diff --git a/include/silk/util/assert.h b/include/silk/util/assert.h
index 58dfba5..a7b764b 100644
--- a/include/silk/util/assert.h
+++ b/include/silk/util/assert.h
@@ -16,12 +16,18 @@ static constexpr bool DebugAssertionsEnabled = false;
 #endif
 
 /**
- * Print a symbolized stack trace, the error message, and optional details, then abort.
+ * Print a symbolized stack trace, the error message, optional printf-formatted
+ * details, and abort. `fmt` is a printf-style format string; pass nullptr (or
+ * omit) to skip the details line.
  */
-[[noreturn]] void assertFail(const char * message, const char * file, int line, const char * details = nullptr) noexcept;
+[[noreturn]] void assertFail(const char * message, const char * file, int line, const char * fmt = nullptr, ...) noexcept
+    __attribute__((format(printf, 4, 5)));
 
 } // namespace silk
 
+// Unconditional abort.
+#define SILK_FAIL(message, ...) silk::assertFail(message, __FILE__, __LINE__ __VA_OPT__(, ) __VA_ARGS__)
+
 // Assertion active in all builds unless DISABLE_ASSERTIONS is defined.
 // Use for invariants that must hold in release -- hard internal contract violations.
 #define SILK_ASSERT(condition, ...) \
@@ -29,7 +35,7 @@ static constexpr bool DebugAssertionsEnabled = false;
     { \
         if (silk::ReleaseAssertionsEnabled && !(condition)) [[unlikely]] \
         { \
-            silk::assertFail("assertion failed: " #condition, __FILE__, __LINE__ __VA_OPT__(, std::format(__VA_ARGS__).c_str())); \
+            silk::assertFail("assertion failed: " #condition, __FILE__, __LINE__ __VA_OPT__(, ) __VA_ARGS__); \
         } \
     } while (0)
 
@@ -40,6 +46,6 @@ static constexpr bool DebugAssertionsEnabled = false;
     { \
         if (silk::DebugAssertionsEnabled && !(condition)) [[unlikely]] \
         { \
-            silk::assertFail("assertion failed: " #condition, __FILE__, __LINE__ __VA_OPT__(, std::format(__VA_ARGS__).c_str())); \
+            silk::assertFail("assertion failed: " #condition, __FILE__, __LINE__ __VA_OPT__(, ) __VA_ARGS__); \
         } \
     } while (0)
diff --git a/include/silk/util/logger.h b/include/silk/util/logger.h
index 7fecc5b..1f9e474 100644
--- a/include/silk/util/logger.h
+++ b/include/silk/util/logger.h
@@ -1,12 +1,12 @@
 #pragma once
 
 #include <atomic>
-#include <format>
-#include <string_view>
 
 namespace silk
 {
 
+// Log severity, ordered low-to-high. A record is emitted when its level is
+// >= the logger's configured minimum (default INFO; change via Logger::setLevel).
 enum class LogLevel
 {
     DEBUG,
@@ -26,20 +26,16 @@ class Logger
 {
 public:
     /** Set the minimum level for messages to be emitted. */
-    static void setLevel(LogLevel level) noexcept;
+    static void setLevel(LogLevel level) noexcept { currentLevel.store(level, std::memory_order_relaxed); }
 
     /** Return true if messages at the given level would be emitted. */
     static bool isEnabled(LogLevel level) noexcept { return level >= currentLevel.load(std::memory_order_relaxed); }
 
-    /** Emit a pre-formatted message. */
-    static void log(LogLevel level, std::string_view file, int line, std::string_view message) noexcept;
+    /** Emit a pre-formatted message verbatim (no printf parsing). */
+    static void log(LogLevel level, const char * file, int line, const char * message) noexcept;
 
-    /** Format and emit a message. */
-    template <typename... Args>
-    static void log(LogLevel level, std::string_view file, int line, std::format_string<Args...> fmt, Args &&... args) noexcept
-    {
-        log(level, file, line, std::format(fmt, std::forward<Args>(args)...));
-    }
+    /** Emit a printf-formatted message. */
+    static void logf(LogLevel level, const char * file, int line, const char * fmt, ...) noexcept __attribute__((format(printf, 4, 5)));
 
 private:
     inline static std::atomic<LogLevel> currentLevel{LogLevel::INFO};
@@ -47,9 +43,22 @@ class Logger
 
 } // namespace silk
 
-// clang-format off
-#define SILK_DEBUG(...) do { if (silk::Logger::isEnabled(silk::LogLevel::DEBUG)) silk::Logger::log(silk::LogLevel::DEBUG, __FILE__, __LINE__, __VA_ARGS__); } while (0)
-#define SILK_INFO(...)  do { if (silk::Logger::isEnabled(silk::LogLevel::INFO))  silk::Logger::log(silk::LogLevel::INFO,  __FILE__, __LINE__, __VA_ARGS__); } while (0)
-#define SILK_WARN(...)  do { if (silk::Logger::isEnabled(silk::LogLevel::WARN))  silk::Logger::log(silk::LogLevel::WARN,  __FILE__, __LINE__, __VA_ARGS__); } while (0)
-#define SILK_ERROR(...) do { if (silk::Logger::isEnabled(silk::LogLevel::ERROR)) silk::Logger::log(silk::LogLevel::ERROR, __FILE__, __LINE__, __VA_ARGS__); } while (0)
-// clang-format on
+// Emit a log record at the given level. Dispatches to Logger::log for plain
+// strings (SILK_INFO("text")) and to Logger::logf for printf-style calls
+// (SILK_INFO("foo %d", x)) -- the latter is the only path that pays vsnprintf.
+// The format string is checked at every call site (-Wformat); formatting only
+// runs when the level is enabled, so a disabled-level call site is one relaxed
+// atomic load.
+#define SILK_LOG(level, fmt, ...) \
+    do \
+    { \
+        if (silk::Logger::isEnabled(level)) \
+        { \
+            silk::Logger::log##__VA_OPT__(f)(level, __FILE__, __LINE__, fmt __VA_OPT__(, ) __VA_ARGS__); \
+        } \
+    } while (0)
+
+#define SILK_DEBUG(fmt, ...) SILK_LOG(silk::LogLevel::DEBUG, fmt __VA_OPT__(, ) __VA_ARGS__)
+#define SILK_INFO(fmt, ...) SILK_LOG(silk::LogLevel::INFO, fmt __VA_OPT__(, ) __VA_ARGS__)
+#define SILK_WARN(fmt, ...) SILK_LOG(silk::LogLevel::WARN, fmt __VA_OPT__(, ) __VA_ARGS__)
+#define SILK_ERROR(fmt, ...) SILK_LOG(silk::LogLevel::ERROR, fmt __VA_OPT__(, ) __VA_ARGS__)
diff --git a/src/fibers/fiber.cpp b/src/fibers/fiber.cpp
index 2597ba8..9b9eb7e 100644
--- a/src/fibers/fiber.cpp
+++ b/src/fibers/fiber.cpp
@@ -22,7 +22,6 @@
 #include <atomic>
 #include <cerrno>
 #include <cstdint>
-#include <format>
 #include <memory>
 #include <mutex>
 #include <thread>
@@ -398,7 +397,7 @@ void Fiber::fiberContextMain(boost::context::detail::transfer_t transfer) noexce
     fiber->result = fiber->fiberMain(fiber->parameters);
     fiber->changeState(FiberState::RUNNING, FiberState::STOPPED);
     fiber->switchToThreadContext(true);
-    SILK_ASSERT(false, "unreachable");
+    SILK_FAIL("unreachable");
 }
 
 void Fiber::changeState(FiberState expectedState, FiberState newState) noexcept
@@ -406,7 +405,7 @@ void Fiber::changeState(FiberState expectedState, FiberState newState) noexcept
     FiberState prevState = state.exchange(newState, std::memory_order_acq_rel);
     SILK_ASSERT(
         prevState == expectedState,
-        "invalid fiber state: expected={}, actual={}",
+        "invalid fiber state: expected=%d, actual=%d",
         static_cast<int>(expectedState),
         static_cast<int>(prevState));
 }
@@ -430,7 +429,7 @@ bool Fiber::tryChangeStateToSuspended() noexcept
                 // runFiber will enqueue the fiber after the callback returns.
                 return false;
             default:
-                SILK_ASSERT(false, "Unexpected fiber state: {}", static_cast<int>(currentState));
+                SILK_FAIL("unexpected fiber state", "state=%d", static_cast<int>(currentState));
         }
     }
 }
@@ -457,7 +456,7 @@ bool Fiber::tryChangeStateToReady() noexcept
                 }
                 break;
             default:
-                SILK_ASSERT(false, "Unexpected fiber state: {}", static_cast<int>(currentState));
+                SILK_FAIL("unexpected fiber state", "state=%d", static_cast<int>(currentState));
         }
     }
 }
diff --git a/src/fibers/mutex.cpp b/src/fibers/mutex.cpp
index e89ac23..a25adaf 100644
--- a/src/fibers/mutex.cpp
+++ b/src/fibers/mutex.cpp
@@ -5,7 +5,6 @@
 #include <silk/util/spinlock.h>
 
 #include <atomic>
-#include <format>
 
 namespace silk
 {
diff --git a/src/perf/fiber-http.cpp b/src/perf/fiber-http.cpp
index 82b361e..b6303ad 100644
--- a/src/perf/fiber-http.cpp
+++ b/src/perf/fiber-http.cpp
@@ -8,7 +8,6 @@
 #include <Poco/Net/StreamSocket.h>
 
 #include <cerrno>
-#include <format>
 
 #include <poll.h>
 #include <unistd.h>
@@ -130,7 +129,7 @@ bool FiberSocketImpl::poll(const Poco::Timespan & timeout, int mode)
 int FiberSocketImpl::sendBytes(const void * buffer, int length, int flags)
 {
     // Poco flags (MSG_NOSIGNAL, MSG_PEEK, etc.) are not plumbed through to io_uring read/write.
-    SILK_ASSERT(flags == 0, "FiberSocketImpl::sendBytes does not support flags (got {})", flags);
+    SILK_ASSERT(flags == 0, "FiberSocketImpl::sendBytes does not support flags (got %d)", flags);
 
     int total = 0;
     const char * ptr = static_cast<const char *>(buffer);
@@ -170,7 +169,7 @@ int FiberSocketImpl::sendBytes(const void * buffer, int length, int flags)
 int FiberSocketImpl::receiveBytes(void * buffer, int length, int flags)
 {
     // Poco flags (MSG_NOSIGNAL, MSG_PEEK, etc.) are not plumbed through to io_uring read/write.
-    SILK_ASSERT(flags == 0, "FiberSocketImpl::receiveBytes does not support flags (got {})", flags);
+    SILK_ASSERT(flags == 0, "FiberSocketImpl::receiveBytes does not support flags (got %d)", flags);
 
 #if defined(USE_IO_URING_RW)
     uint64_t bytesRead = 0;
diff --git a/src/perf/file-perf.cpp b/src/perf/file-perf.cpp
index 37ff7d8..b77e687 100644
--- a/src/perf/file-perf.cpp
+++ b/src/perf/file-perf.cpp
@@ -17,7 +17,6 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
-#include <format>
 #include <iostream>
 #include <memory>
 #include <string>
@@ -221,7 +220,7 @@ void Benchmark::start()
     for (Job & job : jobs)
     {
         int r = silk::FiberScheduler::run(workerFiberMain, {this, &job}, &job.future);
-        SILK_ASSERT(!r, "cannot start fiber: {}", std::strerror(r));
+        SILK_ASSERT(!r, "cannot start fiber: %s", std::strerror(r));
     }
 
     warmupEndCycles.store(silk::Tsc::getCycles() + silk::Tsc::nanosecondsToCycles(cfg.warmupNs), std::memory_order_relaxed);
@@ -280,7 +279,7 @@ int Benchmark::workerFiberMain(WorkerFiberParams * params) noexcept
         int r = slot->future.wait();
         if (r)
         {
-            SILK_ERROR("request failed: {}", std::strerror(r));
+            SILK_ERROR("request failed: %s", std::strerror(r));
             break;
         }
 
@@ -300,7 +299,7 @@ int Benchmark::workerFiberMain(WorkerFiberParams * params) noexcept
     for (uint32_t i = 0; i < benchmark->cfg.iodepth; ++i)
     {
         int r = job->slots[i].future.wait();
-        SILK_ERROR("request failed: {}", std::strerror(r));
+        SILK_ERROR("request failed: %s", std::strerror(r));
     }
 
     return 0;
@@ -403,14 +402,14 @@ int main(int argc, char ** argv)
 
     int openFlags = O_RDWR | O_CREAT | O_TRUNC | O_CLOEXEC | (cfg.direct ? O_DIRECT : 0);
     int fd = ::open(cfg.filename.c_str(), openFlags, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
-    SILK_ASSERT(fd >= 0, "open failed: {}", std::strerror(errno));
+    SILK_ASSERT(fd >= 0, "open failed: %s", std::strerror(errno));
 
     struct stat st;
     int r = ::fstat(fd, &st);
-    SILK_ASSERT(!r, "fstat failed: {}", std::strerror(errno));
+    SILK_ASSERT(!r, "fstat failed: %s", std::strerror(errno));
 
     r = ::fallocate(fd, 0, 0, static_cast<off_t>(cfg.fileSize));
-    SILK_ASSERT(!r, "fallocate failed: {}", std::strerror(errno));
+    SILK_ASSERT(!r, "fallocate failed: %s", std::strerror(errno));
 
     sigset_t mask = blockSignals();
 
@@ -426,8 +425,8 @@ int main(int argc, char ** argv)
     silk::FiberScheduler::initialize(&options);
 
     SILK_INFO(
-        "starting benchmark on: {}  size={:.1f}GiB{}",
-        cfg.filename,
+        "starting benchmark on: %s  size=%.1fGiB%s",
+        cfg.filename.c_str(),
         static_cast<double>(cfg.fileSize) / (1024.0 * 1024 * 1024),
         cfg.direct ? "  direct" : "");
 
@@ -440,13 +439,13 @@ int main(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
diff --git a/src/perf/http-perf.cpp b/src/perf/http-perf.cpp
index d3e1a35..e4196e8 100644
--- a/src/perf/http-perf.cpp
+++ b/src/perf/http-perf.cpp
@@ -34,7 +34,6 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
-#include <format>
 #include <iostream>
 #include <limits>
 #include <memory>
@@ -135,7 +134,7 @@ void Client::start()
         else
         {
             int r = silk::FiberScheduler::run(fiberMain, {this, &conn}, &conn.future);
-            SILK_ASSERT(!r, "cannot start fiber: {}", std::strerror(r));
+            SILK_ASSERT(!r, "cannot start fiber: %s", std::strerror(r));
         }
     }
 
@@ -156,7 +155,7 @@ void Client::stop()
         {
             if (!isExpectedShutdown(e.code()))
             {
-                SILK_ERROR("shutdown failed: {}", e.displayText());
+                SILK_ERROR("shutdown failed: %s", e.displayText().c_str());
             }
         }
     }
@@ -204,7 +203,7 @@ void Client::runLoop(Connection * conn) noexcept
         {
             if (!stopping.load(std::memory_order_relaxed) && !isExpectedShutdown(e.code()))
             {
-                SILK_ERROR("HTTP request failed: {}", e.displayText());
+                SILK_ERROR("HTTP request failed: %s", e.displayText().c_str());
             }
             break;
         }
@@ -308,9 +307,9 @@ static void runClient(int argc, char ** argv)
     }
 
     SILK_INFO(
-        "starting {} http client, host={}:{}, connections={}",
+        "starting %s http client, host=%s:%u, connections=%u",
         cfg.useThreads ? "threaded" : "fiber",
-        cfg.host,
+        cfg.host.c_str(),
         cfg.port,
         cfg.numConnections);
 
@@ -319,13 +318,13 @@ static void runClient(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
@@ -492,7 +491,7 @@ class FiberHTTPServer
 void FiberHTTPServer::start()
 {
     int r = silk::FiberScheduler::run(acceptFiberMain, AcceptFiberParams{this}, &acceptFuture);
-    SILK_ASSERT(r == 0, "spawn accept fiber: {}", std::strerror(r));
+    SILK_ASSERT(r == 0, "spawn accept fiber: %s", std::strerror(r));
 }
 
 void FiberHTTPServer::stop()
@@ -509,12 +508,12 @@ void FiberHTTPServer::stop()
     {
         if (!isExpectedShutdown(e.code()))
         {
-            SILK_ERROR("shutdown failed: {}", e.displayText());
+            SILK_ERROR("shutdown failed: %s", e.displayText().c_str());
         }
     }
 
     int r = acceptFuture.wait();
-    SILK_ASSERT(r == 0, "accept fiber: {}", std::strerror(r));
+    SILK_ASSERT(r == 0, "accept fiber: %s", std::strerror(r));
 
     socket.close();
 
@@ -530,7 +529,7 @@ void FiberHTTPServer::stop()
             {
                 if (!isExpectedShutdown(e.code()))
                 {
-                    SILK_ERROR("shutdown failed: {}", e.displayText());
+                    SILK_ERROR("shutdown failed: %s", e.displayText().c_str());
                 }
             }
         }
@@ -556,7 +555,7 @@ void FiberHTTPServer::acceptLoop() noexcept
         {
             if (!stopping.load(std::memory_order_relaxed) && !isExpectedShutdown(e.code()))
             {
-                SILK_ERROR("accept failed: {}", e.displayText());
+                SILK_ERROR("accept failed: %s", e.displayText().c_str());
             }
             return;
         }
@@ -571,7 +570,7 @@ void FiberHTTPServer::acceptLoop() noexcept
         int r = silk::FiberScheduler::run(connFiberMain, ConnFiberParams{this, clientSocket}, &conn->future);
         if (r != 0)
         {
-            SILK_ERROR("spawn conn fiber: {}", std::strerror(r));
+            SILK_ERROR("spawn conn fiber: %s", std::strerror(r));
             {
                 std::lock_guard lock(connsMutex);
                 conns.remove(conn);
@@ -593,7 +592,7 @@ void FiberHTTPServer::connectionLoop(Poco::Net::StreamSocket socket) noexcept
     {
         if (!stopping.load(std::memory_order_relaxed) && !isExpectedShutdown(e.code()))
         {
-            SILK_ERROR("connection error: {}", e.displayText());
+            SILK_ERROR("connection error: %s", e.displayText().c_str());
         }
     }
 }
@@ -668,11 +667,11 @@ static void runServer(int argc, char ** argv)
     Poco::Net::HTTPRequestHandlerFactory::Ptr factory = new EchoHandlerFactory(handlerCfg);
 
     SILK_INFO(
-        "starting {} http server on port {}, queued={}, delay={}",
+        "starting %s http server on port %u, queued=%u, delay=%s",
         cfg.useThreads ? "threaded" : "fiber",
         cfg.port,
         cfg.maxQueued,
-        formatDuration(cfg.delayNs));
+        formatDuration(cfg.delayNs).c_str());
 
     if (cfg.useThreads)
     {
diff --git a/src/perf/net-perf-asio.cpp b/src/perf/net-perf-asio.cpp
index 7d786b4..13bbf8c 100644
--- a/src/perf/net-perf-asio.cpp
+++ b/src/perf/net-perf-asio.cpp
@@ -115,7 +115,7 @@ asio::awaitable<void> Server::acceptLoop()
         {
             if (!isExpectedShutdown(ec))
             {
-                SILK_ERROR("accept failed: {}", ec.message());
+                SILK_ERROR("accept failed: %s", ec.message().c_str());
             }
             break;
         }
@@ -147,7 +147,7 @@ asio::awaitable<void> Server::handleConnection(Server * server, std::shared_ptr<
             {
                 if (!isExpectedShutdown(ec))
                 {
-                    SILK_ERROR("read failed: {}", ec.message());
+                    SILK_ERROR("read failed: %s", ec.message().c_str());
                 }
                 co_return;
             }
@@ -168,7 +168,7 @@ asio::awaitable<void> Server::handleConnection(Server * server, std::shared_ptr<
             {
                 if (!isExpectedShutdown(ec))
                 {
-                    SILK_ERROR("sleep failed: {}", ec.message());
+                    SILK_ERROR("sleep failed: %s", ec.message().c_str());
                 }
                 co_return;
             }
@@ -183,7 +183,7 @@ asio::awaitable<void> Server::handleConnection(Server * server, std::shared_ptr<
             {
                 if (!isExpectedShutdown(ec))
                 {
-                    SILK_ERROR("write failed: {}", ec.message());
+                    SILK_ERROR("write failed: %s", ec.message().c_str());
                 }
                 co_return;
             }
@@ -297,7 +297,7 @@ asio::awaitable<void> Client::clientConnection(Client * client, Connection * con
             {
                 if (!isExpectedShutdown(ec))
                 {
-                    SILK_ERROR("write failed: {}", ec.message());
+                    SILK_ERROR("write failed: %s", ec.message().c_str());
                 }
                 co_return;
             }
@@ -313,7 +313,7 @@ asio::awaitable<void> Client::clientConnection(Client * client, Connection * con
             {
                 if (!isExpectedShutdown(ec))
                 {
-                    SILK_ERROR("read failed: {}", ec.message());
+                    SILK_ERROR("read failed: %s", ec.message().c_str());
                 }
                 co_return;
             }
@@ -428,7 +428,7 @@ static void runServer(int argc, char ** argv)
     Server server(ioc, cfg);
     server.start();
 
-    SILK_INFO("starting server on {}:{}", cfg.host, cfg.port);
+    SILK_INFO("starting server on %s:%u", cfg.host.c_str(), cfg.port);
 
     int sig = 0;
     sigwait(&mask, &sig);
@@ -515,13 +515,13 @@ static void runClient(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
diff --git a/src/perf/net-perf-epoll.cpp b/src/perf/net-perf-epoll.cpp
index 86b53e5..ecc0a63 100644
--- a/src/perf/net-perf-epoll.cpp
+++ b/src/perf/net-perf-epoll.cpp
@@ -119,7 +119,7 @@ int Event::create(Event * out) noexcept
     if (fd < 0)
     {
         int r = errno;
-        SILK_ERROR("eventfd failed: {}", std::strerror(r));
+        SILK_ERROR("eventfd failed: %s", std::strerror(r));
         return r;
     }
     *out = Event(fd);
@@ -133,7 +133,7 @@ int Event::trigger() noexcept
     if (n != sizeof(value))
     {
         int r = errno;
-        SILK_ERROR("write eventfd failed: {}", std::strerror(r));
+        SILK_ERROR("write eventfd failed: %s", std::strerror(r));
         return r;
     }
     return 0;
@@ -205,7 +205,7 @@ int Epoll::create(Epoll * out) noexcept
     if (fd < 0)
     {
         int r = errno;
-        SILK_ERROR("epoll_create1 failed: {}", std::strerror(r));
+        SILK_ERROR("epoll_create1 failed: %s", std::strerror(r));
         return r;
     }
     *out = Epoll(fd);
@@ -221,7 +221,7 @@ int Epoll::add(int fd, uint32_t events, void * ptr) noexcept
     if (r)
     {
         r = errno;
-        SILK_ERROR("epoll_ctl ADD failed: {}", std::strerror(r));
+        SILK_ERROR("epoll_ctl ADD failed: %s", std::strerror(r));
         return r;
     }
     return 0;
@@ -233,7 +233,7 @@ int Epoll::del(int fd) noexcept
     if (r)
     {
         r = errno;
-        SILK_ERROR("epoll_ctl DEL failed: {}", std::strerror(r));
+        SILK_ERROR("epoll_ctl DEL failed: %s", std::strerror(r));
         return r;
     }
     return 0;
@@ -247,7 +247,7 @@ int Epoll::wait(epoll_event * events, int maxEvents, int timeoutMs, int * outCou
         int r = errno;
         if (r != EINTR)
         {
-            SILK_ERROR("epoll_wait failed: {}", std::strerror(r));
+            SILK_ERROR("epoll_wait failed: %s", std::strerror(r));
         }
         return r;
     }
@@ -334,7 +334,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     int r = ::inet_pton(AF_INET, host, &addr.sin_addr);
     if (r != 1)
     {
-        SILK_ERROR("inet_pton failed: invalid address {}", host);
+        SILK_ERROR("inet_pton failed: invalid address %s", host);
         return EINVAL;
     }
 
@@ -345,7 +345,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (fd < 0)
     {
         r = errno;
-        SILK_ERROR("socket failed: {}", std::strerror(r));
+        SILK_ERROR("socket failed: %s", std::strerror(r));
         return r;
     }
 
@@ -354,7 +354,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (r)
     {
         r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -363,7 +363,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (r)
     {
         r = errno;
-        SILK_ERROR("connect failed: {}", std::strerror(r));
+        SILK_ERROR("connect failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -372,7 +372,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (flags < 0)
     {
         r = errno;
-        SILK_ERROR("fcntl F_GETFL failed: {}", std::strerror(r));
+        SILK_ERROR("fcntl F_GETFL failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -381,7 +381,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (r)
     {
         r = errno;
-        SILK_ERROR("fcntl F_SETFL O_NONBLOCK failed: {}", std::strerror(r));
+        SILK_ERROR("fcntl F_SETFL O_NONBLOCK failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -405,7 +405,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
         int r = ::inet_pton(AF_INET, host, &addr.sin_addr);
         if (r != 1)
         {
-            SILK_ERROR("inet_pton failed: invalid address {}", host);
+            SILK_ERROR("inet_pton failed: invalid address %s", host);
             return EINVAL;
         }
     }
@@ -414,7 +414,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (fd < 0)
     {
         int r = errno;
-        SILK_ERROR("socket failed: {}", std::strerror(r));
+        SILK_ERROR("socket failed: %s", std::strerror(r));
         return r;
     }
 
@@ -423,7 +423,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (r)
     {
         r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -432,7 +432,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (r)
     {
         r = errno;
-        SILK_ERROR("setsockopt SO_REUSEADDR failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt SO_REUSEADDR failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -444,7 +444,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (r)
     {
         r = errno;
-        SILK_ERROR("setsockopt SO_REUSEPORT failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt SO_REUSEPORT failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -453,7 +453,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (r)
     {
         r = errno;
-        SILK_ERROR("bind failed: {}", std::strerror(r));
+        SILK_ERROR("bind failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -462,7 +462,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (r)
     {
         r = errno;
-        SILK_ERROR("listen failed: {}", std::strerror(r));
+        SILK_ERROR("listen failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -485,7 +485,7 @@ int TcpConnection::accept(TcpConnection * out) noexcept
     if (r)
     {
         r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -603,7 +603,7 @@ Server::Server(const ServerConfig & cfg)
     for (Worker & worker : workers)
     {
         int r = TcpConnection::listen(cfg.host.c_str(), cfg.port, LISTEN_BACKLOG, &worker.listener);
-        SILK_ASSERT(!r, "listen failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "listen failed: %s", std::strerror(r));
     }
 }
 
@@ -622,7 +622,7 @@ void Server::start()
     for (Worker & worker : workers)
     {
         int r = Event::create(&worker.stopEvent);
-        SILK_ASSERT(!r, "Event::create failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "Event::create failed: %s", std::strerror(r));
         worker.thread = std::thread(workerMain, WorkerParams{this, &worker});
     }
 
@@ -693,7 +693,7 @@ bool Server::drive(Connection * conn, uint32_t msgSize) noexcept
             }
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("read failed: {}", std::strerror(r));
+                SILK_ERROR("read failed: %s", std::strerror(r));
             }
             return false;
         }
@@ -721,7 +721,7 @@ bool Server::drive(Connection * conn, uint32_t msgSize) noexcept
         }
         if (!isExpectedShutdown(r))
         {
-            SILK_ERROR("write failed: {}", std::strerror(r));
+            SILK_ERROR("write failed: %s", std::strerror(r));
         }
         return false;
     }
@@ -734,7 +734,7 @@ void Server::workerMain(WorkerParams params) noexcept
 
     Epoll epoll;
     int r = Epoll::create(&epoll);
-    SILK_ASSERT(!r, "Epoll::create failed: {}", std::strerror(r));
+    SILK_ASSERT(!r, "Epoll::create failed: %s", std::strerror(r));
 
     ListenerTag listenerTag;
     StopTag stopTag;
@@ -755,7 +755,7 @@ void Server::workerMain(WorkerParams params) noexcept
         {
             continue;
         }
-        SILK_ASSERT(!r, "epoll_wait failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "epoll_wait failed: %s", std::strerror(r));
 
         for (int i = 0; i < eventCount; ++i)
         {
@@ -782,7 +782,7 @@ void Server::workerMain(WorkerParams params) noexcept
                         }
                         if (!isExpectedShutdown(ar))
                         {
-                            SILK_ERROR("accept failed: {}", std::strerror(ar));
+                            SILK_ERROR("accept failed: %s", std::strerror(ar));
                         }
                         break;
                     }
@@ -931,7 +931,7 @@ void Client::start()
     for (Connection & connection : connections)
     {
         int r = TcpConnection::connect(cfg.host.c_str(), cfg.port, &connection.conn);
-        SILK_ASSERT(!r, "connect failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "connect failed: %s", std::strerror(r));
         SILK_ASSERT(cfg.msgSize >= sizeof(uint32_t));
         connection.buf = std::make_unique<char[]>(cfg.msgSize);
         std::memset(connection.buf.get(), 0xAB, cfg.msgSize);
@@ -946,7 +946,7 @@ void Client::start()
     {
         Worker & worker = workers[i];
         int r = Event::create(&worker.stopEvent);
-        SILK_ASSERT(!r, "Event::create failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "Event::create failed: %s", std::strerror(r));
         worker.connections = connections.data() + connIdx;
         worker.connCount = base + (i < extra ? 1 : 0);
         connIdx += worker.connCount;
@@ -1036,7 +1036,7 @@ bool Client::drive(Connection * conn, const ClientConfig & cfg, uint64_t warmupE
             }
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("write failed: {}", std::strerror(r));
+                SILK_ERROR("write failed: %s", std::strerror(r));
             }
             return false;
         }
@@ -1074,7 +1074,7 @@ bool Client::drive(Connection * conn, const ClientConfig & cfg, uint64_t warmupE
         }
         if (!isExpectedShutdown(r))
         {
-            SILK_ERROR("read failed: {}", std::strerror(r));
+            SILK_ERROR("read failed: %s", std::strerror(r));
         }
         return false;
     }
@@ -1087,7 +1087,7 @@ void Client::workerMain(WorkerParams params) noexcept
 
     Epoll epoll;
     int r = Epoll::create(&epoll);
-    SILK_ASSERT(!r, "Epoll::create failed: {}", std::strerror(r));
+    SILK_ASSERT(!r, "Epoll::create failed: %s", std::strerror(r));
 
     StopTag stopTag;
     r = epoll.add(worker->stopEvent.getFd(), EPOLLIN, &stopTag);
@@ -1113,7 +1113,7 @@ void Client::workerMain(WorkerParams params) noexcept
         {
             continue;
         }
-        SILK_ASSERT(!r, "epoll_wait failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "epoll_wait failed: %s", std::strerror(r));
 
         uint64_t warmupEndCycles = client->warmupEndCycles.load(std::memory_order_relaxed);
 
@@ -1231,7 +1231,7 @@ static void runServer(int argc, char ** argv)
     sigset_t mask = blockSignals();
     silk::initialize();
 
-    SILK_INFO("starting server on {}:{} with {} thread(s)", cfg.host, cfg.port, cfg.threads);
+    SILK_INFO("starting server on %s:%u with %u thread(s)", cfg.host.c_str(), cfg.port, cfg.threads);
 
     Server server(cfg);
     server.start();
@@ -1313,7 +1313,8 @@ static void runClient(int argc, char ** argv)
     sigset_t mask = blockSignals();
     silk::initialize();
 
-    SILK_INFO("starting client on {}:{} with {} thread(s) and {} connection(s)", cfg.host, cfg.port, cfg.threads, cfg.numConnections);
+    SILK_INFO(
+        "starting client on %s:%u with %u thread(s) and %u connection(s)", cfg.host.c_str(), cfg.port, cfg.threads, cfg.numConnections);
 
     Client client(cfg);
     client.start();
@@ -1322,13 +1323,13 @@ static void runClient(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
diff --git a/src/perf/net-perf.cpp b/src/perf/net-perf.cpp
index fb5e39c..619d617 100644
--- a/src/perf/net-perf.cpp
+++ b/src/perf/net-perf.cpp
@@ -17,7 +17,6 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
-#include <format>
 #include <iostream>
 #include <memory>
 #include <string>
@@ -115,7 +114,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
 
     if (::inet_pton(AF_INET, host, &addr.sin_addr) != 1)
     {
-        SILK_ERROR("inet_pton failed: invalid address {}", host);
+        SILK_ERROR("inet_pton failed: invalid address %s", host);
         return EINVAL;
     }
 
@@ -123,7 +122,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (fd < 0)
     {
         int r = errno;
-        SILK_ERROR("socket failed: {}", std::strerror(r));
+        SILK_ERROR("socket failed: %s", std::strerror(r));
         return r;
     }
 
@@ -131,7 +130,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (::setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &value, sizeof(value)))
     {
         int r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -142,7 +141,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
         r = errno;
         if (r != EINPROGRESS)
         {
-            SILK_ERROR("connect failed: {}", std::strerror(r));
+            SILK_ERROR("connect failed: %s", std::strerror(r));
             ::close(fd);
             return r;
         }
@@ -150,7 +149,7 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
         r = silk::FiberScheduler::poll(fd, POLLOUT);
         if (r)
         {
-            SILK_ERROR("poll failed: {}", std::strerror(r));
+            SILK_ERROR("poll failed: %s", std::strerror(r));
             ::close(fd);
             return r;
         }
@@ -161,13 +160,13 @@ int TcpConnection::connect(const char * host, uint16_t port, TcpConnection * out
     if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &r, &len))
     {
         r = errno;
-        SILK_ERROR("getsockopt SO_ERROR failed: {}", std::strerror(r));
+        SILK_ERROR("getsockopt SO_ERROR failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
     if (r)
     {
-        SILK_ERROR("connect error: {}", std::strerror(r));
+        SILK_ERROR("connect error: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -188,7 +187,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     }
     else if (::inet_pton(AF_INET, host, &addr.sin_addr) != 1)
     {
-        SILK_ERROR("inet_pton failed: invalid address {}", host);
+        SILK_ERROR("inet_pton failed: invalid address %s", host);
         return EINVAL;
     }
 
@@ -196,7 +195,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (fd < 0)
     {
         int r = errno;
-        SILK_ERROR("socket failed: {}", std::strerror(r));
+        SILK_ERROR("socket failed: %s", std::strerror(r));
         return r;
     }
 
@@ -204,7 +203,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (::setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &value, sizeof(value)))
     {
         int r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -212,7 +211,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (::setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &value, sizeof(value)))
     {
         int r = errno;
-        SILK_ERROR("setsockopt SO_REUSEADDR failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt SO_REUSEADDR failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -220,7 +219,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (::bind(fd, reinterpret_cast<sockaddr *>(&addr), sizeof(addr)))
     {
         int r = errno;
-        SILK_ERROR("bind failed: {}", std::strerror(r));
+        SILK_ERROR("bind failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -228,7 +227,7 @@ int TcpConnection::listen(const char * host, uint16_t port, int backlog, TcpConn
     if (::listen(fd, backlog))
     {
         int r = errno;
-        SILK_ERROR("listen failed: {}", std::strerror(r));
+        SILK_ERROR("listen failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -264,7 +263,7 @@ int TcpConnection::accept(TcpConnection * out) noexcept
     if (::setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &value, sizeof(value)))
     {
         int r = errno;
-        SILK_ERROR("setsockopt TCP_NODELAY failed: {}", std::strerror(r));
+        SILK_ERROR("setsockopt TCP_NODELAY failed: %s", std::strerror(r));
         ::close(fd);
         return r;
     }
@@ -443,7 +442,7 @@ Server::Server(const ServerConfig & cfg)
     : cfg(cfg)
 {
     int r = TcpConnection::listen(cfg.host.c_str(), cfg.port, LISTEN_BACKLOG, &listener);
-    SILK_ASSERT(!r, "listen failed: {}", std::strerror(r));
+    SILK_ASSERT(!r, "listen failed: %s", std::strerror(r));
 }
 
 Server::~Server() = default;
@@ -451,7 +450,7 @@ Server::~Server() = default;
 void Server::start()
 {
     int r = silk::FiberScheduler::run(acceptFiberMain, {this}, &acceptFuture);
-    SILK_ASSERT(!r, "cannot start fiber: {}", std::strerror(r));
+    SILK_ASSERT(!r, "cannot start fiber: %s", std::strerror(r));
 
     acceptStarted = true;
 }
@@ -490,7 +489,7 @@ int Server::acceptFiberMain(AcceptFiberParams * params) noexcept
             delete connection;
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("accept failed: {}", strerror(r));
+                SILK_ERROR("accept failed: %s", strerror(r));
             }
             break;
         }
@@ -500,7 +499,7 @@ int Server::acceptFiberMain(AcceptFiberParams * params) noexcept
         r = silk::FiberScheduler::run(serverFiberMain, {server, connection}, &connection->future);
         if (r)
         {
-            SILK_ERROR("cannot start fiber: {}", std::strerror(r));
+            SILK_ERROR("cannot start fiber: %s", std::strerror(r));
             break;
         }
     }
@@ -523,7 +522,7 @@ int Server::serverFiberMain(ServerFiberParams * params) noexcept
         {
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("read failed: {}", strerror(r));
+                SILK_ERROR("read failed: %s", strerror(r));
             }
             break;
         }
@@ -544,7 +543,7 @@ int Server::serverFiberMain(ServerFiberParams * params) noexcept
         {
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("write failed: {}", strerror(r));
+                SILK_ERROR("write failed: %s", strerror(r));
             }
             break;
         }
@@ -619,13 +618,13 @@ void Client::start()
     for (Connection & connection : connections)
     {
         int r = TcpConnection::connect(cfg.host.c_str(), cfg.port, &connection.conn);
-        SILK_ASSERT(!r, "connect failed: {}", std::strerror(r));
+        SILK_ASSERT(!r, "connect failed: %s", std::strerror(r));
     }
 
     for (Connection & connection : connections)
     {
         int r = silk::FiberScheduler::run(clientFiberMain, {this, &connection}, &connection.future);
-        SILK_ASSERT(!r, "cannot start fiber: {}", std::strerror(r));
+        SILK_ASSERT(!r, "cannot start fiber: %s", std::strerror(r));
     }
 
     warmupEndCycles.store(silk::Tsc::getCycles() + silk::Tsc::nanosecondsToCycles(cfg.warmupNs), std::memory_order_relaxed);
@@ -678,7 +677,7 @@ int Client::clientFiberMain(ClientFiberParams * params) noexcept
         {
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("write failed: {}", strerror(r));
+                SILK_ERROR("write failed: %s", strerror(r));
             }
             break;
         }
@@ -688,7 +687,7 @@ int Client::clientFiberMain(ClientFiberParams * params) noexcept
         {
             if (!isExpectedShutdown(r))
             {
-                SILK_ERROR("read failed: {}", strerror(r));
+                SILK_ERROR("read failed: %s", strerror(r));
             }
             break;
         }
@@ -783,7 +782,7 @@ static void runServer(int argc, char ** argv)
     silk::FiberScheduler::Options options{.enableProfiler = cfg.printCounters};
     silk::FiberScheduler::initialize(&options);
 
-    SILK_INFO("starting server on {}:{}", cfg.host, cfg.port);
+    SILK_INFO("starting server on %s:%u", cfg.host.c_str(), cfg.port);
 
     Server server(cfg);
     server.start();
@@ -869,7 +868,7 @@ static void runClient(int argc, char ** argv)
     silk::FiberScheduler::Options options{.enableProfiler = cfg.printCounters};
     silk::FiberScheduler::initialize(&options);
 
-    SILK_INFO("starting client on {}:{}", cfg.host, cfg.port);
+    SILK_INFO("starting client on %s:%u", cfg.host.c_str(), cfg.port);
 
     Client client(cfg);
     client.start();
@@ -878,13 +877,13 @@ static void runClient(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
diff --git a/src/perf/s3-perf.cpp b/src/perf/s3-perf.cpp
index 83c3152..34822d9 100644
--- a/src/perf/s3-perf.cpp
+++ b/src/perf/s3-perf.cpp
@@ -323,7 +323,7 @@ const char * PocoHttpClient::getMethod(Aws::Http::HttpMethod method)
         case Aws::Http::HttpMethod::HTTP_TRACE:
             return "TRACE";
     }
-    SILK_ASSERT(false);
+    SILK_FAIL("unknown HTTP method", "value=%d", static_cast<int>(method));
 }
 
 void PocoHttpClient::writeHeaders(Poco::Net::HTTPRequest & pocoReq, const Aws::Http::HeaderValueCollection & headers)
@@ -514,7 +514,7 @@ int FiberExecutor::execFiberMain(ExecParams * params) noexcept
         // The SDK wraps tasks in std::packaged_task, which captures exceptions
         // into the promise before rethrowing. The session thread sees the error
         // via callable.get(). We catch here only to satisfy noexcept.
-        SILK_ERROR("fiber executor task exception: {}", e.what());
+        SILK_ERROR("fiber executor task exception: %s", e.what());
     }
     return 0;
 }
@@ -703,7 +703,7 @@ void S3Bench::runSession(Session * session)
             // endpoint or a transient failure every session exits after one bad outcome,
             // and the run produces no useful latency data. failedSessions exposes the
             // count so the caller can warn.
-            SILK_ERROR("S3 request failed: {}", slots[head].error->GetMessage());
+            SILK_ERROR("S3 request failed: %s", slots[head].error->GetMessage().c_str());
             session->failed = true;
             head = (head + 1) % cfg.ioDepth;
             break;
@@ -829,13 +829,13 @@ int main(int argc, char ** argv)
 
     if (cfg.warmupNs > 0)
     {
-        SILK_INFO("warming up for {}...", formatDuration(cfg.warmupNs));
+        SILK_INFO("warming up for %s...", formatDuration(cfg.warmupNs).c_str());
         signalled = sigwaitFor(mask, cfg.warmupNs);
     }
 
     if (!signalled)
     {
-        SILK_INFO("measuring for {}...", formatDuration(cfg.durationNs));
+        SILK_INFO("measuring for %s...", formatDuration(cfg.durationNs).c_str());
         sigwaitFor(mask, cfg.durationNs);
     }
 
@@ -847,7 +847,7 @@ int main(int argc, char ** argv)
     uint32_t failedSessions = bench.failedSessions();
     if (failedSessions > 0)
     {
-        SILK_WARN("{}/{} sessions exited early due to S3 errors", failedSessions, cfg.numJobs);
+        SILK_WARN("%u/%u sessions exited early due to S3 errors", failedSessions, cfg.numJobs);
     }
 
     auto latencies = bench.collectLatencies();
diff --git a/src/profiler/main.cpp b/src/profiler/main.cpp
index a759557..02d9ff8 100644
--- a/src/profiler/main.cpp
+++ b/src/profiler/main.cpp
@@ -120,8 +120,8 @@ int main(int argc, char ** argv)
 
     if (!hasBpf || !hasPerfmon)
     {
-        SILK_ERROR("missing required capabilities (CAP_BPF={}, CAP_PERFMON={})", hasBpf, hasPerfmon);
-        SILK_ERROR("run once: sudo setcap cap_bpf,cap_perfmon,cap_syslog+eip {}", argv[0]);
+        SILK_ERROR("missing required capabilities (CAP_BPF=%d, CAP_PERFMON=%d)", hasBpf, hasPerfmon);
+        SILK_ERROR("run once: sudo setcap cap_bpf,cap_perfmon,cap_syslog+eip %s", argv[0]);
         return 1;
     }
 
@@ -133,7 +133,7 @@ int main(int argc, char ** argv)
     int r = symbolizer.readSelfMappings();
     if (r)
     {
-        SILK_ERROR("readSelfMappings: {}", strerror(r));
+        SILK_ERROR("readSelfMappings: %s", strerror(r));
         return r;
     }
 
@@ -141,7 +141,7 @@ int main(int argc, char ** argv)
     r = symbolizer.readMappings(targetPid);
     if (r)
     {
-        SILK_ERROR("readMappings: {}", strerror(r));
+        SILK_ERROR("readMappings: %s", strerror(r));
         return r;
     }
 
@@ -155,7 +155,7 @@ int main(int argc, char ** argv)
         else
         {
             SILK_WARN("CAP_SYSLOG missing -- kernel frames will appear as hex addresses");
-            SILK_WARN("run once: sudo setcap cap_bpf,cap_perfmon,cap_syslog+eip {}", argv[0]);
+            SILK_WARN("run once: sudo setcap cap_bpf,cap_perfmon,cap_syslog+eip %s", argv[0]);
         }
     }
 
@@ -164,17 +164,17 @@ int main(int argc, char ** argv)
     r = profiler.start();
     if (r)
     {
-        SILK_ERROR("profiler start failed: {}", strerror(r));
+        SILK_ERROR("profiler start failed: %s", strerror(r));
         return r;
     }
 
     if (durationSec > 0)
     {
-        SILK_INFO("profiling pid {} at {} Hz for {} seconds", targetPid, sampleHz, durationSec);
+        SILK_INFO("profiling pid %u at %u Hz for %u seconds", targetPid, sampleHz, durationSec);
     }
     else
     {
-        SILK_INFO("profiling pid {} at {} Hz -- Ctrl+C to stop", targetPid, sampleHz);
+        SILK_INFO("profiling pid %u at %u Hz -- Ctrl+C to stop", targetPid, sampleHz);
     }
 
     uint64_t waitNs = durationSec > 0 ? (uint64_t)durationSec * 1'000'000'000ULL : UINT64_MAX;
@@ -189,7 +189,7 @@ int main(int argc, char ** argv)
     }
     catch (const std::exception & ex)
     {
-        SILK_ERROR("collect: {}", ex.what());
+        SILK_ERROR("collect: %s", ex.what());
         return 1;
     }
 
diff --git a/src/profiler/profiler.cpp b/src/profiler/profiler.cpp
index a57f45d..380cc17 100644
--- a/src/profiler/profiler.cpp
+++ b/src/profiler/profiler.cpp
@@ -148,7 +148,7 @@ int Profiler::start() noexcept
                     // CPU offline
                     continue;
                 }
-                SILK_WARN("perf_event_open cpu {}: {}", cpu, strerror(r));
+                SILK_WARN("perf_event_open cpu %d: %s", cpu, strerror(r));
                 continue;
             }
 
@@ -156,7 +156,7 @@ int Profiler::start() noexcept
             if (!link)
             {
                 int r = errno;
-                SILK_WARN("attach perf_event cpu {}: {}", cpu, strerror(r));
+                SILK_WARN("attach perf_event cpu %d: %s", cpu, strerror(r));
                 ::close(fd);
                 continue;
             }
diff --git a/src/profiler/symbolizer.cpp b/src/profiler/symbolizer.cpp
index b04e86f..099c465 100644
--- a/src/profiler/symbolizer.cpp
+++ b/src/profiler/symbolizer.cpp
@@ -8,7 +8,6 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
-#include <format>
 #include <utility>
 
 #include <backtrace.h>
@@ -21,7 +20,7 @@ static int parseMapsFile(const char * path, Callback callback) noexcept
     if (!f)
     {
         int r = errno;
-        SILK_ERROR("open {}: {}", path, ::strerror(r));
+        SILK_ERROR("open %s: %s", path, ::strerror(r));
         return r;
     }
 
@@ -82,7 +81,7 @@ int Symbolizer::readKallsyms() noexcept
     if (!f)
     {
         int r = errno;
-        SILK_ERROR("open /proc/kallsyms: {}", ::strerror(r));
+        SILK_ERROR("open /proc/kallsyms: %s", ::strerror(r));
         return r;
     }
 
@@ -138,7 +137,9 @@ const std::string & Symbolizer::resolve(uint64_t addr)
         }
         if (result.empty())
         {
-            result = std::format("0x{:x}", addr);
+            char buf[32];
+            std::snprintf(buf, sizeof(buf), "0x%lx", addr);
+            result = buf;
         }
         auto [it2, _] = cache.emplace(addr, std::move(result));
         return it2->second;
@@ -172,7 +173,7 @@ const std::string & Symbolizer::resolve(uint64_t addr)
 
         if (!state)
         {
-            SILK_WARN("0x{:x}: no backtrace state for {}", addr, m.path);
+            SILK_WARN("0x%lx: no backtrace state for %s", addr, m.path.c_str());
             break;
         }
 
@@ -232,18 +233,27 @@ const std::string & Symbolizer::resolve(uint64_t addr)
     {
         if (mappingFound)
         {
-            SILK_DEBUG("0x{:x}: in mapping but not symbolized (elfAddr=0x{:x} {})", addr, mappingElfAddr, mappingPath);
+            SILK_DEBUG(
+                "0x%lx: in mapping but not symbolized (elfAddr=0x%lx %.*s)",
+                addr,
+                mappingElfAddr,
+                static_cast<int>(mappingPath.size()),
+                mappingPath.data());
             std::string_view base = mappingPath;
             if (auto slash = base.rfind('/'); slash != std::string_view::npos)
             {
                 base = base.substr(slash + 1);
             }
-            result = std::format("{}+0x{:x}", base, mappingElfAddr);
+            char buf[256];
+            std::snprintf(buf, sizeof(buf), "%.*s+0x%lx", static_cast<int>(base.size()), base.data(), mappingElfAddr);
+            result = buf;
         }
         else
         {
-            SILK_DEBUG("0x{:x}: no mapping (library loaded after readMappings?)", addr);
-            result = std::format("0x{:x}", addr);
+            SILK_DEBUG("0x%lx: no mapping (library loaded after readMappings?)", addr);
+            char buf[32];
+            std::snprintf(buf, sizeof(buf), "0x%lx", addr);
+            result = buf;
         }
     }
 
@@ -257,11 +267,11 @@ void Symbolizer::backtraceErrorCb(void * data, const char * msg, int errnum)
 
     if (errnum)
     {
-        SILK_ERROR("libbacktrace: {} ({})", msg, ::strerror(errnum));
+        SILK_ERROR("libbacktrace: %s (%s)", msg, ::strerror(errnum));
     }
     else
     {
-        SILK_ERROR("libbacktrace: {}", msg);
+        SILK_ERROR("libbacktrace: %s", msg);
     }
 }
 
diff --git a/src/util/assert.cpp b/src/util/assert.cpp
index e39f2ac..4f3e29c 100644
--- a/src/util/assert.cpp
+++ b/src/util/assert.cpp
@@ -2,15 +2,12 @@
 
 #include <silk/util/platform.h>
 
+#include <cstdarg>
 #include <cstdio>
 #include <cstdlib>
-#include <sstream>
+#include <cstring>
 
 #if defined(SILK_USE_LIBBACKTRACE)
-#    include <cstring>
-#    include <format>
-#    include <string_view>
-
 #    include <backtrace.h>
 #    include <cxxabi.h>
 #endif // SILK_USE_LIBBACKTRACE
@@ -35,14 +32,13 @@ static backtrace_state * btState = backtrace_create_state(nullptr, 1, btCreateEr
 
 struct Context
 {
-    std::ostringstream * out;
     int frame = 0;
 };
 
 static int btCallback(void * data, uintptr_t pc, const char * filename, int lineno, const char * function) noexcept
 {
     Context * ctx = static_cast<Context *>(data);
-    (*ctx->out) << "#" << ctx->frame++ << "  " << std::format("{:#018x}", pc) << " in " << COLOR_YELLOW;
+    std::fprintf(stderr, "#%d  %#018lx in %s", ctx->frame++, static_cast<unsigned long>(pc), COLOR_YELLOW);
 
     int status = 0;
     char * demangled = function ? abi::__cxa_demangle(function, nullptr, nullptr, &status) : nullptr;
@@ -51,24 +47,24 @@ static int btCallback(void * data, uintptr_t pc, const char * filename, int line
         const char * paren = std::strchr(demangled, '(');
         if (paren)
         {
-            (*ctx->out) << std::string_view(demangled, paren - demangled);
-            (*ctx->out) << COLOR_RESET << " " << paren;
+            std::fwrite(demangled, 1, static_cast<size_t>(paren - demangled), stderr);
+            std::fprintf(stderr, "%s %s", COLOR_RESET, paren);
         }
         else
         {
-            (*ctx->out) << demangled << COLOR_RESET << " ()";
+            std::fprintf(stderr, "%s%s ()", demangled, COLOR_RESET);
         }
     }
     else
     {
-        (*ctx->out) << (function ? function : "??") << COLOR_RESET << " ()";
+        std::fprintf(stderr, "%s%s ()", function ? function : "??", COLOR_RESET);
     }
 
     if (filename)
     {
-        (*ctx->out) << " at " << COLOR_GREEN << filename << COLOR_RESET << ":" << lineno;
+        std::fprintf(stderr, " at %s%s%s:%d", COLOR_GREEN, filename, COLOR_RESET, lineno);
     }
-    (*ctx->out) << "\n";
+    std::fputc('\n', stderr);
 
     std::free(demangled);
     return 0;
@@ -76,28 +72,34 @@ static int btCallback(void * data, uintptr_t pc, const char * filename, int line
 
 static void btErrorCallback(void * data, const char * msg, int err) noexcept
 {
-    Context * ctx = static_cast<Context *>(data);
-    (*ctx->out) << "  backtrace error " << err << ": " << msg << "\n";
+    SILK_UNUSED(data);
+    std::fprintf(stderr, "  backtrace error %d: %s\n", err, msg);
 }
 
 #endif // SILK_USE_LIBBACKTRACE
 
-void assertFail(const char * message, const char * file, int line, const char * details) noexcept
+void assertFail(const char * message, const char * file, int line, const char * fmt, ...) noexcept
 {
-    std::ostringstream out;
-    out << COLOR_RED << file << ":" << line << " " << message;
-    if (details)
+    ::flockfile(stderr);
+
+    std::fprintf(stderr, "%s%s:%d %s", COLOR_RED, file, line, message);
+    if (fmt)
     {
-        out << " -- " << details;
+        std::fputs(" -- ", stderr);
+        va_list ap;
+        va_start(ap, fmt);
+        std::vfprintf(stderr, fmt, ap);
+        va_end(ap);
     }
-    out << COLOR_RESET << "\n";
+    std::fprintf(stderr, "%s\n", COLOR_RESET);
 
 #if defined(SILK_USE_LIBBACKTRACE)
-    Context ctx{&out, 0};
+    Context ctx;
     backtrace_full(btState, 0, btCallback, btErrorCallback, &ctx);
 #endif
 
-    std::fputs(out.str().c_str(), stderr);
+    ::funlockfile(stderr);
+
     std::fflush(stderr);
     std::abort();
 }
diff --git a/src/util/logger.cpp b/src/util/logger.cpp
index 53083ee..b34fe2c 100644
--- a/src/util/logger.cpp
+++ b/src/util/logger.cpp
@@ -1,20 +1,16 @@
 #include <silk/util/logger.h>
 
+#include <cstdarg>
 #include <cstdio>
+#include <cstdlib>
 #include <ctime>
-#include <string_view>
 
 namespace silk
 {
 
-static constexpr std::string_view LEVEL_NAMES[] = {"DEBUG", "INFO ", "WARN ", "ERROR"};
+static constexpr const char * LEVEL_NAMES[] = {"DEBUG", "INFO ", "WARN ", "ERROR"};
 
-void Logger::setLevel(LogLevel level) noexcept
-{
-    currentLevel.store(level, std::memory_order_relaxed);
-}
-
-void Logger::log(LogLevel level, std::string_view file, int line, std::string_view message) noexcept
+void Logger::log(LogLevel level, const char * file, int line, const char * message) noexcept
 {
     timespec ts;
     ::clock_gettime(CLOCK_REALTIME, &ts);
@@ -24,7 +20,7 @@ void Logger::log(LogLevel level, std::string_view file, int line, std::string_vi
 
     std::fprintf(
         stderr,
-        "%04d-%02d-%02d %02d:%02d:%02d.%03ld [%s] %.*s:%d: %.*s\n",
+        "%04d-%02d-%02d %02d:%02d:%02d.%03ld [%s] %s:%d: %s\n",
         t.tm_year + 1900,
         t.tm_mon + 1,
         t.tm_mday,
@@ -32,12 +28,46 @@ void Logger::log(LogLevel level, std::string_view file, int line, std::string_vi
         t.tm_min,
         t.tm_sec,
         ts.tv_nsec / 1'000'000,
-        LEVEL_NAMES[static_cast<int>(level)].data(),
-        static_cast<int>(file.size()),
-        file.data(),
+        LEVEL_NAMES[static_cast<int>(level)],
+        file,
         line,
-        static_cast<int>(message.size()),
-        message.data());
+        message);
+}
+
+void Logger::logf(LogLevel level, const char * file, int line, const char * fmt, ...) noexcept
+{
+    // Format into a small stack buffer, falling back to malloc on overflow,
+    // then forward to the pre-formatted log for the actual emission.
+
+    char stackBody[256];
+    char * body = stackBody;
+    char * heapBody = nullptr;
+
+    va_list ap;
+    va_start(ap, fmt);
+    va_list ap2;
+    va_copy(ap2, ap);
+    int n = std::vsnprintf(stackBody, sizeof(stackBody), fmt, ap);
+    va_end(ap);
+    if (n < 0)
+    {
+        n = 0;
+    }
+
+    if (n >= static_cast<int>(sizeof(stackBody)))
+    {
+        heapBody = static_cast<char *>(std::malloc(static_cast<size_t>(n) + 1));
+        if (heapBody)
+        {
+            std::vsnprintf(heapBody, static_cast<size_t>(n) + 1, fmt, ap2);
+            body = heapBody;
+        }
+    }
+    va_end(ap2);
+
+    log(level, file, line, body);
+
+    std::free(heapBody);
 }
 
 } // namespace silk
diff --git a/src/util/tsc.cpp b/src/util/tsc.cpp
index 7c06dfb..6304ad8 100644
--- a/src/util/tsc.cpp
+++ b/src/util/tsc.cpp
@@ -3,7 +3,6 @@
 #include <silk/util/assert.h>
 
 #include <cstring>
-#include <format>
 
 #if defined(__x86_64__)
 #    include <cpuid.h>