diff --git a/apps/api/src/api/tool_instances.py b/apps/api/src/api/tool_instances.py
index 1c4a56d..1f5df7e 100644
--- a/apps/api/src/api/tool_instances.py
+++ b/apps/api/src/api/tool_instances.py
@@ -1962,12 +1962,11 @@ async def start_instance(
             "error": f"Tool type '{instance.tool_type_id}' not found",
         }
 
-    published_port = instance.port or 0
     logger.debug(
-        "Tool type for instance %s: name=%s, published_port=%s, interface_type=%s",
+        "Tool type for instance %s: name=%s, container_port=%s, interface_type=%s",
         instance.id,
         tool_type.name,
-        published_port,
+        tool_type.default_port or 0,
         tool_type.interface_type,
     )
 
@@ -1976,13 +1975,13 @@ async def start_instance(
         # Create temporary Cloudflare tunnel for public access
         try:
             logger.debug(
-                "Creating tunnel for instance %s (port=%d)",
+                "Creating tunnel for instance %s (container_port=%d)",
                 instance.id,
-                published_port,
+                tool_type.default_port or 0,
             )
             tunnel_info = start_tunnel(
                 instance_name=instance.name,
-                published_port=published_port,
+                container_port=tool_type.default_port or 0,
             )
             instance.tunnel_id = tunnel_info["container_name"]
             instance.public_url = tunnel_info["url"]
@@ -2204,7 +2203,7 @@ async def restart_instance(
                 try:
                     tunnel_info = start_tunnel(
                         instance_name=instance.name,
-                        published_port=instance.port or 0,
+                        container_port=tool_type.default_port or 0,
                     )
                     instance.tunnel_id = tunnel_info["container_name"]
                     instance.public_url = tunnel_info["url"]
@@ -2437,10 +2436,17 @@ async def recreate_tunnel_endpoint(
                 "message": "Tunnel is already healthy",
             }
 
+    tool_type = await session.get(ToolType, instance.tool_type_id)
+    if not tool_type:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Tool type not found for this instance",
+        )
+
     try:
         tunnel_info = recreate_tunnel(
             instance_name=instance.name,
-            published_port=instance.port or 0,
+            container_port=tool_type.default_port or 0,
         )
         instance.tunnel_id = tunnel_info["container_name"]
         instance.public_url = tunnel_info["url"]
diff --git a/apps/api/src/services/tunnel.py b/apps/api/src/services/tunnel.py
index 40d6d7f..5432fdc 100644
--- a/apps/api/src/services/tunnel.py
+++ b/apps/api/src/services/tunnel.py
@@ -1,10 +1,10 @@
-"""Clean tunnel service using host-network cloudflared containers.
+"""Clean tunnel service using cloudflared containers on the backend network.
 
 Design:
-- Each tunnel runs as a Docker container on the host network.
-- cloudflared connects to localhost:{published_port}, leveraging Docker's
-  port forwarding. No container name resolution is required.
-- Tunnels are named predictably (tunnel-{instance_name}) for start/stop.
+- Each tunnel runs as a Docker container on the same 'backend' network as the API.
+- cloudflared connects to the tool container by its Docker Compose service name
+  (e.g. http://code-server-headquarter-34837cd3:8443).
+- This avoids host port conflicts and DNS resolution issues.
 """
 
 import logging
@@ -15,6 +15,7 @@ from typing import Any
 logger = logging.getLogger(__name__)
 
 TUNNEL_IMAGE = "cloudflare/cloudflared:latest"
+TUNNEL_NETWORK = "backend"
 
 
 def _tunnel_container_name(instance_name: str) -> str:
@@ -79,13 +80,13 @@ def _get_container_exit_code(tunnel_name: str) -> int | None:
 
 
 def start_tunnel(
-    instance_name: str, published_port: int, timeout: int = 30
+    instance_name: str, container_port: int, timeout: int = 30
 ) -> dict[str, str]:
     """Start a temporary Cloudflare tunnel for an instance.
 
     Args:
-        instance_name: The tool instance name (used to derive tunnel container name).
-        published_port: The host port Docker forwards to the container.
+        instance_name: The tool instance name (used for tunnel naming).
+        container_port: The port the tool container listens on internally.
         timeout: Seconds to wait for the tunnel URL.
 
     Returns:
@@ -96,19 +97,22 @@ def start_tunnel(
     tunnel_name = _tunnel_container_name(instance_name)
     _cleanup_stale_tunnel(tunnel_name)
 
+    # Target the tool container by name on the backend network
+    target_url = f"http://{instance_name.lower()}:{container_port}"
+
     cmd = [
         "docker",
         "run",
         "-d",
         "--network",
-        "host",
+        TUNNEL_NETWORK,
         "--name",
         tunnel_name,
         TUNNEL_IMAGE,
         "tunnel",
         "--no-autoupdate",
         "--url",
-        f"http://localhost:{published_port}",
+        target_url,
     ]
 
     logger.debug("Running: %s", " ".join(cmd))
@@ -148,7 +152,6 @@ def start_tunnel(
         __import__("time").sleep(0.5)
 
     if not url:
-        # Capture final state for debugging
         stdout, stderr = _get_container_logs(tunnel_name)
         combined_logs = stdout + "\n" + stderr
         exit_code = _get_container_exit_code(tunnel_name)
@@ -160,10 +163,10 @@ def start_tunnel(
         )
 
     logger.info(
-        "Tunnel %s started for %s on port %d → %s",
+        "Tunnel %s started for %s → %s (%s)",
         tunnel_name,
         instance_name,
-        published_port,
+        target_url,
         url,
     )
     return {"url": url, "container_name": tunnel_name}
@@ -176,10 +179,10 @@ def stop_tunnel(instance_name: str) -> None:
     logger.debug("Stopped and removed tunnel container %s", tunnel_name)
 
 
-def recreate_tunnel(instance_name: str, published_port: int) -> dict[str, str]:
+def recreate_tunnel(instance_name: str, container_port: int) -> dict[str, str]:
     """Recreate a tunnel for an instance."""
     stop_tunnel(instance_name)
-    return start_tunnel(instance_name, published_port)
+    return start_tunnel(instance_name, container_port)
 
 
 def check_tunnel_health(url: str, timeout: int = 10) -> dict[str, Any]: