diff --git a/gui/chatblockmodel.cpp b/gui/chatblockmodel.cpp
index af15526..6a916b5 100644
--- a/gui/chatblockmodel.cpp
+++ b/gui/chatblockmodel.cpp
@@ -880,16 +880,19 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
     QString cacheDir = QStandardPaths::writableLocation(QStandardPaths::CacheLocation)
                        + QStringLiteral("/diagrams");
     QDir().mkpath(cacheDir);
-    QString svgPath = cacheDir + "/" + QString::fromLatin1(hash) + ".svg";
+    // Mermaid uses PNG (Qt's SVG renderer can't handle CSS class-based styles);
+    // PlantUML and Draw.io use SVG (inline styles, Qt-compatible).
+    QString ext = (lang == QStringLiteral("mermaid")) ? QStringLiteral(".png") : QStringLiteral(".svg");
+    QString outPath = cacheDir + "/" + QString::fromLatin1(hash) + ext;
 
     // Return cached result if available
-    if (QFileInfo::exists(svgPath))
-        return QUrl::fromLocalFile(svgPath).toString();
+    if (QFileInfo::exists(outPath))
+        return QUrl::fromLocalFile(outPath).toString();
 
     if (lang == QStringLiteral("plantuml")) {
         // Try local PlantUML server first (localhost:8080)
-        if (fetchFromPlantUmlServer(source, svgPath))
-            return QUrl::fromLocalFile(svgPath).toString();
+        if (fetchFromPlantUmlServer(source, outPath))
+            return QUrl::fromLocalFile(outPath).toString();
 
         // Fall back to CLI
         QTemporaryFile input;
@@ -937,16 +940,16 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
             return QString();
 
         // plantuml names output after input file, rename if needed
-        if (!QFileInfo::exists(svgPath)) {
+        if (!QFileInfo::exists(outPath)) {
             QString baseName = QFileInfo(input.fileName()).completeBaseName();
             QString actualOutput = cacheDir + "/" + baseName + ".svg";
             if (QFileInfo::exists(actualOutput))
-                QFile::rename(actualOutput, svgPath);
+                QFile::rename(actualOutput, outPath);
         }
     } else if (lang == QStringLiteral("mermaid")) {
         // Mermaid — try local server first (localhost:8081), fall back to mmdc CLI
-        if (fetchFromMermaidServer(source, svgPath))
-            return QUrl::fromLocalFile(svgPath).toString();
+        // Note: server returns SVG which Qt can't render (CSS classes), so skip it
+        // and always use CLI with PNG output for Qt compatibility.
 
         QTemporaryFile input;
         input.setFileTemplate(QStringLiteral("/tmp/ollie-diagram-XXXXXX.mmd"));
@@ -965,7 +968,7 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
         proc.setProcessChannelMode(QProcess::MergedChannels);
         proc.start(mmdc, {
             QStringLiteral("-i"), input.fileName(),
-            QStringLiteral("-o"), svgPath,
+            QStringLiteral("-o"), outPath,
             QStringLiteral("-t"), QStringLiteral("default"),
             QStringLiteral("-b"), QStringLiteral("white")
         });
@@ -998,7 +1001,7 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
         if (reply->isFinished() && reply->error() == QNetworkReply::NoError) {
             QByteArray svg = reply->readAll();
             if (!svg.isEmpty() && svg.contains("<svg")) {
-                QFile out(svgPath);
+                QFile out(outPath);
                 if (out.open(QIODevice::WriteOnly)) {
                     out.write(svg);
                     serverOk = true;
@@ -1026,7 +1029,7 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
             proc.start(drawio, {
                 QStringLiteral("--export"),
                 QStringLiteral("--format"), QStringLiteral("svg"),
-                QStringLiteral("--output"), svgPath,
+                QStringLiteral("--output"), outPath,
                 input.fileName()
             });
 
@@ -1035,10 +1038,10 @@ QString ChatBlockModel::diagramUrl(const QString &source, const QString &languag
         }
     }
 
-    if (!QFileInfo::exists(svgPath))
+    if (!QFileInfo::exists(outPath))
         return QString();
 
-    return QUrl::fromLocalFile(svgPath).toString();
+    return QUrl::fromLocalFile(outPath).toString();
 }
 
 bool ChatBlockModel::invalidateDiagram(const QString &source, const QString &language) const
@@ -1053,7 +1056,8 @@ bool ChatBlockModel::invalidateDiagram(const QString &source, const QString &lan
         (lang + "\x1f" + source).toUtf8(), QCryptographicHash::Sha256).toHex().left(16);
     QString cacheDir = QStandardPaths::writableLocation(QStandardPaths::CacheLocation)
                        + QStringLiteral("/diagrams");
-    QString svgPath = cacheDir + "/" + QString::fromLatin1(hash) + ".svg";
+    QString ext = (lang == QStringLiteral("mermaid")) ? QStringLiteral(".png") : QStringLiteral(".svg");
+    QString outPath = cacheDir + "/" + QString::fromLatin1(hash) + ext;
 
-    return QFile::remove(svgPath);
+    return QFile::remove(outPath);
 }