diff --git a/design.md b/design.md
index d5fb42b..ea5f390 100644
--- a/design.md
+++ b/design.md
@@ -327,6 +327,7 @@ This write-side component is designed to integrate with:
 
 - **Configuration**: All configuration is TOML-based using `config.toml` (see `config.md`)
 - **Testing Strategy**: Run unit tests, benchmarks, and debug tools before submitting changes
+- **Test Design**: Prefer testing through public interfaces rather than implementation details (see `style.md` for detailed testing guidelines)
 - **Build System**: CMake generates gperf hash tables at build time; always use ninja
 - **Test Synchronization**:
   - **ABSOLUTELY NEVER use sleep(), std::this_thread::sleep_for(), or any timeout-based waiting in tests**
diff --git a/style.md b/style.md
index 2e6c7cf..759047f 100644
--- a/style.md
+++ b/style.md
@@ -379,6 +379,28 @@ TEST_CASE("ArenaAllocator basic allocation") {
 }
 ```
 
+### Test Design Principles
+- **Prefer testing through public interfaces** - focus on observable behavior rather than implementation details
+- **Test the contract, not the implementation** - validate what the API promises to deliver
+- **Avoid testing private methods directly** - if private functionality needs testing, consider if it should be public or extracted
+- **Integration over isolation** - test components working together when practical
+- **Mock only external dependencies** - prefer real implementations for internal components
+```cpp
+// Good: Testing through public API
+TEST_CASE("Server accepts connections") {
+  auto config = Config::defaultConfig();
+  auto handler = std::make_unique<TestHandler>();
+  auto server = Server::create(config, std::move(handler));
+
+  // Test observable behavior - server can accept connections
+  auto result = connectToServer(server->getPort());
+  CHECK(result.connected);
+}
+
+// Avoid: Testing internal implementation details
+// TEST_CASE("Server creates epoll instance") { /* implementation detail */ }
+```
+
 ### Test Synchronization
 - **NEVER use timeouts** or sleep-based synchronization
 - **Deterministic synchronization only:**
diff --git a/tests/test_http_handler.cpp b/tests/test_http_handler.cpp
index cc97913..84cc5c0 100644
--- a/tests/test_http_handler.cpp
+++ b/tests/test_http_handler.cpp
@@ -68,7 +68,7 @@ TEST_CASE("HttpHandler route parsing") {
   }
 }
 
-TEST_CASE("HttpHandler route parsing functionality") {
+TEST_CASE("HttpHandler route parsing edge cases") {
   // Test just the static route parsing method since full integration testing
   // would require complex Connection setup with server dependencies
 
diff --git a/tests/test_server_connection_return.cpp b/tests/test_server_connection_return.cpp
index c73d3a9..fb377ca 100644
--- a/tests/test_server_connection_return.cpp
+++ b/tests/test_server_connection_return.cpp
@@ -35,7 +35,8 @@ public:
   }
 };
 
-TEST_CASE("Echo server with connection ownership transfer") {
+TEST_CASE(
+    "Server correctly handles connection ownership transfer via pipeline") {
   weaseldb::Config config;
   config.server.io_threads = 1;
   config.server.epoll_instances = 1;