diff --git a/_codeql_detected_source_root b/_codeql_detected_source_root
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+.
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diff --git a/test/test_formatter.cpp b/test/test_formatter.cpp
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+#include <gtest/gtest.h>
+#include <anyhttp/formatter.hpp>
+
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/beast/http/field.hpp>
+#include <boost/core/detail/string_view.hpp>
+#include <boost/url/authority_view.hpp>
+#include <boost/url/pct_string_view.hpp>
+#include <nghttp2/nghttp2.h>
+
+#include <format>
+#include <thread>
+
+// =================================================================================================
+// Test thread_id formatter
+// =================================================================================================
+
+TEST(FormatterTest, ThreadId)
+{
+   auto tid = std::this_thread::get_id();
+   auto formatted = std::format("{}", tid);
+   EXPECT_FALSE(formatted.empty());
+}
+
+// =================================================================================================
+// Test boost::urls::pct_string_view formatter
+// =================================================================================================
+
+TEST(FormatterTest, PctStringView)
+{
+   boost::urls::pct_string_view psv("hello%20world");
+   auto formatted = std::format("{}", psv);
+   EXPECT_EQ(formatted, "hello%20world");
+}
+
+// =================================================================================================
+// Test boost::urls::authority_view formatter
+// =================================================================================================
+
+TEST(FormatterTest, AuthorityView)
+{
+   boost::urls::authority_view av("user:pass@example.com:8080");
+   auto formatted = std::format("{}", av);
+   EXPECT_EQ(formatted, "user:pass@example.com:8080");
+}
+
+TEST(FormatterTest, AuthorityViewSimple)
+{
+   boost::urls::authority_view av("example.com");
+   auto formatted = std::format("{}", av);
+   EXPECT_EQ(formatted, "example.com");
+}
+
+// =================================================================================================
+// Test boost::core::string_view formatter
+// =================================================================================================
+
+TEST(FormatterTest, BoostStringView)
+{
+   boost::core::string_view sv("test string");
+   auto formatted = std::format("{}", sv);
+   EXPECT_EQ(formatted, "test string");
+}
+
+TEST(FormatterTest, BoostStringViewEmpty)
+{
+   boost::core::string_view sv("");
+   auto formatted = std::format("{}", sv);
+   EXPECT_EQ(formatted, "");
+}
+
+// =================================================================================================
+// Test boost::asio::ip::tcp::endpoint formatter
+// =================================================================================================
+
+TEST(FormatterTest, EndpointIPv4)
+{
+   auto addr = boost::asio::ip::make_address_v4("192.168.1.1");
+   boost::asio::ip::tcp::endpoint endpoint(addr, 8080);
+   auto formatted = std::format("{}", endpoint);
+   EXPECT_EQ(formatted, "192.168.1.1:8080");
+}
+
+TEST(FormatterTest, EndpointIPv6)
+{
+   auto addr = boost::asio::ip::make_address_v6("::1");
+   boost::asio::ip::tcp::endpoint endpoint(addr, 9090);
+   auto formatted = std::format("{}", endpoint);
+   EXPECT_EQ(formatted, "[::1]:9090");
+}
+
+TEST(FormatterTest, EndpointIPv6Full)
+{
+   auto addr = boost::asio::ip::make_address_v6("2001:db8::1");
+   boost::asio::ip::tcp::endpoint endpoint(addr, 443);
+   auto formatted = std::format("{}", endpoint);
+   EXPECT_EQ(formatted, "[2001:db8::1]:443");
+}
+
+// =================================================================================================
+// Test boost::beast::http::field formatter
+// =================================================================================================
+
+TEST(FormatterTest, HttpField)
+{
+   auto field = boost::beast::http::field::content_type;
+   auto formatted = std::format("{}", field);
+   EXPECT_EQ(formatted, "Content-Type");
+}
+
+TEST(FormatterTest, HttpFieldAccept)
+{
+   auto field = boost::beast::http::field::accept;
+   auto formatted = std::format("{}", field);
+   EXPECT_EQ(formatted, "Accept");
+}
+
+TEST(FormatterTest, HttpFieldUserAgent)
+{
+   auto field = boost::beast::http::field::user_agent;
+   auto formatted = std::format("{}", field);
+   EXPECT_EQ(formatted, "User-Agent");
+}
+
+// =================================================================================================
+// Test boost::asio::cancellation_type formatter
+// =================================================================================================
+
+TEST(FormatterTest, CancellationTypeNone)
+{
+   auto ct = boost::asio::cancellation_type::none;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "none");
+}
+
+TEST(FormatterTest, CancellationTypeAll)
+{
+   auto ct = boost::asio::cancellation_type::all;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "all");
+}
+
+TEST(FormatterTest, CancellationTypeTerminal)
+{
+   auto ct = boost::asio::cancellation_type::terminal;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "terminal");
+}
+
+TEST(FormatterTest, CancellationTypePartial)
+{
+   auto ct = boost::asio::cancellation_type::partial;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "partial");
+}
+
+TEST(FormatterTest, CancellationTypeTotal)
+{
+   auto ct = boost::asio::cancellation_type::total;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "total");
+}
+
+TEST(FormatterTest, CancellationTypeCombined)
+{
+   auto ct = boost::asio::cancellation_type::terminal | boost::asio::cancellation_type::partial;
+   auto formatted = std::format("{}", ct);
+   // The order is: terminal, partial, total
+   EXPECT_EQ(formatted, "terminal|partial");
+}
+
+TEST(FormatterTest, CancellationTypeMultipleCombined)
+{
+   auto ct = boost::asio::cancellation_type::terminal | boost::asio::cancellation_type::total;
+   auto formatted = std::format("{}", ct);
+   EXPECT_EQ(formatted, "terminal|total");
+}
+
+// =================================================================================================
+// Test nghttp2_nv formatter
+// =================================================================================================
+
+namespace
+{
+   // Helper to create nghttp2_nv from string literals
+   // Note: const_cast is safe here as the formatter doesn't modify the data
+   nghttp2_nv make_nghttp2_nv(const char* name, const char* value)
+   {
+      return nghttp2_nv{
+         const_cast<uint8_t*>(reinterpret_cast<const uint8_t*>(name)),
+         const_cast<uint8_t*>(reinterpret_cast<const uint8_t*>(value)),
+         strlen(name),
+         strlen(value),
+         NGHTTP2_NV_FLAG_NONE
+      };
+   }
+}
+
+TEST(FormatterTest, NgHttp2NvDefault)
+{
+   auto nv = make_nghttp2_nv("content-type", "text/html");
+   auto formatted = std::format("{}", nv);
+   EXPECT_EQ(formatted, "content-type=text/html");
+}
+
+TEST(FormatterTest, NgHttp2NvNameOnly)
+{
+   auto nv = make_nghttp2_nv("accept-encoding", "gzip, deflate");
+   auto formatted = std::format("{:n}", nv);
+   EXPECT_EQ(formatted, "accept-encoding");
+}
+
+TEST(FormatterTest, NgHttp2NvValueOnly)
+{
+   auto nv = make_nghttp2_nv("user-agent", "Mozilla/5.0");
+   auto formatted = std::format("{:v}", nv);
+   EXPECT_EQ(formatted, "Mozilla/5.0");
+}
+
+TEST(FormatterTest, NgHttp2NvEmptyName)
+{
+   auto nv = make_nghttp2_nv("", "some-value");
+   auto formatted = std::format("{}", nv);
+   EXPECT_EQ(formatted, "=some-value");
+}
+
+TEST(FormatterTest, NgHttp2NvEmptyValue)
+{
+   auto nv = make_nghttp2_nv("some-header", "");
+   auto formatted = std::format("{}", nv);
+   EXPECT_EQ(formatted, "some-header=");
+}