/schemars.

Original Project: This port is based on GREsau/schemars. All design credit and project intent belong to the upstream authors; this repository is a faithful port to Kotlin Multiplatform with no behavioural changes intended.

This is an in-progress port. The goal is feature parity with the upstream Rust crate while providing a native Kotlin Multiplatform API. Every Kotlin file carries a // port-lint: source <path> header naming its upstream Rust counterpart so the AST-distance tool can track provenance.

The text below is reproduced and lightly edited from https://github.com/GREsau/schemars. It is the upstream project's own description and remains under the upstream authors' authorship; links have been rewritten to absolute upstream URLs so they continue to resolve from this repository.

Generate JSON Schema documents from Rust code

For more detailed information, see the full API documentation on docs.rs, and the detailed usage documentation website.

If you don't really care about the specifics, the easiest way to generate a JSON schema for your types is to #[derive(JsonSchema)] and use the schema_for! macro. All fields of the type must also implement JsonSchema - Schemars implements this for many standard library types.

use schemars::{schema_for, JsonSchema};

#[derive(JsonSchema)]
pub struct MyStruct {
    pub my_int: i32,
    pub my_bool: bool,
    pub my_nullable_enum: Option<MyEnum>,
}

#[derive(JsonSchema)]
pub enum MyEnum {
    StringNewType(String),
    StructVariant { floats: Vec<f32> },
}

let schema = schema_for!(MyStruct);
println!("{}", serde_json::to_string_pretty(&schema).unwrap());

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "MyStruct",
  "type": "object",
  "properties": {
    "my_bool": {
      "type": "boolean"
    },
    "my_int": {
      "type": "integer",
      "format": "int32"
    },
    "my_nullable_enum": {
      "anyOf": [
        {
          "$ref": "#/$defs/MyEnum"
        },
        {
          "type": "null"
        }
      ]
    }
  },
  "required": ["my_int", "my_bool"],
  "$defs": {
    "MyEnum": {
      "oneOf": [
        {
          "type": "object",
          "properties": {
            "StringNewType": {
              "type": "string"
            }
          },
          "additionalProperties": false,
          "required": ["StringNewType"]
        },
        {
          "type": "object",
          "properties": {
            "StructVariant": {
              "type": "object",
              "properties": {
                "floats": {
                  "type": "array",
                  "items": {
                    "type": "number",
                    "format": "float"
                  }
                }
              },
              "required": ["floats"]
            }
          },
          "additionalProperties": false,
          "required": ["StructVariant"]
        }
      ]
    }
  }
}

One of the main aims of this library is compatibility with Serde. Any generated schema should match how serde_json would serialize/deserialize to/from JSON. To support this, Schemars will check for any #[serde(...)] attributes on types that derive JsonSchema, and adjust the generated schema accordingly.

use schemars::{schema_for, JsonSchema};
use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(rename_all = "camelCase", deny_unknown_fields)]
pub struct MyStruct {
    #[serde(rename = "myNumber")]
    pub my_int: i32,
    pub my_bool: bool,
    #[serde(default)]
    pub my_nullable_enum: Option<MyEnum>,
}

#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(untagged)]
pub enum MyEnum {
    StringNewType(String),
    StructVariant { floats: Vec<f32> },
}

let schema = schema_for!(MyStruct);
println!("{}", serde_json::to_string_pretty(&schema).unwrap());

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "MyStruct",
  "type": "object",
  "properties": {
    "myBool": {
      "type": "boolean"
    },
    "myNullableEnum": {
      "anyOf": [
        {
          "$ref": "#/$defs/MyEnum"
        },
        {
          "type": "null"
        }
      ],
      "default": null
    },
    "myNumber": {
      "type": "integer",
      "format": "int32"
    }
  },
  "additionalProperties": false,
  "required": ["myNumber", "myBool"],
  "$defs": {
    "MyEnum": {
      "anyOf": [
        {
          "type": "string"
        },
        {
          "type": "object",
          "properties": {
            "floats": {
              "type": "array",
              "items": {
                "type": "number",
                "format": "float"
              }
            }
          },
          "required": ["floats"]
        }
      ]
    }
  }
}

#[serde(...)] attributes can be overriden using #[schemars(...)] attributes, which behave identically (e.g. #[schemars(rename_all = "camelCase")]). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.

If you want a schema for a type that can't/doesn't implement JsonSchema, but does implement serde::Serialize, then you can generate a JSON schema from a value of that type. However, this schema will generally be less precise than if the type implemented JsonSchema - particularly when it involves enums, since schemars will not make any assumptions about the structure of an enum based on a single variant.

use schemars::schema_for_value;
use serde::Serialize;

#[derive(Serialize)]
pub struct MyStruct {
    pub my_int: i32,
    pub my_bool: bool,
    pub my_nullable_enum: Option<MyEnum>,
}

#[derive(Serialize)]
pub enum MyEnum {
    StringNewType(String),
    StructVariant { floats: Vec<f32> },
}

let schema = schema_for_value!(MyStruct {
    my_int: 123,
    my_bool: true,
    my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
});
println!("{}", serde_json::to_string_pretty(&schema).unwrap());

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "MyStruct",
  "examples": [
    {
      "my_bool": true,
      "my_int": 123,
      "my_nullable_enum": {
        "StringNewType": "foo"
      }
    }
  ],
  "type": "object",
  "properties": {
    "my_bool": {
      "type": "boolean"
    },
    "my_int": {
      "type": "integer"
    },
    "my_nullable_enum": true
  }
}

Schemars follows semantic versioning, with the following caveats:

• Increasing MSRV (Minimum Supported Rust Version) is considered a semver-minor change. Schemars aims to support the past year of stable rust versions, but this is not guaranteed.
• External libraries that are supported via optional dependencies (see Feature Flags) may be removed in a minor version change, particularly if a newer semver-incompatible version has been released for a long time.
• The exact structure of generated schemas (both for built-in implementations on standard library types, and for #[derive(JsonSchema)] implementations) may change between versions of schemars - this is not considered a breaking change.
• Exported items that are marked with #[doc(hidden)] and have names beginning with _ are not part of the public API, and may be changed or removed without notice.
• If a bug is found in schemars that causes attributes to be incorrectly processed or silently ignored by #[derive(JsonSchema)], a subsequent version of schemars may instead fail compilation when encountering such attributes. This is considered a bug fix, and not a breaking change.

• std (enabled by default) - implements JsonSchema for types in the rust standard library (JsonSchema is still implemented on types in core and alloc, even when this feature is disabled). Disable this feature to use schemars in no_std environments.
• derive (enabled by default) - provides #[derive(JsonSchema)] macro
• preserve_order - keep the order of struct fields in Schema properties
• raw_value - implements JsonSchema for serde_json::value::RawValue (enables the serde_json raw_value feature)

Schemars can implement JsonSchema on types from several popular crates, enabled via feature flags (dependency versions are shown in brackets):

• arrayvec07 - arrayvec (^0.7)
• bigdecimal04 - bigdecimal (^0.4)
• bytes1 - bytes (^1.0)
• chrono04 - chrono (^0.4)
• either1 - either (^1.3)
• indexmap2 - indexmap (^2.0)
• jiff02 - jiff (^0.2)
• rust_decimal1 - rust_decimal (^1.0)
• semver1 - semver (^1.0.9)
• smallvec1 - smallvec (^1.0)
• smol_str02 - smol_str (^0.2.1)
• smol_str03 - smol_str (^0.3)
• url2 - url (^2.0)
• uuid1 - uuid (^1.0)

Bear in mind that each of these feature flags may be removed in a future semver-minor change of Schemars, particularly if a newer semver-incompatible version of the external library has been released for a long time. This is unfortunately necessary to avoid supporting old/unmaintained libraries indefinitely.

For example, to implement JsonSchema on types from chrono, enable it as a feature in the schemars dependency in your Cargo.toml like so:

[dependencies]
schemars = { version = "1.0", features = ["chrono04"] }

dependencies {
    implementation("io.github.kotlinmania:schemars-kotlin:0.1.2")
}

./gradlew build
./gradlew test

• macOS arm64
• Linux x64
• Windows mingw-x64
• iOS arm64 / simulator-arm64 (Swift export + XCFramework)
• JS (browser + Node.js)
• Wasm-JS (browser + Node.js)
• Android (API 24+)

See AGENTS.md and CLAUDE.md for translator discipline, port-lint header convention, and Rust → Kotlin idiom mapping.

This Kotlin port is distributed under the same MIT license as the upstream GREsau/schemars. See LICENSE (and any sibling LICENSE-* / NOTICE files mirrored from upstream) for the full text.

Original work copyrighted by the schemars authors.
Kotlin port: Copyright (c) 2026 Sydney Renee and The Solace Project.

Thanks to the GREsau/schemars maintainers and contributors for the original Rust implementation. This port reproduces their work in Kotlin Multiplatform; bug reports about upstream design or behavior should go to the upstream repository.