diff --git a/Runner/Helpers/GitHelper.cs b/Runner/Helpers/GitHelper.cs
index 94b0843..3eff0f8 100644
--- a/Runner/Helpers/GitHelper.cs
+++ b/Runner/Helpers/GitHelper.cs
@@ -30,4 +30,19 @@ private static bool ShouldSkipLine(string line)
             span.StartsWith("@@", StringComparison.Ordinal) ||
             span.StartsWith("\\ No newline at end of file", StringComparison.Ordinal);
     }
+
+    public static async Task<List<string>> GetChangedFilesAsync(JobBase job, string baselineRef, string workDir)
+    {
+        List<string> lines = [];
+
+        await job.RunProcessAsync("git",
+            $"diff --name-only {baselineRef}",
+            lines,
+            workDir: workDir,
+            checkExitCode: false,
+            suppressOutputLogs: true,
+            suppressStartingLog: true);
+
+        return lines;
+    }
 }
diff --git a/Runner/Jobs/JitDiffJob.cs b/Runner/Jobs/JitDiffJob.cs
index dc151c0..26df3b5 100644
--- a/Runner/Jobs/JitDiffJob.cs
+++ b/Runner/Jobs/JitDiffJob.cs
@@ -80,7 +80,9 @@ public static async Task BuildAndCopyRuntimeBranchBitsAsync(JobBase job, string
     {
         string arch = IsArm ? "arm64" : "x64";
 
-        await job.RunProcessAsync("bash", $"build.sh clr+libs -c Release {RuntimeHelpers.LibrariesExtraBuildArgs}", logPrefix: $"{branch} release", workDir: "runtime");
+        string targets = await ShouldBuildsLibsAsync() ? "clr+libs" : "clr";
+
+        await job.RunProcessAsync("bash", $"build.sh {targets} -c Release {RuntimeHelpers.LibrariesExtraBuildArgs}", logPrefix: $"{branch} release", workDir: "runtime");
 
         Task copyReleaseBitsTask = RuntimeHelpers.CopyReleaseArtifactsAsync(job, branch, $"artifacts-{branch}");
 
@@ -94,6 +96,25 @@ public static async Task BuildAndCopyRuntimeBranchBitsAsync(JobBase job, string
         }
 
         await copyReleaseBitsTask;
+
+        async Task<bool> ShouldBuildsLibsAsync()
+        {
+            if (job.TryGetFlag("forceRebuildLibs"))
+            {
+                return true;
+            }
+
+            if (branch == "pr")
+            {
+                List<string> changedFiles = await GitHelper.GetChangedFilesAsync(job, "main", "runtime");
+
+                return changedFiles.Any(f =>
+                    !f.StartsWith("src/coreclr/", StringComparison.OrdinalIgnoreCase) ||
+                    f.Contains("/System.Private.CoreLib/", StringComparison.OrdinalIgnoreCase));
+            }
+
+            return true;
+        }
     }
 
     private async Task<string> CollectFrameworksDiffsAsync()