diff --git a/CHANGELOG b/CHANGELOG
index 7d3271c0..6421181e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,9 @@
+Unreleased
+---------
+
+New features and improvements:
+
+- Added optional test-only instrumentation build option `LIB61850_ENABLE_TEST_API` (OFF by default). When enabled exports `MmsConnection_setFileReadArtificialDelay` (marked with `LIB61850_TEST_API`) to inject artificial delays before MMS file read responses for deterministic ObtainFile request timeout testing. Production builds remain unaffected when disabled.
 
 Changes to version 1.6.1
 ------------------------
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5ec9a27f..dccccb35 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -58,6 +58,12 @@ option(CONFIG_IEC61850_R_GOOSE "Build with support for R-GOOSE (mbedtls required
 option(CONFIG_IEC61850_R_SMV "Build with support for R-SMV (mbedtls required)" ON)
 option(CONFIG_IEC61850_SNTP_CLIENT "Build with SNTP client code" ON)
 
+# Option to enable test-only instrumentation APIs (artificial delays, etc.)
+option(LIB61850_ENABLE_TEST_API "Enable test-only instrumentation APIs (e.g. artificial delays)" OFF)
+if(LIB61850_ENABLE_TEST_API)
+    add_compile_definitions(LIB61850_ENABLE_TEST_API)
+endif()
+
 set(CONFIG_IEC61850_SG_RESVTMS 300 CACHE STRING "Configure the maximum number of SG RESVTMS")
 
 set(CONFIG_REPORTING_DEFAULT_REPORT_BUFFER_SIZE "65536" CACHE STRING "Default buffer size for buffered reports in byte" )
@@ -230,6 +236,12 @@ endif(BUILD_EXAMPLES)
 
 add_subdirectory(${CMAKE_CURRENT_LIST_DIR}/src)
 
+# Build tests if requested
+option(BUILD_TESTS "Build unit tests" ON)
+if(BUILD_TESTS)
+    add_subdirectory(${CMAKE_CURRENT_LIST_DIR}/tests)
+endif(BUILD_TESTS)
+
 install(FILES ${API_HEADERS} DESTINATION include/libiec61850 COMPONENT Development)
 
 if(BUILD_PYTHON_BINDINGS)
diff --git a/README.md b/README.md
index 6fea2ba4..6d3f4337 100644
--- a/README.md
+++ b/README.md
@@ -168,6 +168,29 @@ Optionally execute the following stop to install the library and header files in
 
 `sudo make install`
 
+### Optional test-only instrumentation
+
+For deterministic testing (e.g. of MMS file ObtainFile request timeout behavior) the library provides a test-only instrumentation API that can be used to modify the standard behavior. This is disabled by default and excluded from production builds.
+
+Enable it via CMake:
+
+```
+cmake -DLIB61850_ENABLE_TEST_API=ON ..
+```
+
+When enabled some test specific API functions are available (exported with the `LIB61850_TEST_API` attribute).
+
+Usage example:
+
+```c
+#ifdef LIB61850_ENABLE_TEST_API
+  /* Introduce 250 ms delay before each FileRead response */
+  MmsConnection_setFileReadArtificialDelay(conn, 250);
+#endif
+```
+
+If the option is OFF the symbols and related code are omitted entirely and have zero impact on runtime behavior or memory footprint.
+
 
 ### Build on Windows with Visual Studio
 
@@ -229,6 +252,9 @@ Support and commercial license options are provided by MZ Automation GmbH. Pleas
 
 ## Contributing
 
-If you want to contribute to the improvement and development of the library please send me comments, feature requests, bug reports, or patches. For more than trivial contributions I require you to sign a Contributor License Agreement. Please contact info@libiec61850.com.
+If you want to contribute to the improvement and development of the library please send comments, feature requests, bug reports, or patches.
+
+More than trivial contributions require you to sign a Contributor License Agreement. Please contact info@libiec61850.com before you plan such a contribution.
+
+Please note: pull requests to the github repository (https://github.com/mz-automation/libiec61850) are not accepted. The github repository serves as a read-only archive.
 
-Please don't send pull requests before signing the Contributor License Agreement! Such pull requests may be silently ignored.
diff --git a/src/common/inc/libiec61850_common_api.h b/src/common/inc/libiec61850_common_api.h
index a233ccc9..4b0ebf3a 100644
--- a/src/common/inc/libiec61850_common_api.h
+++ b/src/common/inc/libiec61850_common_api.h
@@ -28,6 +28,7 @@
 #if defined _WIN32 || defined __CYGWIN__
     #ifdef EXPORT_FUNCTIONS_FOR_DLL
         #define LIB61850_API __declspec(dllexport)
+        #define LIB61850_TEST_API __declspec(dllexport)
     #else
         #define LIB61850_API
     #endif
@@ -36,10 +37,12 @@
 #else
     #if __GNUC__ >= 4
         #define LIB61850_API __attribute__ ((visibility ("default")))
+        #define LIB61850_TEST_API __attribute__ ((visibility ("default")))
         #define LIB61850_INTERNAL  __attribute__ ((visibility ("hidden")))
     #else
         #define LIB61850_API
         #define LIB61850_INTERNAL
+        #define LIB61850_TEST_API
     #endif
 #endif
 
diff --git a/src/mms/inc/mms_client_connection.h b/src/mms/inc/mms_client_connection.h
index 73e61cb1..09ffda1d 100644
--- a/src/mms/inc/mms_client_connection.h
+++ b/src/mms/inc/mms_client_connection.h
@@ -151,6 +151,24 @@ MmsConnection_setRawMessageHandler(MmsConnection self, MmsRawMessageHandler hand
 LIB61850_API void
 MmsConnection_setFilestoreBasepath(MmsConnection self, const char* basepath);
 
+/* Test-only API: available only when LIB61850_ENABLE_TEST_API is defined at build time */
+#ifdef LIB61850_ENABLE_TEST_API
+/**
+ * \brief (TEST ONLY) Set an artificial delay (in ms) for sending file-read responses (OBTAIN-FILE upload path)
+ *
+ * This is intended for testing timeout behavior. When set > 0 the client waits the specified
+ * time before replying to a file-read-request from the server, potentially triggering the server's
+ * request timeout.
+ *
+ * Requires CMake option LIB61850_ENABLE_TEST_API=ON.
+ *
+ * \param self MmsConnection instance to operate on
+ * \param delayMs delay in milliseconds (0 disables)
+ */
+LIB61850_TEST_API void
+MmsConnection_setFileReadArtificialDelay(MmsConnection self, uint32_t delayMs);
+#endif
+
 /**
  * \brief Set the request timeout in ms for this connection
  *
diff --git a/src/mms/inc_private/mms_client_internal.h b/src/mms/inc_private/mms_client_internal.h
index 5a7b2242..bf233908 100644
--- a/src/mms/inc_private/mms_client_internal.h
+++ b/src/mms/inc_private/mms_client_internal.h
@@ -141,6 +141,10 @@ struct sMmsConnection {
     char* filestoreBasepath;
 #endif
 
+#if defined(LIB61850_ENABLE_TEST_API)
+    /* TEST ONLY: Artificial delay (ms) before sending file-read responses (instrumentation) */
+    uint32_t fileReadArtificialDelayMs;
+#endif
 #endif /* (MMS_OBTAIN_FILE_SERVICE == 1) */
 };
 
diff --git a/src/mms/iso_mms/client/mms_client_connection.c b/src/mms/iso_mms/client/mms_client_connection.c
index 3522fda0..6eb3f76a 100644
--- a/src/mms/iso_mms/client/mms_client_connection.c
+++ b/src/mms/iso_mms/client/mms_client_connection.c
@@ -1649,6 +1649,10 @@ MmsConnection_createInternal(TLSConfiguration tlsConfig, bool createThread)
         self->connectionHandlingThread = NULL;
         self->connectionThreadRunning = false;
 #endif
+
+#if defined(LIB61850_ENABLE_TEST_API)
+        self->fileReadArtificialDelayMs = 0; /* default no artificial delay */
+#endif
     }
 
     return self;
@@ -1737,6 +1741,19 @@ MmsConnection_setFilestoreBasepath(MmsConnection self, const char* basepath)
 #endif
 }
 
+#if defined(LIB61850_ENABLE_TEST_API)
+void
+MmsConnection_setFileReadArtificialDelay(MmsConnection self, uint32_t delayMs)
+{
+#if (MMS_OBTAIN_FILE_SERVICE == 1)
+    self->fileReadArtificialDelayMs = delayMs;
+#else
+    (void)self;
+    (void)delayMs;
+#endif
+}
+#endif
+
 char*
 MmsConnection_getFilestoreBasepath(MmsConnection self)
 {
diff --git a/src/mms/iso_mms/client/mms_client_files.c b/src/mms/iso_mms/client/mms_client_files.c
index 0d8d5006..aae6928f 100644
--- a/src/mms/iso_mms/client/mms_client_files.c
+++ b/src/mms/iso_mms/client/mms_client_files.c
@@ -25,6 +25,7 @@
 #include "stack_config.h"
 #include "mms_common.h"
 #include "mms_client_connection.h"
+#include "hal_thread.h"
 #include "byte_buffer.h"
 
 #include "mms_client_internal.h"
@@ -198,6 +199,11 @@ mmsClient_handleFileReadRequest(
         if (frsm->obtainRequest)
             frsm->obtainRequest->timeout = Hal_getTimeInMs() + connection->requestTimeout;
 
+#if defined(LIB61850_ENABLE_TEST_API)
+        if (connection->fileReadArtificialDelayMs > 0)
+            Thread_sleep(connection->fileReadArtificialDelayMs);
+#endif
+
         mmsMsg_createFileReadResponse(connection->parameters.maxPduSize, invokeId, response, frsm);
     }
     else