diff --git a/src/cli/components/MemfsTreeViewer.tsx b/src/cli/components/MemfsTreeViewer.tsx
index 8992c9e..4cba077 100644
--- a/src/cli/components/MemfsTreeViewer.tsx
+++ b/src/cli/components/MemfsTreeViewer.tsx
@@ -113,7 +113,13 @@ export function MemfsTreeViewer({
     if ((input === "o" || input === "O") && hasGitRepo) {
       showStatus("Opening in browser...", 10000);
       generateAndOpenMemoryViewer(agentId, { agentName })
-        .then(() => showStatus("Opened in browser", 3000))
+        .then((result) => {
+          if (result.opened) {
+            showStatus("Opened in browser", 3000);
+          } else {
+            showStatus(`Run: open ${result.filePath}`, 15000);
+          }
+        })
         .catch((err: unknown) =>
           showStatus(
             err instanceof Error ? err.message : "Failed to open viewer",
diff --git a/src/web/generate-memory-viewer.ts b/src/web/generate-memory-viewer.ts
index 22562e2..a31ff12 100644
--- a/src/web/generate-memory-viewer.ts
+++ b/src/web/generate-memory-viewer.ts
@@ -38,6 +38,7 @@ const RECORD_SEP = "\x1e";
 
 export interface GenerateResult {
   filePath: string;
+  opened: boolean;
 }
 
 // ---------------------------------------------------------------------------
@@ -444,15 +445,17 @@ export async function generateAndOpenMemoryViewer(
   writeFileSync(filePath, html);
   chmodSync(filePath, 0o600);
 
-  // 4. Open in browser
-  try {
-    const { default: openUrl } = await import("open");
-    await openUrl(filePath, { wait: false });
-  } catch (err) {
-    throw new Error(
-      `Failed to open browser. File saved to: ${filePath}${err instanceof Error ? ` (${err.message})` : ""}`,
-    );
+  // 4. Open in browser (skip inside tmux — `open` launches a broken new
+  //    browser instance instead of reusing the running one)
+  const isTmux = Boolean(process.env.TMUX);
+  if (!isTmux) {
+    try {
+      const { default: openUrl } = await import("open");
+      await openUrl(filePath, { wait: false });
+    } catch {
+      throw new Error(`Could not open browser. Run: open ${filePath}`);
+    }
   }
 
-  return { filePath };
+  return { filePath, opened: !isTmux };
 }