Compiler diagnostics give the agent concrete feedback when a candidate violates C++ rules. The checking tool distinguishes failure from successful compilation with or without warnings:

if (r.m_ok && r.m_warnings == 0) {
    return {"OK: the body compiles without errors or warnings.", false};
}
if (r.m_ok) {
    return {formatv("OK with {0} warning(s):\n{1}", r.m_warnings, r.m_text).str(), false};
}
return {formatv("FAILED with {0} error(s):\n{1}", r.m_errors, r.m_text).str(), false};

This is the result-handling part of try_compile. The text contains compiler-generated diagnostics, allowing an agent to revise a missing name, type mismatch, or invalid access using evidence from Clang.

A failed candidate check does not itself consume a submission attempt. The agent can revise within its tool budget before calling submit. Diagnostics still cannot reject every wrong algorithm: returning the wrong numerical result can be perfectly valid C++.

Source: agent_tool_server.cpp.