Fuel Specifications

Icon HelpCircleForumIcon Link

⌘K

Icon HelpCircleForumIcon Link

Docs /

Specs /

ABI /

Json ABI Format

Icon LinkJSON ABI Format

The JSON of an ABI is the human-readable representation of the interface of a Sway contract.

Icon LinkSpec Version

Current specVersion is 1.0

The version above should be updated each time this spec changes and the JSON ABI generator should be updated with the new changes along with an increment in the spec version.

Icon LinkNotation

Before describing the format of the JSON ABI, we provide some definitions that will make the JSON ABI spec easier to read.

Given the example below:

struct Foo { x: bool }
struct Bar<T> { y: T }
 
fn baz(input1: Foo, input2: Bar<u64>); // an ABI function

we define the following expressions:

type concrete declaration: the declaration or definition of a type which can be generic. struct Foo { .. } and struct Bar<T> { .. } in the example above are both type declarations. Note that generic types may have multiple type concrete declaration.
type metadata declaration: the declaration or definition of a type which can be generic. struct Foo { .. } and struct Bar<T> { .. } in the example above are both type declarations. The metadata declaration contains component details about the type. And a single type metadata declaration is generated per type, even for generic types.
type application: the application or use of a type. Foo and Bar<u64> in fn baz(input1: Foo, input2: Bar<u64>); in the example above are both applications of the type declarations struct Foo { .. } and struct Bar<T> { .. } respectively.
type parameter: a generic parameter used in a type declaration. T in struct Bar<T> in the example above is a type parameter.
type argument: an application of a type parameter used in a type application. u64 in input2: Bar<u64> in the example above is a type argument.

Icon LinkJSON ABI Spec

The ABI of a contract is represented as a JSON object containing the following properties:

"specVersion": a string representing the version pointing to this document versioning. specVersion enables the reader of the JSON ABI to find the correct specification for that file, this can be done by comparing it to value in spec version .
"encodingVersion": a string representing the version of the ABIEncode and ABIDecode used in this program.
"programType": a string that can be "script", "contract", "predicate", "library". This is used by the SDK to generate types without having to manually specify the program type.
"concreteTypes": an array describing all the type concrete declarations used (or transitively used) in the ABI. Each type concrete declaration is a JSON object that contains the following properties:
- "type": a string representing the type, the sha256 of this string generates the concreteTypeId.
- "concreteTypeId": a unique string hash based ID. Generated as specified in Hash Based Ids .
- "metadataTypeId": the type metadata declaration ID of this type, if the type metadata has components or is generic, otherwise nonexistent.
- "typeArguments": an array of type concrete declarations hash based IDs of the type parameters of the type, if the type is generic, otherwise nonexistent.
"metadataTypes": an array describing all the type metadata declarations used (or transitively used) in the ABI. Each type metadata declaration is a JSON object that contains the following properties:
- "type": a string representation of the type metadata declaration. The section JSON ABI Format for Each Possible Metadata Type Declaration specifies the format for each possible type.
- "metadataTypeId": a unique integer ID.
- "components": an array of the components of a given type, if any, otherwise nonexistent. Each component is a type application represented as a JSON object that contains the following properties:
  - "name": the name of the component.
  - "typeId": the type metadata declaration ID (number) or type concrete declaration hash based ID (string) of the type of the component.
  - "typeArguments": an array of the type arguments used when applying the type of the component, if the type is generic, otherwise nonexistent. Each type argument is a type application represented as a JSON object that contains the following properties:
    - "typeId": the type metadata declaration ID (number) or type concrete declaration hash based ID (string) of the type of the component.
    - "typeArguments": an array of the type arguments used when applying the type of the type argument, if the type is generic, otherwise nonexistent. The format of the elements of this array recursively follows the rules described in this section.
- "typeParameters": an array of type metadata declaration ID of the type parameters of the type, if the type is generic, otherwise nonexistent. Each type parameter is a type declaration and is represented as described in Generic Type Parameter .
"functions": an array describing all the functions in the ABI. Each function is a JSON object that contains the following properties:
- "name": the name of the function
- "inputs": an array of objects that represents the inputs to the function (i.e. its parameters). Each input is a type application represented as a JSON object that contains the following properties:
  - "name": the name of the input.
  - "concreteTypeId": the type concrete declaration hash based ID of the type of the input.
- "output": the type concrete declaration hash based ID of the type being returned by the function.
- "attributes": an optional array of attributes. Each attribute is explained in the dedicated section and is represented as a JSON object that contains the following properties:
  - "name": the name of the attribute.
  - "arguments": an array of attribute arguments.
"loggedTypes": an array describing all instances of log or logd in the contract's bytecode. Each instance is a JSON object that contains the following properties:
- "logId": a string containing the 64bit hash based decimal ID calculated from the first 8 bytes of the sha256 of a string that represents the type logged as defined in Hash Based Ids . The log and logd instructions must set their $rB register to that ID.
- "concreteTypeId": the type concrete declaration hash based ID of the value being logged.
"messagesTypes": an array describing all instances of smo in the contract's bytecode. Each instance is a JSON object that contains the following properties:
- "message_id": a unique string ID.
- "concreteTypeId": the type concrete declaration hash based ID of the message data being sent.
"configurables": an array describing all configurable variables used in the contract. Each configurable variable is represented as a JSON object that contains the following properties:
- "name": the name of the configurable variable.
- "concreteTypeId": the type concrete declaration hash based ID of the type of the configurable variable.
- "offset": the specific offset within the contract's bytecode, in bytes, to the data section entry for the configurable variable.

Icon InfoCircle
Note: This JSON should be both human-readable and parsable by the tooling around the FuelVM and the Sway programming language. There is a detailed specification for the binary encoding backing this readable descriptor. The Function Selector Encoding section specifies the encoding for the function being selected to be executed and each of the argument types.

Icon LinkAttributes Semantics

Attribute name	Attribute arguments	Semantics
`storage`	`read` and/or `write`	Specifies if a function reads or writes to/from storage
`payable`	None	Specifies if a function can accept coins: a function without `payable` attribute must not accept coins
`test`	None	Specifies if a function is a unit test
`inline`	`never` or `always`, but not both	Specifies if a function should be inlined during code generation
`doc-comment`	String	Documentation comment
`doc`	Not defined yet	Not defined yet

Icon LinkA Simple Example

Below is a simple example showing how the JSON ABI for an example that does not use generic or complex types. We will later go over more complex examples.

abi MyContract {
    fn first_function(arg: u64) -> bool;
    fn second_function(arg: b256);
}

the JSON representation of this ABI looks like:

{
  "concreteTypes": [
    {
      "type": "u64",
      "concreteTypeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0"
    },
    {
      "type": "b256",
      "concreteTypeId": "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b"
    },
    {
      "type": "bool",
      "concreteTypeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903"
    },
    {
      "type": "()",
      "concreteTypeId": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d"
    }
  ],
  "metadataTypes": [],
  "functions": [
    {
      "inputs": [
        {
          "name": "arg",
          "concreteTypeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0"
        }
      ],
      "name": "first_function",
      "output": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903"
    },
    {
      "inputs": [
        {
          "name": "arg",
          "concreteTypeId": "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b"
        }
      ],
      "name": "second_function",
      "output": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d"
    }
  ],
  "loggedTypes": []
}

Icon LinkJSON ABI Format for Each Possible Metadata Type Declaration

Below is a list of the JSON ABI formats for each possible metadata type declaration:

Icon Link`struct`

{
  "metadataTypeId": <id>,
  "type": "struct <struct_name>",
  "components": [
    {
      "name": "<field1_name>",
      "typeId": "<field1_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    {
      "name": "<field2_name>",
      "typeId": "<field2_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    ...
  ],
  "typeParameters": [
    <type_param1_type_id>,
    <type_param2_type_id>,
    ...
  ]
}

Icon Link`enum`

{
  "metadataTypeId": <id>,
  "type": "enum <enum_name>",
  "components": [
    {
      "name": "<variant1_name>",
      "typeId": "<variant1_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    {
      "name": "<variant2_name>",
      "typeId": "<variant2_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    ...
  ],
  "typeParameters": [
    <type_param1_type_id>,
    <type_param2_type_id>,
    ...
  ]
}

Icon Link`array`

{
  "metadataTypeId": <id>,
  "type": "[_; <n>]",
  "components": [
    {
      "name": "__array_element",
      "typeId": "<element_type>",
      "typeArguments": ...
    }
    {
      "name": "__array_element",
      "typeId": "<element_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
  ]
}

<n> is the size of the array.

Icon Link`tuple`

{
  "metadataTypeId": <id>,
  "type": "(_, _, ...)",
  "components": [
    {
      "name": "__tuple_element",
      "typeId": "<field1_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    {
      "name": "__tuple_element",
      "typeId": "<field2_type_id>",
      "typeArguments": [
        {
          "typeId": "<type_arg1_type_id>",
          "typeArguments": ...
        },
        {
          "typeId": "<type_arg2_type_id>",
          "typeArguments": ...
        },
        ...
      ]
    },
    ...
  ]
}

Icon LinkGeneric Type Parameter

{
  "metadataTypeId": <id>,
  "type": "generic <name>"
}

<name> is the name of the generic parameter as specified in the struct or enum declaration that uses it.

Icon LinkSome Complex Examples

Icon LinkAn Example with Non-Generic Custom Types

Given the following ABI declaration:

enum MyEnum {
    Foo: u64,
    Bar: bool,
}
 
struct MyStruct {
    bim: u64,
    bam: MyEnum,
}
 
abi MyContract {
    /// this is a doc comment
    #[payable, storage(read, write)]
    fn complex_function(
        arg1: ([str[5]; 3], bool, b256),
        arg2: MyStruct,
    );
}

its JSON representation would look like:

{
  "concreteTypes": [
    {
      "type": "([str[5]; 3], bool, b256)",
      "concreteTypeId": "625531542be70834dd127e771101ac1014111718451bfae996d97abe700c66a5",
      "metadataTypeId": 1,
    },
    {
      "type": "str[5]",
      "concreteTypeId": "84877f6e98274b9e4721db68b4c0bdb9e52b8e9572c5bd7811c07a41ced882c7",
    },
    {
      "type": "struct MyStruct",
      "concreteTypeId": "392d58c694d2d91f3025f2bccfadacf2a105936f5da881b0899185d49f264522",
      "metadataTypeId": 4,
    },
    {
      "type": "u64",
      "concreteTypeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0"
    },
    {
      "type": "b256",
      "concreteTypeId": "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b"
    },
    {
      "type": "bool",
      "concreteTypeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903"
    },
    {
      "type": "()",
      "concreteTypeId": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d"
    }
  ],
  "metadataTypes": [
    {
      "metadataTypeId": 1,
      "type": "(_, _, _)",
      "components": [
        {
          "name": "__tuple_element",
          "typeId": 2,
        },
        {
          "name": "__tuple_element",
          "typeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903",
        },
        {
          "name": "__tuple_element",
          "typeId": "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b",
        }
      ]
    },
    {
      "metadataTypeId": 2,
      "type": "[_; 3]",
      "components": [
        {
          "name": "__array_element",
          "typeId": "84877f6e98274b9e4721db68b4c0bdb9e52b8e9572c5bd7811c07a41ced882c7",
        }
      ]
    },
    {
      "metadataTypeId": 3,
      "type": "enum MyEnum",
      "components": [
        {
          "name": "Foo",
          "typeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0",
        },
        {
          "name": "Bar",
          "typeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903",
        }
      ]
    },
    {
      "metadataTypeId": 4,
      "type": "struct MyStruct",
      "components": [
        {
          "name": "bim",
          "typeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0",
        },
        {
          "name": "bam",
          "typeId": 3,
        }
      ]
    },
  ],
  "functions": [
    {
      "inputs": [
        {
          "name": "arg1",
          "concreteTypeId": "625531542be70834dd127e771101ac1014111718451bfae996d97abe700c66a5",
        },
        {
          "name": "arg2",
          "concreteTypeId": "392d58c694d2d91f3025f2bccfadacf2a105936f5da881b0899185d49f264522"
        }
      ],
      "name": "complex_function",
      "output": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d",
      "attributes": [
        {
          "name": "doc-comment",
          "arguments": [" this is a doc comment"]
        },
        {
          "name": "payable",
        },
        {
          "name": "storage",
          "arguments": ["read", "write"]
        }
      ]
    }
  ],
  "loggedTypes": []
}

Icon LinkAn Example with Generic Types

Given the following ABI declaration:

enum MyEnum<T, U> {
    Foo: T,
    Bar: U,
}
struct MyStruct<W> {
    bam: MyEnum<W, W>,
}
 
abi MyContract {
    fn complex_function(
        arg1: MyStruct<b256>,
    );
}

its JSON representation would look like:

{
  "concreteTypes": [
    {
      "type": "struct MyStruct<b256>",
      "concreteTypeId": "3ddd5c1768dd7869663dc2f868ea8a8ce68bd6064244dbc4286e2c921c8ce962",
      "metadataTypeId": 5,
      "typeArguments": [
        "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b"
      ]
    },
    {
      "type": "b256",
      "concreteTypeId": "7c5ee1cecf5f8eacd1284feb5f0bf2bdea533a51e2f0c9aabe9236d335989f3b",
    },
    {
      "type": "()",
      "concreteTypeId": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d",
    }
  ],
  "metadataTypes": [
    {
      "metadataTypeId": 1,
      "type": "enum MyEnum",
      "components": [
        {
          "name": "Foo",
          "typeId": 2,
        },
        {
          "name": "Bar",
          "typeId": 3,
        }
      ],
      "typeParameters": [2, 3]
    },
    {
      "metadataTypeId": 2,
      "type": "generic T",
    },
    {
      "metadataTypeId": 3,
      "type": "generic U",
    },
    {
      "metadataTypeId": 4,
      "type": "generic W",
    },
    {
      "metadataTypeId": 5,
      "type": "struct MyStruct",
      "components": [
        {
          "name": "bam",
          "typeId": 1,
          "typeArguments": [
            {
              "typeId": 4,
            },
            {
              "typeId": 4,
            }
          ]
        }
      ],
      "typeParameters": [4]
    }
  ],
  "functions": [
    {
      "inputs": [
        {
          "name": "arg1",
          "concreteTypeId": "3ddd5c1768dd7869663dc2f868ea8a8ce68bd6064244dbc4286e2c921c8ce962"
        }
      ],
      "name": "complex_function",
      "output": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d"
    }
  ],
  "loggedTypes": []
}

Icon LinkAn Example with Logs

Given the following contract:

struct MyStruct<W> {
    x: W,
}
 
abi MyContract {
    fn logging();
}
 
...
 
fn logging() {
    log(MyStruct { x: 42 });
    log(MyStruct { x: true });
}

its JSON representation would look like:

{
  "concreteTypes": [
    {      
      "type": "struct MyStruct<bool>",
      "concreteTypeId": "eca2a040ce95fc19b7cd5f75bac530d052484d0b1a49267a2eb07a7a1b00c389",
      "metadataTypeId": 1,
      "typeArguments": [
        "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903"
      ]
    },
    {      
      "type": "struct MyStruct<u64>",
      "concreteTypeId": "b2fa346d9ca66ceca61951a27dba2977b2a82b8aa8600670604f286a1393dffe",
      "metadataTypeId": 1,
      "typeArguments": [
        "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0"
      ]
    },
    {      
      "type": "bool",
      "concreteTypeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903",
    },
    {      
      "type": "u64",
      "concreteTypeId": "1506e6f44c1d6291cdf46395a8e573276a4fa79e8ace3fc891e092ef32d1b0a0",
    },
    {
      "type": "()",
      "concreteTypeId": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d",
    }
  ],
  "metadataTypes": [
    {
      "metadataTypeId": 1,
      "type": "struct MyStruct",
      "components": [
        {
          "name": "x",
          "typeId": 2,
          "typeArguments": null
        }
      ],
      "typeParameters": [2]
    },
    {
      "metadataTypeId": 2,
      "type": "generic W",
    },
  ],
  "functions": [
    {
      "inputs": [],
      "name": "logging",
      "output": "2e38e77b22c314a449e91fafed92a43826ac6aa403ae6a8acb6cf58239fbaf5d"
    }
  ],
  "loggedTypes": [
    {
      "logId": "12896678128313068780",
      "concreteTypeId": "b2fa346d9ca66ceca61951a27dba2977b2a82b8aa8600670604f286a1393dffe"
    },
    {
      "logId": "16383228984366451899",
      "concreteTypeId": "eca2a040ce95fc19b7cd5f75bac530d052484d0b1a49267a2eb07a7a1b00c389"
    }
  ]
}

The logIds are calculated from:

First 8 bytes of sha256("struct MyStruct<u64>") => "12896678128313068780"
First 8 bytes of sha256("struct MyStruct<bool>") => "16383228984366451899"

Icon LinkJSON ABI Format

Icon LinkSpec Version

Icon LinkNotation

Icon LinkJSON ABI Spec

Icon LinkAttributes Semantics

Icon LinkA Simple Example

Icon LinkJSON ABI Format for Each Possible Metadata Type Declaration

Icon Linkstruct

Icon Linkenum

Icon Linkarray

Icon Linktuple

Icon LinkGeneric Type Parameter

Icon LinkSome Complex Examples

Icon LinkAn Example with Non-Generic Custom Types

Icon LinkAn Example with Generic Types

Icon LinkAn Example with Logs

Icon Link`struct`

Icon Link`enum`

Icon Link`array`

Icon Link`tuple`