diff --git a/public/index.html b/public/index.html
index 4692d6e..c352e76 100644
--- a/public/index.html
+++ b/public/index.html
@@ -1134,6 +1134,14 @@
         // Reset rendered length when collapsing or expanding (element gets recreated)
         session.outputRenderedLength = 0;
         renderSessions();
+
+        // If expanding, resize the PTY to fit the viewport
+        if (session.expanded) {
+          // Wait for DOM to update before measuring
+          setTimeout(() => {
+            handleSessionResize(Number(sessionId));
+          }, 0);
+        }
       }
     };
 
@@ -1978,6 +1986,74 @@
     // Auto-scroll control
     const scrollListeners = new Map();
 
+    // Viewport-based PTY resize
+    let resizeDebounceTimer = null;
+
+    function measureTerminalCols() {
+      // Create a hidden span with monospace font to measure character width
+      const $measure = document.createElement('span');
+      $measure.style.fontFamily = '"SF Mono", Monaco, "Cascadia Code", "Courier New", monospace';
+      $measure.style.fontSize = `${state.settings.fontSize}px`;
+      $measure.style.lineHeight = '1.5';
+      $measure.style.visibility = 'hidden';
+      $measure.style.position = 'absolute';
+      $measure.textContent = 'X'.repeat(100); // Measure 100 chars
+      document.body.appendChild($measure);
+
+      const charWidth = $measure.offsetWidth / 100;
+      document.body.removeChild($measure);
+
+      // Get terminal container width (accounting for padding)
+      const $container = document.querySelector('.session-output');
+      if (!$container) {
+        return 80; // Default fallback
+      }
+
+      const containerWidth = $container.clientWidth - 32; // 16px padding on each side
+      const cols = Math.floor(containerWidth / charWidth);
+
+      // Clamp to reasonable bounds
+      return Math.max(40, Math.min(cols, 300));
+    }
+
+    async function resizeSessionPTY(sessionId, cols, rows) {
+      try {
+        const res = await fetch(`/api/sessions/${sessionId}/resize`, {
+          method: 'POST',
+          headers: {
+            'Content-Type': 'application/json',
+          },
+          body: JSON.stringify({ cols, rows }),
+        });
+        if (!res.ok) {
+          console.error('Failed to resize session PTY:', await res.text());
+        }
+      } catch (error) {
+        console.error('Error resizing session PTY:', error);
+      }
+    }
+
+    function handleSessionResize(sessionId) {
+      const cols = measureTerminalCols();
+      const rows = 24; // Reasonable default row count
+      resizeSessionPTY(sessionId, cols, rows);
+    }
+
+    function handleWindowResize() {
+      if (resizeDebounceTimer) {
+        clearTimeout(resizeDebounceTimer);
+      }
+
+      resizeDebounceTimer = setTimeout(() => {
+        // Resize all expanded sessions
+        state.sessions.forEach((session) => {
+          if (session.expanded) {
+            handleSessionResize(session.id);
+          }
+        });
+      }, 300); // Debounce for 300ms
+    }
+
     function attachScrollListener(sessionId, $outputContainer) {
       // Remove existing listener if present
       const existing = scrollListeners.get(sessionId);
@@ -2041,6 +2117,9 @@
     loadSettings();
     applySettings();
     connectSSE();
+
+    // Set up window resize listener for PTY viewport resize
+    window.addEventListener('resize', handleWindowResize);
   </script>
 </body>
 </html>
diff --git a/src/server.ts b/src/server.ts
index 6cfbac6..d9dc1cd 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -141,6 +141,45 @@ const server = Bun.serve<SessionData>({
       return Response.json(sessions);
     }
 
+    // Resize endpoint for dashboard to resize CLI PTY
+    if (
+      url.pathname.match(/^\/api\/sessions\/\d+\/resize$/) &&
+      req.method === "POST"
+    ) {
+      const parts = url.pathname.split("/");
+      const sessionId = Number.parseInt(parts[3] || "", 10);
+
+      if (Number.isNaN(sessionId)) {
+        return new Response("Invalid session ID", { status: 400 });
+      }
+
+      const body = (await req.json()) as { cols?: unknown; rows?: unknown };
+      if (
+        typeof body.cols !== "number" ||
+        typeof body.rows !== "number" ||
+        body.cols <= 0 ||
+        body.rows <= 0
+      ) {
+        return new Response("Missing or invalid cols/rows", { status: 400 });
+      }
+
+      // Get WebSocket connection for this session
+      const ws = sessionWebSockets.get(sessionId);
+      if (!ws) {
+        return new Response("Session WebSocket not found", { status: 404 });
+      }
+
+      // Send resize command to CLI
+      const message: ServerMessage = {
+        type: "resize",
+        cols: body.cols,
+        rows: body.rows,
+      };
+      ws.send(JSON.stringify(message));
+
+      return Response.json({ success: true });
+    }
+
     // Create prompt for a session
     if (
       url.pathname.match(/^\/api\/sessions\/\d+\/prompts$/) &&