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fmt, the formatting library we use, detects types with conversion to std::string_view (and formats them as strings) and types that support operator<<(std::ostream, const T&) (and performs custom formatting on them). However, if <fmt/ostream.h>, the latter is not done. The problem happens with seastar::sstring, which implements both, and debug mode, which disables inlining. Some translation units do include <fmt/ostream.h>, and so generate code to do custom formatting. exception_utils.cc doesn't, and so generates code to format via string_view conversion. At link time, the compiler picks one of the generated functions and includes it in the final binary; it happened to pick one generated outside exception_utils.cc, using custom formatting. However, there is also code in fmt to encode which path fmt chose - string_view or custom. This code is constexpr and so is evaluated in exception_utils.cc. The result is that the function to perform formatting of seastar::sstring uses custom formatting, while the descriptor containing the method used says it is formatting via string_view. This is enough to cause a crash. The problem is limited to debug mode, since in other modes all this code is inlined, and so is consistent within the translation unit. We need a more general fix (hopefully in fmt), but for now a simple fix is to add the missing include. Ref https://github.com/fmtlib/fmt/issues/1662