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Subscan EVM Contract Verification Guide (Remix & Standard JSON)

Verifying your smart contract source code increases transparency and trust in your project. This guide outlines how to verify contracts on Subscan (applicable to Darwinia, Moonbeam, Astar, etc.), specifically focusing on how Remix users can modify the generated Metadata JSON to meet Subscan's verification requirements.

📍 Example Context

  • Target Network: Darwinia
  • Example Contract: 0x00000000001523057a05d6293c1e5171ee33ee0a
  • Tool: Remix IDE

🛠 Phase 1: Preparation in Remix

Before verification, ensure your local compilation environment in Remix matches the settings used during the actual deployment exactly.

1. Setup Code & Paths

  • Open your project in Remix.
  • Attention to Paths: Ensure your file structure matches the import statements.
  • Example: Upload or edit Quacks.sol.

2. Configure Compiler

  • Navigate to the Solidity Compiler tab.
  • Compiler Version: Select the exact version used for deployment (e.g., 0.8.x).
  • EVM Version: If a specific version was used (e.g., paris, london), select it; otherwise, leave as default.
  • Optimization: Check Enable optimization and set the Runs value (commonly 200). This must match the deployment settings.
  • Click Compile Quacks.sol.

3. Retrieve Metadata JSON

  • After successful compilation, go to the File Explorer (left sidebar).
  • Look inside the artifacts/ folder (or root directory depending on Remix version).
  • Locate the file named: ContractName_metadata.json (e.g., Quacks_metadata.json).

⚙️ Phase 2: Constructing Standard Input JSON (Crucial Step)

Subscan's "Standard JSON" verification method requires the full source code within the JSON file. However, the default metadata.json generated by Remix often contains URLs (IPFS/Swarm) instead of the actual source code content.

You must manually edit this JSON file to inject the source code.

1. Inspect the Generated JSON

Open Quacks_metadata.json. In the sources section, you might see a structure like this (missing actual code):

"sources": {
  "Quacks.sol": {
    "keccak256": "0x...",
    "urls": ["bzz-raw://...", "dweb:/ipfs/..."] // Subscan cannot read these external links
  }
}

2. Modify JSON: Inject Source Code

You need to replace or add to the entry with a "content" field containing the actual Solidity source string.

Example of the Modified JSON Structure:

{
  "language": "Solidity",
  "settings": {
    "optimizer": {
      "enabled": true,
      "runs": 200
    },
    // ... other settings
  },
  "sources": {
    // For the main file
    "Quacks.sol": {
      "content": "// SPDX-License-Identifier: MIT\npragma solidity ^0.8.0;\n\ncontract Quacks { ... PASTE FULL SOURCE CODE HERE ... }"
    },
    // For dependencies (e.g., OpenZeppelin) - EVERY imported file needs this
    "@openzeppelin/contracts-upgradeable/access/AccessControlUpgradeable.sol": {
      "content": "// SPDX-License-Identifier: MIT\n// OpenZeppelin Contracts...\n... PASTE FULL LIBRARY SOURCE HERE ..."
    }
  }
}

💡 Tips:

  • All imported files must be listed in sources.
  • The value of "content" must be a string (watch out for newlines \n and escaping quotes).
  • If you are using Hardhat/Foundry, their generated build-info files already contain the content field, so this manual step is not needed.

3. Save the File

Save your modified JSON file locally as Standard-Input.json.

🚀 Phase 3: Verify on Subscan

1. Navigate to Contract Page

2. Upload Configuration

  • Verification Method: Select Standard-JSON-Input.
  • Compiler Version: Select the version you used in Remix.
  • Upload File: Upload the Standard-Input.json you created in Phase 2.

3. Submit

Click Verify. If the JSON format is correct and the compiled bytecode matches the on-chain bytecode, the verification will succeed.

4. View and Interact After Verification

After verification succeeds, search the contract address on the target network and open the Contract tab to:

❓ Troubleshooting

Issue Possible Cause & Solution
Bytecode mismatch Optimization Runs: Did you use 200 runs in Remix but default in the JSON? Ensure they match.
EVM Version: Ensure the EVM version (e.g., London/Paris) is consistent.
Parsing Error JSON Syntax: Ensure the pasted source code in the JSON doesn't break the JSON string format (escape quotes " with \").
Alternative Method If manual JSON editing is too difficult, install the "Flattener" plugin in Remix. Flatten your code into one file, and use the "Flatten Code" method on Subscan instead.

📚 References

📝 Need help?

If you run into any issues, please reach out via:

Security note: Please do not share private keys, seed phrases, or sensitive credentials in public issues or chats.