diff --git a/src/plugins/clangcodemodel/clangdclient.cpp b/src/plugins/clangcodemodel/clangdclient.cpp
index 1c3c2ad457b..f4596dcb14e 100644
--- a/src/plugins/clangcodemodel/clangdclient.cpp
+++ b/src/plugins/clangcodemodel/clangdclient.cpp
@@ -1315,10 +1315,10 @@ ClangdClient::ClangdClient(Project *project, const Utils::FilePath &jsonDbDir)
     setDocumentChangeUpdateThreshold(d->settings.documentUpdateThreshold);
 
     const auto textMarkCreator = [this](const Utils::FilePath &filePath,
-            const Diagnostic &diag) {
+            const Diagnostic &diag, bool isProjectFile) {
         if (d->isTesting)
             emit textMarkCreated(filePath);
-        return new ClangdTextMark(filePath, diag, this);
+        return new ClangdTextMark(filePath, diag, isProjectFile, this);
     };
     const auto hideDiagsHandler = []{ ClangDiagnosticManager::clearTaskHubIssues(); };
     setDiagnosticsHandlers(textMarkCreator, hideDiagsHandler);
diff --git a/src/plugins/clangcodemodel/clangtextmark.cpp b/src/plugins/clangcodemodel/clangtextmark.cpp
index 16fda9accba..20bef44e3bd 100644
--- a/src/plugins/clangcodemodel/clangtextmark.cpp
+++ b/src/plugins/clangcodemodel/clangtextmark.cpp
@@ -385,6 +385,7 @@ ClangBackEnd::DiagnosticContainer convertDiagnostic(const ClangdDiagnostic &src,
 
 ClangdTextMark::ClangdTextMark(const FilePath &filePath,
                                const Diagnostic &diagnostic,
+                               bool isProjectFile,
                                const Client *client)
     : TextEditor::TextMark(filePath, int(diagnostic.range().start().line() + 1), client->id())
     , m_lspDiagnostic(diagnostic)
@@ -399,7 +400,7 @@ ClangdTextMark::ClangdTextMark(const FilePath &filePath,
     setPriority(isError ? TextEditor::TextMark::HighPriority
                         : TextEditor::TextMark::NormalPriority);
     setIcon(isError ? Icons::CODEMODEL_ERROR.icon() : Icons::CODEMODEL_WARNING.icon());
-    if (client->project()) {
+    if (isProjectFile) {
         setLineAnnotation(diagnostic.message());
         setColor(isError ? Theme::CodeModel_Error_TextMarkColor
                          : Theme::CodeModel_Warning_TextMarkColor);
diff --git a/src/plugins/clangcodemodel/clangtextmark.h b/src/plugins/clangcodemodel/clangtextmark.h
index 15b97ab32f1..2c08e875900 100644
--- a/src/plugins/clangcodemodel/clangtextmark.h
+++ b/src/plugins/clangcodemodel/clangtextmark.h
@@ -72,6 +72,7 @@ class ClangdTextMark : public TextEditor::TextMark
 public:
     ClangdTextMark(const ::Utils::FilePath &filePath,
                    const LanguageServerProtocol::Diagnostic &diagnostic,
+                   bool isProjectFile,
                    const LanguageClient::Client *client);
 
 private:
diff --git a/src/plugins/languageclient/diagnosticmanager.cpp b/src/plugins/languageclient/diagnosticmanager.cpp
index cdaadb7d4a8..381e0f9fc45 100644
--- a/src/plugins/languageclient/diagnosticmanager.cpp
+++ b/src/plugins/languageclient/diagnosticmanager.cpp
@@ -28,6 +28,7 @@
 #include "client.h"
 
 #include <coreplugin/editormanager/documentmodel.h>
+#include <projectexplorer/project.h>
 #include <texteditor/fontsettings.h>
 #include <texteditor/textdocument.h>
 #include <texteditor/texteditor.h>
@@ -74,7 +75,7 @@ private:
 DiagnosticManager::DiagnosticManager(Client *client)
     : m_client(client)
 {
-    m_textMarkCreator = [this](const FilePath &filePath, const Diagnostic &diagnostic) {
+    m_textMarkCreator = [this](const FilePath &filePath, const Diagnostic &diagnostic, bool /*isProjectFile*/) {
         return createTextMark(filePath, diagnostic);
     };
 }
@@ -125,9 +126,11 @@ void DiagnosticManager::showDiagnostics(const DocumentUri &uri, int version)
         QList<QTextEdit::ExtraSelection> extraSelections;
         const VersionedDiagnostics &versionedDiagnostics = m_diagnostics.value(uri);
         if (versionedDiagnostics.version.value_or(version) == version) {
+            const bool isProjectFile = m_client->project()
+                                       && m_client->project()->isKnownFile(filePath);
             for (const Diagnostic &diagnostic : versionedDiagnostics.diagnostics) {
                 extraSelections << toDiagnosticsSelections(diagnostic, doc->document());
-                m_marks[filePath].append(m_textMarkCreator(filePath, diagnostic));
+                m_marks[filePath].append(m_textMarkCreator(filePath, diagnostic, isProjectFile));
             }
         }
 
diff --git a/src/plugins/languageclient/diagnosticmanager.h b/src/plugins/languageclient/diagnosticmanager.h
index a7e2e474852..c4bfabf71a3 100644
--- a/src/plugins/languageclient/diagnosticmanager.h
+++ b/src/plugins/languageclient/diagnosticmanager.h
@@ -43,7 +43,7 @@ namespace LanguageClient {
 class Client;
 
 using TextMarkCreator = std::function<TextEditor::TextMark *(const Utils::FilePath &,
-        const LanguageServerProtocol::Diagnostic &)>;
+        const LanguageServerProtocol::Diagnostic &, bool)>;
 using HideDiagnosticsHandler = std::function<void()>;
 
 class DiagnosticManager