Contract Metadata

The Solidity compiler automatically generates a JSON file, the contract metadata, that contains information about the compiled contract. You can use this file to query the compiler version, the sources used, the ABI and NatSpec documentation to more safely interact with the contract and verify its source code.

The compiler appends by default the IPFS hash of the metadata file to the end of the bytecode (for details, see below) of each contract, so that you can retrieve the file in an authenticated way without having to resort to a centralized data provider. The other available options are the Swarm hash and not appending the metadata hash to the bytecode. These can be configured via the Standard JSON Interface.

You have to publish the metadata file to IPFS, Swarm, or another service so that others can access it. You create the file by using the solc --metadata command together with the --output-dir parameter. Without the parameter, the metadata will be written to standard output. The metadata contains IPFS and Swarm references to the source code, so you have to upload all source files in addition to the metadata file. For IPFS, the hash contained in the CID returned by ipfs add (not the direct sha2-256 hash of the file) shall match with the one contained in the bytecode.

The metadata file has the following format. The example below is presented in a human-readable way. Properly formatted metadata should use quotes correctly, reduce whitespace to a minimum and sort the keys of all objects to arrive at a unique formatting. Comments are not permitted and used here only for explanatory purposes.

  // Required: The version of the metadata format
  "version": "1",
  // Required: Source code language, basically selects a "sub-version"
  // of the specification
  "language": "Solidity",
  // Required: Details about the compiler, contents are specific
  // to the language.
  "compiler": {
    // Required for Solidity: Version of the compiler
    "version": "0.8.2+commit.661d1103",
    // Optional: Hash of the compiler binary which produced this output
    "keccak256": "0x123..."
  // Required: Compilation source files/source units, keys are file paths
    "myDirectory/myFile.sol": {
      // Required: keccak256 hash of the source file
      "keccak256": "0x123...",
      // Required (unless "content" is used, see below): Sorted URL(s)
      // to the source file, protocol is more or less arbitrary, but an
      // IPFS URL is recommended
      "urls": [ "bzz-raw://7d7a...", "dweb:/ipfs/QmN..." ],
      // Optional: SPDX license identifier as given in the source file
      "license": "MIT"
    "destructible": {
      // Required: keccak256 hash of the source file
      "keccak256": "0x234...",
      // Required (unless "url" is used): literal contents of the source file
      "content": "contract destructible is owned { function destroy() { if (msg.sender == owner) selfdestruct(owner); } }"
  // Required: Compiler settings
    // Required for Solidity: Sorted list of import remappings
    "remappings": [ ":g=/dir" ],
    // Optional: Optimizer settings. The fields "enabled" and "runs" are deprecated
    // and are only given for backwards-compatibility.
    "optimizer": {
      "enabled": true,
      "runs": 500,
      "details": {
        // peephole defaults to "true"
        "peephole": true,
        // inliner defaults to "true"
        "inliner": true,
        // jumpdestRemover defaults to "true"
        "jumpdestRemover": true,
        "orderLiterals": false,
        "deduplicate": false,
        "cse": false,
        "constantOptimizer": false,
        "yul": true,
        // Optional: Only present if "yul" is "true"
        "yulDetails": {
          "stackAllocation": false,
          "optimizerSteps": "dhfoDgvulfnTUtnIf..."
    "metadata": {
      // Reflects the setting used in the input json, defaults to "true"
      "appendCBOR": true,
      // Reflects the setting used in the input json, defaults to "false"
      "useLiteralContent": true,
      // Reflects the setting used in the input json, defaults to "ipfs"
      "bytecodeHash": "ipfs"
    // Required for Solidity: File path and the name of the contract or library this
    // metadata is created for.
    "compilationTarget": {
      "myDirectory/myFile.sol": "MyContract"
    // Required for Solidity: Addresses for libraries used
    "libraries": {
      "MyLib": "0x123123..."
  // Required: Generated information about the contract.
    // Required: ABI definition of the contract. See "Contract ABI Specification"
    "abi": [/* ... */],
    // Required: NatSpec developer documentation of the contract.
    "devdoc": {
      "version": 1 // NatSpec version
      "kind": "dev",
      // Contents of the @author NatSpec field of the contract
      "author": "John Doe",
      // Contents of the @title NatSpec field of the contract
      "title": "MyERC20: an example ERC20"
      // Contents of the @dev NatSpec field of the contract
      "details": "Interface of the ERC20 standard as defined in the EIP. See for details",
      "methods": {
        "transfer(address,uint256)": {
          // Contents of the @dev NatSpec field of the method
          "details": "Returns a boolean value indicating whether the operation succeeded. Must be called by the token holder address",
          // Contents of the @param NatSpec fields of the method
          "params": {
            "_value": "The amount tokens to be transferred",
            "_to": "The receiver address"
          // Contents of the @return NatSpec field.
          "returns": {
            // Return var name (here "success") if exists. "_0" as key if return var is unnamed
            "success": "a boolean value indicating whether the operation succeeded"
      "stateVariables": {
        "owner": {
          // Contents of the @dev NatSpec field of the state variable
          "details": "Must be set during contract creation. Can then only be changed by the owner"
      "events": {
         "Transfer(address,address,uint256)": {
           "details": "Emitted when `value` tokens are moved from one account (`from`) toanother (`to`)."
           "params": {
             "from": "The sender address"
             "to": "The receiver address"
             "value": "The token amount"
    // Required: NatSpec user documentation of the contract
    "userdoc": {
      "version": 1 // NatSpec version
      "kind": "user",
      "methods": {
        "transfer(address,uint256)": {
          "notice": "Transfers `_value` tokens to address `_to`"
      "events": {
        "Transfer(address,address,uint256)": {
          "notice": "`_value` tokens have been moved from `from` to `to`"


Since the bytecode of the resulting contract contains the metadata hash by default, any change to the metadata might result in a change of the bytecode. This includes changes to a filename or path, and since the metadata includes a hash of all the sources used, a single whitespace change results in different metadata, and different bytecode.


The ABI definition above has no fixed order. It can change with compiler versions. Starting from Solidity version 0.5.12, though, the array maintains a certain order.

Encoding of the Metadata Hash in the Bytecode

Because we might support other ways to retrieve the metadata file in the future, the mapping {"ipfs": <IPFS hash>, "solc": <compiler version>} is stored CBOR-encoded. Since the mapping might contain more keys (see below) and the beginning of that encoding is not easy to find, its length is added in a two-byte big-endian encoding. The current version of the Solidity compiler usually adds the following to the end of the deployed bytecode

0x64 'i' 'p' 'f' 's' 0x58 0x22 <34 bytes IPFS hash>
0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding>
0x00 0x33

So in order to retrieve the data, the end of the deployed bytecode can be checked to match that pattern and the IPFS hash can be used to retrieve the file (if pinned/published).

Whereas release builds of solc use a 3 byte encoding of the version as shown above (one byte each for major, minor and patch version number), prerelease builds will instead use a complete version string including commit hash and build date.

The commandline flag --no-cbor-metadata can be used to skip metadata from getting appended at the end of the deployed bytecode. Equivalently, the boolean field settings.metadata.appendCBOR in Standard JSON input can be set to false.


The CBOR mapping can also contain other keys, so it is better to fully decode the data instead of relying on it starting with 0xa264. For example, if any experimental features that affect code generation are used, the mapping will also contain "experimental": true.


The compiler currently uses the IPFS hash of the metadata by default, but it may also use the bzzr1 hash or some other hash in the future, so do not rely on this sequence to start with 0xa2 0x64 'i' 'p' 'f' 's'. We might also add additional data to this CBOR structure, so the best option is to use a proper CBOR parser.

Usage for Automatic Interface Generation and NatSpec

The metadata is used in the following way: A component that wants to interact with a contract (e.g. a wallet) retrieves the code of the contract. It decodes the CBOR encoded section containing the IPFS/Swarm hash of the metadata file. With that hash, the metadata file is retrieved. That file is JSON-decoded into a structure like above.

The component can then use the ABI to automatically generate a rudimentary user interface for the contract.

Furthermore, the wallet can use the NatSpec user documentation to display a human-readable confirmation message to the user whenever they interact with the contract, together with requesting authorization for the transaction signature.

For additional information, read Ethereum Natural Language Specification (NatSpec) format.

Usage for Source Code Verification

In order to verify the compilation, sources can be retrieved from IPFS/Swarm via the link in the metadata file. The compiler of the correct version (which is checked to be part of the “official” compilers) is invoked on that input with the specified settings. The resulting bytecode is compared to the data of the creation transaction or CREATE opcode data. This automatically verifies the metadata since its hash is part of the bytecode. Excess data corresponds to the constructor input data, which should be decoded according to the interface and presented to the user.

In the repository sourcify (npm package) you can see example code that shows how to use this feature.