diff --git a/Package.swift b/Package.swift
index 5206b268e..840541887 100644
--- a/Package.swift
+++ b/Package.swift
@@ -468,16 +468,23 @@ extension Array where Element == PackageDescription.CXXSetting {
       .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
     ]
 
-    // Capture the testing library's version as a C++ string constant.
+    // Capture the testing library's commit info as C++ constants.
     if let git {
-      let testingLibraryVersion = if let tag = git.currentTag {
-        tag
-      } else if git.hasUncommittedChanges {
-        "\(git.currentCommit) (modified)"
+      if let tag = git.currentTag {
+        result.append(.define("SWT_TESTING_LIBRARY_VERSION", to: #""\#(tag)""#))
       } else {
-        git.currentCommit
+        result.append(.define("SWT_TESTING_LIBRARY_VERSION", to: "0"))
       }
-      result.append(.define("SWT_TESTING_LIBRARY_VERSION", to: #""\#(testingLibraryVersion)""#))
+
+      result.append(.define("SWT_TESTING_LIBRARY_COMMIT_HASH", to: #""\#(git.currentCommit)""#))
+      if git.hasUncommittedChanges {
+        result.append(.define("SWT_TESTING_LIBRARY_COMMIT_MODIFIED", to: "1"))
+      }
+    } else if let gitHubSHA = Context.environment["GITHUB_SHA"] {
+      // When building in GitHub Actions, the git command may fail to get us the
+      // commit hash, so check if GitHub shared it with us instead.
+      result.append(.define("SWT_TESTING_LIBRARY_VERSION", to: "0"))
+      result.append(.define("SWT_TESTING_LIBRARY_COMMIT_HASH", to: #""\#(gitHubSHA)""#))
     }
 
     return result
diff --git a/Sources/Testing/Support/Versions.swift b/Sources/Testing/Support/Versions.swift
index 62193d6c4..76c5aeaf6 100644
--- a/Sources/Testing/Support/Versions.swift
+++ b/Sources/Testing/Support/Versions.swift
@@ -129,9 +129,26 @@ let simulatorVersion: String = {
 /// an event writer.
 ///
 /// This value is not part of the public interface of the testing library.
-var testingLibraryVersion: String {
-  swt_getTestingLibraryVersion().flatMap(String.init(validatingCString:)) ?? "unknown"
-}
+let testingLibraryVersion: String = {
+  var result = swt_getTestingLibraryVersion().flatMap(String.init(validatingCString:)) ?? "unknown"
+
+  // Get details of the git commit used when compiling the testing library.
+  var commitHash: UnsafePointer<CChar>?
+  var commitModified = CBool(false)
+  swt_getTestingLibraryCommit(&commitHash, &commitModified)
+
+  if let commitHash = commitHash.flatMap(String.init(validatingCString:)) {
+    // Truncate to 15 characters of the hash to match `swift --version`.
+    let commitHash = commitHash.prefix(15)
+    if commitModified {
+      result = "\(result) (\(commitHash) - modified)"
+    } else {
+      result = "\(result) (\(commitHash))"
+    }
+  }
+
+  return result
+}()
 
 /// Get the LLVM target triple used to build the testing library, if available.
 ///
diff --git a/Sources/_TestingInternals/CMakeLists.txt b/Sources/_TestingInternals/CMakeLists.txt
index 16713ab27..2b385cb1b 100644
--- a/Sources/_TestingInternals/CMakeLists.txt
+++ b/Sources/_TestingInternals/CMakeLists.txt
@@ -8,6 +8,7 @@
 
 set(CMAKE_CXX_SCAN_FOR_MODULES 0)
 
+include(GitCommit)
 include(LibraryVersion)
 include(TargetTriple)
 add_library(_TestingInternals STATIC
diff --git a/Sources/_TestingInternals/Versions.cpp b/Sources/_TestingInternals/Versions.cpp
index 97eace99e..8e85f3d06 100644
--- a/Sources/_TestingInternals/Versions.cpp
+++ b/Sources/_TestingInternals/Versions.cpp
@@ -19,6 +19,19 @@ const char *swt_getTestingLibraryVersion(void) {
 #endif
 }
 
+void swt_getTestingLibraryCommit(const char *_Nullable *_Nonnull outHash, bool *outModified) {
+#if defined(SWT_TESTING_LIBRARY_COMMIT_HASH)
+  *outHash = SWT_TESTING_LIBRARY_COMMIT_HASH;
+#else
+  *outHash = nullptr;
+#endif
+#if defined(SWT_TESTING_LIBRARY_COMMIT_MODIFIED)
+  *outModified = (SWT_TESTING_LIBRARY_COMMIT_MODIFIED != 0);
+#else
+  *outModified = false;
+#endif
+}
+
 const char *swt_getTargetTriple(void) {
 #if defined(SWT_TARGET_TRIPLE)
   return SWT_TARGET_TRIPLE;
diff --git a/Sources/_TestingInternals/include/Versions.h b/Sources/_TestingInternals/include/Versions.h
index ed523b86f..b188a4d66 100644
--- a/Sources/_TestingInternals/include/Versions.h
+++ b/Sources/_TestingInternals/include/Versions.h
@@ -38,6 +38,15 @@ static inline uint64_t swt_getSwiftCompilerVersion(void) {
 ///   other conditions. Do not attempt to parse it.
 SWT_EXTERN const char *_Nullable swt_getTestingLibraryVersion(void);
 
+/// Get details of the source control (git) commit from which the testing
+/// library was built.
+///
+/// - Parameters:
+///   - outHash: On return, set to a pointer to a string containing the commit
+///     hash from which the testing library was built.
+///   - outModified: On return, whether or not there were uncommitted changes.
+SWT_EXTERN void swt_getTestingLibraryCommit(const char *_Nullable *_Nonnull outHash, bool *outModified);
+
 /// Get the LLVM target triple used to build the testing library.
 ///
 /// - Returns: A string containing the LLVM target triple used to build the
diff --git a/cmake/modules/GitCommit.cmake b/cmake/modules/GitCommit.cmake
new file mode 100644
index 000000000..86b0b4733
--- /dev/null
+++ b/cmake/modules/GitCommit.cmake
@@ -0,0 +1,35 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2024–2025 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+find_package(Git QUIET)
+if(Git_FOUND)
+  # Get the commit hash corresponding to the current build.
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} rev-parse --verify HEAD
+    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+    OUTPUT_VARIABLE SWT_TESTING_LIBRARY_COMMIT_HASH
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_QUIET)
+
+  # Check if there are local changes.
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} status -s
+    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+    OUTPUT_VARIABLE SWT_TESTING_LIBRARY_COMMIT_MODIFIED
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif()
+
+if(SWT_TESTING_LIBRARY_COMMIT_HASH)
+  message(STATUS "Swift Testing commit hash: ${SWT_TESTING_LIBRARY_COMMIT_HASH}")
+  add_compile_definitions(
+    "$<$<COMPILE_LANGUAGE:CXX>:SWT_TESTING_LIBRARY_COMMIT_HASH=\"${SWT_TESTING_LIBRARY_COMMIT_HASH}\">")
+  if(SWT_TESTING_LIBRARY_COMMIT_MODIFIED)
+    add_compile_definitions(
+      "$<$<COMPILE_LANGUAGE:CXX>:SWT_TESTING_LIBRARY_COMMIT_MODIFIED=1>")
+  endif()
+endif()
diff --git a/cmake/modules/LibraryVersion.cmake b/cmake/modules/LibraryVersion.cmake
index 259ead608..32901365a 100644
--- a/cmake/modules/LibraryVersion.cmake
+++ b/cmake/modules/LibraryVersion.cmake
@@ -10,34 +10,6 @@
 # remember to remove -dev.
 set(SWT_TESTING_LIBRARY_VERSION "6.3-dev")
 
-find_package(Git QUIET)
-if(Git_FOUND)
-  # Get the commit hash corresponding to the current build. Limit length to 15
-  # to match `swift --version` output format.
-  execute_process(
-    COMMAND ${GIT_EXECUTABLE} rev-parse --short=15 --verify HEAD
-    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
-    OUTPUT_VARIABLE GIT_VERSION
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    ERROR_QUIET)
-
-  # Check if there are local changes.
-  execute_process(
-    COMMAND ${GIT_EXECUTABLE} status -s
-    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
-    OUTPUT_VARIABLE GIT_STATUS
-    OUTPUT_STRIP_TRAILING_WHITESPACE)
-  if(GIT_STATUS)
-    set(GIT_VERSION "${GIT_VERSION} - modified")
-  endif()
-endif()
-
-# Combine the hard-coded Swift version with available Git information.
-if(GIT_VERSION)
-set(SWT_TESTING_LIBRARY_VERSION "${SWT_TESTING_LIBRARY_VERSION} (${GIT_VERSION})")
-endif()
-
-# All done!
 message(STATUS "Swift Testing version: ${SWT_TESTING_LIBRARY_VERSION}")
 add_compile_definitions(
   "$<$<COMPILE_LANGUAGE:CXX>:SWT_TESTING_LIBRARY_VERSION=\"${SWT_TESTING_LIBRARY_VERSION}\">")