diff --git a/apps/api/src/api/tool_instances.py b/apps/api/src/api/tool_instances.py
index 0dbf789..ee83ed1 100644
--- a/apps/api/src/api/tool_instances.py
+++ b/apps/api/src/api/tool_instances.py
@@ -45,10 +45,10 @@ from src.services.config_profile_resolver import (
     resolve_profile,
 )
 from src.services.docker import (
-    connect_container_to_network,
     ensure_instance_directory,
     execute_compose_command,
     find_free_port,
+    get_backend_network_name,
     get_container_id,
     get_container_logs,
     get_container_status,
@@ -750,6 +750,49 @@ def _ensure_web_bind_address(
         return
 
 
+def _ensure_backend_network_in_compose(compose_path: str) -> None:
+    """Inject the backend network into the compose file so compose up attaches it.
+
+    Instead of running 'docker network connect' after container creation (which
+    is prone to race conditions and silent failures), we declare the network in
+    the compose file itself. Docker Compose then connects the container to the
+    network atomically during 'docker compose up'.
+    """
+    import yaml
+    from pathlib import Path
+
+    compose_file = Path(compose_path)
+    if not compose_file.exists():
+        return
+
+    content = compose_file.read_text()
+    compose_data = yaml.safe_load(content)
+
+    if not compose_data or "services" not in compose_data:
+        return
+
+    network_name = get_backend_network_name()
+    modified = False
+
+    for svc_config in compose_data["services"].values():
+        existing = svc_config.get("networks", [])
+        if network_name not in existing:
+            svc_config["networks"] = existing + [network_name]
+            modified = True
+        break  # Only modify first service
+
+    # Declare the network as external at the top level
+    if "networks" not in compose_data:
+        compose_data["networks"] = {}
+    if network_name not in compose_data["networks"]:
+        compose_data["networks"][network_name] = {"external": True}
+        modified = True
+
+    if modified:
+        compose_file.write_text(yaml.dump(compose_data, default_flow_style=False))
+        logger.info("Injected backend network '%s' into compose file", network_name)
+
+
 @router.post(
     "/{project_id}/repositories/{repo_id}/instances",
     summary="Create tool instance",
@@ -1690,6 +1733,7 @@ async def start_instance(
 
     # Ensure predictable container name for tunnel connectivity
     _ensure_container_name_in_compose(instance.compose_path, instance.name)
+    _ensure_backend_network_in_compose(instance.compose_path)
 
     # Execute docker compose up with env file
     logger.debug(
@@ -1729,20 +1773,6 @@ async def start_instance(
         "Container name for instance %s: %s", instance.id, expected_container_name
     )
 
-    # Connect container to backend network so API can reach it
-    logger.debug(
-        "Connecting container %s to backend network...", expected_container_name
-    )
-    connected = connect_container_to_network(expected_container_name)
-    if connected:
-        logger.debug(
-            "Successfully connected %s to backend network", expected_container_name
-        )
-    else:
-        logger.warning(
-            "Failed to connect %s to backend network", expected_container_name
-        )
-
     # Verify container reached running state
     if instance.container_id:
         instance.status = "starting"
@@ -2174,6 +2204,7 @@ async def restart_instance(
                 instance.compose_path, tool_type.name, tool_type.default_port
             )
         _ensure_container_name_in_compose(instance.compose_path, instance.name)
+        _ensure_backend_network_in_compose(instance.compose_path)
 
         returncode, stdout, stderr = execute_compose_command(
             instance.compose_path, "restart"