Fix the “mock compiler: source not found” Error in Remix Solidity

First‑time users of Remix for Solidity smart‑contract development may encounter the “mock compiler: source not found” error, which prevents compilation.

Problem

After writing a contract and clicking compile, Remix shows a message like “mock compiler: source not found”. This happens because the Solidity compiler version has not been set in the Remix URL (e.g., https://remix.ethereum.org/#optimize=true&version=builtin lacks a version parameter).

Solution

In the right‑hand panel, open Settings and under “Select new compiler version” choose the appropriate Solidity version. Remix will automatically add a version parameter to the URL.

If you already know the exact version, you can append version=0.8.17 (or any desired version) directly to the URL. Different versions support different syntax features, so choose according to your contract’s requirements.

Low‑Level Errors That Trigger the Same Exception

Sometimes the error is caused by a simple syntax mistake, such as forgetting a semicolon after the pragma line when copying code from elsewhere.

Solidity:ParserError: Expected pragma, import directive or contract/interface/library definition.

Inspect the contract for missing semicolons or other basic syntax errors to resolve the issue.