Error messages versus error codes

style.md

@@ -303,9 +303,20 @@ arena.reset();  // Reset arena memory
 - **Return codes** for expected errors
 - **Exceptions** only for exceptional circumstances
 - **fprintf + abort()** for unrecoverable errors
+- **Error messages are for humans only** - never parse error message strings programmatically
+- **Error codes are the contract** - use enums/codes for programmatic error handling
 ```cpp
 enum class ParseResult { Success, InvalidJson, MissingField };
 
+// Good: Test error codes (part of contract)
+auto result = parser.parse(data);
+if (result == ParseResult::InvalidJson) {
+  // Handle programmatically
+}
+
+// Bad: Don't test or parse error message strings
+// CHECK(parser.get_error() == "Expected '}' at line 5");  // BRITTLE!
+
 if (!memory) {
   std::fprintf(stderr, "ArenaAllocator: Failed to allocate memory\n");
   std::abort();