# Compiler diagnostics (llmcpp)

**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:

```cpp
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](https://github.com/yanevskiv/llmcpp/blob/main/src/llmcpp/agent_tool_server.cpp).