diff --git a/Tests/SourceKitLSPTests/MainFilesProviderTests.swift b/Tests/SourceKitLSPTests/MainFilesProviderTests.swift
index 9923bbd47..779c8c05a 100644
--- a/Tests/SourceKitLSPTests/MainFilesProviderTests.swift
+++ b/Tests/SourceKitLSPTests/MainFilesProviderTests.swift
@@ -214,9 +214,19 @@ final class MainFilesProviderTests: XCTestCase {
 
     // 'MyFancyLibrary.c' now also includes 'shared.h'. Since it lexicographically preceeds MyLibrary, we should use its
     // build settings.
-    let postEditDiags = try await project.testClient.nextDiagnosticsNotification()
-    XCTAssertEqual(postEditDiags.diagnostics.count, 1)
-    let postEditDiag = try XCTUnwrap(postEditDiags.diagnostics.first)
-    XCTAssertEqual(postEditDiag.message, "Unused variable 'fromMyFancyLibrary'")
+    // `clangd` may return diagnostics from the old build settings sometimes (I believe when it's still building the
+    // preamble for shared.h when the new build settings come in). Check that it eventually returns the correct
+    // diagnostics.
+    var receivedCorrectDiagnostic = false
+    for _ in 0..<Int(defaultTimeout) {
+      let refreshedDiags = try await project.testClient.nextDiagnosticsNotification(timeout: 1)
+      if let diagnostic = refreshedDiags.diagnostics.only,
+        diagnostic.message == "Unused variable 'fromMyFancyLibrary'"
+      {
+        receivedCorrectDiagnostic = true
+        break
+      }
+    }
+    XCTAssert(receivedCorrectDiagnostic)
   }
 }