diff --git a/AGENTS.md b/AGENTS.md
index 7358b25..ca48772 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -91,6 +91,8 @@ to the custom scripts instead of system-installed packages.
 - Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`,
   etc.)
 - Separate subject from body with a blank line
+- Do **not** add yourself as a co-author (`Co-Authored-By:` trailers are
+  forbidden)
 
 Example:
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 25c4b2f..9153b43 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -16,7 +16,7 @@ add_subdirectory(ctdd)
 add_executable(main main.c)
 target_include_directories(main PRIVATE .)
 target_compile_features(main PRIVATE c_std_23)
-target_link_libraries(main PRIVATE ctdd_str ctdd_report ctdd_logger)
+target_link_libraries(main PRIVATE ctdd_str ctdd_report ctdd_logger ctdd_log_write)
 
 # Testing
 enable_testing()
diff --git a/ctdd/CMakeLists.txt b/ctdd/CMakeLists.txt
index 23dd818..9977380 100644
--- a/ctdd/CMakeLists.txt
+++ b/ctdd/CMakeLists.txt
@@ -2,12 +2,17 @@ add_library(ctdd_str STATIC str.c)
 target_include_directories(ctdd_str PUBLIC "${CMAKE_SOURCE_DIR}")
 target_compile_features(ctdd_str PRIVATE c_std_23)
 
-# Reporter — calls log_message(); symbol resolved by the final binary
+# Reporter — calls log_info(); symbol resolved by the final binary
 add_library(ctdd_report STATIC report.c)
 target_include_directories(ctdd_report PUBLIC "${CMAKE_SOURCE_DIR}")
 target_compile_features(ctdd_report PRIVATE c_std_23)
 
-# Real log_message implementation — linked into production binaries only
+# Level-filtering logger — calls log_write(); symbol resolved by the final binary
 add_library(ctdd_logger STATIC logger.c)
 target_include_directories(ctdd_logger PUBLIC "${CMAKE_SOURCE_DIR}")
 target_compile_features(ctdd_logger PRIVATE c_std_23)
+
+# Real log_write implementation — linked into production binaries only
+add_library(ctdd_log_write STATIC log_write.c)
+target_include_directories(ctdd_log_write PUBLIC "${CMAKE_SOURCE_DIR}")
+target_compile_features(ctdd_log_write PRIVATE c_std_23)
diff --git a/ctdd/log_write.c b/ctdd/log_write.c
new file mode 100644
index 0000000..3558d81
--- /dev/null
+++ b/ctdd/log_write.c
@@ -0,0 +1,6 @@
+#include "ctdd/log_write.h"
+#include <stdio.h>
+
+void log_write(char const* msg) {
+    printf("%s\n", msg);
+}
diff --git a/ctdd/log_write.h b/ctdd/log_write.h
new file mode 100644
index 0000000..87ea5ce
--- /dev/null
+++ b/ctdd/log_write.h
@@ -0,0 +1,3 @@
+#pragma once
+
+void log_write(char const* msg);
diff --git a/ctdd/logger.c b/ctdd/logger.c
index 5ec1106..13cbee6 100644
--- a/ctdd/logger.c
+++ b/ctdd/logger.c
@@ -1,6 +1,18 @@
 #include "ctdd/logger.h"
+#include "ctdd/log_write.h"
 #include <stdio.h>
 
-void log_message(char const* msg) {
-    printf("[LOG] %s\n", msg);
+static log_level s_level = LOG_DEBUG;
+
+void logger_set_level(log_level level) { s_level = level; }
+
+static void emit(char const* prefix, char const* msg) {
+    char buf[512];
+    snprintf(buf, sizeof(buf), "[%s] %s", prefix, msg);
+    log_write(buf);
 }
+
+void log_debug(char const* msg) { if (s_level <= LOG_DEBUG) emit("DEBUG", msg); }
+void log_info(char const* msg)  { if (s_level <= LOG_INFO)  emit("INFO",  msg); }
+void log_warn(char const* msg)  { if (s_level <= LOG_WARN)  emit("WARN",  msg); }
+void log_err(char const* msg)   { if (s_level <= LOG_ERROR) emit("ERROR", msg); }
diff --git a/ctdd/logger.h b/ctdd/logger.h
index 919a6e2..10127bc 100644
--- a/ctdd/logger.h
+++ b/ctdd/logger.h
@@ -1,3 +1,9 @@
 #pragma once
 
-void log_message(char const* msg);
+typedef enum { LOG_DEBUG = 0, LOG_INFO, LOG_WARN, LOG_ERROR, LOG_NONE } log_level;
+
+void logger_set_level(log_level level);
+void log_debug(char const* msg);
+void log_info(char const* msg);
+void log_warn(char const* msg);
+void log_err(char const* msg);
diff --git a/ctdd/report.c b/ctdd/report.c
index 4232430..f33a7db 100644
--- a/ctdd/report.c
+++ b/ctdd/report.c
@@ -5,5 +5,5 @@
 void report_value(char const* label, int value) {
     char buf[256];
     snprintf(buf, sizeof(buf), "%s: %d", label, value);
-    log_message(buf);
+    log_info(buf);
 }
diff --git a/main.c b/main.c
index 528b65c..03f1a18 100644
--- a/main.c
+++ b/main.c
@@ -13,7 +13,7 @@ int main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) {
 
     char upper[32];
     str_upper(upper, "hello, tdd", sizeof(upper));
-    log_message(upper);
+    log_info(upper);
 
     return 0;
 }
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 87e5851..d61e1d6 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -25,8 +25,6 @@ function(cmock_generate_mock target header)
         VERBATIM
     )
     target_sources("${target}" PRIVATE "${mock_src}")
-    # MOCK_GEN_DIR for the generated header; header_dir so the generated
-    # #include "logger.h" resolves without the ctdd/ prefix
     target_include_directories("${target}" PRIVATE "${MOCK_GEN_DIR}" "${header_dir}")
 endfunction()
 
@@ -37,10 +35,18 @@ target_link_libraries(test_str PRIVATE ctdd_str Unity::Unity)
 target_compile_features(test_str PRIVATE c_std_23)
 add_test(NAME test_str COMMAND test_str)
 
-# report tests — CMock-generated mock for log_message
+# report tests — mocks logger.h so log_info calls are intercepted
 add_executable(test_report test_report.c)
 target_include_directories(test_report PRIVATE "${CMAKE_SOURCE_DIR}")
 target_link_libraries(test_report PRIVATE ctdd_report Unity::Unity CMock::CMock)
 target_compile_features(test_report PRIVATE c_std_23)
 cmock_generate_mock(test_report "${CMAKE_SOURCE_DIR}/ctdd/logger.h")
 add_test(NAME test_report COMMAND test_report)
+
+# logger tests — mocks log_write.h so output calls are intercepted
+add_executable(test_logger test_logger.c)
+target_include_directories(test_logger PRIVATE "${CMAKE_SOURCE_DIR}")
+target_link_libraries(test_logger PRIVATE ctdd_logger Unity::Unity CMock::CMock)
+target_compile_features(test_logger PRIVATE c_std_23)
+cmock_generate_mock(test_logger "${CMAKE_SOURCE_DIR}/ctdd/log_write.h")
+add_test(NAME test_logger COMMAND test_logger)
diff --git a/tests/test_logger.c b/tests/test_logger.c
new file mode 100644
index 0000000..0dee2a8
--- /dev/null
+++ b/tests/test_logger.c
@@ -0,0 +1,73 @@
+#include "unity.h"
+#include "ctdd/logger.h"
+#include "Mocklog_write.h"
+
+void setUp(void)    { Mocklog_write_Init(); logger_set_level(LOG_DEBUG); }
+void tearDown(void) { Mocklog_write_Verify(); Mocklog_write_Destroy(); }
+
+void test_log_debug_emits_at_debug_level(void) {
+    log_write_Expect("[DEBUG] hello");
+    log_debug("hello");
+}
+
+void test_log_info_emits_at_debug_level(void) {
+    log_write_Expect("[INFO] world");
+    log_info("world");
+}
+
+void test_log_warn_emits_at_warn_level(void) {
+    logger_set_level(LOG_WARN);
+    log_write_Expect("[WARN] alert");
+    log_warn("alert");
+}
+
+void test_log_err_emits_at_warn_level(void) {
+    logger_set_level(LOG_WARN);
+    log_write_Expect("[ERROR] fatal");
+    log_err("fatal");
+}
+
+void test_log_debug_suppressed_at_info_level(void) {
+    logger_set_level(LOG_INFO);
+    log_debug("silent");
+}
+
+void test_log_info_suppressed_at_warn_level(void) {
+    logger_set_level(LOG_WARN);
+    log_info("silent");
+}
+
+void test_log_warn_suppressed_at_error_level(void) {
+    logger_set_level(LOG_ERROR);
+    log_warn("silent");
+}
+
+void test_log_none_suppresses_all(void) {
+    logger_set_level(LOG_NONE);
+    log_debug("silent");
+    log_info("silent");
+    log_warn("silent");
+    log_err("silent");
+}
+
+void test_level_can_be_raised_then_lowered(void) {
+    logger_set_level(LOG_ERROR);
+    log_info("silent");
+    logger_set_level(LOG_DEBUG);
+    log_write_Expect("[INFO] now visible");
+    log_info("now visible");
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_log_debug_emits_at_debug_level);
+    RUN_TEST(test_log_info_emits_at_debug_level);
+    RUN_TEST(test_log_warn_emits_at_warn_level);
+    RUN_TEST(test_log_err_emits_at_warn_level);
+    RUN_TEST(test_log_debug_suppressed_at_info_level);
+    RUN_TEST(test_log_info_suppressed_at_warn_level);
+    RUN_TEST(test_log_warn_suppressed_at_error_level);
+    RUN_TEST(test_log_none_suppresses_all);
+    RUN_TEST(test_level_can_be_raised_then_lowered);
+    return UNITY_END();
+}
diff --git a/tests/test_report.c b/tests/test_report.c
index 6f9a0a9..bdb936a 100644
--- a/tests/test_report.c
+++ b/tests/test_report.c
@@ -6,22 +6,22 @@ void setUp(void)    { Mocklogger_Init(); }
 void tearDown(void) { Mocklogger_Verify(); Mocklogger_Destroy(); }
 
 void test_report_formats_label_and_value(void) {
-    log_message_Expect("count: 42");
+    log_info_Expect("count: 42");
     report_value("count", 42);
 }
 
 void test_report_negative_value(void) {
-    log_message_Expect("score: -5");
+    log_info_Expect("score: -5");
     report_value("score", -5);
 }
 
 void test_report_zero(void) {
-    log_message_Expect("total: 0");
+    log_info_Expect("total: 0");
     report_value("total", 0);
 }
 
 void test_report_calls_log_exactly_once(void) {
-    log_message_Expect("x: 1");
+    log_info_Expect("x: 1");
     report_value("x", 1);
 }